Thermal effect of a chemical reaction and its significance. Thermal effect of a chemical reaction and its practical application

Here T (\displaystyle T)- absolute temperature, S (\displaystyle S)- entropy, P (\displaystyle P)- pressure, V (\displaystyle V)- volume , n i (\displaystyle n_(i))- quantity (or mass) i (\displaystyle i) th substance composing the system, μ i (\displaystyle \mu _(i))- chemical potential of this substance (see Entropy of an open system).

Thus, the heat of an infinitesimal quasi-static isochoric process q V (\displaystyle q_(V)) equal to

and the heat of an infinitesimal quasi-static isobaric process q P (\displaystyle q_(P)) equal to

The energy effect of a chemical reaction is always considered in relation to a specific thermochemical equation, which may not be relevant to the actual chemical process. The thermochemical equation only shows which sets of initial and final individual substances, located in certain states and quantitative ratios, disappear and are formed. In the initial state, only the starting substances (reactants) should be present, and in the final state, only the products of the chemical reaction. The only condition when writing a thermochemical equation is compliance with the material and charge balance. Substances in a dissolved or adsorbed state are also considered individual compounds; if the solvent or adsorbent does not directly participate in a chemical reaction and does not react with the solute, then it is considered simply as a factor influencing the thermodynamic properties of the substance in question. Finally, the thermochemical equation may include particles that are not capable of independent existence (electrons, protons, ions, radicals, atomic simple substances).

The energy effect of a real process with a chemical reaction depends on the conditions of the process and cannot serve as a standard characteristic of a specific chemical reaction. Chemical thermodynamics requires an indicator related to the energy of a chemical reaction, but independent of the conditions of its implementation. Let us show how the indicator we are interested in can be obtained. To do this, consider the following thought experiment. Let us take pure individual starting substances in molar quantities corresponding to the stoichiometric coefficients of the thermochemical equation of interest to us, and located at a certain temperature and pressure. If these substances are brought into contact, then the enthalpy of the resulting nonequilibrium system at the initial moment of time will be equal to the sum of the enthalpies of the initial substances. Now let us consider the final state of the system under study under the assumption that the reactants have reacted completely and the reaction products are at the same temperature and the same pressure as the reactants. The enthalpy of a system (in the general case, nonequilibrium) of the products of a chemical reaction will be equal to the sum of the enthalpies of these substances. Since enthalpy is a function of state, the enthalpy difference Δ H (\displaystyle \Delta H) system at the end and beginning of the considered thought experiment does not depend on the conditions of the chemical reaction. This enthalpy difference is called the isobaric thermal effect (thermochemical heat) of a chemical reaction corresponding to a certain thermochemical equation. It is important that the real feasibility of the considered thought experiment, the hypothetical conditions for its implementation and the nonequilibrium of the initial and final states of the thermochemical system do not affect the definition of the thermal effect of a chemical reaction.

Often the thermal effect of a chemical reaction is referred to 1 mole of one of the reaction products.

Let us summarize what has been said: the heat of the process associated with the actual occurrence of a chemical reaction and the energetic effect of a chemical reaction are by no means the same thing, and the definition of the thermal effect of a chemical reaction does not at all imply the actual implementation of the reaction corresponding to the thermochemical equation under consideration.

Both internal energy and enthalpy are functions of state, therefore the thermal effect of a chemical reaction depends on the nature and state of the starting materials and final products, but does not depend on the reaction path, that is, on the number and nature of intermediate stages (Hess's law).

The thermal effect of a chemical reaction occurring at constant pressure, and equal to the change in enthalpy of the system in a process corresponding to the thermochemical equation, is called isobaric thermal effect or enthalpy of a chemical reaction. The thermal effect of a chemical reaction occurring at a constant volume and equal to the change in the internal energy of the system in a process corresponding to the thermochemical equation is called isochoric thermal effect .

For certain types of chemical reactions, instead of the general term “thermal effect of a chemical reaction,” special (abbreviated) terms are used: heat of formation, calorific value and so on.

Definitions of thermal effects must be supplemented by an indication of the starting points for the values ​​of energy and enthalpy. To compare thermal effects and simplify thermodynamic calculations, all values ​​of the thermal effects of reactions are referred to standard conditions (all substances are in standard states). If a reaction - actually or hypothetically - is carried out under standard conditions ( T= 298.15 K = 25 °C And P= 1 bar = 100 kPa) then the thermal effect is called standard reaction heat effect or standard enthalpy of reaction Δ Ho
r.

Chemical reactions accompanied by an increase in temperature are called exothermic, while a decrease in temperature is called endothermic. In the thermodynamic system of symbols, the thermal effect of an exothermic reaction ( ΔU< 0 {\displaystyle \Delta U<0} or ΔH< 0 {\displaystyle \Delta H<0} ) is considered negative, endothermic ( Δ U > 0 (\displaystyle \Delta U>0) or Δ H > 0 (\displaystyle \Delta H>0)) - positive. In the outdated and not recommended for use thermochemical system of signs, on the contrary, the thermal effect of an exothermic reaction is considered positive, and negative - endothermic.

The thermal effects of chemical reactions are important for theoretical chemistry and are necessary in calculating the equilibrium compositions of mixtures, the yield of reaction products, the specific thrust of jet engine fuels and for solving many other applied problems.

The study of the thermal effects of chemical reactions is the most important task of thermochemistry. To calculate the standard thermal effects of chemical reactions, tables of standard heats of formation or combustion are used.

Standard enthalpy of formation (standard heat of formation)

The standard heat of formation is understood as the thermal effect of the reaction of the formation of one mole of a substance from simple substances and its components that are in stable standard states.

For example, the standard enthalpy of formation of 1 mole of methane from carbon and hydrogen is equal to the thermal effect of the reaction:

C(tv) + 2H 2 (g) = CH 4 (g) + 74.9 kJ/mol.

The standard enthalpy of formation is denoted by Δ Ho
f. Here the index f means formation(formation), and the sign “O” in the superscript indicates that the value refers to the standard state of matter: one mole of an individual chemical compound, taken in pure form under standard conditions in that state of aggregation that is stable under these conditions (unless there is a special clause ) . Sometimes a crossed out superscript "O" is used to indicate the standard state; According to the IUPAC guidelines for the use of notation in physical chemistry, the crossed out and uncrossed "O" symbols used to represent the standard state are equally acceptable. In the literature, another designation for standard enthalpy is often found - Δ H o
298.15, where the sign "O" indicates pressure equal to one atmosphere (or, somewhat more precisely, standard conditions), and 298.15 is temperature. Sometimes the index "O" is used for quantities related to pure substance, stipulating that it is possible to designate standard thermodynamic quantities with it only when a pure substance is chosen as the standard state. For example, the state of a substance in an extremely dilute solution can also be accepted as standard.

