Autonomous heat sources (individual heating devices). (review)

Example 1. How many times should the hydrogen concentration in the system be increased?

N 2  3H 2  2NH 3

so that the reaction rate increases 100 times?

Solution. Expressing the rate of a given reaction

v = k 3 .

At the initial moment of time

v 0 = k 0 0 3 .

Let us take the increase in hydrogen concentration to be x,

v 1 =k 0 [xH 2 ] 3 = x 3 k 0 0 3 = x 3 v 0 = 100v 0,

and then the increase in hydrogen concentration should be

Example 2. How will the rate of the forward reaction change if the pressure in the system is increased threefold?

N 2  3H 2  2NH 3

Solution. A threefold increase in pressure is equivalent to a threefold decrease in volume and, accordingly, a threefold increase in the concentration of all substances.

Reaction rate at the initial moment of time:

v 0 = k 0 0 3 ;

after increasing pressure

v 1 = k 3 = 3 3 3 k 0 0 3 = 81v 0 ,

i.e., the rate of the direct reaction will increase by 81 times.

Example 3. Increasing the temperature from 50 0 C to 70 0 C causes an increase in the reaction rate by 9 times. Find the temperature coefficient of the reaction.

Solution. Let us express the temperature coefficient of the reaction from the Van't Hoff equation:

γ (t 1- t 2)/10 = v 2 /v 1,

and we get

γ (70-50)/10 = 9, γ 2 = 9, γ = 3.

Example 4. Calculate the activation energy and rate constant of the chemical

what kind of reaction

CO + H 2 O  H 2 + CO 2

at 303 K (T 3), if the reaction rate constants at 288 K (T 1) and 313 K (T 2), respectively, are 3.1 10 -4 and 8.15 10 -3 mol/l.

Solution. From the Arrhenius equation it follows

Еа = 2.3RT 1 T 2 log(k 2 /k 1)/(T 2 T 1).

Substituting the obtained values, we get:

Ea = 2.3 8.31 288 313 log(8.15. 10 -3 /3.1 10 -4)/(313288) = 97848 J/mol.

The reaction rate constant at 303 K can be found from the relation

log(k 3 /k 1) = Ea(T 3 -T 1)/(2.3RT 3 T 1) or log(k 2 /k 3) = Ea(T 2 -T 3)/(2.3RT 2 T 3).

Substituting the available values ​​into any of these formulas, we get:

k 3 = 2.34 10 -3 l mol -1 min -1 .

Example 5. At a temperature of 10 0 C, the reaction ends after 120 seconds, and at 30 0 C - after 30 seconds. Find the activation energy.

Solution. It is obvious that k (30) /k (10) =  (10) / (30), and then, substituting the values ​​in the formula Ea = 2.3RT 1 T 2 log(k 2 /k 1)/(T 2 T 1), we get:

Еа = 2.3 8.31(273+10)(273+30)log(120/30)/(3010) = 49336 J/mol

or 49.3 kJ/mol.

Example 6. The saponification constant of ethyl acetic acid with sodium hydroxide at 100C is 2.38 l/mol. min. Determine the time required for saponification of 90 ethyl acetate if you mix 1 liter of a 0.05 molar solution of ether with 1 liter of a 0.05 molar solution of NaOH at this temperature.

Solution. Reaction

CH 3 COOC 2 H 5 + NaOH = CH 3 COONa + C 2 H 5 OH

is a second order reaction; the concentrations of ether and alkali are equal and you can use the equation:

k = x/( a(ax)),

 = x/(k a(ax)).

Taking into account the mutual dilution of the solutions by a factor of two and the conversion of acetic ethyl ether by 90, we obtain:

a = 0.05/2 = 0.025 mol/l; x = 0.05 0.9/2 = 0.0225 mol/l.

Then the reaction time will be

 = 0.0225/((2.38 0.025(0.0250.225)) = 151.2 min.

2. Chemical equilibrium

2.1. Reversible and irreversible reactions

One of the most important characteristics chemical reaction is the depth (degree) of transformation, showing how much the starting substances are converted into reaction products. The larger it is, the more economically the process can be carried out. The depth of transformation, among other factors, depends on the reversibility of the reaction.

Reversible reactions , unlike irreversible, do not proceed to the end: none of the reacting substances is completely consumed. At the same time, the reaction products interact with the formation of starting substances.

