One of the most important tasks in chemistry is the correct composition of chemical formulas. A chemical formula is a written representation of the composition of a chemical substance using the Latin element designation and indices. To correctly compile the formula, we will definitely need the periodic table and knowledge of simple rules. They are quite simple and even children can remember them.
How to make chemical formulas
The main concept when drawing up chemical formulas is “valency”. Valency is the property of one element to hold a certain number of atoms in a compound. The valence of a chemical element can be viewed in the periodic table, and you also need to remember and be able to apply simple general rules.
- The valence of a metal is always equal to the group number, provided that it is in the main subgroup. For example, potassium has a valency of 1, and calcium has a valency of 2.
- Non-metals are a little more complicated. A non-metal can have higher and lower valency. The highest valence is equal to the group number. The lowest valency can be determined by subtracting the element's group number from eight. When combined with metals, nonmetals always have the lowest valence. Oxygen always has a valence of 2.
- In a compound of two non-metals, the chemical element that is located to the right and higher in the periodic table has the lowest valence. However, fluorine always has a valence of 1.
- And one more important rule when setting odds! The total number of valencies of one element must always be equal to the total number of valencies of another element!
Let's consolidate the knowledge gained using the example of a compound of lithium and nitrogen. The metal lithium has a valence of 1. The non-metal nitrogen is located in group 5 and has a higher valency of 5 and a lower valence of 3. As we already know, in compounds with metals, non-metals always have a lower valence, so nitrogen in this case will have a valence of three. We arrange the coefficients and get the required formula: Li 3 N.
So, quite simply, we learned how to compose chemical formulas! And for better memorization of the algorithm for composing formulas, we have prepared its graphical representation.
Key words: Chemistry 8th grade. All formulas and definitions, symbols of physical quantities, units of measurement, prefixes for designating units of measurement, relationships between units, chemical formulas, basic definitions, briefly, tables, diagrams.
1. Symbols, names and units of measurement
some physical quantities used in chemistry
| Physical quantity | Designation | Unit |
| Time | t | With |
| Pressure | p | Pa, kPa |
| Quantity of substance | ν | mole |
| Mass of substance | m | kg, g |
| Mass fraction | ω | Dimensionless |
| Molar mass | M | kg/mol, g/mol |
| Molar volume | Vn | m 3 /mol, l/mol |
| Volume of substance | V | m 3, l |
| Volume fraction | Dimensionless | |
| Relative atomic mass | A r | Dimensionless |
| M r | Dimensionless | |
| Relative density of gas A to gas B | D B (A) | Dimensionless |
| Density of matter | R | kg/m 3, g/cm 3, g/ml |
| Avogadro's constant | N A | 1/mol |
| Absolute temperature | T | K (Kelvin) |
| Temperature in Celsius | t | °C (degrees Celsius) |
| Thermal effect of a chemical reaction | Q | kJ/mol |
2. Relationships between units of physical quantities
3. Chemical formulas in 8th grade
4. Basic definitions in 8th grade
- Atom- the smallest chemically indivisible particle of a substance.
- Chemical element- a certain type of atom.
- Molecule- the smallest particle of a substance that retains its composition and chemical properties and consists of atoms.
- Simple substances- substances whose molecules consist of atoms of the same type.
- Complex substances- substances whose molecules consist of atoms of different types.
- Qualitative composition of the substance shows which atoms of elements it consists of.
- Quantitative composition of the substance shows the number of atoms of each element in its composition.
- Chemical formula- conventional recording of the qualitative and quantitative composition of a substance using chemical symbols and indices.
- Atomic mass unit(amu) - a unit of measurement of atomic mass, equal to the mass of 1/12 of a carbon atom 12 C.
- Mole- the amount of a substance that contains a number of particles equal to the number of atoms in 0.012 kg of carbon 12 C.
- Avogadro's constant (Na = 6*10 23 mol -1) - the number of particles contained in one mole.
- Molar mass of a substance (M ) is the mass of a substance taken in an amount of 1 mole.
- Relative atomic mass element A r - the ratio of the mass of an atom of a given element m 0 to 1/12 of the mass of a carbon atom 12 C.
- Relative molecular weight substances M r - the ratio of the mass of a molecule of a given substance to 1/12 of the mass of a carbon atom 12 C. The relative molecular mass is equal to the sum of the relative atomic masses of the chemical elements forming the compound, taking into account the number of atoms of a given element.
- Mass fraction chemical element ω(X) shows what part of the relative molecular mass of substance X is accounted for by a given element.
ATOMIC-MOLECULAR TEACHING
1. There are substances with molecular and non-molecular structure.
2. There are gaps between the molecules, the sizes of which depend on the state of aggregation of the substance and temperature.
3. Molecules are in continuous motion.
4. Molecules are made up of atoms.
6. Atoms are characterized by a certain mass and size.
During physical phenomena, molecules are preserved; during chemical phenomena, as a rule, they are destroyed. Atoms rearrange during chemical phenomena, forming molecules of new substances.
LAW OF CONSTANT COMPOSITION OF MATTER
Each chemically pure substance of molecular structure, regardless of the method of preparation, has a constant qualitative and quantitative composition.
VALENCE
Valence is the property of an atom of a chemical element to attach or replace a certain number of atoms of another element.
CHEMICAL REACTION
A chemical reaction is a phenomenon as a result of which other substances are formed from one substance. Reactants are substances that enter into a chemical reaction. Reaction products are substances formed as a result of a reaction.
Signs of chemical reactions:
1. Release of heat (light).
2. Change in color.
3. Odor appears.
4. Formation of sediment.
5. Gas release.
- Chemical equation- recording a chemical reaction using chemical formulas. Shows which substances and in what quantities react and are obtained as a result of the reaction.
LAW OF CONSERVATION OF MASS
The mass of substances that entered into a chemical reaction is equal to the mass of substances formed as a result of the reaction. As a result of chemical reactions, atoms do not disappear or appear, but they are rearranged.
The most important classes of inorganic substances
Lesson summary “Chemistry 8th grade. All formulas and definitions."
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2.1. Chemical language and its parts
Humanity uses many different languages. Except natural languages(Japanese, English, Russian - more than 2.5 thousand in total), there are also artificial languages, for example, Esperanto. Among artificial languages there are languages various sciences. So, in chemistry they use their own, chemical language.
Chemical language– a system of symbols and concepts designed for a brief, succinct and visual recording and transmission of chemical information.
A message written in most natural languages is divided into sentences, sentences into words, and words into letters. If we call sentences, words and letters parts of language, then we can identify similar parts in chemical language (Table 2).
Table 2.Parts of chemical language
It is impossible to master any language immediately; this also applies to a chemical language. Therefore, for now you will only get acquainted with the basics of this language: learn some “letters”, learn to understand the meaning of “words” and “sentences”. At the end of this chapter you will be introduced to names chemical substances are an integral part of the chemical language. As you study chemistry, your knowledge of chemical language will expand and deepen.
CHEMICAL LANGUAGE.
1.What artificial languages do you know (other than those mentioned in the text of the textbook)?
2.How do natural languages differ from artificial ones?
3. Do you think it is possible to describe chemical phenomena without using chemical language? If not, why not? If so, what would be the advantages and disadvantages of such a description?
2.2. Chemical element symbols
The symbol for a chemical element represents the element itself or one atom of that element.
Each such symbol is an abbreviated Latin name of a chemical element, consisting of one or two letters of the Latin alphabet (for the Latin alphabet, see Appendix 1). The symbol is written with a capital letter. Symbols, as well as Russian and Latin names of some elements, are given in Table 3. Information about the origin of the Latin names is also given there. There is no general rule for the pronunciation of symbols, therefore Table 3 also shows the “reading” of the symbol, that is, how this symbol is read in the chemical formula.
It is impossible to replace the name of an element with a symbol in oral speech, but in handwritten or printed texts this is allowed, but not recommended. Currently, 110 chemical elements are known, 109 of them have names and symbols approved by the International Union of Pure and Applied Chemistry (IUPAC).