The enthalpy of formation of simple substances is taken equal to zero, and the zero value of the enthalpy of formation refers to a state of aggregation that is stable at T= 298.15 K. For example, for iodine in the crystalline state Δ H o (I 2, tv) = 0 kJ/mol, and for liquid iodine Δ H o (I 2, l) = 22 kJ/mol. The enthalpies of formation of simple substances under standard conditions are their main energy characteristics.

The thermal effect of any reaction is found as the difference between the sum of the heats of formation of all products and the sum of the heats of formation of all reactants in a given reaction (a consequence of Hess’s law):

Thermochemical effects can be incorporated into chemical reactions. Chemical equations that indicate the amount of heat released or absorbed are called thermochemical equations. Reactions accompanied by the release of heat into the environment have a negative thermal effect and are called exothermic. Reactions accompanied by the absorption of heat have a positive thermal effect and are called endothermic. The thermal effect usually refers to one mole of reacted starting material whose stoichiometric coefficient is maximum.

Temperature dependence of the thermal effect (enthalpy) of the reaction

To calculate the temperature dependence of the enthalpy of a reaction, it is necessary to know the molar heat capacities of the substances participating in the reaction. Change in reaction enthalpy with increasing temperature from T 1 to T 2 is calculated according to Kirchhoff’s law (it is assumed that in a given temperature range the molar heat capacities do not depend on temperature and there are no phase transformations):

Δ H (T 2) = Δ H (T 1) + ∫ 1 2 Δ C p (T 1, T 2) d (T). (\displaystyle \Delta (H(T_(2)))=\Delta (H(T_(1)))+\int \limits _(1)^(2)(\Delta (C_(p))(T_ (1)(,)T_(2))d(T)).)

If phase transformations occur in a given temperature range, then in the calculation it is necessary to take into account the heats of the corresponding transformations, as well as the change in the temperature dependence of the heat capacity of substances that have undergone such transformations:

Where Δ C p(T 1 , T φ) - change in heat capacity in the temperature range from T 1 to the phase transition temperature; Δ C p(T φ , T 2) - change in heat capacity in the temperature range from the phase transition temperature to the final temperature, and Tφ - phase transition temperature.

Standard enthalpy of combustion - Δ H O
mountains, the thermal effect of the combustion reaction of one mole of a substance in oxygen to the formation of oxides in the highest oxidation state. The heat of combustion of non-combustible substances is assumed to be zero.

Standard enthalpy of solution - Δ H O
sol., the thermal effect of the process of dissolving 1 mole of a substance in an infinitely large amount of solvent. It is composed of the heat of destruction of the crystal lattice and the heat of hydration (or the heat of solvation for non-aqueous solutions), released as a result of the interaction of solvent molecules with molecules or ions of the solute with the formation of compounds of variable composition - hydrates (solvates). The destruction of the crystal lattice is usually an endothermic process - Δ H decide > 0, and hydration of ions is exothermic, Δ H hydr.< 0 . Depending on the ratio of Δ values H decide and Δ H hydr. the enthalpy of dissolution can have both positive and negative values. Thus, the dissolution of crystalline potassium hydroxide is accompanied by the release of heat:

Under the enthalpy of hydration Δ H hydr. refers to the heat that is released when 1 mole of ions passes from vacuum to solution.

Standard enthalpy of neutralization Δ H O
neutral - enthalpy of the reaction of strong acids and bases with the formation of 1 mole of water under standard conditions:

HCl + NaOH = NaCl + H 2 O H + + OH − = H 2 O, Δ H O
neutral = −55.9 kJ/mol

The standard enthalpy of neutralization for concentrated solutions of strong electrolytes depends on the ion concentration due to changes in the value of Δ H O
hydration of ions upon dilution.

see also

Comments

Notes

1. , With. 450.
2. , With. 16.
3. , With. 522-523.
4. , With. 290.
5. , With. 21.
6. , With. 17, 63.
7. , With. 311.
8. , With. 174.
9. , With. 6.
10. The state of a simple thermodynamic system (gases and isotropic liquids in a situation where surface effects and the presence of external force fields can be neglected) is completely determined by its volume, pressure in the system and the masses of the substances composing the system.
11. , With. 143.
12. , With. 103.
13. Infinitesimal ( elementary, infinitesimal) is a process for which the difference between the initial and final states of the system is infinitesimal.
14. Heat here refers to the change in energy of the system as a result of heat transfer through the boundary surface (see Heat).
15. , With. 8.
16. , With. 114.
17. , With. 54.
18. , p. 14.
19. , With. 31.
20. , p. 36.
21. , With. 257.
22. , With. 125.
23. , With. eleven.
24. , With. 87.
25. , With. 10.
26. , With. 127.
27. , With. 128.
28. The fact that the final state may be unattainable in reality is irrelevant for the purposes of this discussion.
29. , With. 130.
30. , With. 24.
31. , With. 17.

FOR V - const and p = const

The thermal effect of a chemical reaction occurring at constant volume is called isochoric thermal effect and is denoted Q V.

Substituting into equation (43) Q V, taking into account that V = const, we get

Consequently, the isochoric thermal effect of a reaction (occurring in an isochoric-isothermal process) is equal to the change in the internal energy of the system.

The thermal effect of a reaction occurring at constant pressure is called isobaric thermal effect Qp. Substituting into equation (43) the value Qp, we get

(45)

Replacing the expression U 2 + pV 2 on H 2, A U 1 + pV 1 on H 1, we get

Q p = ΔН = Н 2 -Н 1. (46)

Consequently, the isobaric thermal effect of a reaction (occurring in an isobaric-isothermal process) is equal to the change in enthalpy of the system.

Thus, isobaric and isochoric thermal effects are equal to changes in state functions (44) and (46). Consequently, they do not depend on the transition path, but are determined by the initial and final states of the system. In general, the heat of reaction depends on the nature of the process.

§ 5. DEPENDENCE BETWEEN THERMAL EFFECTS Q v AND Q p

To derive the equation of dependence between Qv And Qp let's use the relation

Q p = ΔН = ΔU p + Δ (pV),

where ΔU p is the change in the internal energy of the thermodynamic system during an isobaric process. In the general case, this change differs from the change in internal energy in an isochoric process, i.e. ΔU P ≠ ΔU V, because

V≠const. Hence, . Therefore, when replacing ΔU V by Q V equation (45) can be rewritten as follows:

.

In condensed systems the difference between Qp And Qv is insignificant and it can be accepted that Q p = Q v. However, if there are gaseous substances in the system, the difference is significant.

