Humanity began to actively use metals as early as 3000-4000 BC. Then people got acquainted with the most common of them, these are gold, silver, copper. These metals were very easy to find on the surface of the earth. A little later, they learned chemistry and began to isolate from them such species as tin, lead and iron. In the Middle Ages, very toxic types of metals gained popularity. Arsenic was in common use, with which more than half of the royal court in France was poisoned. It is the same, which helped to cure various diseases of those times, ranging from tonsillitis to the plague. Already before the twentieth century, more than 60 metals were known, and at the beginning of the XXI century - 90. Progress does not stand still and leads humanity forward. But the question arises, which metal is heavy and surpasses all others in weight? And in general, what are these heaviest metals in the world?
Many mistakenly think that gold and lead are the heaviest metals. Why exactly did it happen? Many of us grew up with old movies and saw how the main character uses a lead plate to protect himself from vicious bullets. In addition, lead plates are still used today in some types of body armor. And at the word gold, many people have a picture with heavy ingots of this metal. But to think that they are the heaviest is wrong!
To determine the heaviest metal, its density must be taken into account, because the greater the density of a substance, the heavier it is.
TOP 10 heaviest metals in the world
- Osmium (22.62 g / cm 3),
- Iridium (22.53 g / cm 3),
- Platinum (21.44 g / cm 3),
- Rhenium (21.01 g / cm 3),
- Neptunium (20.48 g / cm 3),
- Plutonium (19.85 g / cm 3),
- Gold (19.85 g/cm3)
- Tungsten (19.21 g / cm 3),
- Uranium (18.92 g / cm 3),
- Tantalum (16.64 g/cm3).
And where is the lead? And it is located much lower in this list, in the middle of the second ten.
Osmium and iridium are the heaviest metals in the world
Consider the main heavyweights who share 1st and 2nd places. Let's start with iridium and at the same time say thanks to the English scientist Smithson Tennat, who in 1803 obtained this chemical element from platinum, where it was present along with osmium as an impurity. Iridium from ancient Greek can be translated as "rainbow". The metal has a white color with a silver tint and can be called not only heavy, but also the most durable. There is very little of it on our planet and only up to 10,000 kg of it is mined per year. It is known that most deposits of iridium can be found at the sites of meteorite impacts. Some scientists come to the conclusion that this metal was previously widespread on our planet, however, due to its weight, it constantly squeezed itself closer to the center of the Earth. Iridium is now widely in demand in industry and is used to generate electrical energy. Paleontologists also like to use it, and with the help of iridium they determine the age of many finds. In addition, this metal can be used to coat some surfaces. But it's difficult to do so.
Next, consider osmium. It is the heaviest in the periodic table of Mendeleev, well, respectively, and the heaviest metal in the world. Osmium is tin-white with a blue tint and was also discovered by Smithson Tennat at the same time as iridium. Osmium is almost impossible to process and is mainly found at the sites of meteorite impacts. It smells unpleasant, the smell is similar to a mixture of chlorine and garlic. And from ancient Greek it is translated as "smell". The metal is quite refractory and is used in light bulbs and other appliances with refractory metals. For just one gram of this element, you have to pay more than 10,000 dollars, from which it is clear that the metal is very rare.
Osmium Like it or not, the heaviest metals are very rare and therefore they are expensive. And we must remember for the future that neither gold nor lead are the heaviest metals in the world! Iridium and osmium are the winners in weight!
Consisting of atoms of one chemical element. In the periodic table, the metallic properties of the elements increase from right to left. All pure metals (as elements) are simple substances.
Crystalline silicon - semiconductor 
photoelectric effect Distinguish between physical and chemical metal properties. In general, the properties of metals are quite diverse. Distinguish metals alkaline, alkaline earth, black, colored, lanthanides(or rare earth - close in chemical properties to alkaline earth), actinides(most of them are radioactive elements), noble and platinum metals. In addition, individual metals exhibit both metallic and non-metallic properties. Such metals are amphoteric (or, as they say, transitional).
Almost all metals have some common properties: metallic luster, crystal lattice structure, the ability to exhibit the properties of a reducing agent in chemical reactions, while being oxidized. In chemical reactions, ions of dissolved metals, when interacting with acids, form salts; when interacting with water (depending on the activity of the metal), they form an alkali or a base.
Why Do Metals Shine?
The nodes of the crystal lattice of metals contain atoms. Electrons moving around atoms form an "electron gas" that can move freely in different directions. This property explains the high electrical and thermal conductivity of metals.
Electron gas reflects almost all light rays. That is why metals are so shiny and most often have a gray or white color. The bonds between the individual metal layers are small, which makes it possible to move these layers under load in different directions (in other words, to deform the metal). Pure gold is a unique metal. By forging pure gold, you can make a foil with a thickness of 0.002 mm! such a thin sheet of metal is translucent and has a green tint if you look through it into sunlight.
Electrophysical property of metals expressed in terms of its electrical conductivity. It is generally accepted that all metals have a high electrical conductivity, that is, conduct current well! But this is not so, and besides, it all depends on the temperature at which the current is measured. Imagine a crystal lattice of a metal, in which the current is transmitted by the movement of electrons. Electrons move from one node of the crystal lattice to another. One electron "pushes" another electron out of the lattice site, which continues to move towards another lattice site, and so on. That is, electrical conductivity also depends on how easily electrons can move between lattice sites. We can say that the electrical conductivity of the metal depends on the crystal structure of the lattice and the density of particles in it. The particles at the lattice sites have oscillations, and these oscillations are the greater, the higher the temperature of the metal. Such vibrations significantly impede the movement of electrons in the crystal lattice. Thus, the lower the temperature of the metal, the higher its ability to conduct current!
From this comes the concept superconductivity, which occurs in the metal at a temperature close to absolute zero! At absolute zero (-273 0 C), the vibrations of particles in the crystal lattice of the metal are completely damped!
Electrophysical property of metals associated with the passage of current is called temperature coefficient of electrical resistance!
Electrophysical property of metals Electrophysical property of metals
An interesting fact has been established that, for example, in lead (Pb) and mercury (Hg) at a temperature that is only a few degrees above absolute zero, electrical resistance almost completely disappears, that is, the condition of superconductivity sets in.
Silver (Ag) has the highest electrical conductivity, followed by copper (Cu), followed by gold (Au) and aluminum (Al). The high electrical conductivity of these metals is associated with their use in electrical engineering. Sometimes, it is gold (gold-plated contacts) that is used to ensure chemical resistance and anti-corrosion properties.
It should be noted that the electrical conductivity of metals is much higher than the electrical conductivity of non-metals. For example, carbon (C - graphite) or silicon (Si) have an electrical conductivity 1000 times less than, for example, that of mercury. In addition, non-metals, for the most part, are not conductors of electricity. But among non-metals there are semiconductors: germanium (Ge), crystalline silicon, as well as some oxides, phosphites (chemical compounds of metal with phosphorus) and sulfides (chemical compounds of metal and sulfur).
You are probably familiar with the phenomenon - this is the property of metals to give off electrons under the influence of temperature or light.
As for the thermal conductivity of metals, it can be estimated from the periodic table - it is distributed in exactly the same way as the electronegativity of metals. (Metals at the top left have the highest electronegativity, for example, the electronegativity of sodium Na is -2.76 V). In turn, the thermal conductivity of metals is explained by the presence of free electrons, which carry thermal energy.
Edelman V. Metals // Kvant. - 1992. - No. 2. - S. 2-9.
By special agreement with the editorial board and the editors of the journal "Kvant"
What are metals?
“Metal is a light body that can be forged,” Lomonosov wrote in 1763. Take a look at your chemistry textbook and you will see that metals have a characteristic metallic luster (“bright body”) and are good conductors of heat and electricity. True, right there you will read that there are elements that exhibit the properties of both metals and non-metals. In other words, there is no clear line separating one from the other. The chemist, who is primarily interested in chemical reactions and for whom each element is its own special world, is not very embarrassed by such ambiguity. But physics is not satisfied. If physics divides bodies into metals and non-metals, then you need to understand what their fundamental difference is. Therefore, it is necessary to define what a metal is in such a way that, as in other cases in the field of exact sciences, two requirements are met:
- all metals must possess all, without exception, the attributes attributed to them;
- other objects should not have at least one of these features.
Armed with these considerations, let us see whether all metals, without exception, have all the properties attributed to them by the textbook. Let's start with "you can forge", that is, with plasticity, in modern terms. And then, by consonance, we recall plastics: after all, it is not for nothing that they are so named, many of them are characterized by plasticity - the ability to irreversibly change shape without destruction. Of course, copper, iron, aluminum are easy to forge, even easier with lead, indium is a rather rare and expensive metal - it can be crushed almost like wax (and wax is not a metal!), alkali metals are even softer. And try to hit on ordinary cast iron - and it will shatter into pieces! Well, then metallurgists will say: this is because cast iron is not a simple substance. It consists of iron crystals separated by interlayers of carbon, i.e. graphite. It is on these layers that cast iron breaks. Well, that's all right. Only here is the trouble - brittle graphite, as it turns out, modern physics refers to metals! Yes, and more than one graphite: for example, arsenic, antimony and bismuth are listed among the metals, but they can be forged with the same success as glass - they shatter into small pieces!
Do this simple experiment: break the balloon of a burned-out lamp, remove the tungsten coil from there and try to spin it. Nothing will come of it, it will crumble into dust! But somehow they managed to twist it at the factory? This means that it can be something like this - either it can be deformed, or it can’t, depending on what happened to the sample in the past. Well, it is necessary, apparently, to part with this sign - plasticity. Moreover, it is inherent in many non-metals; after all, the same glass - heat it, and it will become soft and pliable.
So, we shorten the wording and move on.
Next in line is “brilliance”, or, in scientific terms, optical properties. There are many shiny objects: water, glass, polished stones, and you never know what else. So just “brilliance” is not enough, so they say: metals are characterized by a metallic luster. Well, this is quite good: it turns out that metal is metal. True, intuitively we feel that polished copper, gold, silver, and iron glisten with a metallic sheen. And the widespread mineral pyrite - doesn't it shine like metals? There is no need to talk about typical semiconductors germanium and silicon, in appearance they cannot be distinguished from metals. On the other hand, not so long ago they learned how to obtain good crystals of such compounds as molybdenum dioxide; these crystals are brown-violet and bear little resemblance to ordinary metal. It turns out that this substance should be considered a metal. Why - it will be clear a little further.
So the shine as a purely "metallic" sign disappears.
Next up is thermal conductivity. Perhaps this sign can be discarded immediately - without exception, all bodies conduct heat. True, it is said about metals that they well conduct heat. But, I'm afraid, to the question "what is good and what is bad?" in this case, no dad will answer.
Does copper conduct heat well? Let's look at the table and immediately run into a counter question: what kind of copper and at what temperature? If you take pure copper, for example, that from which wires for radio devices are made, and heat it to a red heat, that is, anneal it, then at room temperature it, and even pure silver, will conduct heat better than any other metal. But bend such a copper sample, hit it or clamp it in a vise - and its thermal conductivity will become noticeably worse. And what happens if a piece of annealed copper starts to cool? First, the thermal conductivity will increase, increase tenfold at a temperature of about 10 K, and then it will begin to fall rapidly and, upon reaching absolute zero, it should become zero (Fig. 1).
