The principle of displacement of dynamic chemical equilibrium. Shift in chemical equilibrium

Chemical equilibrium and the principles of its displacement (Le Chatelier's principle)

In reversible reactions, under certain conditions, a state of chemical equilibrium can occur. This is the state in which the rate of the reverse reaction becomes equal to the rate of the forward reaction. But in order to shift the equilibrium in one direction or another, it is necessary to change the conditions for the reaction. The principle of shifting equilibrium is Le Chatelier's principle.

Basic provisions:

1. An external impact on a system that is in a state of equilibrium leads to a shift in this equilibrium in the direction in which the effect of the produced impact is weakened.

2. With an increase in the concentration of one of the reacting substances, the equilibrium shifts towards the consumption of this substance, with a decrease in concentration, the equilibrium shifts towards the formation of this substance.

3. With an increase in pressure, the equilibrium shifts towards a decrease in the amount of gaseous substances, that is, towards a decrease in pressure; when pressure decreases, the equilibrium shifts in the direction of increasing amounts of gaseous substances, that is, in the direction of increasing pressure. If the reaction proceeds without changing the number of molecules of gaseous substances, then the pressure does not affect the equilibrium position in this system.

4. With an increase in temperature, the equilibrium shifts towards an endothermic reaction, with a decrease in temperature - towards an exothermic reaction.

For the principles, we thank the manual "The Beginnings of Chemistry" Kuzmenko N.E., Eremin V.V., Popkov V.A.

USE assignments for chemical equilibrium (formerly A21)

Task number 1.

H2S(g) ↔ H2(g) + S(g) - Q

1. Pressurizing

2. Temperature rise

3. pressure reduction

Explanation: to begin with, consider the reaction: all substances are gases and on the right side there are two molecules of products, and on the left side there is only one, the reaction is also endothermic (-Q). Therefore, consider the change in pressure and temperature. We need the equilibrium to shift towards the products of the reaction. If we increase the pressure, then the equilibrium will shift towards a decrease in volume, that is, towards the reagents - this does not suit us. If we increase the temperature, then the equilibrium will shift towards the endothermic reaction, in our case towards the products, which is what was required. The correct answer is 2.

Task number 2.

Chemical equilibrium in the system

SO3(g) + NO(g) ↔ SO2(g) + NO2(g) - Q

will shift towards the formation of reagents at:

1. Increasing NO concentration

2. Increasing SO2 concentration

3. Temperature rise

4. Increasing pressure

Explanation: all substances are gases, but the volumes on the right and left sides of the equation are the same, so the pressure will not affect the equilibrium in the system. Consider a change in temperature: as the temperature rises, the equilibrium shifts towards an endothermic reaction, just towards the reactants. The correct answer is 3.

Task number 3.

In system

2NO2(g) ↔ N2O4(g) + Q

the shift of equilibrium to the left will contribute to

1. Pressure increase

2. Increasing the concentration of N2O4

3. Lowering the temperature

4. Catalyst introduction

Explanation: Let us pay attention to the fact that the volumes of gaseous substances in the right and left parts of the equation are not equal, therefore, a change in pressure will affect the equilibrium in this system. Namely, with an increase in pressure, the equilibrium shifts towards a decrease in the amount of gaseous substances, that is, to the right. It doesn't suit us. The reaction is exothermic, therefore, a change in temperature will also affect the equilibrium of the system. As the temperature decreases, the equilibrium will shift towards the exothermic reaction, that is, also to the right. With an increase in the concentration of N2O4, the equilibrium shifts towards the consumption of this substance, that is, to the left. The correct answer is 2.

Task number 4.

In reaction

2Fe(t) + 3H2O(g) ↔ 2Fe2O3(t) + 3H2(g) - Q

equilibrium will shift towards the products of the reaction

1. Pressurizing

2. Adding a catalyst

3. Addition of iron

4. Adding water

Explanation: the number of molecules in the right and left sides is the same, so a change in pressure will not affect the equilibrium in this system. Consider an increase in the concentration of iron - the equilibrium should shift towards the consumption of this substance, that is, to the right (towards the reaction products). The correct answer is 3.

Task number 5.