If we assume ideal gases, then equation (45) can be written in the form

Q P = Qv + pΔV= Q V + pV 2 - pV 1.

Replacing in this expression pV 2 on n 2 RT And pV 1 on n 1 RT, Where n 1 And n 2- the number of kilomoles of gaseous substances before and after the reaction, from equation (3) we obtain

Q p = Q v + Δ nRT(47)

Q v = Q p -Δ nRT,(48)

Where Δn- change in the number of kilomoles of gaseous reaction products. At Δn > 0

Q V< Q P .

An example of such a reaction is the formation of carbon monoxide

2C + O 2 = 2CO, wherein Δn= 2 - 1 = 1 and Q v = Q p - RT, i.e. Q v< Q p . In this case, the thermodynamic system performs expansion work by reducing the internal energy of the system.

At Δn<0 Q V >Qp. An example of such a reaction would be the following reactions: CO + 0.5O 2 = CO 2 or H 2 + 0.5O 2 = H 2 O, in which Δn = 1 - 1.5 = -0.5, i.e. Δn< 0 . Then Qv = Qp + 0.5RT, i.e. Q v > Q p.

In this case, work is done on the thermodynamic system by the external environment and the system receives additional heat.

When Δn = 0, thermal effects Q v = Q p. An example of such a reaction would be the reaction CO + H 2 O = CO 2 + H 2, in which Δn = 2 - 2 = 0. Therefore, Qv = Qp.

HESS'S LAW

The independence of the thermal effect of the reaction from the intermediate stages of chemical processes was established by the Russian scientist Academician G. I. Hess in 1840 on the basis of experimental data. This is true for reactions occurring at V, T = const or p, T = const. This statement is, in essence, the law of conservation of energy as applied to chemical reactions. It should be noted that Hess's law, the fundamental law of chemical thermodynamics, was discovered even before the first law of thermodynamics was formulated. Hess's law states that the thermal effect of a chemical reaction does not depend on the path of transition of a system from one state to another, but is determined only by its initial and final states.

Thus, the previously derived relations

Q V =U 2 -U 1 and Q p =H 2 - H 1

are algebraic expressions of Hess's law.

Calculations of the thermal effects of chemical reactions are described in the works of M.V. Lomonosov, Lavoisier, and Laplace. Significant experimental material was obtained by G. I. Hess, N. N. Beketov, Berthelot, Thomson, I. A. Kablukov and other scientists. Extensive research to determine the thermal effects of chemical reactions was carried out by V.F. Luginin and his students.

To determine the thermal effects of chemical reactions, special instruments are used - calorimeters.

Hess's law is of great practical importance, since it can be used to calculate the thermal effects of chemical reactions, the experimental determination of which is difficult or practically impossible. Let's explain this with an example

Let us assume that the substance A turns into matter IN three ways: directly from the substance A into substance IN with thermal effect Q 1; through stages C, D with thermal effects Q 2, Q 3, Q 4, through stages E, N, M with thermal effects Q 5, Q 6, Q 7 And Q 8. According to Hess's law, the total thermal effects are the same, therefore

Q 1 =Q 2 +Q 3 +Q 4 ;

Q 1= Q 5 +Q 6 +Q 7 +Q 8.

Q 2 +Q 3 +Q 4 =Q 5 +Q 6 +Q 7 +Q 8.

Using these relationships, it is easy to calculate the thermal effect of any chemical reaction, which cannot be obtained experimentally. For example, the thermal effect

Q 8 =Q 1 -Q 5 -Q 6 -Q 7.

As a rule, the experimental determination of thermal effects at all stages is carried out with great care; all prerequisites arising from Hess’s law are observed (conditions to which the initial and final combustion products are reduced, the same chemical composition of the initial products, etc.) and are reduced to a minimum errors and inaccuracies associated with the conditions of heat exchange between experimental equipment and the environment, methods of measuring temperatures, etc., i.e., irreversible losses associated with the conversion of mechanical energy directly into thermal energy are practically absent.

Using Hess's law, it is possible to make calculations using the so-called thermochemical equations, which are stoichiometric equations of chemical reactions, in which, along with the chemical formulas of the substances participating in the reaction, thermal effects (referred to the same conditions) are written down. You can perform algebraic operations with these equations in the same way as with any algebraic equations.

Stoichiometric equations or ratios are numerical relationships between the quantities of reacting substances that comply with the laws of stoichiometry, the main provisions of which follow from the laws of Avogadro, Gay-Lussac, constancy of composition, multiple ratios, etc.

From the stoichiometric ratio, for example,

2H 2 + O 2 = 2H 2 O

it follows that when water is formed, there is one oxygen molecule per two hydrogen molecules, or in general terms

x a A+x b B=x a D, during education xd molecules of matter D on x a molecules of matter A required x b molecules of matter IN. Odds x a, x b And xd- the number of molecules of the starting substances and those obtained in the reaction are called stoichiometric coefficients.

The number of kilomoles of starting and resulting substances in a chemical reaction is proportional to the stoichiometric coefficients. In gas reactions, the volumes and partial pressures of the reactants and reaction products are also proportional to the stoichiometric coefficients.

Since thermal effects depend on the physical state of the reacting substances and the conditions under which the reaction occurs, then in order to carry out thermochemical calculations, the thermal effects introduced into thermochemical equations must be related to some identical conditions, otherwise they are incomparable. Such conditions are taken to be the conditions under which a reaction occurs between substances in certain standard states.

The standard states of individual liquid and solid substances are taken to be their stable state at a given temperature and pressure p = 1 atm = 760 mmHg Art., or 1,013- 10 5 Pa, and for individual gases - this is their state when at pressure p = 760 mmHg Art. and at a given temperature they obey the equation of state of an ideal gas.

Thermal effects widely cited in reference books are usually attributed to pressure p = 1 physical atmosphere ( 1.013 10 5 Pa) and temperature t = 25° C(298.15 K) and denote Q 0 V 298 And Q 0 P 298

or ΔQ 0 298 And ΔH 0 298 .

Corollaries of great practical importance follow from Hess's law.

1. Thermal effect of decomposition reaction Q pa from a chemical compound is equal in magnitude and opposite in sign to the thermal effect of formation Q o 6p of this compound from decomposition products:

Q diff =- Q arr.