Let's look at examples:

1) equal volumes of gaseous iodine and hydrogen are introduced into a closed vessel at a certain temperature. If collisions of the molecules of these substances occur with the required orientation and sufficient energy, then the chemical bonds can rearrange with the formation of an intermediate compound (activated complex, see section 1.3.1). Further rearrangement of bonds can lead to the breakdown of the intermediate compound into two molecules of hydrogen iodide. Reaction equation:

H 2  I 2  2HI

But hydrogen iodide molecules will also randomly collide with hydrogen molecules, iodine molecules, and with each other. When HI molecules collide, nothing will prevent the formation of an intermediate compound, which can then decompose into iodine and hydrogen. This process is expressed by the equation:

2HI  H 2 + I 2

Thus, in this system two reactions will occur simultaneously - the formation of hydrogen iodide and its decomposition. They can be expressed by one general equation

H 2 + I 2  2HI

The reversibility of the process shows the sign .

The reaction directed in this case towards the formation of hydrogen iodide is called direct, and the opposite is called reverse.

2) if you mix two moles of sulfur dioxide with one mole of oxygen, create conditions in the system that are favorable for the reaction to occur, and after a period of time carry out an analysis gas mixture, then the results will show that the system will contain both SO 3 - the reaction product, and the starting substances - SO 2 and O 2. If, under the same conditions, sulfur oxide (+6) is placed as a starting substance, then it will be found that part of it will decompose into oxygen and sulfur oxide (+4), and the final ratio between the quantities of all three substances will be the same as in the case where they started from a mixture of sulfur dioxide and oxygen.

Thus, the interaction of sulfur dioxide with oxygen is also one of the examples of reversible chemical reaction and is expressed by the equation

2SO 2 + O 2  2SO 3

3) interaction of iron with hydrochloric acid proceeds according to the equation:

Fe + 2HCL  FeCL 2 + H 2

If there is enough hydrochloric acid, the reaction will end when

all the iron will be used up. In addition, if you try to carry out this reaction in the opposite direction - passing hydrogen through a solution of ferric chloride, then metallic iron and hydrochloric acid will not work - this reaction cannot go in the opposite direction. Thus, the interaction of iron with hydrochloric acid is an irreversible reaction.

However, it should be borne in mind that theoretically any irreversible process can be represented as occurring reversibly under certain conditions, i.e. In principle, all reactions can be considered reversible. But very often one of the reactions clearly predominates. This happens in cases where interaction products are removed from the reaction sphere: a precipitate forms, gas is released, and practically non-dissociating products are formed during ion exchange reactions; or when, due to a clear excess of starting substances, the opposite process is practically suppressed. Thus, natural or artificial exclusion of the possibility of a reverse reaction allows the process to be completed almost to completion.

Examples of such reactions include the interaction of sodium chloride with silver nitrate in solution

NaCL + AgNO 3  AgCl + NaNO 3,

copper bromide with ammonia

CuBr 2 + 4NH 3  Br 2,

neutralization of hydrochloric acid with sodium hydroxide solution

HCl + NaOH  NaCl + H 2 O.

These are all examples only practically irreversible processes, since silver chloride is somewhat soluble, and the complex cation 2+ is not absolutely stable, and water dissociates, although to an extremely insignificant extent.

Chemical elements that make up living and inanimate nature, are in constant movement, because the substances that consist of these elements are constantly changing.

Chemical reactions (from the Latin reaction - opposition, resistance) are the response of substances to the influence of other substances and physical factors (temperature, pressure, radiation, etc.).

However, this definition also corresponds to physical changes occurring with substances - boiling, melting, condensation, etc. Therefore, it is necessary to clarify that chemical reactions are processes as a result of which old chemical bonds and new ones arise and, as a consequence, new substances are formed from the original substances.

Chemical reactions continuously occur both inside our body and in the world around us. Countless reactions are usually classified according to various criteria. Let's remember from the 8th grade course the signs that you are already familiar with. To do this, let's turn to laboratory experiment.

Laboratory experiment No. 3
Substitution of iron for copper in a solution of copper (II) sulfate

We have come to a very important concept in chemistry - “the rate of a chemical reaction.” It is known that some chemical reactions occur very quickly, others over significant periods of time. When a solution of silver nitrate is added to a solution of sodium chloride, a white cheesy precipitate precipitates almost instantly:

AgNO 3 + NaCl = NaNO 3 + AgCl↓.

Reactions occur at enormous speeds, accompanied by an explosion (Fig. 11, 1). On the contrary, stalactites and stalagmites grow slowly in stone caves (Fig. 11, 2), steel products corrode (rust) (Fig. 11, 3), and are destroyed by exposure to acid rain palaces and statues (Fig. 11, 4).

Rice. 11.
Chemical reactions occurring at enormous speeds (1) and very slowly (2-4)

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 for the rate of a chemical reaction - 1 mol/(l s).

A special branch of chemistry studies the rate of chemical reactions, which is called chemical kinetics.

Knowing its laws allows you to control a chemical reaction, making it proceed faster or slower.

What factors determine the rate of a chemical reaction?