Rice. 1. Dependence of thermal conductivity on temperature for various substances. (Specific thermal conductivity is the amount of heat that flows between opposite faces of a cube with a side of 1 cm at a temperature difference between these faces of 1 K in 1 s.)
Let us now take another metal - bismuth. The picture for him is very similar to that which we saw for copper, only the maximum thermal conductivity lies at 3 K, and at room temperature bismuth conducts heat poorly, not much better than a quartz crystal. But quartz is not a metal! And the same quartz, as can be seen from Figure 1, sometimes turns out to be no worse than copper in terms of its heat-conducting properties. And fused quartz, i.e. quartz glass, conducts heat poorly, like stainless steel.
Quartz is no exception. All good quality crystals behave in a similar way, only the numbers will be slightly different. Diamond, for example, already at room temperature has better thermal conductivity than copper.
We reject thermal conductivity with a pure heart and we will not regret it. And not only because it is not so easy to distinguish a metal from a non-metal on this basis, but also because, it turns out, the specific features in the thermal conductivity of metals (and there are such) are a consequence of its electrical conductivity - the last remaining property.
And again, in the wording given at the beginning of the article, the clarification is not just electrical conductivity, but good electrical conductivity. But when it came to thermal conductivity, the epithet “good” alerted us and, as it turned out, not in vain. What - and the last property under suspicion? It is imperative to save it, otherwise we will be left without metals at all, and at the same time without semiconductors, without insulators. This is how science works! In most cases, any schoolchild will say without hesitation what he is dealing with, but they dug deeper - they stopped in bewilderment.
And there is something from. Let's take tables of physical quantities and look at the numbers. Here, for example, at room temperature, the resistivity ρ (Ohm cm) copper ~1.55 10 -6 ; at bismuth ρ ~ 10 -4 ; graphite ρ ~ 10 -3 ; for pure silicon and germanium ρ ~ 10 2 (but by adding impurities, it can be brought up to ~ 10 -3); at the marble ρ = 10 7 - 10 11; by the glass ρ = 10 10 ; and somewhere at the end of the list - amber with a resistivity of up to 1019. And where do the conductor metals end and the dielectrics begin? And we haven't mentioned electrolytes yet. Ordinary sea water conducts current well. What - and consider it a metal?
Let's see if the temperature helps us. If you increase the temperature, then the differences between the substances will begin to smooth out: for copper, the resistance will begin to increase, for glass, for example, to decrease. So, it is necessary to follow what happens during cooling. And here we finally see qualitative differences. Look at Figure 2: at liquid helium temperatures, near absolute zero, substances split into two groups. For some, the resistance remains small, for alloys or for not very pure metals ρ almost does not change upon cooling, in pure metals the resistance decreases greatly. The purer and more perfect the crystal, the greater this change. Sometimes u at a temperature close to absolute zero is hundreds of thousands of times less than at room temperature. In other substances, such as semiconductors, as the temperature decreases, the resistance begins to increase rapidly, and the lower the temperature, the greater it is. If it were possible to get to absolute zero, then ρ would become infinitely large. However, it is enough that the resistance actually becomes so large that it can no longer be measured by any modern instrument.
So, we got to the answer: metals are substances that conduct electricity at any temperature.
Rice. Fig. 2. Dependence of the resistivity of pure metals (copper and platinum) and a semiconductor (pure germanium) on temperature.
In contrast, dielectrics cease to conduct current when they are cooled to absolute zero. Using this definition, both graphite and molybdenum dioxide are metals. But where to put semiconductors? If a we are talking about pure, perfect crystals, then they are, strictly speaking, dielectrics. But if they contain a lot of impurities, then they can become metals, i.e., retain conductivity at the lowest temperatures.
What do we have left in the end? We managed to identify the only one an essential sign, guided by which we can, if not in everyday practice, then at least in principle, always distinguish a metal from a non-metal. And since this sign is the only one, then both conditions are automatically satisfied, the fulfillment of which we demanded at the beginning of the article.
Why do metals conduct current?
It has long been noted that some elements, such as copper, gold, silver, iron, lead, tin, both in pure form and when fused with each other, form metals. Others, such as phosphorus, sulfur, chlorine, nitrogen, oxygen, not only are not metals themselves, but when combined with metals, they turn them into dielectrics. An example of this is common salt. NaCl . Therefore, in chemistry, the division of elements into metals and non-metals appeared.
Such a classification, however, is nothing more than a statement of facts, although at first glance it claims to explain the properties of substances based only on the structure of atoms. In fact, let's look at the periodic table. Elements located in the same column are very similar in their chemical properties. But will crystals or alloys made from them conduct electric current? Looking at the table, it is impossible to answer this question. So, all elements of the first group are metals, with the exception of the first - hydrogen. But a law that someone is allowed to break is no longer a law. True, things are better in the second group: here all the elements are familiar metals; and in the third group there is again a failure: boron is a semiconductor, and aluminum is a wonderful metal. Further even worse. The first element of the fourth group is carbon; we have already mentioned that graphite, the so-called carbon crystal, is a metal. But diamond is also a crystal composed of carbon atoms, but arranged differently than in graphite - an insulator. Silicon and germanium are classical semiconductors. Tin - it would seem, a typical metal. However... If the familiar white shiny tin is held for a long time at a temperature of -30 ° C, then its crystal structure will change, and outwardly it will turn gray. And this tin - they call it “grey tin” - is a semiconductor! And lead is always a metal.
If you start mixing different elements, then the picture will become completely complicated. Take, for example, and fuse two metals indium and antimony - in a ratio of one to one. We get a semiconductor widely used in technology InSb . On the other hand, we have already said that molybdenum dioxide MoO 2 at T≈ 0 K conducts current, i.e. MoO 2 - metal. (AND WO 2 , and Re 2 O 3 and some other oxides are also metals.) And if the crystals resulting from atoms are strongly compressed, squeezed, then it turns out that almost all substances become metals, even such typical metalloids as sulfur. True, for it the pressure of transition to the metallic state is very high - several hundred thousand atmospheres (and even more for hydrogen).
It seems that separating elements into metals and non-metals is not such an easy task. In any case, it is clear that, considering individual atoms, we cannot say whether a substance composed of these atoms will conduct current at T≈ 0 K, because the way the atoms are located relative to each other plays a huge role. Therefore, to answer the question "why do metals conduct current?" it is necessary to study how atoms interact with each other, forming a solid body.
Let's see how things stand with the simplest of metals - lithium. Serial number Li - three. This means that the nucleus of an atom Li contains three protons and the positive charge of the nucleus compensates for three electrons. Two of them form a filled s-shell closest to the nucleus and are strongly bound to the nucleus. The remaining electron is located on the second s-shell. It could fit one more electron, but lithium does not have it. All other allowed states of energy are free, and electrons enter them only when the atom is excited (for example, when lithium vapor is strongly heated). The scheme of levels in the lithium atom is shown in Figure 3.
Rice. 3. Scheme of the energy levels of the lithium atom and their transformation into zones when atoms combine into a crystal. Busy states are marked in red.
Consider now the set of lithium atoms located in a limited volume. They can form a gas (steam), liquid or solid. At a sufficiently low temperature, the forces of mutual attraction prevent the thermal motion of atoms, and a crystal is formed. This certainly occurs at absolute zero temperature, when all known substances, except for helium, are crystals.
So, it is known from experience that at low temperatures a solid is a stable state for lithium. But, as is known, such a state of matter is always stable, in which its internal energy is less than in other possible states of aggregation at the same temperature. The total decrease in energy during the transition from one state to another is easy to measure - after all, this is the heat of evaporation or melting.
From a microscopic point of view, at low temperatures, the internal energy of a substance is, first of all, the sum of the energies of the electrons of the atoms that make up the body. But electrons in atoms occupy strictly defined energy levels. This means that we can expect that when the atoms approach each other, the energy levels will change. In this case, the distribution of electrons over levels should turn out to be such that their total energy is less than the sum of the energies of electrons in the same number of atoms isolated from each other.
What will happen to the levels can be understood based on the analogy of the movement of an electron in an atom with any oscillatory system, for example, with a pendulum. Suppose we have two completely identical pendulums. As long as they do not interact with each other, the oscillation frequency of both pendulums is the same. Let us now introduce the interaction between them - we will connect them, for example, with a soft spring. And immediately, instead of one frequency, two will appear. Look at Figure 4: coupled pendulums can oscillate in phase, or they can oscillate towards each other. Obviously, in the latter case, their movement will be faster, i.e., the frequency of oscillations of such a system is higher than the natural frequency of oscillations of one pendulum. Thus, coupling leads to frequency splitting. If you connect three pendulums, then there will already be three natural frequencies, a system of four connected pendulums has four natural frequencies, and so on ad infinitum.
Rice. 4. Oscillations of coupled pendulums.
The behavior of any other oscillatory system is similar. If we replace pendulums, for example, with electric oscillatory circuits, then, as radio amateurs are well aware, when a connection is introduced between them, their natural frequencies are also split. Electrons in an atom are also a kind of oscillatory system. Like a pendulum, electrons have mass, there is a Coulomb force that returns them to their equilibrium position; and this determines the motion of the electrons in the atom, which, according to quantum mechanics, is characterized by its own frequency. For electrons, the inclusion of interaction during mutual approach leads to the fact that the frequencies that were previously the same become slightly different.
In quantum mechanics, there is a direct relationship between energy and oscillation frequency, expressed by the formula \(~E = h \nu\), where h\u003d 6.6 10 -34 J s - Planck's constant, and ν - oscillation frequency. Therefore, it should be expected that when two lithium atoms approach each of the levels shown in Figure 3, it will split into two. Each new level of energy will correspond to its own electron shell, now not of a single atom, but of a “molecule”. Shells are filled with electrons according to the same rule as for an atom - two electrons per shell. That pair of shells, which turned out from the lowest level, will be completely filled with electrons. Indeed, four electrons can be placed on them, and two lithium atoms have six of them. Two electrons remain, which will now be located on the lower level of the second pair. Notice the qualitative leap that has taken place: previously, these two electrons occupied two of the four states that had the same energy. Now they have the opportunity to choose, and they positioned themselves so that their total energy was smaller. It is not hard to imagine what will happen when the following atoms are added: for three atoms, each initial level will split into three (see Fig. 3). Nine electrons will be located as follows: six on the lower triad of levels that have arisen from the level of the inner filled shell of the atom closest to the nucleus; two more electrons - at the lower level of the next triad; the remaining electron is at the middle level of the same triad. One more place on this level remains free, and the upper level is completely empty. If you take n atoms (\(~n \gg 1\)), then each level splits into n closely spaced levels forming, as they say, a band or zone of allowed energy values. In the lower band, all states are occupied, and in the second - only half, and precisely those whose energy is lower. The next lane is completely empty.