2. If the same end products are formed from two chemical systems in two different ways, then the difference between the values ​​of the thermal effects of chemical reactions is equal to the thermal effect of converting one chemical system into another. So, for example, for the reaction of formation of substance B from substances A And WITH(Fig. 7), according to Hess’s law,

Q 1 = Q 2 + Q 3,

where does the thermal effect of substance transformation come from? A V WITH

Q 3 = Q 1 - Q 2

3. If systems of identical chemical composition are converted in two ways into different final products, then the difference between the values ​​of thermal effects is equal to the heat obtained from the transformation of one final product of a chemical reaction into another. So, when formed from a substance A substances IN And WITH(Fig. 8), according to Hess’s law, Q 1 = Q 2 + Q 3, where does the thermal effect of the transition of matter come from WITH into substance IN

Q 3 =Q 1 - Q 2.

In thermochemical calculations, two types of thermal effects of chemical reactions are of particular importance: the heat of formation of compounds and the heat of combustion.

The heat of formation is usually called the thermal effect of the reaction of formation of a given compound from the corresponding simple substances under standard conditions.

The standard state of simple substances is taken to be their stable state at a pressure equal to one physical atmosphere (760 mm Hg, or 1.013-10 5 Pa) and a temperature of 298.15 K.

An example is the reaction of the formation of benzene: from substances in standard states - "■ solid carbon and gaseous hydrogen, liquid benzene is obtained

6S TV + ZN 2 = S 6 N 6zh.

The indices “g” and “tv”, respectively, refer to the liquid and solid phases. The subscript “g” refers to a gaseous substance, but it is usually omitted in calculation equations.

The heat of formation corresponding to standard conditions is called standard. Data on the heat of formation, along with other physicochemical quantities, are given in reference books.

Since in thermodynamic calculations it is not the absolute values ​​of internal energy and enthalpy that are determined, but their change, then when determining the heat of formation of any compound, the starting point for the internal energy or enthalpy can be chosen arbitrarily. So, for example, in reference books for various simple substances under standard conditions it is assumed that the enthalpy is zero. Such substances include C, H2, O2, Cl2(g), F2(g), etc.

Thus, the thermal effect of the formation of compounds from these substances, for example, Q p, turns out to be equal to the enthalpy of the compound under the desired conditions.

The heat of formation can be applied to any quantity of a substance. In reference books, as a rule, it is referred to as 1 kmol or 1 kg of compound.

In table Table 1 shows the heat of formation of substances for some common chemical compounds.

Heat of combustion. Combustion is a complex, rapidly occurring chemical transformation, accompanied by the release of a significant amount of heat and, as a rule, a bright glow.

Table 1. Thermal effects of the formation of compounds from simple substances under standard conditions

Rice. 9. Diagram of a calorimetric “bomb”:

1 – cylinder; 2 – cover; 3 – calyx; 4 - spiral

The thermal effect of a combustion reaction, called the heat of combustion, is usually measured calorimetrically.

The heat of combustion of a compound is the thermal effect of the oxidation reaction of a given compound with oxygen with the formation of limiting higher oxides of the corresponding elements. So, for example, in organic compounds, which are the main fuel in heat engines, carbon is oxidized to carbon dioxide, hydrogen to water vapor, and other substances included in the compound in small quantities to their final oxidation products.

The heat of combustion is significantly influenced by temperature and pressure. To be able to use the heat of combustion in thermochemical ratios, it must be brought to standard conditions. The heat of combustion in this case is called standard. The calorific value found from the reference book is used to determine the thermal effects of reactions.

In Fig. Figure 9 shows a diagram of a calorimetric bomb in which the heat of combustion is experimentally determined. The calorimetric bomb is a thick-walled steel cylinder 1, coated on the inside with platinum. Cover 2 is screwed onto the cylinder. Inside the cylinder there is a cup 3 for weighing the test substance. Oxygen is pumped into the cylinder under high pressure. Using wire 4, heated by electric current, the substance under study is ignited. The bomb is placed in a calorimeter, through which the heat of combustion of the substance under study is determined. The temperature of the combustion products is “brought” to the temperature in the bomb before ignition.

The heat of combustion of organic compounds, often called the heat of combustion of fuel, is the initial value in calculating the operating processes of heat engines. It is defined as the amount of heat (in J or kcal) released during the complete combustion of 1 kg of mass, 1 m 3 of volume or 1 kmol of fuel.

The heat of combustion of the fuel, if determined in the manner described above, in a calorimetric bomb will be the heat of combustion for the process at V = const, i.e. it will be the thermal effect Q V .

There are higher and lower calorific values ​​of fuel.

The higher calorific value of fuel Q B is the total amount of heat released during the combustion of combustible parts of the fuel under the condition of condensation of water vapor.

The lower calorific value of fuel combustion Q H is the difference between the total amount of heat released and the latent heat of vaporization of water, both present in the fuel as an impurity and resulting from the combustion of hydrogen.

The highest Q B and the lowest Q H heat of combustion of fuel are related to each other by the relation

-Q h = -Q B +r b (9H + W) = -Q b + 2.512 10 6 (9H+W), Jfkg, (49)

where r b is the latent heat of vaporization (for technical calculations, r b ≈ 2.512· 10 6 Jfkg is taken); 9H - the amount of water vapor generated during the combustion of H (kg) of hydrogen contained in 1 kg of fuel; W is the amount of moisture contained in 1 kg of fuel, kg.

In calculations of internal combustion engine operating processes, the lower calorific value is taken as the calorific value, since the combustion products removed from the engine through the exhaust system usually have a temperature exceeding the condensation temperature of the water vapor contained in them.

In table Table 2 shows the values ​​of the lower heating value of fuels.

Based on Hess's law and its consequences, a thermochemical equation can be constructed to determine the thermal effect of a reaction through the thermal effects of the formation of reactants.

So, for example, if the reaction takes place bB + dD = eE + gG, where B, D, E, G, b, d, e, g - starting materials and reaction products

table 2

Lower heating value of fuels

and their stoichiometric coefficients, respectively, then the thermal effect of this reaction

Q p =(eQ arr +gQ arrG) – (bQ arrB +dQ arrD)

Hence the equation in general form

(50)

where Q arrB, Q arrD, Q arrE and Q o 6pG are the heat of formation of the starting materials and reaction products, respectively; n i - numbers of kilomoles (from 1 to t), proportional to the stoichiometric coefficients of the reacting substances.

Consequently, the thermal effect of the reaction is equal to the difference between the heat of formation of the reaction products and the heat of formation of the starting substances, taken with the corresponding stoichiometric coefficients.

Using Hess's law and its consequences, it is also possible to construct a thermochemical equation to calculate the thermal effect if the heat of combustion of the substances involved in the reaction is known.

In general

i.e., the thermal effect of the reaction is equal to the difference between the heat of combustion of the starting substances and the heat of combustion of the reaction products (taking into account their stoichiometric coefficients).