Table 3 provides information on only 33 elements. These are the elements that you will encounter first when studying chemistry. Russian names (in alphabetical order) and symbols of all elements are given in Appendix 2.
Table 3.Names and symbols of some chemical elements
Name |
||||
Latin |
Writing |
|||
| - | Writing |
Origin |
- | - |
| Nitrogen | N itrogenium | From Greek "giving birth to saltpeter" | "en" | |
| Aluminum | Al uminium | From Lat. "alum" | "aluminum" | |
| Argon | Ar gon | From Greek "inactive" | "argon" | |
| Barium | Ba rium | From Greek " heavy" | "barium" | |
| Bor | B orum | From Arabic "white mineral" | "boron" | |
| Bromine | Br omum | From Greek "smelly" | "bromine" | |
| Hydrogen | H hydrogenium | From Greek "giving birth to water" | "ash" | |
| Helium | He lium | From Greek " Sun" | "helium" | |
| Iron | Fe rrum | From Lat. "sword" | "ferrum" | |
| Gold | Au rum | From Lat. "burning" | "aurum" | |
| Iodine | I odum | From Greek " violet" | " iodine" | |
| Potassium | K alium | From Arabic "lye" | "potassium" | |
| Calcium | Ca lcium | From Lat. "limestone" | "calcium" | |
| Oxygen | O xygenium | From Greek "acid-generating" | " O" | |
| Silicon | Si licium | From Lat. "flint" | "silicium" | |
| Krypton | Kr ypton | From Greek "hidden" | "krypton" | |
| Magnesium | M a g nesium | From the name Magnesia Peninsula | "magnesium" | |
| Manganese | M a n ganum | From Greek "cleansing" | "manganese" | |
| Copper | Cu prum | From Greek name O. Cyprus | "cuprum" | |
| Sodium | Na trium | From Arabic, "detergent" | "sodium" | |
| Neon | Ne on | From Greek " new" | "neon" | |
| Nickel | Ni ccolum | From him. "St. Nicholas Copper" | "nickel" | |
| Mercury | H ydrar g yrum | Lat. "liquid silver" | "hydrargyrum" | |
| Lead | P lum b um | From Lat. names of an alloy of lead and tin. | "plumbum" | |
| Sulfur | S ulfur | From Sanskrit "combustible powder" | "es" | |
| Silver | A r g entum | From Greek " light" | "argentum" | |
| Carbon | C arboneum | From Lat. " coal" | "tse" | |
| Phosphorus | P hosphorus | From Greek "bringer of light" | "peh" | |
| Fluorine | F luorum | From Lat. verb "to flow" | "fluorine" | |
| Chlorine | Cl orum | From Greek "greenish" | "chlorine" | |
| Chromium | C h r omium | From Greek " dye" | "chrome" | |
| Cesium | C ae s ium | From Lat. "sky blue" | "cesium" | |
| Zinc | Z i n cum | From him. "tin" | "zinc" | |
2.3. Chemical formulas
Used to designate chemical substances chemical formulas.
For molecular substances, a chemical formula can denote one molecule of this substance.
Information about a substance may vary, so there are different types of chemical formulas.
Depending on the completeness of the information, chemical formulas are divided into four main types: protozoa,
molecular, structural And spatial.
Subscripts in the simplest formula do not have a common divisor.
The index "1" is not used in formulas.
Examples of the simplest formulas: water - H 2 O, oxygen - O, sulfur - S, phosphorus oxide - P 2 O 5, butane - C 2 H 5, phosphoric acid - H 3 PO 4, sodium chloride (table salt) - NaCl.
The simplest formula of water (H 2 O) shows that the composition of water includes the element hydrogen(H) and element oxygen(O), and in any portion (a portion is a part of something that can be divided without losing its properties.) of water, the number of hydrogen atoms is twice the number of oxygen atoms.
Number of particles, including number of atoms, denoted by a Latin letter N. Denoting the number of hydrogen atoms – N H, and the number of oxygen atoms is N O, we can write that
Or N H: N O=2:1.
The simplest formula of phosphoric acid (H 3 PO 4) shows that phosphoric acid contains atoms hydrogen, atoms phosphorus and atoms oxygen, and the ratio of the numbers of atoms of these elements in any portion of phosphoric acid is 3:1:4, that is
NH: N P: N O=3:1:4.
The simplest formula can be compiled for any individual chemical substance, and for a molecular substance, in addition, it can be compiled molecular formula.
Examples of molecular formulas: water - H 2 O, oxygen - O 2, sulfur - S 8, phosphorus oxide - P 4 O 10, butane - C 4 H 10, phosphoric acid - H 3 PO 4.
Non-molecular substances do not have molecular formulas.
The sequence of writing element symbols in simple and molecular formulas is determined by the rules of chemical language, which you will become familiar with as you study chemistry. The information conveyed by these formulas is not affected by the sequence of symbols.
Of the signs reflecting the structure of substances, we will only use for now valence stroke("dash"). This sign shows the presence between the atoms of the so-called covalent bond(what type of connection this is and what its features are, you will soon find out).
In a water molecule, an oxygen atom is connected by simple (single) bonds to two hydrogen atoms, but the hydrogen atoms are not connected to each other. This is precisely what the structural formula of water clearly shows. 
Another example: the sulfur molecule S8. In this molecule, 8 sulfur atoms form an eight-membered ring, in which each sulfur atom is connected to two other atoms by simple bonds. Compare the structural formula of sulfur with the three-dimensional model of its molecule shown in Fig. 3. Please note that the structural formula of sulfur does not convey the shape of its molecule, but only shows the sequence of connection of atoms by covalent bonds.
The structural formula of phosphoric acid shows that in the molecule of this substance one of the four oxygen atoms is connected only to the phosphorus atom by a double bond, and the phosphorus atom, in turn, is connected to three more oxygen atoms by single bonds. Each of these three oxygen atoms is also connected by a simple bond to one of the three hydrogen atoms present in the molecule.
Compare the following three-dimensional model of a methane molecule with its spatial, structural and molecular formula:
|
|
|
In the spatial formula of methane, wedge-shaped valence strokes, as if in perspective, show which of the hydrogen atoms is “closer to us” and which is “further from us”.
Sometimes the spatial formula indicates bond lengths and angles between bonds in a molecule, as is shown in the example of a water molecule.
Non-molecular substances do not contain molecules. For the convenience of chemical calculations in a non-molecular substance, the so-called formula unit.
Examples of the composition of formula units of some substances: 1) silicon dioxide (quartz sand, quartz) SiO 2 – a formula unit consists of one silicon atom and two oxygen atoms; 2) sodium chloride (table salt) NaCl – the formula unit consists of one sodium atom and one chlorine atom; 3) iron Fe - a formula unit consists of one iron atom. Like a molecule, a formula unit is the smallest portion of a substance that retains its chemical properties.
Table 4
Information conveyed by different types of formulas
Formula type |
Information conveyed by the formula. |
|
| The simplest Molecular Structural Spatial |
|
|
Let us now consider, using examples, what information different types of formulas give us.
1. Substance: acetic acid. The simplest formula is CH 2 O, molecular formula is C 2 H 4 O 2, structural formula
The simplest formula tells us that
1) acetic acid contains carbon, hydrogen and oxygen;
2) in this substance the number of carbon atoms relates to the number of hydrogen atoms and the number of oxygen atoms, as 1: 2: 1, that is N H: N C: N O = 1:2:1.
Molecular formula adds that
3) in a molecule of acetic acid there are 2 carbon atoms, 4 hydrogen atoms and 2 oxygen atoms.
Structural formula adds that
4, 5) in a molecule two carbon atoms are connected to each other by a simple bond; one of them, in addition, is connected to three hydrogen atoms, each with a single bond, and the other to two oxygen atoms, one with a double bond and the other with a single bond; the last oxygen atom is still connected by a simple bond to the fourth hydrogen atom.