The distance between adjacent levels in the zone is easy to estimate. It is natural to assume that when the atoms approach each other, the change in the energy of the electrons of the atom is approximately equal to the heat of evaporation of the substance, recalculated per one atom. It is usually several electronvolts for metals, and hence the total band width Δ E, determined by the interaction of neighboring atoms, must have the same scale, i.e., Δ E~ 1 eV ≈ 10 -19 J. For the distance between levels we get \(~\delta E \sim \dfrac(\Delta E)(n)\), where n is the number of atoms in the sample. This number is extremely large: the interatomic distance is only a few angstroms, and the volume per atom is only ~ 10 -22 cm 3 . If our sample has, for definiteness, a volume of 1 cm 3, then for it n≈ 10 22 . Therefore, numerically it turns out δ E≈ 10 -22 Δ E≈ 10 -41 J. This value is so small that one can always neglect the energy quantization within the zone and assume that any energy values are allowed within the zone.
So, in a crystal, the energy levels are smeared into zones having a width comparable to the distance between them. Allowed for electrons are states inside the band, and here the electrons can have almost any energy (of course, within the band width). But it is very important that the number of places in each zone is strictly limited and equal to twice the number of atoms that make up the crystal. And this circumstance, together with the principle of minimum energy, determines the distribution of electrons over the zones. Now we are all set to finally understand why lithium conducts current. Let's look again at Figure 3. What happened? While the atoms were on their own, all the electrons were in well-defined states, strictly the same for all atoms. Now the atoms have combined into a crystal. The atoms themselves in a crystal are not only the same, but also exactly the same located relative to their neighbors (with the exception, of course, of those that hit the surface of the crystal). And all the electrons now have different energies. This can only be the case if the electrons no longer belong to individual atoms, but each electron has been "divided" among all the atoms. In other words, electrons move freely inside an ideal crystal, forming, as it were, a liquid that fills the entire volume of the sample. And electric current is a directed flow of this liquid, similar to water flowing through pipes.
To force water to flow through a pipe, a pressure difference must be created at the ends of the pipe. Then, under the action of external forces, the molecules will acquire a directed velocity - water will flow. The appearance of a directed velocity is very important here, because the molecules themselves move chaotically at tremendous speeds - at room temperature, the average velocity of the thermal motion of a molecule is about 10 3 m/s. So the additional energy acquired by the molecule in the flow is small compared to the energy of thermal motion.
The additional energy that must be imparted to an electron so that it participates in the general directed motion of electrons in a crystal (and this is the current) is also small compared to the self-energy of the electron. This is easy to verify. We have already said that the energy of an electron is equal in order of magnitude to 1 eV = 1.6 10 -19 J. If we recall that for a free electron \(~E = \dfrac(m \upsilon^2)(2)\) and m\u003d 9.1 10 -31 kg, then it is easy to find the speed: υ ~ 10 6 m/s. Suppose that all electrons participate in the current, and they are in 1 m 3 of the conductor n ~ 10 28 Z (Z is the nuclear charge). Then in a wire with a cross section S\u003d 10 -6 m 2 at current I≈ 10 A (at a higher current the wire will melt) the directional velocity of the electrons is \(~\upsilon_H = \dfrac(I)(neS) \approx 10^(-2) - 10^(-3)\) m/s. This means that the energy of the electron participating in the current is greater than the energy E free electron by only 10 -8 E, i.e. by 1.6 10 -27 J.
And here we are faced with a surprising fact: it turns out that electrons that are located in the lower band, usually called the valence band, cannot change their energy by a small amount. After all, if some electron increases its energy, this means that it must move to another level, and all neighboring levels in the valence band are already occupied. There are only vacancies in the next zone. But to get there, the electron must change its energy by several electron volts at once. This is how electrons sit in the valence band and wait for the pie in the sky - an energetic quantum. And the quanta of the required energy are in visible or ultraviolet light.
So, there is liquid, but it cannot flow. And if lithium had only two electrons in an atom, that is, if we built a picture for lithium atoms, then we would get an insulator. But solid helium is indeed an insulator, so we can already congratulate ourselves on some success: we have not yet explained why current can flow in metals, but we understand why dielectrics, where there are a lot of electrons and they are all “smeared” throughout the crystal , do not conduct current.
But what about lithium? Why, he has a second zone, which is only half filled. The energy separating the occupied and free levels within this band is called the Fermi energy E f. As we have already said, the energy difference between the levels in the band is very small. It is enough for an electron located in the zone near the Fermi level to slightly increase its energy - and it is free, where the states are not occupied. Nothing prevents the electrons from the boundary strip from increasing their energy under the action of an electric field and acquiring a directed velocity. But this is the current! But it is just as easy for these electrons to lose their directional velocity when they collide with impurity atoms (which are always there) or with other violations of the ideal crystal structure. This explains the current resistance.
It seems clear why helium is an insulator and lithium is a conductor. Let's try to apply our ideas to the next element - beryllium. And here - a misfire, the model did not work. Beryllium has four electrons, and it would seem that the first and second zones should be completely occupied, and the third should be empty. It turns out an insulator, while beryllium is a metal.
The point is this. If the width of the zones is large enough, then they can overlap each other. About such a phenomenon they say that the zones overlap. This is exactly what happens in beryllium: the minimum energy of electrons in the third zone is less than the maximum energy in the second. Therefore, it turns out to be energetically favorable for electrons to leave the empty part of the second band and occupy states at the bottom of the third. This is where the metal comes in.
What will happen to the other elements? Whether the zones overlap or not, it is impossible to say in advance, this requires cumbersome computer calculations, and it is not always possible to obtain a reliable answer. But here is what is remarkable: from our scheme it follows that if we take elements with an odd number of electrons, then a metal should always be obtained, if only a single atom is a structural unit in a crystal. But hydrogen, for example, nitrogen and fluorine, do not want to crystallize into such a lattice. They prefer to unite first in pairs, and already the molecules containing an even number of electrons line up in a crystal. And the laws of quantum mechanics do not prevent him from being a dielectric.
So, we now know what a metal is from the point of view of physics, and figured out the very essence of the phenomenon, understanding why insulators and conductors exist in principle. We have seen that there is no easy way to explain why a particular substance is an insulator or a metal. This can be done only by armed with all the power of the apparatus of modern quantum mechanics and computer technology, but this is already the task of specialists.
You know that most of the chemical elements are classified as metals - 92 of the 114 known elements.
Metals - these are chemical elements, the atoms of which donate electrons of the outer (and some of the pre-outer) electron layer, turning into positive ions.
This property of metal atoms, as you know, is determined by the fact that they have relatively large radii and a small number of electrons (mainly from 1 to 3) on the outer layer.
The only exceptions are 6 metals: atoms of germanium, tin, lead on the outer layer have 4 electrons, atoms of antimony, bismuth -5, polonium atoms - 6.
Metal atoms are characterized by low electronegativity values (from 0.7 to 1.9) and exclusively reducing properties, that is, the ability to donate electrons.
You already know that in the Periodic Table of Chemical Elements of D. I. Mendeleev, metals are below the boron-astatine diagonal, I am also above it in side subgroups. In the periods and clay subgroups, there are regularities known to you in changing the metallic, and hence the reducing properties of the atoms of the elements.
Chemical elements located near the boron-astat diagonal have dual properties: in some of their compounds they behave like metals, in others they exhibit the properties of a non-metal.
In secondary subgroups, the reducing properties of metals most often decrease with increasing serial number. Compare the activity of the group I metals of the side subgroup known to you: Cu, Ag, Au; II group of a secondary subgroup - and you will see for yourself.
Simple substances formed by chemical elements - metals, and complex metal-containing substances play an important role in the mineral and organic "life" of the Earth. Suffice it to recall that the atoms (nones) of metal elements are an integral part of the compounds that determine the metabolism in the body of humans, animals, and plants.
For example, sodium ions regulate the water content in the body, the transmission of nerve impulses. Its deficiency leads to headache, weakness, poor memory, loss of appetite, and its excess leads to increased blood pressure, hypertension, and heart disease. Nutrition experts recommend consuming no more than 5 g (1 teaspoon) of table salt (NaCl) per adult per day. The influence of metals on the condition of animals and plants can be found in Table 16.
Simple substances - metals
With the development of the production of metals (simple substances) and alloys, the emergence of civilization (“Bronze Age”, Iron Age) was connected.
Figure 38 shows a diagram of the crystal lattice of sodium metal. In it, each sodium atom is surrounded by eight neighboring ones. Sodium atoms, like all metals, have many free valence orbitals and few valence electrons.
The only valence electron of the sodium atom Zs 1 can occupy any of the nine free orbitals, because they do not differ much in energy level. When the atoms approach each other, when a crystal lattice is formed, the valence orbitals of neighboring atoms overlap, due to which the electrons move freely from one orbital to another, making a connection between all the atoms of the metal crystal.
This type of chemical bond is called a metallic bond. A metallic bond is formed by elements whose atoms on the outer layer have few valence electrons compared to a large number of outer energetically close orbitals. Their valence electrons are weakly held in the atom. The electrons that carry out the connection are socialized and move throughout the crystal lattice of the neutral metal as a whole.
Substances with a metallic bond are characterized by metallic crystal lattices, which are usually depicted schematically as a tick, as shown in the figure, the nodes are cations and metal atoms. Shared electrons electrostatically attract metal cations located at the nodes of their crystal lattice, ensuring its stability and strength (shared electrons are depicted as small black balls).
A metallic bond is a bond in metals and alloys between metal atom-ions located at the nodes of the crystal lattice, which is carried out by socialized valence electrons.
Some metals crystallize in two or more crystalline forms. This property of substances - to exist in several crystalline modifications - is called polymorphism. Polymorphism for simple substances is known to you as allotropy.
Tin has two crystalline modifications:
. alpha - stable below 13.2 ºС with density р - 5.74 g/cm3. This is gray tin. It has a crystal lattice like diamond (atomic):
. betta - stable above 13.2 ºС with a density p - 6.55 g/cm3. This is white tin.
White tin is a very soft metal. When cooled below 13.2 ºС, it crumbles into a gray powder, since at the transition | 1 » n its specific volume increases significantly. This phenomenon is called the tin plague. Of course, a special type of chemical bond and the type of crystal lattice of metals should determine and explain them. physical properties.
What are they? These are metallic luster, plasticity, high electrical conductivity and thermal conductivity, an increase in electrical resistance with increasing temperature, as well as such practically significant properties as density, melting and boiling points, hardness, and magnetic properties.
Let's try to explain the reasons that determine the basic physical properties of metals. Why are metals plastic?
Mechanical action on a crystal with a metal crystal lattice causes the layers of ion-atoms to shift relative to each other, since electrons move throughout the crystal, bonds are not broken, therefore, metals are characterized by greater plasticity.
A similar effect on a solid substance with covalent bonds (atomic crystal lattice) leads to the breaking of covalent bonds. Breaking bonds in the ionic lattice leads to mutual repulsion of like-charged ions (Fig. 40). Therefore, substances with atomic and ionic crystal lattices are fragile.
The most plastic metals are Au, Af, Cu, Sn, Pb, Zn. They are easily drawn into wire, amenable to forging, pressing, rolling into sheets. For example, gold foil 0.008 nm thick can be made from gold, and a thread 1 km long can be drawn from 0.5 g of this metal.
Even mercury, which, as you know, is liquid at room temperature, becomes malleable like lead at low temperatures in the solid state. Only Bi and Mn do not have plasticity, they are brittle.