1. Nature of 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 using the example of the interaction of acids with metals

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 using the example of the interaction of zinc with hydrochloric acid of various concentrations

It's easy to conclude: The higher the concentration of reactants, the higher the rate of interaction between them.

Concentration gaseous substances for homogeneous production processes increase, increasing the pressure. For example, this is done in the production of sulfuric acid, ammonia, and ethyl alcohol.

The factor of 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 activity, for example in medicine. Patients with lung diseases, in whom the rate of interaction of blood hemoglobin with oxygen in the air is low, breathe easier with the help of oxygen pillows.

3. Contact area of ​​reacting substances. 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
Dependence of the rate of a chemical reaction on the area of ​​contact of the reacting substances

For heterogeneous reactions: how larger area contact of reacting substances, the higher the reaction rate.

You could verify this from personal experience. To light a fire, you put small wood chips under the wood, and under them - crumpled paper, from which the whole fire caught fire. On the contrary, extinguishing a fire with water involves reducing the area of ​​contact of burning objects with air.

In production, this factor is taken into account specifically; the so-called fluidized bed is used. To increase the reaction rate, the solid substance 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
Dependence of the rate of a chemical reaction on the temperature of the reacting substances using the example of the interaction of copper (II) oxide with a solution of sulfuric acid at different temperatures

It is easy to conclude: the higher the temperature, the greater the reaction rate.

First laureate Nobel Prize 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 cast 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 speed of a chemical reaction depends is catalysts. You will meet him in the next paragraph.

New words and concepts

1. Chemical reactions and their classification.
2. Signs of classification of chemical reactions.
3. The rate of a chemical reaction and the factors on which it depends.

Tasks for independent work

1. What is a chemical reaction? What is the essence of chemical processes?
2. Give a complete classification description of the following chemical processes:
   • a) combustion of phosphorus;
   • b) the interaction of a sulfuric acid solution with aluminum;
   • c) neutralization reactions;
   • d) the formation of nitric oxide (IV) from nitric oxide (II) and oxygen.
3. Based on personal experience Give examples of chemical reactions occurring at different rates.
4. What is the rate of a chemical reaction? What factors does it depend on?
5. Give examples of influence various factors on biochemical and industrial chemical processes.
6. Based on personal experience, give examples of the influence of various factors on chemical reactions that occur in everyday life.
7. Why is food stored in the refrigerator?
8. 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?

The effect of temperature on the rate of a chemical reaction is approximately determined van't Hoff's rule. When the temperature increases by 10 0 C, the rate of the chemical reaction increases 2-4 times.

Mathematical notation of van't Hoff's rule: γ - temperature coefficient of reaction rate or Van't Hoff coefficient for most reactions lies within 2-4.

Task. How many times will the rate of a chemical reaction occurring in the gas phase change if the temperature changes from 80 0 C to 120 0 C ( γ = 3)?

In accordance with Van't Hoff's rule, we write:

The increase in the rate of a chemical reaction with increasing temperature is explained not only by an increase kinetic energy interacting molecules. For example, the number of molecular collisions increases in proportion to the square root of 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 approximately 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, whose energy 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 reserve that molecules must have to carry out a chemical reaction. If E A< 40 кДж/моль – реакции протекают быстро, если Е А >120 kJ/mol - reactions do not occur, 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.

Established a more accurate dependence of the rate of a chemical reaction on temperature C. Arrhenius: the reaction rate constant is proportional to the base natural logarithm, raised to the power (–Е А /RT). ,

A – pre-exponential factor, determines the number of active

collisions;

e – exponent (base of natural logarithm).

SOME INFORMATION ABOUT OPERATING PRINCIPLES

Charcoal hot water bottles.
About 90 years ago, inventive thought turned to the most common exothermic process - the combustion reaction. Devices have appeared in which a smoldering coal rod wrapped in special paper was placed in a metal case, and the latter in a cloth case. Such heating pads weighed relatively little and lasted 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 heating pads.
During the First World War, millions of soldiers froze in the trenches, and during the four war years, inventors in the USA, Japan and England patented several versions of pocket liquid heating pads. 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 wool and a platinum gasket. Holes were drilled in the housing to supply air to the catalyst and remove 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 warmers is their limited service life: impurities contained in the fuel quickly poison the catalyst and the warming cigarette case becomes useless.

Warmers using the lime slaking reaction.

Back in the 20s in Germany, it was proposed to use the heat released when slaking quicklime with water to heat food in the field. However, the insufficiently large thermal effect of the reaction prevented the practical application this idea. A step forward was the combination of two reactions: lime slaking and lime neutralization. For this purpose, crystalline hydrates of oxalic or citric acid. 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 you can portable device obtain a temperature of 100 to 300 degrees Celsius. In addition, the use of acid crystal hydrates allows you to run a heating pad a small amount water, and the water released during neutralization will react with subsequent portions of lime.