2. Substance: sodium chloride.
The simplest formula is NaCl.
1) Sodium chloride contains sodium and chlorine.
2) In this substance, the number of sodium atoms is equal to the number of chlorine atoms.
3. Substance: iron.
The simplest formula is Fe.
1) This substance contains only iron, that is, it is a simple substance.
4. Substance: trimetaphosphoric acid . The simplest formula is HPO 3, molecular formula is H 3 P 3 O 9, structural formula
1) Trimetaphosphoric acid contains hydrogen, phosphorus and oxygen.
2) N H: N P: N O = 1:1:3.
3) The molecule consists of three hydrogen atoms, three phosphorus atoms and nine oxygen atoms.
4, 5) Three phosphorus atoms and three oxygen atoms, alternating, form a six-membered cycle. All connections in the cycle are simple. Each phosphorus atom is, in addition, connected to two more oxygen atoms, one with a double bond and the other with a single bond. Each of the three oxygen atoms connected by simple bonds to phosphorus atoms is also connected by a simple bond to a hydrogen atom.
| Phosphoric acid – H 3 PO 4(another name is orthophosphoric acid) is a transparent, colorless, crystalline substance of molecular structure that melts at 42 o C. This substance dissolves very well in water and even absorbs water vapor from the air (hygroscopic). Phosphoric acid is produced in large quantities and is used primarily in the production of phosphate fertilizers, but also in the chemical industry, in the production of matches and even in construction. In addition, phosphoric acid is used in the manufacture of cement in dental technology and is included in many medicines. This acid is quite cheap, so in some countries, such as the United States, very pure phosphoric acid, highly diluted with water, is added to refreshing drinks to replace the expensive citric acid. |
| Methane - CH 4. If you have a gas stove at home, then you encounter this substance every day: the natural gas that burns in the burners of your stove consists of 95% methane. Methane is a colorless and odorless gas with a boiling point of –161 o C. When mixed with air, it is explosive, which explains the explosions and fires that sometimes occur in coal mines (another name for methane is firedamp). The third name for methane - swamp gas - is due to the fact that bubbles of this particular gas rise from the bottom of swamps, where it is formed as a result of the activity of certain bacteria. In industry, methane is used as fuel and raw material for the production of other substances. Methane is the simplest hydrocarbon. This class of substances also includes ethane (C 2 H 6), propane (C 3 H 8), ethylene (C 2 H 4), acetylene (C 2 H 2) and many other substances. |
Table 5.Examples of different types of formulas for some substances-
Chemistry is the science of substances, their properties and transformations
.
That is, if nothing happens to the substances around us, then this does not apply to chemistry. But what does “nothing happens” mean? If a thunderstorm suddenly caught us in the field, and we were all wet, as they say, “to the skin,” then isn’t this a transformation: after all, the clothes were dry, but they became wet.
If, for example, you take an iron nail, file it, and then assemble iron filings (Fe) , then isn’t this also a transformation: there was a nail - it became powder. But if you then assemble the device and carry out obtaining oxygen (O 2): heat up potassium permanganate(KMpO 4) and collect oxygen in a test tube, and then place these red-hot iron filings into it, then they will flare up with a bright flame and after combustion will turn into a brown powder. And this is also a transformation. So where is the chemistry? Despite the fact that in these examples the shape (iron nail) and the condition of the clothing (dry, wet) change, these are not transformations. The fact is that the nail itself was a substance (iron), and remained so, despite its different shape, and our clothes absorbed the water from the rain and then evaporated it into the atmosphere. The water itself has not changed. So what are transformations from a chemical point of view?
From a chemical point of view, transformations are those phenomena that are accompanied by a change in the composition of a substance. Let's take the same nail as an example. It doesn’t matter what shape it took after being filed, but after the pieces collected from it iron filings placed in an oxygen atmosphere - it turned into iron oxide(Fe 2 O 3 ) . So, something has changed after all? Yes, it has changed. There was a substance called a nail, but under the influence of oxygen a new substance was formed - element oxide gland. Molecular equation This transformation can be represented by the following chemical symbols:
4Fe + 3O 2 = 2Fe 2 O 3 (1)
For someone uninitiated in chemistry, questions immediately arise. What is "molecular equation", what is Fe? Why are the numbers “4”, “3”, “2”? What are the little numbers “2” and “3” in the formula Fe 2 O 3? This means it’s time to sort everything out in order.
Signs of chemical elements.
Despite the fact that chemistry begins to be studied in the 8th grade, and some even earlier, many people know the great Russian chemist D.I. Mendeleev. And of course, his famous “Periodic Table of Chemical Elements”. Otherwise, more simply, it is called the “Periodical Table”.
In this table, the elements are arranged in the appropriate order. To date, about 120 of them are known. The names of many elements have been known to us for a long time. These are: iron, aluminum, oxygen, carbon, gold, silicon. Previously, we used these words without thinking, identifying them with objects: an iron bolt, an aluminum wire, oxygen in the atmosphere, a gold ring, etc. etc. But in fact, all these substances (bolt, wire, ring) consist of their corresponding elements. The whole paradox is that the element cannot be touched or picked up. How so? They are in the periodic table, but you can’t take them! Yes exactly. A chemical element is an abstract (that is, abstract) concept, and is used in chemistry, as well as in other sciences, for calculations, drawing up equations, and solving problems. Each element differs from the other in that it has its own characteristic electronic configuration of an atom. The number of protons in the nucleus of an atom is equal to the number of electrons in its orbitals. For example, hydrogen is element No. 1. Its atom consists of 1 proton and 1 electron. Helium is element #2. Its atom consists of 2 protons and 2 electrons. Lithium is element #3. Its atom consists of 3 protons and 3 electrons. Darmstadtium – element No. 110. Its atom consists of 110 protons and 110 electrons.
Each element is designated by a certain symbol, Latin letters, and has a certain reading translated from Latin. For example, hydrogen has the symbol "N", read as "hydrogenium" or "ash". Silicon has the symbol "Si" read as "silicium". Mercury has a symbol "Hg" and is read as "hydrargyrum". And so on. All these notations can be found in any 8th grade chemistry textbook. The main thing for us now is to understand that when composing chemical equations, it is necessary to operate with the indicated symbols of the elements.
Simple and complex substances.
Denoting various substances with single symbols of chemical elements (Hg mercury, Fe iron, Cu copper, Zn zinc, Al aluminum) we essentially denote simple substances, that is, substances consisting of atoms of the same type (containing the same number of protons and neutrons in an atom). For example, if the substances iron and sulfur interact, then the equation will take the following writing form:
Fe + S = FeS (2)
Simple substances include metals (Ba, K, Na, Mg, Ag), as well as non-metals (S, P, Si, Cl 2, N 2, O 2, H 2). Moreover, one should pay attention
special attention to the fact that all metals are designated by single symbols: K, Ba, Ca, Al, V, Mg, etc., and non-metals are either simple symbols: C, S, P or may have different indices that indicate their molecular structure: H 2, Cl 2, O 2, J 2, P 4, S 8. In the future, this will be very important when composing equations. It is not at all difficult to guess that complex substances are substances formed from atoms of different types, for example,
1). Oxides:
aluminium oxide Al 2 O 3, 
sodium oxide Na2O,
copper oxide CuO,
zinc oxide ZnO,
titanium oxide Ti2O3,
carbon monoxide or carbon monoxide (+2) CO,
sulfur oxide (+6) SO 3
2). Reasons:
iron hydroxide(+3) Fe(OH) 3,
copper hydroxide Cu(OH)2,
potassium hydroxide or alkali potassium KOH,
sodium hydroxide NaOH.
3). Acids:
hydrochloric acid HCl,
sulfurous acid H2SO3,
Nitric acid HNO3
4). Salts:
sodium thiosulfate Na 2 S 2 O 3 ,
sodium sulfate or Glauber's salt Na2SO4,
calcium carbonate or limestone CaCO 3,
copper chloride CuCl2
5). Organic matter:
sodium acetate CH 3 COONa,
methane CH 4,
acetylene C 2 H 2,
glucose C 6 H 12 O 6
Finally, after we have figured out the structure of various substances, we can begin to write chemical equations.