Why do metals have a characteristic luster and are also opaque?
Electrons filling the interatomic space reflect light rays (and do not transmit, like glass), and most metals equally scatter all the rays of the visible part of the spectrum. Therefore, they have a silvery white or gray color. Strontium, gold and copper absorb short wavelengths (close to violet) to a greater extent and reflect long wavelengths of the light spectrum, therefore they have light yellow, yellow and copper colors, respectively.
Although in practice, you know, metal does not always seem to us to be a light body. First, its surface can oxidize and lose its luster. Therefore, native copper looks like a greenish stone. And secondly, even pure metal may not shine. Very thin sheets of silver and gold have a completely unexpected appearance - they have a bluish-green color. And fine metal powders appear dark gray, even black.
Silver, aluminum, palladium have the highest reflectivity. They are used in the manufacture of mirrors, including spotlights.
Why do metals have high electrical conductivity and thermal conductivity?
Chaotically moving electrons in a metal under the influence of an applied electrical voltage acquire a directed motion, that is, they conduct an electric current. With an increase in the meta-aphid temperature, the vibration amplitudes of the atoms and ions located at the nodes of the crystal lattice increase. This makes it difficult for electrons to move, and the electrical conductivity of the metal decreases. At low temperatures, the oscillatory motion, on the contrary, greatly decreases and the electrical conductivity of metals increases sharply. Near absolute zero, there is practically no resistance in metals, and superconductivity appears in most metals.
It should be noted that non-metals with electrical conductivity (for example, graphite), at low temperatures, on the contrary, do not conduct electric current due to the absence of free electrons. And only with an increase in temperature and the destruction of some covalent bonds, their electrical conductivity begins to increase.
Silver, copper, as well as gold, aluminum have the highest electrical conductivity, manganese, lead, and mercury have the lowest.
Most often, with the same regularity as the electrical conductivity, the thermal conductivity of metals changes.
They are due to the high mobility of free electrons, which, colliding with vibrating ions and atoms, exchange energy with them. Therefore, there is an equalization of temperature throughout the piece of metal.
The mechanical strength, density, melting point of metals are very different. Moreover, with an increase in the number of electrons that bind ions-atoms, and a decrease in the interatomic distance in crystals, the indicators of these properties increase.
So, alkali metals, whose atoms have one valence electron, are soft (cut with a knife), with low density (lithium is the lightest metal with p - 0.53 g / cm3) and melt at low temperatures (for example, the melting point of cesium is 29 "C) The only metal that is liquid under normal conditions - mercury - has a melting point of 38.9 "C.
Calcium, which has two electrons in the outer energy level of atoms, is much harder and melts at a higher temperature (842º C).
Even more arched is the crystal lattice formed by scandium atoms, which have three valence electrons.
But the strongest crystal lattices, high densities and melting points are observed in metals of secondary subgroups of groups V, VI, VII, VIII. This is explained by. that for metals of side subgroups having unsaved valence electrons at the d-sublevel, the formation of very strong covalent bonds between atoms is characteristic, in addition to the metallic one, carried out by electrons of the outer layer from s-orbitals.
Remember that the heaviest metal is osmium (a component of superhard and wear-resistant alloys), the most refractory metal is tungsten (used to make lamp filaments), the hardest metal is chromium Cr (scratches glass). They are part of the materials from which metal-cutting tools, brake pads of heavy machines, etc. are made.
Metals differ with respect to magnetic fields. But this sign they are divided into three groups:
. ferromagnetic Able to be magnetized under the influence of even weak magnetic fields (iron - alpha form, cobalt, nickel, gadolinium);
Paramagnetic exhibit a weak ability to magnetize (aluminum, chromium, titanium, almost all lanthanides);
Diamagnetic are not attracted to the magnet, even slightly repelled from it (tin, stranded, bismuth).
Recall that when considering the electronic structure of metals, we subdivided metals into metals of the main subgroups (k- and p-elements) and metals of secondary subgroups.
In engineering, it is customary to classify metals according to various physical properties:
a) density - light (p< 5 г/см3) и тяжелые (все остальные);
b) melting point - fusible and refractory.
There are classifications of metals according to chemical properties.
Metals with low chemical activity are called noble (silver, gold, platinum and its analogues - osmium, iridium, ruthenium, palladium, rhodium).
According to the proximity of chemical properties, alkali (group I metals of the main subgroup), alkaline earth (calcium, strontium, barium, radium), as well as rare earth metals (scandium, yttrium, lanthanum and lanthanides, actinium and actinides) are distinguished.
General chemical properties of metals
Metal atoms give up valence electrons relatively easily and pass into positively charged nons, that is, they are oxidized. This, as you know, is the main common property of both atoms and simple metal substances.
Metals in chemical reactions are always a reducing agent. The reducing ability of atoms of simple substances - metals, formed by chemical elements of one period or one main subgroup of the Periodic system of D. I. Mendeleev, changes naturally.
The reducing activity of a metal in chemical reactions that occur in aqueous solutions reflects its position in the electrochemical series of metal voltages.
1. The further to the left the metal is in this row, the stronger the reducing agent it is.
2. Each metal is able to displace (restore) from salts in solution those metals that are after it (to the right) in a series of voltages.
3. Metals that are in the series of voltages to the left of hydrogen are able to displace it from acids in solution.
4. Metals, which are the strongest reducing agents (alkaline and alkaline earth), in any aqueous solutions interact primarily with water.
The reducing activity of a metal, determined from the electrochemical series, does not always correspond to its position in the Periodic Table. This is explained by. That when determining the position of a metal in a series of voltages, not only the energy of detachment of electrons from individual atoms is taken into account, but also the energy expended on the destruction of the crystal lattice, as well as the energy released during the hydration of ions.
Having considered the general provisions characterizing the reducing properties of metals, we turn to specific chemical reactions.
Interaction with simple non-metal substances
1. With oxygen, most metals form oxides - basic and amphoteric.
Lithium and alkaline earth metals react with atmospheric oxygen to form basic oxides.
2. With halogens, metals form salts of hydrohalic acids.
3. With hydrogen, the most active metals form hydrides - ionic salts, one common substance in which hydrogen has an oxidation state of -1, for example: calcium hydride.
4. Metals form salts with sulfur - sulfides.
5. Metals react with nitrogen somewhat more difficult, since the chemical bond in the nitrogen molecule Г^r is very strong, and nitrides are formed. At ordinary temperatures, only lithium interacts with nitrogen.
Interaction with complex substances
1. With water. Alkali and alkaline earth metals under normal conditions displace hydrogen from water and form soluble alkali bases.
Other metals, standing in a series of voltages up to hydrogen, can also, under certain conditions, displace hydrogen from water. But aluminum reacts violently with water only if the oxide film is removed from its surface.
Magnesium interacts with water only when boiling, and hydrogen is also released. If burning magnesium is added to water, then combustion continues, as the reaction proceeds: hydrogen burns. Iron interacts with water only when heated.
2. Metals that are in the series of voltages up to hydrogen interact with acids in solution. This produces salt and hydrogen. But lead (and some other metals), despite its position in the voltage series (to the left of hydrogen), almost does not dissolve in dilute sulfuric acid, since the resulting lead sulfate PbSO is insoluble and creates a protective film on the metal surface.
3. With salts of less active metals in solution. As a result of such a reaction, a salt of a more active metal is formed and a less active metal is released in a free form.
4. With organic substances. Interaction with organic acids is similar to reactions with mineral acids. Alcohols, on the other hand, can exhibit weak acidic properties when interacting with alkali metals.
Metals participate in reactions with haloalkanes, which are used to obtain lower cycloalkanes and for syntheses, during which the carbon skeleton of the molecule becomes more complex (A. Wurtz reaction):
5. Metals whose hydroxides are amphoteric interact with alkalis in solution.
6. Metals can form chemical compounds with each other, which are collectively called intermetallic compounds. They most often do not show the oxidation states of atoms, which are characteristic of compounds of metals with non-metals.
Intermetallic compounds usually do not have a constant composition, the chemical bond in them is mainly metallic. The formation of these compounds is more typical for metals of secondary subgroups.
Metal oxides and hydroxides
Oxides formed by typical metals are classified as salt-forming, basic in nature of properties.
The oxides and hydroxides of some metals are amphoteric, that is, they can exhibit both basic and acidic properties, depending on the substances with which they interact.
For example:
Many metals of secondary subgroups, which have a variable oxidation state in compounds, can form several oxides and hydroxides, the nature of which depends on the oxidation state of the metal.
For example, chromium in compounds exhibits three oxidation states: +2, +3, +6, therefore it forms three series of oxides and hydroxides, and with an increase in the oxidation state, the acid character increases and the basic character weakens.
Corrosion of metals
When metals interact with environmental substances, compounds are formed on their surface that have completely different properties than the metals themselves. In a normal vein, we often use the words "rust", "rusting", seeing a brown-red coating on products made of iron and its alloys. Rusting is a common form of corrosion.
Corrosion- this is the process of spontaneous destruction of metals and alloys under the influence of the external environment (from lat. - corrosion).
However, almost all metals undergo destruction, as a result of which many of their properties deteriorate (or are completely lost): strength, ductility, gloss decrease, electrical conductivity decreases, and friction between moving machine parts increases, the dimensions of parts change, etc.
Corrosion of metals can be continuous and local.
The most common types of corrosion are chemical and electrochemical.
I. Chemical corrosion occurs in a non-conductive environment. This type of corrosion manifests itself in the case of the interaction of metals with dry gases or liquids - non-electrolytes (gasoline, kerosene, etc.). Parts and components of engines, gas turbines, rocket launchers are subjected to such destruction. Chemical corrosion is often observed during the processing of metals at high temperatures.
Most metals are oxidized by atmospheric oxygen, forming oxide films on the surface. If this film is strong, dense, well bonded to the metal, then it protects the metal from further destruction. In iron, it is loose, porous, easily separated from the surface and therefore is not able to protect the metal from further destruction.
II. Electrochemical corrosion occurs in a conductive medium (electrolyte) with the appearance of an electric current inside the system. As a rule, metals and alloys are heterogeneous and contain inclusions of various impurities. When they come into contact with electrolytes, some parts of the surface begin to play the role of an anode (donate electrons), while others play the role of a cathode (accept electrons).
In one case, gas evolution (Hg) will be observed. In the other - the formation of rust.
So, electrochemical corrosion is a reaction that occurs in media that conduct current (in contrast to chemical corrosion). The process occurs when two metals come into contact or on the surface of a metal containing inclusions that are less active conductors (it may also be a non-metal).
At the anode (a more active metal), metal atoms are oxidized to form cations (dissolution).
At the cathode (a less active conductor), hydrogen ions or oxygen molecules are reduced with the formation of H2 or OH- hydroxide ions, respectively.
Hydrogen cations and dissolved oxygen are the most important oxidizing agents that cause electrochemical corrosion.
The corrosion rate is the greater, the more the metals (metal and impurities) differ in their activity (for metals, the farther apart they are located in a series of voltages). Corrosion increases significantly with increasing temperature.
The electrolyte can be sea water, river water, condensed moisture and, of course, well-known electrolytes - solutions of salts, acids, alkalis.