This can be illustrated by the example of the combustion of methyl alcohol CH 3 OH (Fig. 10). Heat of combustion of 1 kmol of methyl alcohol

Q 2сг = - 726.49·10 6 J/kmol;

the heats of combustion of C in CO 2 and H 2 in H 2 O F are respectively equal

Q" 1 c g = -393.51·10 6 J/kmol;

Q" 1 c g = -285.84·10 6 J/kmol;

Q lc g = -965.19 ·10 6 J/kmol.

Rice. 10. Heat determination scheme effect upon combustion of methyl alcohol

Let us write down the thermochemical equations of combustion reactions:

C + O 2 = CO 2 + Q" 1 c g;

2H 2 + O 2 = 2H 2 O F + 2Q" 1 c g;

CH 3 OH F + 1.5O 2 = CO 2 + 2H 2 O + Q 2.

To determine the heat of formation of methyl alcohol from the equation C + 2H 2 + 0.5O 2 = CH 3 OH + Q 3, add the two equations written above and subtract the third. After some transformations we get

C + 2H 2 + 0.5O 2 = CH 3 OH + (Q lcr - Q 2cr),

Comparing the last two equations, we conclude that the required heat of formation of 1 kmol of liquid methyl alcohol is

Q 3rev = -238.7·10 6 Jfkmol.

Related information.

Any chemical processes, as well as a number of physical transformations of substances (evaporation, condensation, melting, polymorphic transformations, etc.) are always accompanied by a change in the internal energy reserve of systems. Thermochemistry is a branch of chemistry that studies the change in the amount of heat during a process. One of the founders of thermochemistry is the Russian scientist G. I. Hess.

Thermal effect of a chemical reaction is the heat that is released or absorbed during a chemical reaction. Standard thermal effect of a chemical reaction is the heat that is released or absorbed during a chemical reaction under standard conditions. All chemical processes can be divided into two groups: exothermic and endothermic.

Exothermic- These are reactions in which heat is released into the environment. In this case, the reserve of internal energy of the starting substances (U 1) is greater than that of the resulting products (U 2). Therefore, ∆U< 0, а это приводит к образованию термодинамически устойчивых веществ.

Endothermic These are reactions in which heat is absorbed from the environment. In this case, the reserve of internal energy of the starting substances (U 1) is less than that of the resulting products (U 2). Consequently, ∆U > 0, and this leads to the formation of thermodynamically unstable substances. Unlike thermodynamics, in thermochemistry the released heat is considered positive, and the absorbed heat is considered negative. Heat in thermochemistry is denoted by Q. The unit of heat is J/mol or kJ/mol. Depending on the conditions of the process, isochoric and isobaric thermal effects are distinguished.

Isochoric (Q V) The thermal effect is the amount of heat that is released or absorbed during a given process at a constant volume (V = const) and equal temperatures of the final and initial states (T 1 = T 2).

Isobaric (Q p) The thermal effect is the amount of heat that is released or absorbed during a given process at constant pressure (p = const) and equal temperatures of the final and initial states (T 1 = T 2).

For liquid and solid systems, the change in volume is small and it can be assumed that Q p » Q V . For gaseous systems

Q р = Q V – ∆nRT, (4.3)

where ∆n is the change in the number of moles of gaseous reaction participants

∆n = ån cont. reactions – ån ref. substances. (4.4)

In all cases, the conversion of part of the internal (chemical) energy into thermal (or other types) and vice versa, thermal into chemical, occurs in strict accordance with the law of conservation of energy and the first law of thermodynamics.

In thermochemistry it is common to use thermochemical equations These are equations of chemical reactions, in which the starting substances are given on the left side of the equation, and the reaction products plus (or minus), the thermal effect are shown on the right, and the aggregative state of the substances and their crystalline forms are also shown. For example,

C graphite + O 2 = CO 2 (g) + 393.77 kJ

H 2 + 1/2O 2 = H 2 O (l) + 289.95 kJ

C (diamond) + 2S (diamond) = CS 2 (g) – 87.9 kJ

With thermochemical equations, you can perform all algebraic operations: addition, subtraction, multiplication, transfer terms, etc.

The thermal effects of many chemical and physical processes are determined experimentally (calorimetry) or calculated theoretically using the values ​​of the heats of formation (decomposition) and heats of combustion of certain chemical compounds.

The heat of formation of a given compound is the amount of heat released or absorbed when 1 mole of it is formed from simple substances in kJ. The heats of formation of simple substances that are in a stable state under standard conditions are taken to be zero. In reactions

K (tv) + 1/2Cl (g) = KS1 (tv) + 442.13 kJ

C (tv) + 1/2H 2 (g) + 1/2N (g) = HCN (g) – 125.60 kJ

the thermal effects of 442.13 kJ and -125.60 kJ are the heats of formation of KCl and HCN, respectively. Heat of decomposition of the indicated compounds into simple substances, according to the law of conservation of energy, are equal in absolute value, but opposite in sign, i.e. for KCl the heat of decomposition is -442.13 kJ, and for HCN it is +125.60 kJ.

The more heat is released during the formation of a compound, the more heat must be spent on its decomposition, and the stronger the given compound is under normal conditions. Chemically stable and durable substances are: SiO 2, A1 2 O 3, P 2 O 5, KCl, NaCl, etc. Substances formed with the absorption of heat are unstable (for example, NO, CS 2, C 2 H 2, HCN and all explosives). The heat of formation of organic compounds cannot be determined experimentally. They are calculated theoretically based on the values ​​of the heat of combustion of these compounds, found experimentally.

Heat of combustion is the heat released during the complete combustion of 1 mole of a substance in a stream of oxygen. Heat of combustion is determined in a calorimetric installation, the main parts of which are: an oxygen cylinder, a calorimetric bomb, a calorimeter with a weighed amount of water and a stirrer, and an electrical ignition device.

The magnitude of the thermal effects of chemical reactions depends on many factors: the nature of the reacting substances, the state of aggregation of the initial and final substances, the conditions of the reaction (temperature, pressure, volume of systems, concentration).

THERMAL EFFECT, heat released or absorbed thermodynamically. system when chemical flows through it. districts. It is determined under the condition that the system does not perform any work (except for possible expansion work), and the t-ry and products are equal. Since heat is not a state function, i.e. during the transition between states depends on the transition path, then in the general case the thermal effect cannot serve as a characteristic of a specific district. In two cases, an infinitesimal amount of heat (elementary heat) d Q coincides with the total differential of the state function: with constant volume d Q = = dU (U is the internal energy of the system), and with constant d Q = dH (H - enthalpy of the system).