Chemical equation.
The word “equation” itself is derived from the word “equalize”, i.e. divide something into equal parts. In mathematics, equations constitute almost the very essence of this science. For example, you can give a simple equation in which the left and right sides will be equal to “2”:
40: (9 + 11) = (50 x 2) : (80 – 30);
And in chemical equations the same principle: the left and right sides of the equation must correspond to the same numbers of atoms and elements participating in them. Or, if an ionic equation is given, then in it number of particles must also meet this requirement. A chemical equation is a conventional representation of a chemical reaction using chemical formulas and mathematical symbols. A chemical equation inherently reflects one or another chemical reaction, that is, the process of interaction of substances, during which new substances arise. For example, it is necessary write a molecular equation reactions in which they take part barium chloride BaCl 2 and sulfuric acid H 2 SO 4. As a result of this reaction, an insoluble precipitate is formed - barium sulfate BaSO 4 and hydrochloric acid HCl:
BaCl 2 + H 2 SO 4 = BaSO 4 + 2HCl (3)
First of all, it is necessary to understand that the large number “2” standing in front of the substance HCl is called a coefficient, and the small numbers “2”, “4” under the formulas BaCl 2, H 2 SO 4, BaSO 4 are called indices. Both coefficients and indices in chemical equations act as multipliers, not summands. To write a chemical equation correctly, you need assign coefficients in the reaction equation. Now let's start counting the atoms of the elements on the left and right sides of the equation. On the left side of the equation: the substance BaCl 2 contains 1 barium atom (Ba), 2 chlorine atoms (Cl). In the substance H 2 SO 4: 2 hydrogen atoms (H), 1 sulfur atom (S) and 4 oxygen atoms (O). On the right side of the equation: in the BaSO 4 substance there is 1 barium atom (Ba), 1 sulfur atom (S) and 4 oxygen atoms (O), in the HCl substance: 1 hydrogen atom (H) and 1 chlorine atom (Cl). It follows that on the right side of the equation the number of hydrogen and chlorine atoms is half as much as on the left side. Therefore, before the HCl formula on the right side of the equation, it is necessary to put the coefficient “2”. If we now add up the numbers of atoms of the elements participating in this reaction, both on the left and on the right, we obtain the following balance:
In both sides of the equation, the numbers of atoms of the elements participating in the reaction are equal, therefore it is composed correctly.
Chemical equation and chemical reactions
As we have already found out, chemical equations are a reflection of chemical reactions. Chemical reactions are those phenomena during which the transformation of one substance into another occurs. Among their diversity, two main types can be distinguished:
1). Compound reactions
2). Decomposition reactions.
The overwhelming majority of chemical reactions belong to addition reactions, since changes in its composition can rarely occur with an individual substance if it is not exposed to external influences (dissolution, heating, exposure to light). Nothing characterizes a chemical phenomenon or reaction better than the changes that occur during the interaction of two or more substances. Such phenomena can occur spontaneously and be accompanied by an increase or decrease in temperature, light effects, color changes, sediment formation, release of gaseous products, and noise.
For clarity, we present several equations reflecting the processes of compound reactions, during which we obtain sodium chloride(NaCl), zinc chloride(ZnCl2), silver chloride precipitate(AgCl), aluminum chloride(AlCl 3)
Cl 2 + 2Nа = 2NaCl (4)
CuCl 2 + Zn = ZnCl 2 + Cu (5)
AgNO 3 + KCl = AgCl + 2KNO 3 (6)
3HCl + Al(OH) 3 = AlCl 3 + 3H 2 O (7)
Among the reactions of the compound, special mention should be made of the following: : substitution (5), exchange (6), and as a special case of an exchange reaction - the reaction neutralization (7).
Substitution reactions include those in which atoms of a simple substance replace atoms of one of the elements in a complex substance. In example (5), zinc atoms replace copper atoms from the CuCl 2 solution, while zinc passes into the soluble salt ZnCl 2, and copper is released from the solution in the metallic state.
Exchange reactions include those reactions in which two complex substances exchange their constituent parts. In the case of reaction (6), the soluble salts AgNO 3 and KCl, when both solutions are merged, form an insoluble precipitate of the AgCl salt. At the same time, they exchange their constituent parts - cations and anions. Potassium cations K + are added to the NO 3 anions, and silver cations Ag + are added to the Cl - anions.
A special, special case of exchange reactions is the neutralization reaction. Neutralization reactions include those reactions in which acids react with bases, resulting in the formation of salt and water. In example (7), hydrochloric acid HCl reacts with the base Al(OH) 3 to form the salt AlCl 3 and water. In this case, aluminum cations Al 3+ from the base are exchanged with Cl - anions from the acid. What happens in the end neutralization of hydrochloric acid.
Decomposition reactions include those in which two or more new simple or complex substances, but of a simpler composition, are formed from one complex substance. Examples of reactions include those in the process of which 1) decomposes. Potassium nitrate(KNO 3) with the formation of potassium nitrite (KNO 2) and oxygen (O 2); 2). Potassium permanganate(KMnO 4): potassium manganate (K 2 MnO 4) is formed, manganese oxide(MnO 2) and oxygen (O 2); 3). Calcium carbonate or marble; in the process are formed carbonicgas(CO2) and calcium oxide(CaO)
2KNO 3 = 2KNO 2 + O 2 (8)
2KMnO 4 = K 2 MnO 4 + MnO 2 + O 2 (9)
CaCO 3 = CaO + CO 2 (10)
In reaction (8), one complex and one simple substance are formed from a complex substance. In reaction (9) there are two complex and one simple. In reaction (10) there are two complex substances, but simpler in composition
All classes of complex substances are subject to decomposition:
1). Oxides: silver oxide 2Ag 2 O = 4Ag + O 2 (11)
2). Hydroxides: iron hydroxide 2Fe(OH) 3 = Fe 2 O 3 + 3H 2 O (12)
3). Acids: sulfuric acid H 2 SO 4 = SO 3 + H 2 O (13)
4). Salts: calcium carbonate CaCO 3 = CaO + CO 2 (14)
5). Organic matter: alcoholic fermentation of glucose
C 6 H 12 O 6 = 2C 2 H 5 OH + 2CO 2 (15)
According to another classification, all chemical reactions can be divided into two types: reactions that release heat are called exothermic, and reactions that occur with the absorption of heat - endothermic. The criterion for such processes is thermal effect of the reaction. As a rule, exothermic reactions include oxidation reactions, i.e. interaction with oxygen, for example methane combustion:
CH 4 + 2O 2 = CO 2 + 2H 2 O + Q (16)
and to endothermic reactions - decomposition reactions already given above (11) - (15). The Q sign at the end of the equation indicates whether heat is released (+Q) or absorbed (-Q) during the reaction:
CaCO 3 = CaO+CO 2 - Q (17)
You can also consider all chemical reactions according to the type of change in the degree of oxidation of the elements involved in their transformations. For example, in reaction (17), the elements participating in it do not change their oxidation states:
Ca +2 C +4 O 3 -2 = Ca +2 O -2 +C +4 O 2 -2 (18)
And in reaction (16), the elements change their oxidation states:
2Mg 0 + O 2 0 = 2Mg +2 O -2
Reactions of this type are redox . They will be considered separately. To compose equations for reactions of this type, you must use half-reaction method and apply electronic balance equation.
After presenting the various types of chemical reactions, you can proceed to the principle of composing chemical equations, or, in other words, selecting coefficients on the left and right sides.
Mechanisms for composing chemical equations.
Whatever type a chemical reaction belongs to, its recording (chemical equation) must correspond to the condition that the number of atoms before and after the reaction is equal.
There are equations (17) that do not require equalization, i.e. placement of coefficients. But in most cases, as in examples (3), (7), (15), it is necessary to take actions aimed at equalizing the left and right sides of the equation. What principles should be followed in such cases? Is there any system for selecting odds? There is, and not only one. Such systems include:
1). Selection of coefficients according to given formulas.
