The influence of temperature on the rate of a chemical reaction is approximately determined by van't Hoff's rule. With an increase in temperature by 10 0 C, the rate of a chemical reaction increases by 2-4 times.
Mathematical notation of the van't Hoff rule: γ - reaction rate temperature coefficient or van't Hoff coefficient for most reactions lies in the range of 2-4.
A task. How many times will the rate of a chemical reaction occurring in the gas phase change if the temperature changes from 80 0 С to 120 0 С ( γ = 3)?
In accordance with the van't Hoff rule, we write:
An increase in the rate of a chemical reaction with an increase in temperature is explained not only by an increase in the kinetic energy of the interacting molecules. For example, the number of collisions of molecules increases in proportion to the square root of the absolute temperature. When substances are heated from zero to one hundred degrees Celsius, the speed of movement of molecules increases by 1.2 times, and the speed of a chemical reaction increases by about 59 thousand times. Such a sharp increase in the reaction rate with increasing temperature is explained by the proportion of active molecules whose collisions lead to chemical interaction. According to the theory of active collisions, only active molecules, the energy of which exceeds the average energy of the molecules of a given substance, i.e. molecules with activation energy.
Activation energy (E A)- this is the excess energy compared to the average supply that molecules must have in order to carry out a chemical reaction. If E A< 40 кДж/моль – реакции протекают быстро, если Е А >120 kJ / mol - reactions do not go if E A = 40-120 kJ / mol - reactions proceed under normal conditions. An increase in temperature reduces the activation energy, makes substances more reactive, and the rate of interaction increases.
A more accurate dependence of the rate of a chemical reaction on temperature was established by C. Arrhenius: The reaction rate constant is proportional to the base of the natural logarithm raised to the power (-E A / RT). ,
A - pre-exponential factor, determines the number of active
collisions;
e is the exponent (the base of the natural logarithm).
inspired by the weatherSOME INFORMATION ABOUT THE PRINCIPLES OF WORK
Charcoal heaters.
Even 90 years ago, inventive thought turned to the most common exothermic process - the combustion reaction. Devices appeared in which a smoldering carbon rod wrapped in special paper was placed in a metal case, and the latter in a cloth case. Such heaters weighed relatively little, and acted for 5-6 hours. On the surface of the case, the temperature was from 60 to 100 degrees Celsius.
C + O2 --> CO2 + 94 kcal/mol
catalytic heaters.
During the First World War, millions of soldiers froze in the trenches, and during the four war years, the inventors of the United States, Japan and England patented several versions of pocket liquid heaters. The principle of their operation was simple: catalytic flameless oxidation of alcohol or gasoline. Platinum served as the catalyst in all cases. The Japanese heating pad looked like a cigarette case, inside of which there was a reservoir filled with cotton and a platinum gasket. Holes were drilled in the housing for air supply to the catalyst and removal of combustion gases. To start the heating pad, alcohol was poured into the tank, which soaked the cotton wool. Then the catalyst was heated with a match flame and the reaction began. The main disadvantage of catalytic heaters is their limited service life: impurities contained in the fuel quickly poison the catalyst and the heating cigarette case becomes useless.
Heating pads using lime slaking reaction.
Back in the 1920s in Germany, for heating food in the field, it was proposed to use the heat released when extinguishing quicklime with water. However, the insufficiently large thermal effect of the reaction prevented at first the practical application of this idea. A step forward was the combination of two reactions: lime slaking and lime neutralization. To do this, oxalic or citric acid crystalline hydrates were introduced into lime. The reactions in the heating pad went according to the following scheme.
CaO + H2 O --> Ca(OH)2 + 10.6 kcal.
2Ca (OH) 2 + H2C2O4 + 2 H2O --> CaC2O4 + 4H2O + 31 kcal
With these two reactions, it is possible to obtain temperatures from 100 to 300 degrees Celsius in a portable device. In addition, the use of acid hydrates allows you to start the heating pad with a small amount of water, and the water released during neutralization will react with the next portions of lime.
Heating pads using metal oxidation reactions.