Two types of thermal effects are practically important: isothermal-isobaric (at constant temperatures T and p) and isothermal-isochoric (at constant T and volume V). There are differential and integral thermal effects. The differential thermal effect is determined by the expressions:

where u i, h i -resp. partial molar extr. energy and ; v i -stoichiometric coefficient (v i > 0 for products, v i<0 для ); x = (n i - n i 0)/v i ,-хим. переменная, определяющая состав системы в любой момент протекания р-ции (n i и n i0 - числа i-го компонента в данный момент времени и в начале хим. превращения соотв.). Размерность дифференциального теплового эффекта реакции-кДж/ . Если u T,V , h T,p >0, district called endothermic, with the opposite sign of the effect - exothermic. The two types of effects are related by:

The temperature dependence of the thermal effect is given, the application of which, strictly speaking, requires knowledge of the partial molars of all substances involved in the solution, but in most cases these quantities are unknown. Since for processes occurring in real solutions and other thermodynamically non-ideal environments, thermal effects, like others, significantly depend on the composition of the system and the experiment. conditions, an approach has been developed that facilitates the comparison of different districts and the taxonomy of thermal effects. This purpose is served by the concept of standard thermal effect (denoted). By standard we mean a thermal effect carried out (often hypothetically) under conditions when all the entities participating in the district are in the given conditions. Differential and integral standard thermal effects are always numerically the same. The standard thermal effect can be easily calculated using tables of standard heats of formation or heats of combustion (see below). For non-ideal media, there is a large discrepancy between the actually measured and standard thermal effects, which must be kept in mind when using thermal effects in thermodynamic calculations. For example, for alkaline diacetimide [(CH 3 CO) 2 NH (sol) + H 2 O (l) = CH 3 SOKH 2 (sol) + CH 3 COOH (l) +] in 0.8 n. NaOH solution in aqueous (58% by weight) at 298 K measured thermal effect DH 1 = - 52.3 kJ/. For the same district under standard conditions, = - 18.11 kJ/ was obtained. It means so much. the difference is explained by the thermal effects accompanying the substance in the specified solution (heat). For solid, liquid acetic acid and heat are equal, respectively: D H 2 = 13.60; D H 3 = - 48.62; D H 4 = - 0.83 kJ/, so = D H 1 - D H 2 - D H 3 + D H 4. From the example viewbut that when studying thermal effects, it is important to measure the thermal effects of the accompanying physical-chemical. processes.

The study of thermal effects is a very important task. Basic let's experiment method - calorimetry. Modern The equipment makes it possible to study thermal effects in gas, liquid and solid phases, at the interface, as well as in complex ones. systems. The range of typical values ​​of measured thermal effects is from hundreds of J/ to hundreds of kJ/. In table calorimetric data are given. measurements of thermal effects of certain districts. Measuring thermal effects, dilution, and heat allows one to move from actually measured thermal effects to standard ones.

An important role belongs to thermal effects of two types - the heat of formation of the compound. from simple substances and the heat of combustion of pure substances with the formation of higher elements, of which the substance consists. These thermal effects are brought to standard conditions and tabulated. With their help it is easy to calculate any thermal effect; it is equal to algebraic. the sum of the heats of formation or heats of combustion of all the substances involved in the process:

Application of tabular values allowscalculate thermal effects plural. thousand rubles, although these values ​​themselves are known only for several. thousand connections. This method of calculation is unsuitable, however, for districts with small thermal effects, since the calculated small value obtained as an algebraic amount several large values, characterized by an error, edges in abs. may exceed the thermal effect. Calculation of thermal effects using quantities based on the fact that there is a state function. This makes it possible to compose thermochemical systems. equations to determine the thermal effect of the required solution (see). Standard thermal effects are almost always calculated. In addition to the method discussed above, the calculation of thermal effects is carried out using the temperature dependence of -eq.

Introduction

The thermal effects of chemical reactions are necessary for many technical calculations. They find wide application in many industries, as well as in military developments.

The purpose of this course work is to study the practical application of the thermal effect. We will look at some options for its use, and find out how important it is to use the thermal effects of chemical reactions in the context of the development of modern technologies.

Thermal effect of a chemical reaction

Each substance stores a certain amount of energy. We encounter this property of substances already at breakfast, lunch or dinner, since food allows our body to use the energy of a wide variety of chemical compounds contained in food. In the body, this energy is converted into movement, work, and is used to maintain a constant (and quite high!) body temperature.

One of the most famous scientists working in the field of thermochemistry is Berthelot. Berthelot - professor of chemistry at the Higher Pharmaceutical School in Paris (1859). Minister of Education and Foreign Affairs.

Beginning in 1865, Berthelot was actively involved in thermochemistry and conducted extensive calorimetric research, which led, in particular, to the invention of the “calorimetric bomb” (1881); he owns the concepts of “exothermic” and “endothermic” reactions. Berthelot obtained extensive data on the thermal effects of a huge number of reactions, on the heat of decomposition and formation of many substances.

Berthelot studied the effect of explosives: explosion temperature, combustion speed and blast wave propagation, etc.

The energy of chemical compounds is concentrated mainly in chemical bonds. It takes energy to break a bond between two atoms. When a chemical bond is formed, energy is released.

Any chemical reaction consists of breaking some chemical bonds and forming others.

When, as a result of a chemical reaction during the formation of new bonds, more energy is released than was required to destroy the “old” bonds in the starting substances, the excess energy is released in the form of heat. An example is combustion reactions. For example, natural gas (methane CH 4) burns in oxygen in the air, releasing a large amount of heat (Fig. 1a). Such reactions are exothermic.

Reactions that occur with the release of heat exhibit a positive thermal effect (Q>0, DH<0) и называются экзотермическими.

In other cases, the destruction of bonds in the original substances requires more energy than can be released during the formation of new bonds. Such reactions occur only when energy is supplied from outside and are called endothermic.

Reactions that occur with the absorption of heat from the environment (Q<0, DH>0), i.e. with a negative thermal effect, are endothermic.

An example is the formation of carbon monoxide (II) CO and hydrogen H2 from coal and water, which occurs only when heated (Fig. 1b).

Rice. 1a,b. Depiction of chemical reactions using molecular models: a) exothermic reaction, b) endothermic reaction. The models clearly show how, with a constant number of atoms between them, old chemical bonds are destroyed and new chemical bonds arise.

Thus, any chemical reaction is accompanied by the release or absorption of energy. Most often, energy is released or absorbed in the form of heat (less often in the form of light or mechanical energy). This heat can be measured. The measurement result is expressed in kilojoules (kJ) for one mole of reactant or (less commonly) for one mole of reaction product. This quantity is called the thermal effect of the reaction.

Thermal effect is the amount of heat released or absorbed by a chemical system when a chemical reaction occurs in it.