You obviously remember that in winter, technical salt (sodium chloride, sometimes calcium chloride, etc.) is used to remove snow and ice from sidewalks. The resulting solutions drain into sewer pipelines, thereby creating a favorable environment for electrochemical corrosion of underground utilities.
Corrosion protection methods
Already in the design of metal structures, their manufacture provides for measures to protect against corrosion.
1. Sanding the surfaces of the product so that moisture does not linger on them.
2. The use of alloyed alloys containing special additives: chromium, nickel, which at high temperatures form a stable oxide layer on the metal surface. Alloy steels are well-known - stainless steels, from which household items (sheathed forks, spoons), machine parts, and tools are made.
3. Application of protective coatings. Consider their types.
Non-metallic - non-oxidizing oils, special varnishes, paints. True, they are short-lived, but they are cheap.
Chemical - artificially created surface films: oxide, citric, silicide, polymer, etc. For example, all small arms The details of many precision instruments are burnished - this is the process of obtaining the thinnest film of iron oxides on the surface of a steel product. The resulting artificial oxide film is very durable and gives the product a beautiful black color and blue tint. Polymer coatings are made from polyethylene, polyvinyl chloride, polyamide resins. They are applied in two ways: a heated product is placed in a polymer powder, which melts and welds to the metal, or the metal surface is treated with a polymer solution in a low-temperature solvent, which quickly evaporates, and the polymer film remains on the product.
Metallic coatings are coatings with other metals, on the surface of which stable protective films are formed under the action of oxidizing agents.
The application of chromium to the surface - chromium plating, nickel - nickel plating, zinc - zinc plating, tin - tinning, etc. The coating can also serve as a chemically passive metal - gold, silver, copper.
4. Electrochemical methods of protection.
Protective (anodic) - a piece of more active metal (protector) is attached to the protected metal structure, which serves as an anode and is destroyed in the presence of an electrolyte. Magnesium, aluminum, zinc are used as a protector when protecting ship hulls, pipelines, cables and other stylish products;
Cathode - the metal structure is connected to the cathode of an external current source, which eliminates the possibility of its anode destruction
5. Special treatment of the electrolyte or the environment in which the protected metal structure is located.
It is known that Damascus craftsmen for descaling and
rust used solutions of sulfuric acid with the addition of brewer's yeast, flour, starch. These bring and were among the first inhibitors. They did not allow the acid to act on the weapon metal, as a result, only scale and rust were dissolved. Ural gunsmiths used pickling soups for this purpose - solutions of sulfuric acid with the addition of flour bran.
Examples of the use of modern inhibitors: during transportation and storage, hydrochloric acid is perfectly "tamed" by butylamine derivatives. and sulfuric acid - nitric acid; volatile diethylamine is injected into various containers. Note that inhibitors act only on the metal, making it passive with respect to the medium, for example, to an acid solution. More than 5 thousand corrosion inhibitors are known to science.
Removal of oxygen dissolved in water (deaeration). This process is used in the preparation of water entering boiler plants.
Methods for obtaining metals
Significant chemical activity of metals (interaction with atmospheric oxygen, other non-metals, water, salt solutions, acids) leads to the fact that they are found in the earth's crust mainly in the form of compounds: oxides, sulfides, sulfates, chlorides, carbonates, etc.
In free form, there are metals located in the series of voltages to the right of hydrogen, although much more often copper and mercury can be found in nature in the form of compounds.
Minerals and rocks containing metals and their compounds, from which the extraction of pure metals is technically possible and economically feasible, are called ores.
Obtaining metals from ores is the task of metallurgy.
Metallurgy is also the science of industrial methods for obtaining metals from ores. and industry sector.
Any metallurgical process is a process of reduction of metal ions with the help of various reducing agents.
To implement this process, it is necessary to take into account the activity of the metal, select a reducing agent, consider technological feasibility, economic and environmental factors. In accordance with this, there are the following methods for obtaining metals: pyrometallurgical. hydrometallurgical, electrometallurgical.
Pyrometallurgy- recovery of metals from ores at high temperatures using carbon, carbon monoxide (II). hydrogen, metals. - aluminum, magnesium.
For example, tin is reduced from cassiterite, and copper from cuprite by calcination with coal (coke). Sulfide ores are preliminarily roasted with air access, and then the resulting oxide is reduced with coal. Metals are also isolated from carbonate ores by pumping a with coal, since carbonates decompose when heated, turning into oxides, and the latter are reduced by coal.
Hydrometallurgy is the reduction of metals to them by their salts in solution. The process takes place in 2 stages: 1) a natural compound is dissolved in a suitable reagent to obtain a solution of a salt of this metal; 2) from the resulting solution, this metal is displaced by a more active one or restored by electrolysis. For example, to obtain copper from ores containing copper oxide, CuO, it is treated with dilute sulfuric acid.
Copper is extracted from the salt solution either by electrolysis or displaced from sulfate with iron. Silver, zinc, molybdenum, gold, uranium are obtained in this way.
Electrometallurgy— recovery of metals in the process of electrolysis of solutions or melts of their compounds.
Electrolysis
If the electrodes are lowered into the electrolyte solution or melt and a constant electric current is passed through, then the ions will move in a direction: cations - to the cathode (negatively charged electrode), anions - to the anode (positively charged electrode).
At the cathode, cations accept electrons and are reduced at the anode, anions donate electrons and are oxidized. This process is called electrolysis.
Electrolysis is a redox process that occurs on electrodes when an electric current passes through an electrolyte solution or solution.
The simplest example of such processes is the electrolysis of molten salts. Consider the process of electrolysis of a sodium chloride melt. The process of thermal dissociation takes place in the melt. Under the action of an electric current, cations move towards the cathode and receive electrons from it.
Sodium metal is formed at the cathode, and chlorine gas is formed at the anode.
The main thing to remember is that in the process of electrolysis, a chemical reaction is carried out due to electrical energy, which cannot go on spontaneously.
The situation is more complicated in the case of electrolysis of electrolyte solutions.
In a salt solution, in addition to metal ions and an acidic residue, there are water molecules. Therefore, when considering processes on electrodes, it is necessary to take into account their participation in electrolysis.
The following rules exist for determining the electrolysis products of aqueous solutions of electrolytes.
1. The process on the cathode does not depend on the material of the cathode on which it is made, but on the position of the metal (electrolyte cation) in the electrochemical series of voltages, and if:
1.1. The electrolyte cation is located in the series of voltages at the beginning of the series (along with Al inclusive), then the process of water reduction is going on at the cathode (hydrogen is released). Metal cations are not reduced, they remain in solution.
1.2. The electrolyte cation is in a series of voltages between aluminum and hydrogen, then both metal nones and water molecules are reduced at the cathode.
1.3. The electrolyte cation is in a series of voltages after hydrogen, then metal cations are reduced at the cathode.
1.4. The solution contains cations of different metals, then the downloaded metal cation is restored, standing in a series of voltages
These rules are shown in Figure 10.
2. The process at the anode depends on the material of the anode and on the nature of the anode (Scheme 11).
2.1. If the anode is dissolved (iron, zinc, copper, silver and all metals that are oxidized during electrolysis), then the anode metal is oxidized, regardless of the nature of the anion. 2. If the anode does not dissolve (it is called inert - graphite, gold, platinum), then:
a) during the electrolysis of solutions of salts of anoxic acids (prome fluorides), the anion is oxidized at the anode;
b) during the electrolysis of solutions of salts of oxygen-containing acid and fluorides at the anode, the process of water oxidation occurs. Anions are not oxidized, they remain in solution;
Electrolysis of melts and solutions of substances is widely used in industry:
1. To obtain metals (aluminum, magnesium, sodium, cadmium are obtained only by electrolysis).
2. To obtain hydrogen, halogens, alkalis.
3. For the purification of metals - refining (purification of copper, nickel, lead is carried out by the electrochemical method).
4. To protect metals from corrosion - applying protective coatings in the form of a thin layer of another metal that is resistant to corrosion (chromium, nickel, copper, silver, gold) - electroplating.
5. Obtaining metal copies, records - electroplating.
1. How are the structure of metals related to their location in the main and secondary subgroups of the Periodic Table of Chemical Elements of D. I. Mendeleev?
2. Why do alkali and alkaline earth metals have a single oxidation state in compounds: (+1) and (+2), respectively, while metals of secondary subgroups, as a rule, exhibit different oxidation states in compounds? 8. What oxidation states can manganese exhibit? What oxides and hydroxides correspond to manganese in these oxidation states? What is their character?
4. Compare the electronic structure of the atoms of the elements of group VII: manganese and chlorine. Explain the difference in their chemical properties and the presence of different degrees of oxidation of atoms in both elements.
5. Why does the position of metals in the electrochemical series of voltages not always correspond to their position in the Periodic system of D. I. Mendeleev?
9. Make equations for the reactions of sodium and magnesium with acetic acid. In which case and why will the reaction rate be faster?
11. What methods of obtaining metals do you know? What is the essence of all methods?
14. What is corrosion? What types of corrosion do you know? Which one is a physical and chemical process?
15. Can the following processes be considered corrosion: a) oxidation of iron during electric welding, b) interaction of zinc with hydrochloric acid in obtaining etched acid for soldering? Give a reasoned answer.
17. The manganese product is in water and does not come into contact with the copper product. Will both remain unchanged?
18. Will an iron structure be protected from electrochemical corrosion in water if a plate of another metal is strengthened on it: a) magnesium, b) lead, c) nickel?
19. For what purpose is the surface of tanks for storing petroleum products (gasoline, kerosene) painted with silver - a mixture of aluminum powder with one of the vegetable oils?
General information about metals
You know that most of the chemical elements are classified as metals - 92 of the 114 known elements.
Metals are chemical elements whose atoms donate electrons from the outer (and some from the outer) electron layer, turning into positive ions.
This property of metal atoms, as you know, is determined by the fact that they have relatively large radii and a small number of electrons (mainly from 1 to 3) on the outer layer.
The only exceptions are 6 metals: atoms of germanium, tin, lead on the outer layer have 4 electrons, atoms of antimony, bismuth -5, polonium atoms - 6.
Metal atoms are characterized by low electronegativity values (from 0.7 to 1.9) and exclusively reducing properties, that is, the ability to donate electrons.
You already know that in the Periodic Table of Chemical Elements of D. I. Mendeleev, metals are below the boron-astatine diagonal, I am also above it in secondary subgroups. In the periods and clay subgroups, there are regularities known to you in changing the metallic, and hence the reducing properties of the atoms of the elements.
Chemical elements located near the boron-astatine diagonal have dual properties: in some of their compounds they behave like metals, in others they exhibit the properties of a non-metal.
In secondary subgroups, the reducing properties of metals most often decrease with increasing serial number. Compare the activity of the group I metals of the side subgroup known to you: Cu, Ag, Au; II group of a secondary subgroup - and you will see for yourself.
This can be explained by the fact that the strength of the bond of valence electrons with the nucleus of the atoms of these metals is more affected by the value of the charge of the nucleus, and not by the radius of the atom. The value of the charge of the nucleus increases significantly, the attraction of electrons to the nucleus increases. In this case, although the radius of the atom increases, it is not as significant as that of the metals of the main subgroups.