Chemical equilibrium

H2O(g) + C(t) ↔ H2(g) + CO(g) - Q

will shift towards the formation of products in the case of

1. Pressure boost

2. Temperature rise

3. Increasing the process time

4. Catalyst Applications

Explanation: a change in pressure will not affect the equilibrium in a given system, since not all substances are gaseous. As the temperature rises, the equilibrium shifts towards the endothermic reaction, that is, to the right (in the direction of the formation of products). The correct answer is 2.

Task number 6.

As the pressure increases, the chemical equilibrium will shift towards the products in the system:

1. CH4(g) + 3S(t) ↔ CS2(g) + 2H2S(g) - Q

2. C(t) + CO2(g) ↔ 2CO(g) - Q

3. N2(g) + 3H2(g) ↔ 2NH3(g) + Q

4. Ca(HCO3)2(t) ↔ CaCO3(t) + CO2(g) + H2O(g) - Q

Explanation: the change in pressure does not affect reactions 1 and 4, therefore not all the substances involved are gaseous, in equation 2 the number of molecules on the right and left sides is the same, so the pressure will not be affected. Equation 3 remains. Let's check: with an increase in pressure, the equilibrium should shift towards a decrease in the amount of gaseous substances (4 molecules on the right, 2 molecules on the left), that is, towards the reaction products. The correct answer is 3.

Task number 7.

Does not affect balance shift

H2(g) + I2(g) ↔ 2HI(g) - Q

1. Pressurizing and adding catalyst

2. Increasing the temperature and adding hydrogen

3. Lowering the temperature and adding hydrogen iodine

4. Addition of iodine and addition of hydrogen

Explanation: in the right and left parts, the amounts of gaseous substances are the same, therefore, a change in pressure will not affect the equilibrium in the system, and the addition of a catalyst will also not affect, because as soon as we add a catalyst, the direct reaction will accelerate, and then immediately the reverse and the equilibrium in the system will be restored . The correct answer is 1.

Task number 8.

To shift the equilibrium to the right in the reaction

2NO(g) + O2(g) ↔ 2NO2(g); ∆H°<0

required

1. Catalyst introduction

2. Lowering the temperature

3. Pressure reduction

4. Decreased oxygen concentration

Explanation: a decrease in the oxygen concentration will lead to a shift in the equilibrium towards the reactants (to the left). A decrease in pressure will shift the equilibrium in the direction of decreasing the amount of gaseous substances, that is, to the right. The correct answer is 3.

Task number 9.

Yield of product in exothermic reaction

2NO(g) + O2(g) ↔ 2NO2(g)

with simultaneous increase in temperature and decrease in pressure

1. Increase

2. Decrease

3. Will not change

4. First increase, then decrease

Explanation: when the temperature rises, the equilibrium shifts towards an endothermic reaction, that is, towards the products, and when the pressure decreases, the equilibrium shifts towards an increase in the amount of gaseous substances, that is, also to the left. Therefore, the yield of the product will decrease. The correct answer is 2.

Task number 10.

Increasing the yield of methanol in the reaction

CO + 2H2 ↔ CH3OH + Q

promotes

1. Temperature rise

2. Catalyst introduction

3. Introduction of an inhibitor

4. Pressure increase

Explanation: when the pressure increases, the equilibrium shifts towards an endothermic reaction, that is, towards the reactants. An increase in pressure shifts the equilibrium towards a decrease in the amount of gaseous substances, that is, towards the formation of methanol. The correct answer is 4.

Tasks for independent decision (answers below)

1. In the system

CO(g) + H2O(g) ↔ CO2(g) + H2(g) + Q

a shift in the chemical equilibrium towards the products of the reaction will contribute to

1. Reduce pressure

2. Increasing temperature

3. Increasing the concentration of carbon monoxide

4. Increasing the concentration of hydrogen

2. In which system, with increasing pressure, does the equilibrium shift towards the reaction products

1. 2CO2(g) ↔ 2CO(g) + O2(g)

2. С2Н4 (g) ↔ С2Н2 (g) + Н2 (g)