2). Compilation by valences of reacting substances.
3). Arrangement of reacting substances according to oxidation states.
In the first case, it is assumed that we know the formulas of the reacting substances both before and after the reaction. For example, given the following equation:
N 2 + O 2 →N 2 O 3 (19)
It is generally accepted that until equality is established between the atoms of the elements before and after the reaction, the equal sign (=) is not placed in the equation, but is replaced by an arrow (→). Now let's get down to the actual adjustment. On the left side of the equation there are 2 nitrogen atoms (N 2) and two oxygen atoms (O 2), and on the right side there are two nitrogen atoms (N 2) and three oxygen atoms (O 3). There is no need to equalize it in terms of the number of nitrogen atoms, but in terms of oxygen it is necessary to achieve equality, since before the reaction there were two atoms involved, and after the reaction there were three atoms. Let's make the following diagram:
before reaction after reaction
O 2 O 3
Let's determine the smallest multiple between the given numbers of atoms, it will be “6”.
O 2 O 3
\ 6 /
Let's divide this number on the left side of the oxygen equation by “2”. We get the number “3” and put it into the equation to be solved:
N 2 + 3O 2 →N 2 O 3
We also divide the number “6” for the right side of the equation by “3”. We get the number “2”, and also put it in the equation to be solved:
N 2 + 3O 2 → 2N 2 O 3
The numbers of oxygen atoms on both the left and right sides of the equation became equal, respectively, 6 atoms each:
But the number of nitrogen atoms on both sides of the equation will not correspond to each other:
The left one has two atoms, the right one has four atoms. Therefore, in order to achieve equality, it is necessary to double the amount of nitrogen on the left side of the equation, setting the coefficient to “2”:
Thus, equality in nitrogen is observed and, in general, the equation takes the form:
2N 2 + 3О 2 → 2N 2 О 3
Now in the equation you can put an equal sign instead of an arrow:
2N 2 + 3О 2 = 2N 2 О 3 (20)
Let's give another example. The following reaction equation is given:
P + Cl 2 → PCl 5
On the left side of the equation there is 1 phosphorus atom (P) and two chlorine atoms (Cl 2), and on the right side there is one phosphorus atom (P) and five oxygen atoms (Cl 5). There is no need to equalize it in terms of the number of phosphorus atoms, but in terms of chlorine it is necessary to achieve equality, since before the reaction there were two atoms involved, and after the reaction there were five atoms. Let's make the following diagram:
before reaction after reaction
Cl 2 Cl 5
Let's determine the smallest multiple between the given numbers of atoms, it will be “10”.
Cl 2 Cl 5
\ 10 /
Divide this number on the left side of the chlorine equation by “2”. Let’s get the number “5” and put it into the equation to be solved:
P + 5Cl 2 → PCl 5
We also divide the number “10” for the right side of the equation by “5”. We get the number “2”, and also put it in the equation to be solved:
P + 5Cl 2 → 2РCl 5
The numbers of chlorine atoms on both the left and right sides of the equation became equal, respectively, 10 atoms each:
But the number of phosphorus atoms on both sides of the equation will not correspond to each other:
Therefore, in order to achieve equality, it is necessary to double the amount of phosphorus on the left side of the equation by setting the coefficient “2”:
Thus, equality for phosphorus is observed and, in general, the equation takes the form:
2Р + 5Cl 2 = 2РCl 5 (21)
When composing equations by valencies must be given valency determination and set values for the most famous elements. Valence is one of the previously used concepts, but is currently not used in a number of school programs. But with its help it is easier to explain the principles of drawing up equations of chemical reactions. Valence is understood as the number of chemical bonds that an atom can form with another or other atoms . Valency does not have a sign (+ or -) and is indicated by Roman numerals, usually above the symbols of chemical elements, for example:
Where do these values come from? How to use them when writing chemical equations? The numerical values of the valences of the elements coincide with their group number of the Periodic Table of Chemical Elements by D.I. Mendeleev (Table 1).
For other elements valence values may have other values, but never greater than the number of the group in which they are located. Moreover, for even group numbers (IV and VI), the valences of elements take only even values, and for odd ones they can have both even and odd values (Table 2).
Of course, there are exceptions to the valence values for some elements, but in each specific case these points are usually specified. Now let's consider the general principle of composing chemical equations based on given valences for certain elements. Most often, this method is acceptable in the case of drawing up equations of chemical reactions of compounds of simple substances, for example, when interacting with oxygen ( oxidation reactions). Let's say you need to display an oxidation reaction aluminum. But let us recall that metals are designated by single atoms (Al), and non-metals in the gaseous state are designated by the indices “2” - (O 2). First, let's write the general reaction scheme:
Al + О 2 →AlО
At this stage, it is not yet known what the correct spelling should be for aluminum oxide. And it is precisely at this stage that knowledge of the valences of elements will come to our aid. For aluminum and oxygen, let’s put them above the expected formula of this oxide:
III II
Al O
After that, “cross”-on-“cross” for these element symbols we will put the corresponding indices at the bottom:
III II
Al 2 O 3
Composition of a chemical compound Al 2 O 3 determined. The further diagram of the reaction equation will take the form:
Al+ O 2 →Al 2 O 3
All that remains is to equalize its left and right parts. Let us proceed in the same way as in the case of composing equation (19). Let's equalize the numbers of oxygen atoms by finding the smallest multiple:
before reaction after reaction
O 2 O 3
\ 6 /
Let's divide this number on the left side of the oxygen equation by “2”. Let’s get the number “3” and put it into the equation being solved. We also divide the number “6” for the right side of the equation by “3”. We get the number “2”, and also put it in the equation to be solved:
Al + 3O 2 → 2Al 2 O 3
To achieve equality in aluminum, it is necessary to adjust its quantity on the left side of the equation by setting the coefficient to “4”:
4Al + 3O 2 → 2Al 2 O 3
Thus, equality for aluminum and oxygen is observed and, in general, the equation will take its final form:
4Al + 3O 2 = 2Al 2 O 3 (22)
Using the valence method, you can predict what substance is formed during a chemical reaction and what its formula will look like. Let’s assume that the compound reacted with nitrogen and hydrogen with the corresponding valences III and I. Let’s write the general reaction scheme:
N 2 + N 2 → NH
For nitrogen and hydrogen, let’s put the valencies above the expected formula of this compound:
As before, “cross”-on-“cross” for these element symbols, let’s put the corresponding indices below:
III I
NH 3
The further diagram of the reaction equation will take the form:
N 2 + N 2 → NH 3
Equating in a well-known way, through the smallest multiple for hydrogen equal to “6”, we obtain the required coefficients and the equation as a whole:
N 2 + 3H 2 = 2NH 3 (23)
When composing equations according to oxidation states reactants, it is necessary to recall that the oxidation state of a particular element is the number of electrons accepted or given up during a chemical reaction. Oxidation state in compounds Basically, it numerically coincides with the valence values of the element. But they differ in sign. For example, for hydrogen, the valence is I, and the oxidation state is (+1) or (-1). For oxygen, the valence is II, and the oxidation state is -2. For nitrogen, the valences are I, II, III, IV, V, and the oxidation states are (-3), (+1), (+2), (+3), (+4), (+5), etc. . The oxidation states of the elements most often used in equations are given in Table 3.
In the case of compound reactions, the principle of compiling equations by oxidation states is the same as when compiling by valences. For example, let us give the equation for the oxidation of chlorine with oxygen, in which chlorine forms a compound with an oxidation state of +7. Let's write down the proposed equation:
Cl 2 + O 2 → ClO
Let us place the oxidation states of the corresponding atoms over the proposed compound ClO:
As in previous cases, we establish that the required compound formula will take the form:
7 -2
Cl 2 O 7
The reaction equation will take the following form:
Cl 2 + O 2 → Cl 2 O 7
Equating for oxygen, finding the smallest multiple between two and seven, equal to “14,” we ultimately establish the equality:
2Cl 2 + 7O 2 = 2Cl 2 O 7 (24)
A slightly different method must be used with oxidation states when composing exchange, neutralization, and substitution reactions. In some cases, it is difficult to find out: what compounds are formed during the interaction of complex substances?