Under normal conditions, the corrosion of metals in air proceeds, fortunately, slowly. The presence of salts dramatically speeds up the process. At the end of the 20s, an "iron" heating pad was recommended for heating the soldiers of the Red Army - in addition to iron filings, potassium permanganate and fillers - coal and sand were placed in a bag of rubberized fabric. After adding water, the temperature of 100 degrees Celsius is maintained on the surface of the heater for 10-20 hours.
4Fe + 2H2 O + 3O2 --> 2(Fe2O3 * H2O) + 390.4 kcal/mol
Instead of iron in corrosive heating pads, it is better to use aluminum. Much more heat is released in this reaction than during the oxidation of iron:
8Al + 3Fe3O4 ---> 4Al2O3 + 9Fe + 795 kcal/mol
Heating pads using metal displacement reactions.
In 1940, the USSR developed a heating belt - a copper tank covered with leather, which was attached to a trouser belt. The reservoir was filled with 200 g of the reaction mixture - aluminum powder of copper chloride, taken in a stoichiometric ratio. Water in an amount of 100-120 ml. was added to the tank from a spray bottle in the breast pocket. The water supply was regulated by a simple thermal relay. The belt could keep warm for 8 hours. This chemical heating pad was new not only in form, but also in content: for the first time, the heat generated by the displacement of one metal by another, more electronegative, was used. In Leningrad, during the blockade winter of 1942, heating pads filled with a mixture of copper chloride and iron shavings were used. From one filling with water, such heating pads worked 60-70 hours.
Crystallization heaters.
Crystallization heaters use substances with low melting points and relatively high heats of fusion. Such a thermal accumulator gives off heat, which is released during the crystallization or solidification of a preheated and molten substance. The classic working body of heaters-accumulators is paraffin. You can also use stearic acid, low melting crystalline hydrates, for example, Glauber's salt Na2 SO4 * 10H2O or sodium acetate trihydrate CH3COONa * 3H2O. Small additions to hydrated calcium chloride, sodium thiosulfite or glycerin can slow down the crystallization process and thereby increase the duration of the heating pad. The heating pad heats up in 15 seconds. up to 55 °C and the process of heat release lasts 25-30 minutes. The heating pad has a sufficiently high heat capacity and for another 25-30 minutes is able to give off heat in the cooling mode. A heating pad of a crystallization type is good as a therapeutic and prophylactic agent for inflammatory processes, for patients with various forms of sciatica, for liver tubage and other procedures in stationary conditions (at home or in a hospital).
The use of crystallization heaters in emergency situations in the field is limited by the short duration of the heat release mode of the heaters.
The main advantage of crystallization-type heating pads is the possibility of repeated use: to restore the initial state of the heating pad, it is enough to boil it in water for 15-20 minutes.
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TEST TUBE HEATER
When hiking, fishing, especially in bad weather, there is often a need for an ordinary heating pad. Of course, ordinary rubber is also good, but it has one significant drawback: water is heated very slowly for it at the stake.
Let's try to make a chemical heating pad. To do this, we need the most common reagents.
Let's start with a simple experiment. Go to the kitchen and take a pack of table salt. However, you don't need a pack. 20 g (2 teaspoons) will be enough. Then look into the locker, where all kinds of household preparations and materials are stored. Surely there was preserved after the repair of the apartment a little copper sulphate. It will need 40 g (3 teaspoons). Wood chips and a piece of aluminum wire, presumably, will also be found. If so, you're all set. Pound vitriol and salt in a mortar so that the size of the crystals does not exceed 1 mm (of course, by eye). Add 30 g (5 tablespoons) of sawdust to the resulting mixture and mix thoroughly. Bend a piece of wire with a spiral or snake, put it in a mayonnaise jar. Pour the prepared mixture there so that the backfill level is 1-1.5 cm below the neck of the jar. The heating pad is in your hands. To put it into action, it is enough to pour 50 ml (a quarter cup) of water into a jar. After 3-4 minutes, the temperature of the heating pad will rise to 50-60°C.