Thermal effect is indicated by the symbols Q or DH (Q = -DH). Its value corresponds to the difference between the energies of the initial and final states of the reaction:

DH = Hfin. - Nish. = Efin. - Eout.

Icons (d), (g) indicate the gaseous and liquid states of substances. There are also designations (tv) or (k) - solid, crystalline substance, (aq) - substance dissolved in water, etc.

The designation of the state of aggregation of a substance is important. For example, in the combustion reaction of hydrogen, water is initially formed in the form of steam (gaseous state), upon condensation of which some more energy can be released. Consequently, for the formation of water in the form of a liquid, the measured thermal effect of the reaction will be slightly greater than for the formation of only steam, since when the steam condenses, another portion of heat will be released.

A special case of the thermal effect of the reaction is also used - the heat of combustion. From the name itself it is clear that the heat of combustion serves to characterize the substance used as fuel. The heat of combustion is referred to 1 mole of a substance that is a fuel (a reducing agent in an oxidation reaction), for example:

The energy (E) stored in molecules can be plotted on the energy scale. In this case, the thermal effect of the reaction ( E) can be shown graphically (Fig. 2).

Rice. 2. Graphic representation of the thermal effect (Q =  E): a) exothermic reaction of hydrogen combustion; b) endothermic reaction of water decomposition under the influence of electric current. The reaction coordinate (horizontal axis of the graph) can be considered, for example, as the degree of conversion of substances (100% is the complete conversion of the starting substances).

Chemical Reaction Equations

Equations of chemical reactions in which the thermal effect of the reaction is written along with the reagents and products are called thermochemical equations.

The peculiarity of thermochemical equations is that when working with them, you can transfer the formulas of substances and the magnitude of thermal effects from one part of the equation to another. As a rule, this cannot be done with ordinary equations of chemical reactions.

Term-by-term addition and subtraction of thermochemical equations is also allowed. This may be necessary to determine the thermal effects of reactions that are difficult or impossible to measure experimentally.

Let's give an example. In the laboratory, it is extremely difficult to carry out the “pure form” reaction of producing CH4 methane by direct combination of carbon with hydrogen:

C + 2H 2 = CH 4

But you can learn a lot about this reaction through calculations. For example, find out whether this reaction will be exo- or endothermic, and even quantitatively calculate the magnitude of the thermal effect.

The thermal effects of the combustion reactions of methane, carbon and hydrogen are known (these reactions occur easily):

a) CH 4 (g) + 2O 2 (g) = CO 2 (g) + 2H 2 O (l) + 890 kJ

b) C(tv) + O 2 (g) = CO 2 (g) + 394 kJ

c) 2H 2 (g) + O 2 (g) = 2H 2 O (l) + 572 kJ

Let us subtract the last two equations (b) and (c) from equation (a). We will subtract the left sides of the equations from the left, and the right sides from the right. In this case, all molecules O 2, CO 2 and H 2 O will contract. We get:

CH 4 (g) - C (tv) - 2H 2 (g) = (890 - 394 - 572) kJ = -76 kJ

This equation looks somewhat unusual. Let's multiply both sides of the equation by (-1) and move CH 4 to the right side with the opposite sign. We get the equation we need for the formation of methane from coal and hydrogen:

C(tv) + 2H 2 (g) = CH 4 (g) + 76 kJ/mol

So, our calculations showed that the thermal effect of the formation of methane from carbon and hydrogen is 76 kJ (per mole of methane), and this process must be exothermic (energy will be released in this reaction).

It is important to pay attention to the fact that only substances that are in identical states of aggregation can be added, subtracted and reduced term by term in thermochemical equations, otherwise we will make a mistake in determining the thermal effect on the value of the heat of transition from one state of aggregation to another.

Basic laws of thermochemistry

The branch of chemistry that studies the transformation of energy in chemical reactions is called thermochemistry.

There are two most important laws of thermochemistry. The first of them, the Lavoisier–Laplace law, is formulated as follows:

The thermal effect of a forward reaction is always equal to the thermal effect of a reverse reaction with the opposite sign.

This means that during the formation of any compound, the same amount of energy is released (absorbed) as is absorbed (released) during its decomposition into the original substances. For example:

2H 2 (g) + O 2 (g) = 2H 2 O (l) + 572 kJ (combustion of hydrogen in oxygen)

2 H 2 O (l) + 572 kJ = 2H 2 (g) + O 2 (g) (decomposition of water by electric current)

Lavoisier–Laplace's law is a consequence of the law of conservation of energy.

The second law of thermochemistry was formulated in 1840 by Russian academician G. I. Hess:

The thermal effect of a reaction depends only on the initial and final states of the substances and does not depend on the intermediate stages of the process.

This means that the total thermal effect of a series of successive reactions will be the same as that of any other series of reactions if the starting and ending substances are the same at the beginning and at the end of these series. These two basic laws of thermochemistry give thermochemical equations some similarity to mathematical ones, when in reaction equations it is possible to transfer terms from one part to another, to add, subtract and reduce the formulas of chemical compounds term by term. In this case, it is necessary to take into account the coefficients in the reaction equations and not to forget that the substances being added, subtracted or reduced by moles must be in the same state of aggregation.

Application of the thermal effect in practice

The thermal effects of chemical reactions are needed for many technical calculations. For example, consider the powerful Russian Energia rocket, capable of launching spacecraft and other payloads into orbit. The engines of one of its stages operate on liquefied gases - hydrogen and oxygen.

Suppose we know the work (in kJ) that will have to be spent to deliver a rocket with cargo from the surface of the Earth to orbit, we also know the work to overcome air resistance and other energy costs during the flight. How to calculate the required supply of hydrogen and oxygen, which (in a liquefied state) are used in this rocket as fuel and oxidizer?

Without the help of the thermal effect of the reaction of the formation of water from hydrogen and oxygen, this is difficult to do. After all, the thermal effect is the very energy that should launch the rocket into orbit. In the combustion chambers of a rocket, this heat is converted into the kinetic energy of molecules of hot gas (steam), which escapes from the nozzles and creates jet thrust.

In the chemical industry, thermal effects are needed to calculate the amount of heat to heat reactors in which endothermic reactions occur. In the energy sector, thermal energy production is calculated using the heat of combustion of fuel.

Dietitians use the thermal effects of food oxidation in the body to create proper diets not only for patients, but also for healthy people - athletes, workers in various professions. Traditionally, calculations here use not joules, but other energy units - calories (1 cal = 4.1868 J). The energy content of food is referred to any mass of food products: 1 g, 100 g, or even standard packaging of the product. For example, on the label of a jar of condensed milk you can read the following inscription: “calorie content 320 kcal/100 g.”