How to find out: what will happen in the reaction process?
Indeed, how do you know what reaction products may arise during a particular reaction? For example, what is formed when barium nitrate and potassium sulfate react?
Ba(NO 3) 2 + K 2 SO 4 → ?
Maybe BaK 2 (NO 3) 2 + SO 4? Or Ba + NO 3 SO 4 + K 2? Or something else? Of course, during this reaction the following compounds are formed: BaSO 4 and KNO 3. How is this known? And how to write the formulas of substances correctly? Let's start with what is most often overlooked: the very concept of “exchange reaction.” This means that in these reactions substances change their constituent parts with each other. Since exchange reactions are mostly carried out between bases, acids or salts, the parts with which they will be exchanged are metal cations (Na +, Mg 2+, Al 3+, Ca 2+, Cr 3+), H + ions or OH -, anions - acid residues, (Cl -, NO 3 2-, SO 3 2-, SO 4 2-, CO 3 2-, PO 4 3-). In general, the exchange reaction can be given in the following notation:
Kt1An1 + Kt2An1 = Kt1An2 + Kt2An1 (25)
Where Kt1 and Kt2 are metal cations (1) and (2), and An1 and An2 are their corresponding anions (1) and (2). In this case, it is necessary to take into account that in compounds before and after the reaction, cations are always installed in first place, and anions are in second place. Therefore, if the reaction occurs potassium chloride And silver nitrate, both in dissolved state
KCl + AgNO 3 →
then in its process the substances KNO 3 and AgCl are formed and the corresponding equation will take the form:
KCl + AgNO 3 =KNO 3 + AgCl (26)
During neutralization reactions, protons from acids (H +) will combine with hydroxyl anions (OH -) to form water (H 2 O):
HCl + KOH = KCl + H 2 O (27)
The oxidation states of metal cations and the charges of anions of acidic residues are indicated in the table of solubility of substances (acids, salts and bases in water). The horizontal line shows metal cations, and the vertical line shows the anions of acid residues.
Based on this, when drawing up an equation for an exchange reaction, it is first necessary to establish on the left side the oxidation states of the particles receiving in this chemical process. For example, you need to write an equation for the interaction between calcium chloride and sodium carbonate. Let’s create the initial diagram of this reaction:
CaCl + NaCO 3 →
Ca 2+ Cl - + Na + CO 3 2- →
Having performed the already known “cross”-on-“cross” action, we determine the real formulas of the starting substances:
CaCl 2 + Na 2 CO 3 →
Based on the principle of exchange of cations and anions (25), we will establish preliminary formulas for the substances formed during the reaction:
CaCl 2 + Na 2 CO 3 → CaCO 3 + NaCl
Let us place the corresponding charges above their cations and anions:
Ca 2+ CO 3 2- + Na + Cl -
Substance formulas written correctly, in accordance with the charges of cations and anions. Let's create a complete equation, equalizing its left and right sides for sodium and chlorine:
CaCl 2 + Na 2 CO 3 = CaCO 3 + 2NaCl (28)
As another example, here is the equation for the neutralization reaction between barium hydroxide and phosphoric acid:
VaON + NPO 4 →
Let us place the corresponding charges over the cations and anions:
Ba 2+ OH - + H + PO 4 3- →
Let's determine the real formulas of the starting substances:
Ba(OH) 2 + H 3 PO 4 →
Based on the principle of exchange of cations and anions (25), we will establish preliminary formulas for the substances formed during the reaction, taking into account that during an exchange reaction one of the substances must necessarily be water:
Ba(OH) 2 + H 3 PO 4 → Ba 2+ PO 4 3- + H 2 O
Let us determine the correct notation for the formula of the salt formed during the reaction:
Ba(OH) 2 + H 3 PO 4 → Ba 3 (PO 4) 2 + H 2 O
Let's equalize the left side of the equation for barium:
3Ba (OH) 2 + H 3 PO 4 → Ba 3 (PO 4) 2 + H 2 O
Since on the right side of the equation the orthophosphoric acid residue is taken twice, (PO 4) 2, then on the left it is also necessary to double its amount:
3Ba (OH) 2 + 2H 3 PO 4 → Ba 3 (PO 4) 2 + H 2 O
It remains to match the number of hydrogen and oxygen atoms on the right side of water. Since on the left the total number of hydrogen atoms is 12, on the right it must also correspond to twelve, therefore before the formula of water it is necessary set the coefficient“6” (since the water molecule already has 2 hydrogen atoms). For oxygen, equality is also observed: on the left is 14 and on the right is 14. So, the equation has the correct written form:
3Ba (OH) 2 + 2H 3 PO 4 → Ba 3 (PO 4) 2 + 6H 2 O (29)
Possibility of chemical reactions
The world consists of a great variety of substances. The number of variants of chemical reactions between them is also incalculable. But can we, having written this or that equation on paper, say that a chemical reaction will correspond to it? There is a misconception that if it is correct set the odds in the equation, then it will be feasible in practice. For example, if we take sulfuric acid solution and put it in it zinc, then you can observe the process of hydrogen evolution:
Zn+ H 2 SO 4 = ZnSO 4 + H 2 (30)
But if copper is dropped into the same solution, then the process of gas evolution will not be observed. The reaction is not feasible.
Cu+ H 2 SO 4 ≠
If concentrated sulfuric acid is taken, it will react with copper:
Cu + 2H 2 SO 4 = CuSO 4 + SO 2 + 2H 2 O (31)
In reaction (23) between the gases nitrogen and hydrogen, we observe thermodynamic equilibrium, those. how many molecules ammonia NH 3 is formed per unit time, the same amount of them will decompose back into nitrogen and hydrogen. Chemical equilibrium shift can be achieved by increasing pressure and decreasing temperature
N 2 + 3H 2 = 2NH 3
If you take potassium hydroxide solution and pour it on him sodium sulfate solution, then no changes will be observed, the reaction will not be feasible:
KOH + Na 2 SO 4 ≠
Sodium chloride solution when interacting with bromine, it will not form bromine, despite the fact that this reaction can be classified as a substitution reaction:
NaCl + Br 2 ≠
What are the reasons for such discrepancies? The point is that it is not enough just to correctly determine compound formulas, it is necessary to know the specifics of the interaction of metals with acids, skillfully use the table of solubility of substances, and know the rules of substitution in the activity series of metals and halogens. This article outlines only the most basic principles of how assign coefficients in reaction equations, How write molecular equations, How determine the composition of a chemical compound.
Chemistry, as a science, is extremely diverse and multifaceted. The above article reflects only a small part of the processes occurring in the real world. Types, thermochemical equations, electrolysis, processes of organic synthesis and much, much more. But more on that in future articles.
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Well, to complete our acquaintance with alcohols, I will also give the formula of another well-known substance - cholesterol. Not everyone knows that it is a monohydric alcohol!
|`/`\\`|<`|w>`\`/|<`/w$color(red)HO$color()>\/`|0/`|/\<`|w>|_q_q_q<-dH>:a_q|0<|dH>`/<`|wH>`\|dH; #a_(A-72)<_(A-120,d+)>-/-/<->`\
I marked the hydroxyl group in it in red.