Simple substances formed by chemical elements - metals, and complex metal-containing substances play an important role in the mineral and organic "life" of the Earth. Suffice it to recall that the atoms (nones) of metal elements are an integral part of the compounds that determine the metabolism in the body of humans, animals, and plants. For example, 76 elements were found in human blood, and only 14 of them are not metals. In the human body, some metal elements (calcium, potassium, sodium, magnesium) are present in large quantities, that is, they are macronutrients. And metals such as chromium, manganese, iron, cobalt, copper, zinc, molybdenum are present in small quantities, that is, these are trace elements. If a person weighs 70 kg, then his body contains (in grams): calcium - 1700, potassium - 250, sodium - 70, magnesium - 42, iron - 5. zinc - 3. All metals are extremely important, health problems arise and in their deficiency and excess.
For example, sodium ions regulate the water content in the body, the transmission of nerve impulses. Its deficiency leads to headache, weakness, poor memory, loss of appetite, and its excess leads to increased blood pressure, hypertension, and heart disease. Nutrition experts recommend consuming no more than 5 g (1 teaspoon) of table salt (NaCl) per adult per day. The influence of metals on the condition of animals and plants can be found in Table 16.
Simple substances - metals
With the development of the production of metals (simple substances) and alloys, the emergence of civilization (“Bronze Age”, Iron Age) was connected.
The scientific and technological revolution that began about 100 years ago, affecting both industry and the social sphere, is also closely connected with the production of metals. On the basis of tungsten, molybdenum, titanium and other metals began to create corrosion-resistant, superhard, refractory alloys, the use of which greatly expanded the possibilities of mechanical engineering. In nuclear and space technology, tungsten and rhenium alloys are used to make parts operating at temperatures up to 3000 ºС. in medicine, surgical instruments made of tantalum and platinum alloys, unique ceramics based on titanium and zirconium oxides are used.
And of course, we should not forget that in most alloys the long-known iron metal is used (Fig. 37), and the basis of many light alloys is relatively “young” metals: aluminum and magnesium.
Supernovae are composite materials representing, for example, a polymer or ceramics, which inside (like concrete with iron bars) are reinforced with metal fibers, which can be made of tungsten, molybdenum, steel and other metals and alloys - it all depends on the goal that is necessary to achieve it material properties.
You already have an idea about the nature of the chemical bond in metal crystals. Recall, using the example of one of them - sodium, how it is formed.
Figure 38 shows a diagram of the crystal lattice of sodium metal. In it, each sodium atom is surrounded by eight neighboring ones. Sodium atoms, like all metals, have many free valence orbitals and few valence electrons.
The only valence electron of the sodium atom Zs 1 can occupy any of the nine free orbitals, because they do not differ much in energy level. When atoms approach each other, when a crystal lattice is formed, the valence orbitals of neighboring atoms overlap, due to which electrons do not freely move from one orbital to another, making a connection between all atoms of the metal crystal.
This type of chemical bond is called a metallic bond. A metallic bond is formed by elements whose atoms on the outer layer have few valence electrons compared to a large number of outer energetically close orbitals. Their valence electrons are weakly held in the atom. The electrons that carry out the connection are socialized and move throughout the crystal lattice of the neutral metal as a whole.
Substances with a metallic bond are characterized by metallic crystal lattices, which are usually depicted schematically as a tick, as shown in the figure, the nodes are cations and metal atoms. Shared electrons electrostatically attract metal cations located in the vicinity of their crystal lattice, ensuring its stability and strength (shared electrons are depicted as black small balls).
A metallic bond is a bond in metals and alloys between metal atom-ions located in the crystal lattice, which is carried out by socialized valence electrons.
Some metals crystallize in two or more crystalline forms. This property of substances - to exist in several crystalline modifications - is called polymorphism. Polymorphism for simple substances is known to you as allotropy.
Tin has two crystalline modifications:
alpha - stable below 13.2 ºС with density р - 5.74 g/cm3. This is gray tin. It has an almaav (atomic) crystal lattice:
betta - stable above 13.2 ºС with density p - 6.55 g/cm3. This is white tin.
White tin is a very soft metal. When cooled below 13.2 ºС, it crumbles into a gray powder, since at the transition | 1 » n its specific volume increases significantly. This phenomenon is called the tin plague. Of course, a special type of chemical bond and the type of crystal lattice of metals should determine and explain their physical properties.
What are they? These are metallic luster, plasticity, high electrical conductivity and thermal conductivity, an increase in electrical resistance with increasing temperature, as well as such practically significant properties as density, melting and boiling points, hardness, and magnetic properties.
Let's try to explain the reasons that determine the basic physical properties of metals. Why are metals plastic?
Mechanical action on a crystal with a metal crystal lattice causes the layers of ion-atoms to shift relative to each other, since electrons move throughout the crystal, bonds are not broken, therefore, metals are characterized by greater plasticity.
A similar effect on a solid substance with connline bonds (atomic crystal lattice) leads to the breaking of covalent bonds. Breaking bonds in the ionic lattice leads to mutual repulsion of like-charged ions (Fig. 40). Therefore, substances with atomic and ionic crystal lattices are fragile.
The most ductile metals are Au, Af, Cu, Sn, Pb, Zn. They are easily drawn into wire, amenable to forging, pressing, rolling into sheets. For example, gold foil 0.008 nm thick can be made from gold, and a thread 1 km long can be drawn from 0.5 g of this metal.
Even mercury, which, as you know, is liquid at room temperature, becomes malleable like lead at low temperatures in the solid state. Only Bi and Mn do not have plasticity, they are brittle.
Why do metals have a characteristic luster and are also opaque?
Electrons filling the interatomic space reflect light rays (and do not transmit, like glass), and most metals equally scatter all the rays of the visible part of the spectrum. Therefore, they have a silvery white or gray color. Strontium, gold and copper absorb short wavelengths (close to violet) to a greater extent and reflect long wavelengths of the light spectrum, therefore they have light yellow, yellow and copper colors, respectively.
Although in practice, you know, metal does not always seem to us to be a light body. First, its surface can oxidize and lose its luster. Therefore, native copper looks like a greenish stone. And secondly, even pure metal may not shine. Very thin sheets of silver and gold have a completely unexpected appearance - they have a bluish-green color. And fine metal powders appear dark gray, even black.
Silver, aluminum, palladium have the highest reflectivity. They are used in the manufacture of mirrors, including spotlights.
Why do metals have high electrical conductivity and thermal conductivity?
Chaotically moving electrons in a metal under the influence of an applied electric voltage acquire a directed movement, that is, they conduct an electric current. With an increase in the meta-aphid temperature, the vibration amplitudes of the atoms and ions located at the nodes of the crystal lattice increase. This makes it difficult for electrons to move, and the electrical conductivity of the metal decreases. At low temperatures, the oscillatory motion, on the contrary, greatly decreases and the electrical conductivity of metals increases sharply. Near absolute zero, there is practically no resistance in metals, and superconductivity appears in most metals.
It should be noted that non-metals with electrical conductivity (for example, graphite), at low temperatures, on the contrary, do not conduct electric current due to the absence of free electrons. And only with an increase in temperature and the destruction of some covalent bonds, their electrical conductivity begins to increase.
Silver, copper, as well as gold, aluminum have the highest electrical conductivity, manganese, lead, and mercury have the lowest.
Most often, with the same regularity as the electrical conductivity, the thermal conductivity of metals changes.
They are due to the high mobility of free electrons, which, colliding with vibrating ions and atoms, exchange energy with them. Therefore, there is an equalization of temperature throughout the piece of metal.
The mechanical strength, density, melting point of metals are very different. Moreover, with an increase in the number of oekgrons. binding ion-atoms, and by decreasing the interatomic distance in crystals, the indicators of these properties increase.
So, alkali metals, whose atoms have one valence electron, are soft (cut with a knife), with a low density (lithium is the lightest metal with p - 0.53 g / cm3) and melt at low temperatures (for example, the melting point of cesium is 29 "C) The only metal that is liquid under ordinary conditions - mercury - has a melting point of 38.9 "C.
Calcium, which has two electrons in the outer energy level of atoms, is much harder and melts at a higher temperature (842º C).
Even more arched is the crystal lattice formed by scandium atoms, which have three valence electrons.
But the strongest crystal lattices, high densities and melting points are observed in metals of secondary subgroups V, VI, VII, MP groups. This is explained by. that for metals of side subgroups having unsaved valence electrons at the d-sublevel, the formation of very strong covalent bonds between atoms is characteristic, in addition to the metallic one, carried out by electrons of the outer layer from s-orbitals.
Remember that the heaviest metal is osmium (a component of superhard and wear-resistant alloys), the most refractory metal is tungsten (used to make lamp filaments), the hardest metal is chromium Cr (scratches glass). They are part of the materials from which metal-cutting tools, brake pads of heavy machines, etc. are made.
Metals differ with respect to magnetic fields. But this sign they are divided into three groups:
Ferromagnetic Able to be magnetized under the influence of even weak magnetic fields (iron - alpha form, cobalt, nickel, gadolinium);
Paramagnetic exhibit a weak ability to magnetize (aluminum, chromium, titanium, almost all lanthanides);
Diamagnetic are not attracted to the magnet, even slightly repelled from it (tin, stranded, bismuth).
Recall that when considering the electronic structure of metals, we subdivided metals into metals of the main subgroups (k- and p-elements) and metals of secondary subgroups.
In engineering, it is customary to classify metals according to various physical properties:
a) density - light (p< 5 г/см3) и тяжелые (все остальные);
b) melting point - fusible and refractory.
Classifications of metals by chemical properties
Metals with low chemical activity are called noble (silver, gold, platinum and its analogues - osmium, iridium, ruthenium, palladium, rhodium).
According to the proximity of chemical properties, alkali (group I metals of the main subgroup), alkaline earth (calcium, strontium, barium, radium), as well as rare earth metals (scandium, yttrium, lanthanum and lanthanides, actinium and actinides) are distinguished.
General chemical properties of metals
Metal atoms give up valence electrons relatively easily and pass into positively charged nons, that is, they are oxidized. This, as you know, is the main common property of both atoms and simple metal substances.
Metals in chemical reactions are always a reducing agent. The reducing ability of atoms of simple substances - metals, formed by chemical elements of one period or one main subgroup of the Periodic system of D. I. Mendeleev, changes naturally.
The reducing activity of a metal in chemical reactions that occur in aqueous solutions reflects its position in the electrochemical series of metal voltages.
1. The further to the left the metal is in this row, the stronger the reducing agent it is.
2. Each metal is able to displace (restore) and is salty in solution those metals that are after it (to the right) in a series of voltages.
3. Metals that are in the series of voltages to the left of hydrogen are able to displace it from acids in solution.
4. Metals, which are the strongest reducing agents (alkaline and alkaline earth), in any aqueous solutions interact primarily with water.
The reducing activity of a metal, determined from the electrochemical series, does not always correspond to its position in the Periodic Table. This is explained by. That when determining the position of a metal in a series of voltages, not only the energy of detachment of electrons from individual atoms is taken into account, but also the energy expended on the destruction of the crystal lattice, as well as the energy released during the hydration of ions.