The thermal effect is calculated when producing monomethylaniline, which belongs to the class of substituted aromatic amines. The main area of ​​application of monomethylaniline is as an anti-knock additive for gasoline. It is possible to use monomethylaniline in the production of dyes. Commercial monomethylaniline (N-methylaniline) is isolated from the catalyzate by periodic or continuous rectification. Thermal effect of the reaction ∆Н= -14±5 kJ/mol.

Heat-resistant coatings

The development of high-temperature technology necessitates the creation of particularly heat-resistant materials. This problem can be solved by using refractory and heat-resistant metals. Intermetallic coatings are attracting increasing attention because they have many valuable qualities: resistance to oxidation, aggressive melts, heat resistance, etc. Of interest is also the significant exothermicity of the formation of these compounds from their constituent elements. There are two possible ways to use the exothermicity of the reaction for the formation of intermetallic compounds. The first is the production of composite, two-layer powders. When heated, the components of the powder interact, and the heat of the exothermic reaction compensates for the cooling of the particles, reaching the protected surface in a completely molten state and forming a low-porosity coating firmly adhered to the base. Another option would be to apply a mechanical mixture of powders. When the particles are heated sufficiently, they interact already in the coating layer. If the magnitude of the thermal effect is significant, then this can lead to self-melting of the coating layer, the formation of an intermediate diffusion layer that increases the adhesion strength, and obtaining a dense, low-porosity coating structure. When choosing a composition that forms an intermetallic coating with a great thermal effect and has many valuable qualities - corrosion resistance, sufficient heat resistance and wear resistance, nickel aluminides, in particular NiAl and Ni 3 Al, attract attention. The formation of NiAl is accompanied by a maximum thermal effect.

Thermochemical method of diamond processing

The “thermochemical” method received its name due to the fact that it occurs at elevated temperatures, and is based on the use of the chemical properties of diamond. The method is carried out as follows: the diamond is brought into contact with a metal capable of dissolving carbon, and in order for the dissolution or processing process to proceed continuously, it is carried out in a gas atmosphere that interacts with carbon dissolved in the metal, but does not react directly with the diamond. During the process, the magnitude of the thermal effect takes on a high value.

To determine the optimal conditions for thermochemical processing of diamond and identify the capabilities of the method, it was necessary to study the mechanisms of certain chemical processes, which, as shown by an analysis of the literature, have not been studied at all. A more specific study of the thermochemical processing of diamond was hampered, first of all, by insufficient knowledge of the properties of the diamond itself. They were afraid of ruining it with heat. Research on the thermal stability of diamond has only been carried out in recent decades. It has been established that diamonds that do not contain inclusions can be heated to 1850 “C” in a neutral atmosphere or in a vacuum without any harm to them, and only higher.

Diamond is the best blade material due to its unique hardness, elasticity and low friction against biological tissue. Operating with diamond knives facilitates operations and reduces the healing time of incisions by 2-3 times. According to microsurgeons at the MNTK for eye microsurgery, knives sharpened by thermochemical methods are not only not inferior, but also superior in quality to the best foreign samples. Thousands of operations have already been performed with thermochemically sharpened knives. Diamond knives of different configurations and sizes can be used in other areas of medicine and biology. Thus, microtomes are used to make preparations in electron microscopy. The high resolution of the electron microscope places special demands on the thickness and quality of the section of specimens. Diamond microtomes, sharpened by thermochemical method, make it possible to produce sections of the required quality.

Technogenic raw materials for cement production

Further intensification of cement production involves the widespread introduction of energy and resource-saving technologies using waste from various industries.

When processing skarn-magnetite ores, dry magnetic separation (DMS) tailings are released, which are crushed stone material with a grain size of up to 25 mm. SMS tailings have a fairly stable chemical composition, wt.%:

SiO 2 40…45,

Al 2 O 3 10…12,

Fe 2 O 3 15…17,

CaO 12…13,

MgO 5…6,

The possibility of using SMS tailings in the production of Portland cement clinker has been proven. The resulting cements are characterized by high strength properties.

The thermal effect of clinker formation (TEC) is defined as the algebraic sum of the heats of endothermic processes (decarbonization of limestone, dehydration of clay minerals, formation of a liquid phase) and exothermic reactions (oxidation of pyrite introduced by CMS tailings, formation of clinker phases).

The main advantages of using skarn-magnetite ore enrichment waste in cement production are:

Expansion of the raw material base due to man-made sources;

Saving natural raw materials while maintaining cement quality;

Reducing fuel and energy costs for clinker firing;

Possibility of producing low-energy active low-basic clinkers;

Solving environmental problems through rational waste disposal and reducing gas emissions into the atmosphere during clinker firing.

Biosensors

Biosensors are sensors based on immobilized enzymes. They allow you to quickly and efficiently analyze complex, multicomponent mixtures of substances. Currently, they are increasingly used in a number of branches of science, industry, agriculture and healthcare. The basis for the creation of automatic enzymatic analysis systems was the latest advances in the field of enzymology and engineering enzymology. The unique qualities of enzymes - specificity of action and high catalytic activity - contribute to the simplicity and high sensitivity of this analytical method, and the large number of enzymes known and studied to date makes it possible to constantly expand the list of analyzed substances.

Enzyme microcalorimetric sensors - use the thermal effect of an enzymatic reaction. It consists of two columns (measuring and control), filled with a carrier with an immobilized enzyme and equipped with thermistors. When the analyzed sample is passed through the measuring column, a chemical reaction occurs, which is accompanied by a recorded thermal effect. This type of sensor is interesting for its versatility.

Conclusion

So, after analyzing the practical application of the thermal effect of chemical reactions, we can conclude: the thermal effect is closely related to our everyday life, it is constantly being studied and is finding new applications in practice.

With the development of modern technologies, the warm effect has found its application in various industries. Chemical, military, construction, food, mining and many other industries use the thermal effect in their developments. It is used in internal combustion engines, refrigeration units and various combustion devices, as well as in the production of surgical instruments, heat-resistant coatings, new types of building materials and so on.

In modern conditions of constantly developing science, we are seeing the emergence of more and more new developments and discoveries in the field of production. This entails more and more new areas of application of the thermal effect of chemical reactions.

Chernykh E. A.

Bibliography

Musabekov Yu. S., Marcelin Berthelot, M., 1965; Centenaire de Marcelin Berthelot, 1827-1927, P., 1929.

Patent 852586 Russian Federation. MKI V 28 D 5/00. Method for dimensional processing of diamond / A.P.Grigoriev, S.H.Lifshits, P.P.Shamaev (Russian Federation). - 2 s.