Carboxylic acids
Any winemaker knows that wine should be stored without access to air. Otherwise it will turn sour. But chemists know the reason - if you add another oxygen atom to an alcohol, you get an acid.Let's look at the formulas of acids that are obtained from alcohols already familiar to us:
| Substance | Skeletal formula | Gross formula | ||
|---|---|---|---|---|
| Methane acid (formic acid) |
H/C`|O|\OH | HCOOH | O//\OH | |
| Ethanoic acid (acetic acid) |
H-C-C\O-H; H|#C|H | CH3-COOH | /`|O|\OH | |
| Propanic acid (methylacetic acid) |
H-C-C-C\O-H; H|#2|H; H|#3|H | CH3-CH2-COOH | \/`|O|\OH | |
| Butanoic acid (butyric acid) |
H-C-C-C-C\O-H; H|#2|H; H|#3|H; H|#4|H | CH3-CH2-CH2-COOH | /\/`|O|\OH | |
| Generalized formula | (R)-C\O-H | (R)-COOH or (R)-CO2H | (R)/`|O|\OH |
A distinctive feature of organic acids is the presence of a carboxyl group (COOH), which gives such substances acidic properties.
Anyone who has tried vinegar knows that it is very sour. The reason for this is the presence of acetic acid in it. Typically table vinegar contains between 3 and 15% acetic acid, with the rest (mostly) water. Consumption of acetic acid in undiluted form poses a danger to life.
Carboxylic acids can have multiple carboxyl groups. In this case they are called: dibasic, tribasic etc...
Food products contain many other organic acids. Here are just a few of them:
The name of these acids corresponds to the food products in which they are contained. By the way, please note that here there are acids that also have a hydroxyl group, characteristic of alcohols. Such substances are called hydroxycarboxylic acids(or hydroxy acids).
Below, under each of the acids, there is a sign specifying the name of the group of organic substances to which it belongs.
Radicals
Radicals are another concept that has influenced chemical formulas. The word itself is probably known to everyone, but in chemistry radicals have nothing in common with politicians, rebels and other citizens with an active position.
Here these are just fragments of molecules. And now we will figure out what makes them special and get acquainted with a new way of writing chemical formulas.
Generalized formulas have already been mentioned several times in the text: alcohols - (R)-OH and carboxylic acids - (R)-COOH. Let me remind you that -OH and -COOH are functional groups. But R is a radical. It’s not for nothing that he is depicted as the letter R.
To be more specific, a monovalent radical is a part of a molecule lacking one hydrogen atom. Well, if you subtract two hydrogen atoms, you get a divalent radical.
Radicals in chemistry received their own names. Some of them even received Latin designations similar to the designations of the elements. And besides, sometimes in formulas radicals can be indicated in abbreviated form, more reminiscent of gross formulas.
All this is demonstrated in the following table.
| Name | Structural formula | Designation | Brief formula | Example of alcohol | ||
|---|---|---|---|---|---|---|
| Methyl | CH3-() | Me | CH3 | (Me)-OH | CH3OH | |
| Ethyl | CH3-CH2-() | Et | C2H5 | (Et)-OH | C2H5OH | |
| I cut through | CH3-CH2-CH2-() | Pr | C3H7 | (Pr)-OH | C3H7OH | |
| Isopropyl | H3C\CH(*`/H3C*)-() | i-Pr | C3H7 | (i-Pr)-OH | (CH3)2CHOH | |
| Phenyl | `/`=`\//-\\-{} | Ph | C6H5 | (Ph)-OH | C6H5OH | |
I think everything is clear here. I just want to draw your attention to the column where examples of alcohols are given. Some radicals are written in a form that resembles the gross formula, but the functional group is written separately. For example, CH3-CH2-OH turns into C2H5OH.
And for branched chains like isopropyl, structures with brackets are used.
There is also such a phenomenon as free radicals. These are radicals that, for some reason, have separated from functional groups. In this case, one of the rules with which we began studying the formulas is violated: the number of chemical bonds no longer corresponds to the valency of one of the atoms. Well, or we can say that one of the connections becomes open at one end. Free radicals usually live for a short time as the molecules tend to return to a stable state.
Introduction to nitrogen. Amines
I propose to get acquainted with another element that is part of many organic compounds. This nitrogen.
It is denoted by the Latin letter N and has a valency of three.
Let's see what substances are obtained if nitrogen is added to the familiar hydrocarbons:
| Substance | Expanded structural formula | Simplified structural formula | Skeletal formula | Gross formula |
|---|---|---|---|---|
| Aminomethane (methylamine) |
H-C-N\H;H|#C|H | CH3-NH2 | \NH2 | |
| Aminoethane (ethylamine) |
H-C-C-N\H;H|#C|H;H|#3|H | CH3-CH2-NH2 | /\NH2 | |
| Dimethylamine | H-C-N<`|H>-C-H; H|#-3|H; H|#2|H | $L(1.3)H/N<_(A80,w+)CH3>\dCH3 | /N<_(y-.5)H>\ | |
| Aminobenzene (Aniline) |
H\N|C\\C|C<\H>`//C<|H>`\C<`/H>`||C<`\H>/ | NH2|C\\CH|CH`//C<_(y.5)H>`\HC`||HC/ | NH2|\|`/`\`|/_o | |
| Triethylamine | $slope(45)H-C-C/N\C-C-H;H|#2|H; H|#3|H; H|#5|H;H|#6|H; #N`|C<`-H><-H>`|C<`-H><-H>`|H | CH3-CH2-N<`|CH2-CH3>-CH2-CH3 | \/N<`|/>\| |
As you probably already guessed from the names, all these substances are united under the general name amines. The functional group ()-NH2 is called amino group. Here are some general formulas of amines:
In general, there are no special innovations here. If these formulas are clear to you, then you can safely engage in further study of organic chemistry using a textbook or the Internet.
But I would also like to talk about formulas in inorganic chemistry. You will see how easy it is to understand them after studying the structure of organic molecules.
Rational formulas
It should not be concluded that inorganic chemistry is easier than organic chemistry. Of course, inorganic molecules tend to look much simpler because they don't tend to form complex structures like hydrocarbons. But then we have to study more than a hundred elements that make up the periodic table. And these elements tend to combine according to their chemical properties, but with numerous exceptions.
So, I won’t tell you any of this. The topic of my article is chemical formulas. And with them everything is relatively simple.
Most often used in inorganic chemistry rational formulas. And now we’ll figure out how they differ from those already familiar to us.
First, let's get acquainted with another element - calcium. This is also a very common element.
It is designated Ca and has a valency of two. Let's see what compounds it forms with the carbon, oxygen and hydrogen we know.
| Substance | Structural formula | Rational formula | Gross formula |
|---|---|---|---|
| Calcium oxide | Ca=O | CaO | |
| Calcium hydroxide | H-O-Ca-O-H | Ca(OH)2 | |
| Calcium carbonate | $slope(45)Ca`/O\C|O`|/O`\#1 | CaCO3 | |
| Calcium bicarbonate | HO/`|O|\O/Ca\O/`|O|\OH | Ca(HCO3)2 | |
| Carbonic acid | H|O\C|O`|/O`|H | H2CO3 |
At first glance, you can see that the rational formula is something between a structural and a gross formula. But it is not yet very clear how they are obtained. To understand the meaning of these formulas, you need to consider the chemical reactions in which substances participate.
Calcium in its pure form is a soft white metal. It does not occur in nature. But it is quite possible to buy it at a chemical store. It is usually stored in special jars without access to air. Because in air it reacts with oxygen. Actually, that’s why it doesn’t occur in nature.
So, the reaction of calcium with oxygen:
2Ca + O2 -> 2CaO
The number 2 before the formula of a substance means that 2 molecules are involved in the reaction.
Calcium and oxygen produce calcium oxide. This substance also does not occur in nature because it reacts with water:
CaO + H2O -> Ca(OH2)
The result is calcium hydroxide. If you look closely at its structural formula (in the previous table), you can see that it is formed by one calcium atom and two hydroxyl groups, with which we are already familiar.
These are the laws of chemistry: if a hydroxyl group is added to an organic substance, an alcohol is obtained, and if it is added to a metal, a hydroxide is obtained.
But calcium hydroxide does not occur in nature due to the presence of carbon dioxide in the air. I think everyone has heard about this gas. It is formed during the respiration of people and animals, the combustion of coal and petroleum products, during fires and volcanic eruptions. Therefore, it is always present in the air. But it also dissolves quite well in water, forming carbonic acid:
CO2 + H2O<=>H2CO3
Sign<=>indicates that the reaction can proceed in both directions under the same conditions.