For example, lithium is more active in aqueous solutions than sodium (although Na is a more active metal in terms of its position in the Periodic Table). The fact is that the hydration energy of Li+ ions is much greater than the hydration energy of Na+ ions. therefore, the first process is energetically more favorable.
Having considered the general provisions characterizing the reducing properties of metals, we turn to specific chemical reactions.
Interaction with simple non-metal substances
1. With oxygen, most metals form oxides - basic and amphoterpy. Acid transition metal oxides, such as chromium oxide or manganese oxide, are not formed by direct oxidation of the metal with oxygen. They are obtained indirectly.
Alkali metals Na, K actively react with atmospheric oxygen, forming peroxides.
Sodium oxide is obtained indirectly, by calcining peroxides with the corresponding metals:
Lithium and alkaline earth metals react with atmospheric oxygen to form basic oxides.
Other metals, except for gold and platinum metals, which are not oxidized at all by atmospheric oxygen, interact with it less actively or when heated.
2. With halogens, metals form salts of hydrohalic acids.
3. With hydrogen, the most active metals form hydrides - ionic salts, one common substances in which hydrogen has an oxidation state of -1, for example:
calcium hydride.
Many transition metals form hydrides of a special type with hydrogen - there is a kind of dissolution or introduction of hydrogen into the crystal lattice of metals between atoms and ions, while the metal retains its appearance, but increases in volume. Absorbed hydrogen is in the metal, apparently in atomic form. There are also intermediate metal hydrides.
4. Metals form salts with sulfur - sulfides.
5. Metals react with nitrogen somewhat more difficult, since the chemical bond in the nitrogen molecule Г^r is very strong, and nitrides are formed. At ordinary temperatures, only lithium interacts with nitrogen.
Interaction with complex substances
1. With water. Alkali and alkaline earth metals under normal conditions displace hydrogen from water and form soluble alkali bases.
Other metals, standing in a series of voltages up to hydrogen, can also, under certain conditions, displace hydrogen from water. But aluminum reacts violently with water only if the oxide film is removed from its surface.
Magnesium interacts with water only when boiling, and hydrogen is also released. If burning magnesium is added to water, then combustion continues, as the reaction proceeds: hydrogen burns. Iron interacts with water only when heated.
2. Metals that are in the series of voltages up to hydrogen interact with acids in solution. This produces salt and hydrogen. But lead (and some other metals), despite its position in the voltage series (to the left of hydrogen), almost does not dissolve in dilute sulfuric acid, since the resulting lead sulfate PbSO is insoluble and creates a protective film on the metal surface.
3. With salts of less active metals in solution. As a result of such a reaction, a salt of a more active metal is formed and a less active metal is released in a free form.
It must be remembered that the reaction proceeds in cases where the resulting salt is soluble. The displacement of metals from their compounds by other metals was first studied in detail by N. N. Beketov, a prominent Russian physical chemist. He arranged the metals according to their chemical activity in the "expressive series", which became the prototype of the series of metal stresses.
4. With organic substances. Interaction with organic acids is similar to reactions with mineral acids. Alcohols, on the other hand, can exhibit weak acidic properties when interacting with alkali metals.
Metals participate in reactions with haloalkanes, which are used to obtain lower cycloalkanes and for syntheses, during which the carbon skeleton of the molecule becomes more complex (A. Wurtz reaction):
5. Metals whose hydroxides are amphoteric interact with alkalis in solution.
6. Metals can form chemical compounds with each other, which are collectively called intermetallic compounds. They most often do not show the oxidation states of atoms, which are characteristic of compounds of metals with non-metals.
Intermetallic compounds usually do not have a constant composition, the chemical bond in them is mainly metallic. The formation of these compounds is more typical for metals of secondary subgroups.
Metal oxides and hydroxides
Oxides formed by typical metals are classified as salt-forming, basic in nature of properties. As you know, they correspond to hydroxides. which are bases, which in the case of alkali and alkaline earth metals are soluble in water, are strong electrolytes and are called alkalis.
The oxides and hydroxides of some metals are amphoteric, that is, they can exhibit both basic and acidic properties, depending on the substances with which they interact.
For example:
Many metals of secondary subgroups, which have a variable oxidation state in compounds, can form several oxides and hydroxides, the nature of which depends on the oxidation state of the metal.
For example, chromium in compounds exhibits three oxidation states: +2, +3, +6, therefore it forms three series of oxides and hydroxides, and with an increase in the degree of oxidation, the acid character increases and the basic character weakens.
Corrosion of metals
When metals interact with environmental substances, compounds appear on their surfaces that have completely different properties than the metals themselves. In a normal vein, we often use the words "rust", "rusting", seeing a brown-red coating on products made of iron and its alloys. Rusting is a common form of corrosion.
Corrosion is a process of spontaneous destruction of metals and a splat not) aliaishsm of the current environment (from lat. - corrosive).
However, almost all metals undergo destruction, as a result of which many of their properties deteriorate (or are completely lost): strength, ductility, gloss decrease, electrical conductivity decreases, friction between moving parts of the machine also increases, the dimensions of parts change, etc.
Corrosion of metals can be continuous and local.
Nerven is not as dangerous as the second, its manifestations can be taken into account when designing structures and apparatus. Local corrosion is much more dangerous, although metal losses here can be small. One of its most dangerous types is point. They consist in the formation of through lesions, that is, point cavities - pitting, while the strength of individual sections decreases, the reliability of structures, apparatus, and structures decreases.
Corrosion of metals causes great economic harm. Mankind bears huge material losses in the aftermath of the destruction of pipelines, machine parts, ships, bridges, and various equipment.
Corrosion leads to a decrease in the reliability of metal structures. Taking into account possible destruction, it is necessary to overestimate the strength of some products (for example, aircraft parts, turbine blades), which means increasing metal consumption, and this requires additional economic costs.
Corrosion leads to production downtime due to the replacement of failed equipment, to the loss of raw materials and products as a result of the destruction of halo, oil and water pipelines. It is impossible not to take into account the damage to nature, and hence to human health, caused by the leakage of oil products and other chemicals. Corrosion can lead to contamination) of products, and consequently, to a decrease in its quality. The costs of compensating for losses associated with corrosion are enormous. They make up about 30% of the annual production of metals worldwide.
From all that has been said, it follows that a very important problem is to find ways to protect metals and alloys from corrosion.
They are very varied. But for their selection it is necessary to know and take into account the chemical essence of corrosion processes.
But the chemical nature of corrosion is a redox process. Depending on the environment in which it occurs, there are several types of corrosion.
The most common types of corrosion are chemical and electrochemical.
I. Chemical corrosion occurs in a non-conductive environment. This type of corrosion manifests itself in the case of the interaction of metals with dry gases or liquids - non-electrolytes (gasoline, kerosene, etc.). Parts and components of engines, gas turbines, rocket launchers are subjected to such destruction. Chemical corrosion is often observed during the processing of metals at high temperatures.
Most metals are oxidized by atmospheric oxygen, forming oxide films on the surface. If this film is strong, dense, well bonded to the metal, then it protects the metal from further destruction. In iron, it is loose, porous, easily separated from the surface and therefore is not able to protect the metal from further destruction.
II. Electrochemical corrosion occurs in a conductive medium (electrolyte) with the appearance of an electric current inside the system. As a rule, metals and alloys are heterogeneous and contain inclusions of various impurities. When they come into contact with electrolytes, some parts of the surface begin to play the role of an anode (donate electrons), while others act as a cathode (accept electrons).
In one case, gas evolution (Hg) will be observed. In the other - the formation of rust.
So, electrochemical corrosion is a reaction that occurs in media that conduct current (as opposed to chemical corrosion). The process occurs when two metals come into contact or on the surface of a metal containing inclusions that are less active conductors (it may also be a non-metal).
At the anode (a more active metal), metal atoms are oxidized to form cations (dissolution).
At the cathode (a less active conductor), hydrogen ions or oxygen molecules are reduced with the formation of H2 or OH- hydroxide ions, respectively.
Hydrogen cations and dissolved oxygen are the most important oxidizing agents that cause electrochemical corrosion.
The corrosion rate is the greater, the more the metals (metal and impurities) differ in their activity (for metals, the farther apart they are located in a series of voltages). Corrosion increases significantly with increasing temperature.
The electrolyte can be sea water, river water, condensed moisture and, of course, well-known electrolytes - solutions of salts, acids, alkalis.
You obviously remember that in winter, technical salt (sodium chloride, sometimes calcium chloride, etc.) is used to remove snow and ice from sidewalks. The resulting solutions drain into sewer pipelines, thereby creating a favorable environment for electrochemical corrosion of underground utilities.
Corrosion protection methods
Already in the design of metal structures, their manufacture provides for measures to protect against corrosion.
1. Sanding the surfaces of the product so that moisture does not linger on them.
2. The use of alloyed alloys containing special additives: chromium, nickel, which at high temperatures form a stable oxide layer on the metal surface. Alloy steels are well known - stainless steels, from which household items (sheathed forks, spoons), machine parts, tools are made.
3. Application of protective coatings.
Consider their types.
Non-metallic - non-oxidizing oils, special varnishes, paints. True, they are short-lived, but they are cheap.
Chemical - artificially created surface films: oxide, citric, silicide, polymer, etc. For example, all small arms The parts of many precision instruments are burnished - this is the process of obtaining the thinnest film of iron oxides on the surface of a steel product. The resulting artificial oxide film is very durable and gives the product a beautiful black color and blue tint. Polymer coatings are made from polyethylene, polyvinyl chloride, polyamide resins. They are applied in two ways: a heated product is placed in a polymer powder, which melts and welds to the metal, or the metal surface is treated with a polymer solution in a low-temperature solvent, which quickly evaporates, and the polymer film remains on the product.
Metallic coatings are coatings with other metals, on the surface of which stable protective films are formed under the action of oxidizing agents.
The application of chromium to the surface - chromium plating, nickel - nickel plating, zinc - zinc plating, tin - tinning, etc. A chemically passive metal - gold, silver, copper can also serve as a coating.
4. Electrochemical methods of protection.
Protective (anodic) - a piece of a more active metal (protector) is attached to the protected metal structure, which serves as an anode and is destroyed in the presence of an electrolyte. Magnesium, aluminum, zinc are used as a protector when protecting ship hulls, pipelines, cables and other stylish products;
Cathode - the metal structure is connected to the cathode of an external current source, which eliminates the possibility of its anode destruction
5. Special treatment of the electrolyte or the environment in which the protected metal structure is located.
It is known that Damascus craftsmen for descaling and
rust used solutions of sulfuric acid with the addition of brewer's yeast, flour, starch. These bring and were among the first inhibitors. They did not allow the acid to act on the weapon metal, as a result, only scale and rust were dissolved. Ural gunsmiths used pickling soups for these purposes - solutions of sulfuric acid with the addition of flour bran.
Examples of the use of modern inhibitors: during transportation and storage, hydrochloric acid is perfectly "tamed" by butylamine derivatives. and sulfuric acid - nitric acid; volatile diethylamine is injected into various containers. Note that inhibitors act only on the metal, making it passive with respect to the medium, for example, to an acid solution. More than 5 thousand corrosion inhibitors are known to science.
Removal of oxygen dissolved in water (deaeration). This process is used in the preparation of water entering boiler plants.