Where does the heat come from in the jar, and what role does each of the components play? Let's look at the reaction equation:
CuSO4+2NaCl > Na2SO4+CuCl2
As a result of the interaction of copper sulfate with common salt, sodium sulfate and copper chloride are formed. It is she who interests us. If we calculate the heat balance of the reaction, it turns out that the formation of one gram-molecule of copper chloride releases 4700 calories of heat. Plus, the heat of dissolution in the initial resulting preparations is 24,999 calories. Total: Approximately 29,600 calories.
Immediately after formation, copper chloride interacts with aluminum wire:
2Al+3CuCl2 > 2AlCl3+3Cu
In this case, approximately 84,000 calories are released (also in terms of 1 g-mol of copper chloride).
As you can see, as a result of the process, the total amount of heat released exceeds 100,000 calories per gram-molecule of the substance. So there is no mistake or deceit: the heating pad is real.
What about sawdust? Not taking any part in chemical reactions, they at the same time play a very important role. Greedily absorbing water, sawdust slows down the course of reactions, stretches the work of the heating pad in time. In addition, wood has a fairly low thermal conductivity: it kind of accumulates the released heat and then constantly gives it away. In a tightly sealed container, heat is retained for at least two hours.
And the last remark: the bank, of course, is not the best vessel for a heating pad. We only needed it for demonstration purposes. So think for yourself about the shape and material for the tank in which to place the heating mixture.
The chemical elements that make up animate and inanimate nature are in constant motion, because the substances that consist of these elements are constantly changing.
Chemical reactions (from Latin reaction - counteraction, repulse) - this is the response of substances to the influence of other substances and physical factors (temperature, pressure, radiation, etc.).
However, this definition also corresponds to the physical changes that occur with substances - boiling, melting, condensation, etc. Therefore, it is necessary to clarify that chemical reactions are processes that destroy old chemical bonds and create new ones and, as a result, from new substances are formed.
Chemical reactions are constantly taking place both inside our body and in the world around us. Countless reactions are usually classified according to various criteria. Let's recall from the 8th grade course the signs with which you are already familiar. To do this, we turn to a laboratory experiment.
Lab experience #3
Substitution of iron for copper in copper (II) sulfate solution
Pour 2 ml of copper (II) sulfate solution into a test tube and place a push pin or paper clip in it. What are you watching? Write down the reaction equations in molecular and ionic forms. Consider redox processes. Based on the molecular equation, assign this reaction to one or another group of reactions based on the following features:
Now check yourself. CuSO 4 + Fe \u003d FeSO 4 + Cu.
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We have come to a very important concept in chemistry - "the rate of a chemical reaction." It is known that some chemical reactions proceed very quickly, others - for considerable periods of time. When a solution of silver nitrate is added to a solution of sodium chloride, a white cheesy precipitate almost instantly precipitates:
AgNO 3 + NaCl \u003d NaNO 3 + AgCl ↓.
Reactions proceed with great speeds, accompanied by an explosion (Fig. 11, 1). On the contrary, stalactites and stalagmites slowly grow in stone caves (Fig. 11, 2), steel products corrode (rust) (Fig. 11, 3), palaces and statues are destroyed under the action of acid rains (Fig. 11, 4).
Rice. eleven.
Chemical reactions occurring at great speeds (1) and very slowly (2-4)
| The rate of a chemical reaction is understood as the change in the concentration of reactants per unit time: V p \u003d C 1 - C 2 /t. |
In turn, concentration is understood as the ratio of the amount of a substance (as you know, it is measured in moles) to the volume that it occupies (in liters). From here it is not difficult to derive the unit of measurement of the rate of a chemical reaction - 1 mol / (l s).
The study of the rate of a chemical reaction is a special branch of chemistry called chemical kinetics.
Knowing its patterns allows you to control a chemical reaction, making it proceed faster or slower.
What factors affect the rate of a chemical reaction?