Thus, calcium hydroxide, dissolved in water, reacts with carbonic acid and turns into slightly soluble calcium carbonate:
Ca(OH)2 + H2CO3 -> CaCO3"|v" + 2H2O
A down arrow means that as a result of the reaction the substance precipitates.
With further contact of calcium carbonate with carbon dioxide in the presence of water, a reversible reaction occurs to form an acidic salt - calcium bicarbonate, which is highly soluble in water
CaCO3 + CO2 + H2O<=>Ca(HCO3)2
This process affects the hardness of the water. When the temperature rises, bicarbonate turns back into carbonate. Therefore, in regions with hard water, scale forms in kettles.
Chalk, limestone, marble, tuff and many other minerals are largely composed of calcium carbonate. It is also found in corals, mollusk shells, animal bones, etc...
But if calcium carbonate is heated over very high heat, it will turn into calcium oxide and carbon dioxide.
This short story about the calcium cycle in nature should explain why rational formulas are needed. So, rational formulas are written so that the functional groups are visible. In our case it is:
In addition, individual elements - Ca, H, O (in oxides) - are also independent groups.Ions
I think it's time to get acquainted with ions. This word is probably familiar to everyone. And after studying the functional groups, it doesn’t cost us anything to figure out what these ions are.
In general, the nature of chemical bonds is usually that some elements give up electrons while others gain them. Electrons are particles with a negative charge. An element with a full complement of electrons has zero charge. If he gave away an electron, then its charge becomes positive, and if he accepted it, then it becomes negative. For example, hydrogen has only one electron, which it gives up quite easily, turning into a positive ion. There is a special entry for this in chemical formulas:
H2O<=>H^+ + OH^-
Here we see that as a result electrolytic dissociation water breaks down into a positively charged hydrogen ion and a negatively charged OH group. The OH^- ion is called hydroxide ion. It should not be confused with the hydroxyl group, which is not an ion, but part of some kind of molecule. The + or - sign in the upper right corner shows the charge of the ion.
But carbonic acid never exists as an independent substance. In fact, it is a mixture of hydrogen ions and carbonate ions (or bicarbonate ions):
H2CO3 = H^+ + HCO3^-<=>2H^+ + CO3^2-
The carbonate ion has a charge of 2-. This means that two electrons have been added to it.
Negatively charged ions are called anions. Typically these include acidic residues.
Positively charged ions - cations. Most often these are hydrogen and metals.
And here you can probably fully understand the meaning of rational formulas. The cation is written in them first, followed by the anion. Even if the formula does not contain any charges.
You probably already guess that ions can be described not only by rational formulas. Here is the skeletal formula of the bicarbonate anion:
Here the charge is indicated directly next to the oxygen atom, which received an extra electron and therefore lost one line. Simply put, each extra electron reduces the number of chemical bonds depicted in the structural formula. On the other hand, if some node of the structural formula has a + sign, then it has an additional stick. As always, this fact needs to be demonstrated with an example. But among the substances familiar to us there is not a single cation that consists of several atoms.
And such a substance is ammonia. Its aqueous solution is often called ammonia and is included in any first aid kit. Ammonia is a compound of hydrogen and nitrogen and has the rational formula NH3. Consider the chemical reaction that occurs when ammonia is dissolved in water:
NH3 + H2O<=>NH4^+ + OH^-
The same thing, but using structural formulas:
H|N<`/H>\H + H-O-H<=>H|N^+<_(A75,w+)H><_(A15,d+)H>`/H + O`^-# -H
On the right side we see two ions. They were formed as a result of one hydrogen atom moving from a water molecule to an ammonia molecule. But this atom moved without its electron. The anion is already familiar to us - it is a hydroxide ion. And the cation is called ammonium. It exhibits properties similar to metals. For example, it may combine with an acidic residue. The substance formed by combining ammonium with a carbonate anion is called ammonium carbonate: (NH4)2CO3.
Here is the reaction equation for the interaction of ammonium with a carbonate anion, written in the form of structural formulas:
2H|N^+<`/H><_(A75,w+)H>_(A15,d+)H + O^-\C|O`|/O^-<=>H|N^+<`/H><_(A75,w+)H>_(A15,d+)H`|0O^-\C|O`|/O^-|0H_(A-15,d-)N^+<_(A105,w+)H><\H>`|H
But in this form the reaction equation is given for demonstration purposes. Typically equations use rational formulas:
2NH4^+ + CO3^2-<=>(NH4)2CO3
Hill system
So, we can assume that we have already studied structural and rational formulas. But there is another issue that is worth considering in more detail. How do gross formulas differ from rational ones?
We know why the rational formula of carbonic acid is written H2CO3, and not some other way. (The two hydrogen cations come first, followed by the carbonate anion.) But why is the gross formula written CH2O3?
In principle, the rational formula of carbonic acid may well be considered a true formula, because it has no repeating elements. Unlike NH4OH or Ca(OH)2.
But an additional rule is very often applied to gross formulas, which determines the order of elements. The rule is quite simple: carbon is placed first, then hydrogen, and then the remaining elements in alphabetical order.
So CH2O3 comes out - carbon, hydrogen, oxygen. This is called the Hill system. It is used in almost all chemical reference books. And in this article too.
A little about the easyChem system
Instead of a conclusion, I would like to talk about the easyChem system. It is designed so that all the formulas that we discussed here can be easily inserted into the text. Actually, all the formulas in this article are drawn using easyChem.
Why do we even need some kind of system for deriving formulas? The thing is that the standard way to display information in Internet browsers is hypertext markup language (HTML). It is focused on processing text information.
Rational and gross formulas can be depicted using text. Even some simplified structural formulas can also be written in text, for example alcohol CH3-CH2-OH. Although for this you would have to use the following entry in HTML: CH 3-CH 2-OH.
This of course creates some difficulties, but you can live with them. But how to depict the structural formula? In principle, you can use a monospace font:
H H | | H-C-C-O-H | | H H Of course it doesn’t look very nice, but it’s also doable.
The real problem comes when trying to draw benzene rings and when using skeletal formulas. There is no other way left except connecting a raster image. Rasters are stored in separate files. Browsers can include images in gif, png or jpeg format.
To create such files, a graphic editor is required. For example, Photoshop. But I have been familiar with Photoshop for more than 10 years and I can say for sure that it is very poorly suited for depicting chemical formulas.
Molecular editors cope with this task much better. But with a large number of formulas, each of which is stored in a separate file, it is quite easy to get confused in them.
For example, the number of formulas in this article is . They are displayed in the form of graphic images (the rest using HTML tools).
The easyChem system allows you to store all formulas directly in an HTML document in text form. In my opinion, this is very convenient.
In addition, the gross formulas in this article are calculated automatically. Because easyChem works in two stages: first the text description is converted into an information structure (graph), and then various actions can be performed on this structure. Among them, the following functions can be noted: calculation of molecular weight, conversion to a gross formula, checking for the possibility of output as text, graphic and text rendering.
Thus, to prepare this article, I only used a text editor. Moreover, I didn’t have to think about which of the formulas would be graphic and which would be text.
Here are a few examples that reveal the secret of preparing the text of an article: Descriptions from the left column are automatically turned into formulas in the second column.
In the first line, the description of the rational formula is very similar to the displayed result. The only difference is that the numerical coefficients are displayed interlinearly.
In the second line, the expanded formula is given in the form of three separate chains separated by a symbol; I think it is easy to see that the textual description is in many ways reminiscent of the actions that would be required to depict the formula with a pencil on paper.
The third line demonstrates the use of slanted lines using the \ and / symbols. The ` (backtick) sign means the line is drawn from right to left (or bottom to top).
There is much more detailed documentation on using the easyChem system here.
Let me finish this article and wish you good luck in studying chemistry.