3. PCl3(g) + Cl2(g) ↔ PCl5(g)

4. H2(g) + Cl2(g) ↔ 2HCl(g)

3. Chemical equilibrium in the system

2HBr(g) ↔ H2(g) + Br2(g) - Q

will shift towards the reaction products at

1. Pressurizing

2. Temperature rise

3. pressure reduction

4. Using a catalyst

4. Chemical equilibrium in the system

C2H5OH + CH3COOH ↔ CH3COOC2H5 + H2O + Q

shifts towards the reaction products at

1. Adding water

2. Reducing the concentration of acetic acid

3. Increasing the concentration of ether

4. When removing the ester

5. Chemical equilibrium in the system

2NO(g) + O2(g) ↔ 2NO2(g) + Q

shifts towards the formation of the reaction product at

1. Pressurizing

2. Temperature rise

3. pressure reduction

4. Catalyst application

6. Chemical equilibrium in the system

CO2 (g) + C (tv) ↔ 2CO (g) - Q

will shift towards the reaction products at

1. Pressurizing

2. Lowering the temperature

3. Increasing CO concentration

4. Temperature rise

7. Pressure change will not affect the state of chemical equilibrium in the system

1. 2NO(g) + O2(g) ↔ 2NO2(g)

2. N2(g) + 3H2(g) ↔ 2NH3(g)

3. 2CO(g) + O2(g) ↔ 2CO2(g)

4. N2(g) + O2(g) ↔ 2NO(g)

8. In which system, with increasing pressure, will the chemical equilibrium shift towards the starting materials?

1. N2(g) + 3H2(g) ↔ 2NH3(g) + Q

2. N2O4(g) ↔ 2NO2(g) - Q

3. CO2(g) + H2(g) ↔ CO(g) + H2O(g) - Q

4. 4HCl(g) + O2(g) ↔ 2H2O(g) + 2Cl2(g) + Q

9. Chemical equilibrium in the system

C4H10(g) ↔ C4H6(g) + 2H2(g) - Q

will shift towards the reaction products at

1. Temperature rise

2. Lowering the temperature

3. Using a catalyst

4. Reducing the concentration of butane

10. On the state of chemical equilibrium in the system

H2(g) + I2(g) ↔ 2HI(g) -Q

does not affect

1. Pressure increase

2. Increasing the concentration of iodine

3. Increasing temperature

4. Temperature decrease

Tasks for 2016

1. Establish a correspondence between the equation of a chemical reaction and the shift in chemical equilibrium with increasing pressure in the system.

Reaction equation Chemical equilibrium shift

A) N2 (g) + O2 (g) ↔ 2NO (g) - Q 1. Shifts towards the direct reaction

B) N2O4 (g) ↔ 2NO2 (g) - Q 2. Shifts towards the reverse reaction

C) CaCO3 (tv) ↔ CaO (tv) + CO2 (g) - Q 3. There is no equilibrium shift

D) Fe3O4(s) + 4CO(g) ↔ 3Fe(s) + 4CO2(g) + Q

2. Establish a correspondence between external influences on the system:

CO2 (g) + C (tv) ↔ 2CO (g) - Q

and shifting chemical equilibrium.

A. Increasing the concentration of CO 1. Shifts towards the direct reaction

B. Decrease in pressure 3. There is no shift in equilibrium

3. Establish a correspondence between external influences on the system

HCOOH(l) + C5H5OH(l) ↔ HCOOC2H5(l) + H2O(l) + Q

External influence Displacement of chemical equilibrium

A. Addition of HCOOH 1. Shifts towards forward reaction

B. Dilution with water 3. No shift in equilibrium occurs

D. Rise in temperature

4. Establish a correspondence between external influences on the system

2NO(g) + O2(g) ↔ 2NO2(g) + Q

and a shift in chemical equilibrium.

External influence Displacement of chemical equilibrium

A. Decrease in pressure 1. Shifts towards direct reaction

B. Increasing temperature 2. Shifting towards the reverse reaction

B. Increase in NO2 temperature 3. No equilibrium shift occurs

D. O2 addition

5. Establish a correspondence between external influences on the system

4NH3(g) + 3O2(g) ↔ 2N2(g) + 6H2O(g) + Q

and a shift in chemical equilibrium.

External influence Displacement of chemical equilibrium

A. Decrease in temperature 1. Shift towards direct reaction

B. Increase in pressure 2. Shifts towards the reverse reaction

B. Increasing the concentration in ammonia 3. There is no shift in equilibrium

D. Water vapor removal

6. Establish a correspondence between external influences on the system

WO3(s) + 3H2(g) ↔ W(s) + 3H2O(g) + Q

and a shift in chemical equilibrium.