1. Nature of the reactants. Let's turn to the experiment.
Laboratory experiment No. 4
Dependence of the rate of a chemical reaction on the nature of the reactants on the example of the interaction of acids with metals
| Pour 1-2 ml of hydrochloric acid into two test tubes and place: in the 1st - a zinc granule, in the 2nd - a piece of iron of the same size. The nature of what reagent affects the rate of interaction between an acid and a metal? Why? Write down the reaction equations in molecular and ionic forms. Consider them from the standpoint of oxidation-reduction. Then place in two other test tubes on the same zinc granule and add solutions of acids of the same concentration to them: in the 1st - hydrochloric acid, in the 2nd - acetic. The nature of what reagent affects the rate of interaction between an acid and a metal? Why? Write down the reaction equations in molecular and ionic forms. Consider them from the standpoint of oxidation-reduction. |
2. Concentration of reactants. Let's turn to the experiment.
Laboratory experiment No. 5
Dependence of the rate of a chemical reaction on the concentration of reactants on the example of the interaction of zinc with hydrochloric acid of various concentrations
It is easy to conclude: the higher the concentration of reactants, the higher the rate of interaction between them.
The concentration of gaseous substances for homogeneous production processes is increased by increasing the pressure. For example, this is done in the production of sulfuric acid, ammonia, ethyl alcohol.
The factor of the dependence of the rate of a chemical reaction on the concentration of reacting substances is taken into account not only in production, but also in other areas of human life, for example, in medicine. Patients with lung diseases, in whom the rate of interaction of blood hemoglobin with atmospheric oxygen is low, facilitate breathing with the help of oxygen pillows.
3. Contact area of reactants. An experiment illustrating the dependence of the rate of a chemical reaction on this factor can be performed using the following experiment.
Laboratory experiment No. 6
The dependence of the rate of a chemical reaction on the area of contact of the reactants
For heterogeneous reactions: the larger the contact area of the reactants, the faster the reaction rate.
You could see this from personal experience. To kindle a fire, you put small chips under the firewood, and under them - crumpled paper, from which the whole fire caught fire. On the contrary, extinguishing a fire with water is to reduce the area of contact of burning objects with air.
In production, this factor is taken into account on purpose, the so-called fluidized bed is used. To increase the rate of the reaction, the solid is crushed almost to the state of dust, and then a second substance, usually gaseous, is passed through it from below. Passing it through a finely divided solid creates a boiling effect (hence the name of the method). The fluidized bed is used, for example, in the production of sulfuric acid and petroleum products.
Laboratory experiment No. 7
Fluidized Bed Modeling
4. Temperature. Let's turn to the experiment.
Laboratory experiment No. 8
The dependence of the rate of a chemical reaction on the temperature of the reacting substances on the example of the interaction of copper oxide (II) with a solution of sulfuric acid at different temperatures
It is easy to conclude that the higher the temperature, the faster the reaction rate.
The first Nobel Prize winner, the Dutch chemist J. X. Van't Hoff, formulated the rule:
In production, as a rule, high-temperature chemical processes are used: in the smelting of iron and steel, the melting of glass and soap, the production of paper and petroleum products, etc. (Fig. 12).
Rice. 12.
High-temperature chemical processes: 1 - iron smelting; 2 - glass melting; 3 - production of petroleum products
The fifth factor on which the rate of a chemical reaction depends is catalysts. You will meet him in the next paragraph.
New words and concepts
- Chemical reactions and their classification.
- Signs of the classification of chemical reactions.
- The rate of a chemical reaction and the factors on which it depends.
Tasks for independent work
- What is a chemical reaction? What is the essence of chemical processes?
- Give a complete classification of the following chemical processes:
- a) the burning of phosphorus;
- b) the interaction of a solution of sulfuric acid with aluminum;
- c) neutralization reactions;
- d) the formation of nitric oxide (IV) from nitric oxide (II) and oxygen.
- Based on personal experience, give examples of chemical reactions occurring at different rates.
- What is the rate of a chemical reaction? What factors does it depend on?
- Give examples of the influence of various factors on biochemical and industrial chemical processes.
- Based on personal experience, give examples of the influence of various factors on chemical reactions that occur in everyday life.
- Why is food stored in the refrigerator?
- The chemical reaction was started at a temperature of 100°C, then raised to 150°C. The temperature coefficient of this reaction is 2. How many times will the rate of the chemical reaction increase?