Heating pads using metal oxidation reactions.
Under normal conditions, 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 Red Army soldiers - in a bag made of rubberized fabric, in addition to iron filings, potassium permanganate and fillers - coal and sand - were placed. After adding water, the surface of the heating pad is maintained at a temperature of 100 degrees Celsius for 10-20 hours.

4Fe + 2H2 O + 3O2 --> 2(Fe2O3 * H2O) + 390.4 kcal/mol

Instead of iron, it is better to use aluminum in corrosive heating pads. Much more heat is released in this reaction than during the oxidation of iron:

8Аl + 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. 200 g of the reaction mixture - aluminum powder and copper chloride, taken in a stoichiometric ratio, were poured into the tank. Water in the amount of 100-120 ml. added to the reservoir from a can located in the chest pocket. The water supply was regulated by a simple thermal relay. The belt could provide warmth for 8 hours. This chemical heating pad was new not only in form, but also in content: for the first time, the heat that occurs when one metal is replaced by another - more electronegative - was used. In Leningrad, during the blockade winter of 1942, they used heating pads filled with a mixture of copper chloride and iron filings. From one filling with water, such heating pads worked for 60-70 hours.

Crystallization warmers.
Crystallization warmers use substances with low temperatures melting and relatively high heats of fusion. Such a thermal accumulator gives off heat, which is released during crystallization or solidification of a preheated and molten substance. The classic working fluid of accumulator heating pads is paraffin. You can also use stearic acid, low-melting crystal hydrates, for example, Glauber's salt Na2 SO4 * 10H2O or sodium acetate trihydrate CH3COONa * 3H2O. Small additions to crystalline hydrates of calcium chloride, sodium thiosulfite or glycerin can slow down the crystallization process and thereby increase the operating time of the heating pad. The heating pad warms up in 15 seconds. up to 55 °C and the process of heat release lasts 25-30 minutes. The heating pad has a fairly high heat capacity and is capable of releasing heat for another 25-30 minutes in the cooling mode. A crystallization-type heating pad is good as a therapeutic and prophylactic agent for inflammatory processes, for patients with various forms radiculitis, for liver tubing and other procedures in a hospital setting (at home or in the hospital).

Using crystallization warmers in emergency situations in field conditions it is limited by the short duration of the heat release mode of the heating pads.

The main advantage of crystallization-type heating pads is the possibility of repeated use: to restore the heating pad to its original state, it is enough to boil it in water for 15-20 minutes.

http://umcsa.narod.ru/rus/umcsa/projects/ait.htm

HEATER FROM A TEST TUBE
When hiking, fishing, especially in bad weather, you often need an ordinary heating pad. Of course, an ordinary rubber one is not bad, but it has one significant drawback: it heats up water very slowly over a fire.

Let's try to make a chemical heating pad. For this we need the most common reagents.

First, let's do a simple experiment. Go to the kitchen and take a pack of table salt. However, you won't need a pack. 20 g (2 teaspoons) will be enough. Then look into the cabinet where all kinds of household preparations and materials are stored. Surely there was some copper sulfate left there after the apartment was renovated. You will need 40 g (3 teaspoons). Wood chips and a piece of aluminum wire, presumably, will also be found. If so, you're done. Grind the vitriol and salt in a mortar so that the size of the crystals does not exceed 1 mm (by eye, of course). Add 30 g (5 tablespoons) to the resulting mixture. sawdust and mix thoroughly. Bend a piece of wire into a spiral or snake and place it in a mayonnaise jar. Pour the prepared mixture there so that the filling level is 1-1.5 cm below the neck of the jar. The heating pad is in your hands. To activate it, just pour 50 ml (a quarter cup) of water into the jar. After 3-4 minutes, the temperature of the heating pad will rise to 50-60° C.

Where does the heat in the jar come from, and what role does each component play? Let's look at the reaction equation:

CuSO4+2NaCl > Na2SO4+CuCl2

As a result of the interaction of copper sulfate with table 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 drugs - 24999 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 calculated per 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 deception: the heating pad is real.

What about sawdust? Without taking any part in chemical reactions, they at the same time play a very important role. By greedily absorbing water, sawdust slows down the course of reactions and extends the operation of the heating pad over time. In addition, wood has a fairly low thermal conductivity: it seems to accumulate the generated heat and then constantly releases it. A tightly sealed container will retain heat for at least two hours.

One final note: a jar is, of course, not the best vessel for a heating pad. We only needed it for demonstration. So think about the shape and material for the tank in which to place the heating mixture.