Methods for obtaining metals
Significant chemical activity of metals (interaction with atmospheric oxygen, other non-metals, water, salt solutions, acids) leads to the fact that they are found in the earth's crust mainly in the form of compounds: oxides, sulfides, sulfates, chlorides, carbonates, etc.
In free form, there are metals located in the series of voltages to the right of hydrogen, although much more often copper and mercury can be found in nature in the form of compounds.
Minerals and rocks containing metals and their compounds, from which the extraction of pure metals is technically possible and economically feasible, are called ores.
Obtaining metals from ores is the task of metallurgy.
Metallurgy is also the science of industrial methods for obtaining metals from ores. and industry sector.
Any metallurgical process is a process of reduction of metal ions with the help of various reducing agents.
To implement this process, it is necessary to take into account the activity of the metal, select a reducing agent, consider technological feasibility, economic and environmental factors. In accordance with this, there are the following methods for obtaining metals: pyrometallurgical. hydrometallurgical, electrometallurgical.
Pyrometallurgy is the recovery of metals from ores at high temperatures using carbon, carbon monoxide (II). hydrogen, metals - aluminum, magnesium.
For example, tin is reduced from cassiterite, and copper from cuprite by calcination with coal (coke). Sulfide ores are preliminarily roasted with air access, and then the resulting oxide is reduced with coal. Metals are also isolated from carbonate ores by pumping a with coal, since carbonates decompose when heated, turning into oxides, and the latter are reduced by coal.
Hydrometallurgy is the reduction of metals to their salts in solution. The process takes place in 2 stages:
1) the natural compound is dissolved in a suitable reagent to obtain a solution of the metal salt;
2) this metal is displaced from the obtained solution by a more active one or restored by electrolysis. For example, to obtain copper for ores containing copper oxide, CuO, it is treated with dilute sulfuric acid.
The copper is then removed from the salt solution either by electrolysis or by displacing the sulfate with iron. Silver, zinc, molybdenum, gold, uranium are obtained in this way.
Electrometallurgy is the reduction of metals in the process of electrolysis of solutions or melts of their compounds.
Electrolysis
If the electrodes are lowered into the electrolyte solution or melt and a constant electric current is passed through, then the ions will move in a direction: cations - to the cathode (negatively charged electrode), anions - to the anode (positively charged electrode).
At the cathode, cations accept electrons and are reduced at the anode, anions donate electrons and are oxidized. This process is called electrolysis.
Electrolysis is an oxidation-reduction process that occurs on an electrical system during the passage of an electric current, chsrse, or an electrolyte solution.
The simplest example of such processes is the electrolysis of molten salts. Consider the process of electrolysis of a sodium chloride melt. The process of thermal dissociation takes place in the melt. Under the action of an electric current, cations move towards the cathode and receive electrons from it.
Sodium metal is formed at the cathode, and chlorine gas is formed at the anode.
The main thing to remember is that in the process of electrolysis, a chemical reaction is carried out due to electrical energy, which cannot go on spontaneously.
The situation is more complicated in the case of electrolysis of electrolyte solutions.
In a salt solution, in addition to metal ions and an acidic residue, there are water molecules. Therefore, when considering processes on electrodes, it is necessary to take into account their participation in electrolysis.
To determine the products of electrolysis of aqueous solutions of electrolytes, there are the following rules.
1. The process on the cathode does not depend on the material of the cathode on which it is made, but on the position of the metal (electrolyte cation) in the electrochemical series of voltages, and if:
1.1. The electrolyte cation is located in the series of voltages at the beginning of the series (along with Al inclusive), then the process of water reduction is going on at the cathode (hydrogen is released). Metal cations are not reduced, they remain in solution.
1.2. The electrolyte cation is in a series of voltages between aluminum and hydrogen, then both metal nones and water molecules are reduced at the cathode.
1.3. The electrolyte cation is in a series of voltages after hydrogen, then metal cations are reduced at the cathode.
1.4. The solution contains cations of different metals, then the downloaded metal cation is restored, standing in a series of voltages
These rules are shown in Figure 10.
2. The process at the anode depends on the material of the anode and on the nature of the annon (Scheme 11).
2.1. If the anode is dissolved (iron, zinc, copper, silver and all metals that are oxidized during electrolysis), then the anode metal is oxidized, regardless of the nature of the anion. 2.2. If the anode does not dissolve (it is called inert - graphite, gold, platinum), then:
a) during the electrolysis of solutions of salts of anoxic acids (prome fluorides), the anion is oxidized at the anode;
b) during the electrolysis of solutions of salts of oxygen-containing acid and fluorides at the anode, the process of water oxidation occurs. Anions are not oxidized, they remain in solution;
Electrolysis of melts and solutions of substances is widely used in industry:
1. To obtain metals (aluminum, magnesium, sodium, cadmium are obtained only by electrolysis).
2. To obtain hydrogen, halogens, alkalis.
3. For the purification of metals - refining (purification of copper, nickel, lead is carried out by the electrochemical method).
4. To protect metals from corrosion - applying protective coatings in the form of a thin layer of another metal resistant to corrosion (chromium, nickel, copper, silver, gold) - electroplating.
5. Obtaining metal copies, records - electroplating.
Practical task
1. How are the structure of metals related to their location in the main and secondary subgroups of the Periodic Table of Chemical Elements of D. I. Mendeleev?
2. Why do alkali and alkaline earth metals have a single oxidation state in compounds: (+1) and (+2), respectively, while metals of secondary subgroups, as a rule, exhibit different oxidation states in compounds?
3. What oxidation states can manganese exhibit? What oxides of hydrokenda correspond to manganese in these oxidation states? What is their character?
4. Compare the electronic structure of the atoms of the elements of group VII: manganese and chlorine. Explain the difference in their chemical properties and the presence of different degrees of oxidation of atoms in both elements.
5. Why does the position of metals in the electrochemical series of voltages not always correspond to their position in the Periodic system of D. I. Mendeleev?
9. Make equations for the reactions of sodium and magnesium with acetic acid. In which case and why will the reaction rate be faster?
11. What methods of obtaining metals do you know? What is the essence of all methods?
14. What is corrosion? What types of corrosion do you know? Which of them is a physical and chemical process?
15. Can the following processes be considered corrosion: a) oxidation of iron during electric welding, b) interaction of zinc with hydrochloric acid in obtaining etched acid for soldering? Give a reasoned answer.
17. The manganese product is in water and does not come into contact with the copper product. Will both remain unchanged?
18. Will an iron structure be protected from electrochemical corrosion in water if a plate of another metal is stolen on it: a) magnesium, b) lead, c) nickel?
19. For what purpose is the surface of tanks for storing petroleum products (gasoline, kerosene) painted with silver - a mixture of aluminum powder with one of the vegetable oils?
20. On the surface of the acidified soil of the garden plot there are iron pipes with inserted brass taps. What will corrode: pipe yiyang faucet? Where is the destruction most pronounced?
21. What is the difference between the electrolysis of melts and the electrolysis of aqueous solutions?
22*. What metals can be obtained by electrolysis of melts of their salts and cannot be obtained by electrolysis of aqueous solutions of these substances?
23*. Make the equations for the electrolysis of barium chloride in: a) melt, b) solution
28. To a solution containing 27 g of copper (II) chloride, 1-4 g of iron filings were added. What mass of copper was released as a result of this reaction?
Answer: 12.8 g.
29. What mass of zinc sulfate can be obtained by reacting excess zinc with 500 ml of a 20% sulfuric acid solution with a density of 1.14 g/ml?
Answer: 187.3
31. When treating 8 g of a mixture of magnesium and magnesium oxide with hydrochloric acid, 5.6 liters of hydrogen (n, w.) were released. What is the mass fraction (in %) of JUNE in the initial mixture?
Answer: 75%.
34. Determine the mass fraction (in percent) of carbon in steel (an alloy of iron with carbon), if 0.28 l of carbon monoxide (IV) (n.a.) was collected during the combustion of its sample weighing 10 g in an oxygen stream.
Answer: 1.5%.
35. A sample of sodium weighing 0.5 g was placed in water. Neither the neutralization of the resulting solution spent 29.2 g of 1.5% hydrochloric acid. What is the mass fraction (in percent) of sodium in the sample?
Answer: 55.2%.
36. An alloy of copper and aluminum was treated with an excess of sodium hydroxide solution, and a gas with a volume of 1.344 liters (n.a.) was released, the residue after the reaction was dissolved in nitric acid, then the solution was evaporated and calcined to a constant mass, which turned out to be 0.4 g. alloy composition? Answer: 1.08 g Al 0.32 g Cu or 77.14% Al 22.86% Cu.
37. What mass of cast iron containing 94% iron can be obtained from 1 ton of red iron ore (Fe2O3) containing 20% impurities?
Answer: 595.74 kg.
Metals in nature
If you carefully studied chemistry in previous classes, then you know that the periodic table has more than ninety types of metals, and approximately sixty of them can be found in the natural environment.
Naturally occurring metals can be roughly divided into the following groups:
Metals that can be found in nature in free form;
metals occurring in the form of compounds;
metals that can be found in mixed form, that is, they can be both in free form and in the form of compounds.
Unlike other chemical elements, metals are quite often found in nature in the form of simple substances. They usually have a native state. Such metals, which are presented in the form of simple substances, include gold, silver, copper, platinum, mercury and others.
But not all metals found in the natural environment are presented in a native state. Some metals can be found in the form of compounds and are called minerals.
In addition, such chemical elements as silver, mercury and copper can be found both in the native state and in the state having the form of compounds.
All those minerals from which metals can later be obtained are called ores. In nature, there are ore, which includes iron. This compound is called iron ore. And if the composition contains copper, but accordingly, such a compound is called copper ore.
Of course, the most common in nature are metals that actively interact with oxygen and sulfur. They are called metal oxides and sulfides.
One such common element that forms a metal is aluminum. Aluminum is found in clay and is also found in gemstones such as sapphire and ruby.
The second most popular and widespread metal is iron. It is usually found in nature in the form of compounds, and in its native form it can only be found in the composition of meteorite stones.
The next most common in the natural environment, or rather in the earth's crust, are metals such as magnesium, calcium, sodium, potassium.
Holding coins in your hand, you probably noticed that a characteristic smell emanates from them. But, it turns out that this is not the smell of metal, but the smell that comes from the compounds that form when the metal comes into contact with human sweat.
Did you know that in Switzerland there is a production of gold bars in the form of a chocolate bar, which can be broken into slices and used as a gift or means of payment? The company produces such chocolate bars from gold, silver, platinum and palladium. If such a tile is broken into slices, then each of them weighs only one gram.
And yet, such a metal alloy as nitinol has a rather interesting property. It is unique in that it has a memory effect and when heated, a deformed product made of this alloy is able to return to its original shape. Such peculiar materials with the so-called memory are used for the manufacture of bushings. They have the ability to shrink at low temperatures, and at room temperature these bushings straighten out and this connection is even more reliable than welding. And this phenomenon occurs due to the fact that these alloys have a thermoelastic structure.
Have you ever wondered why it is customary to add an alloy of silver or copper to gold jewelry? It turns out that this is because pure gold is very soft and easy to scratch even with a fingernail.