External influence Displacement of chemical equilibrium

A. Increasing temperature 1. Shifts towards direct reaction

B. Increase in pressure 2. Shifts towards the reverse reaction

B. Use of a catalyst 3. No equilibrium shift occurs

D. Water vapor removal

7. Establish a correspondence between external influences on the system

С4Н8(g) + Н2(g) ↔ С4Н10(g) + Q

and a shift in chemical equilibrium.

External influence Displacement of chemical equilibrium

A. Increasing the concentration of hydrogen 1. Shifts towards a direct reaction

B. Temperature increase 2. Shifts towards the reverse reaction

B. Increase in pressure 3. There is no shift in equilibrium

D. Use of a catalyst

8. Establish a correspondence between the equation of a chemical reaction and a simultaneous change in the parameters of the system, leading to a shift in chemical equilibrium towards a direct reaction.

Reaction equation Changing system parameters

A. H2(g) + F2(g) ↔ 2HF(g) + Q 1. Increasing temperature and hydrogen concentration

B. H2(g) + I2(tv) ↔ 2HI(g) -Q 2. Decrease in temperature and hydrogen concentration

B. CO(g) + H2O(g) ↔ CO2(g) + H2(g) + Q 3. Increase in temperature and decrease in hydrogen concentration

D. C4H10(g) ↔ C4H6(g) + 2H2(g) -Q 4. Temperature decrease and hydrogen concentration increase

9. Establish a correspondence between the equation of a chemical reaction and the shift in chemical equilibrium with increasing pressure in the system.

Reaction equation Direction of displacement of chemical equilibrium

A. 2HI(g) ↔ H2(g) + I2(tv) 1. Shifts towards the direct reaction

B. C(g) + 2S(g) ↔ CS2(g) 2. Shifts towards the reverse reaction

B. C3H6(g) + H2(g) ↔ C3H8(g) 3. There is no equilibrium shift

H. H2(g) + F2(g) ↔ 2HF(g)

10. Establish a correspondence between the equation of a chemical reaction and a simultaneous change in the conditions for its implementation, leading to a shift in chemical equilibrium towards a direct reaction.

Reaction equation Changing conditions

A. N2(g) + H2(g) ↔ 2NH3(g) + Q 1. Increasing temperature and pressure

B. N2O4 (g) ↔ 2NO2 (g) -Q 2. Decrease in temperature and pressure

B. CO2 (g) + C (solid) ↔ 2CO (g) + Q 3. Increasing temperature and decreasing pressure

D. 4HCl(g) + O2(g) ↔ 2H2O(g) + 2Cl2(g) + Q 4. Temperature decrease and pressure increase

Answers: 1 - 3, 2 - 3, 3 - 2, 4 - 4, 5 - 1, 6 - 4, 7 - 4, 8 - 2, 9 - 1, 10 - 1

1. 3223

2. 2111

3. 1322

4. 2221

5. 1211

6. 2312

7. 1211

8. 4133

9. 1113

10. 4322

For the tasks we thank the collections of exercises for 2016, 2015, 2014, 2013 authors:

Kavernina A.A., Dobrotina D.Yu., Snastina M.G., Savinkina E.V., Zhiveinova O.G.

1. Among all known reactions, reversible and irreversible reactions are distinguished. When studying ion exchange reactions, the conditions under which they proceed to completion were listed. ().

There are also known reactions that do not go to completion under given conditions. So, for example, when sulfur dioxide is dissolved in water, the reaction occurs: SO 2 + H 2 O→ H2SO3. But it turns out that only a certain amount of sulfurous acid can be formed in an aqueous solution. This is due to the fact that sulfurous acid is fragile, and the reverse reaction occurs, i.e. decomposition into sulfur oxide and water. Therefore, this reaction does not go to the end because two reactions occur simultaneously - straight(between sulfur oxide and water) and reverse(decomposition of sulfuric acid). SO 2 + H 2 O↔H2SO3.

Chemical reactions proceeding under given conditions in mutually opposite directions are called reversible.

2. Since the rate of chemical reactions depends on the concentration of the reactants, then at first the rate of the direct reaction ( υ pr) should be maximum, and the rate of the reverse reaction ( υ arr) equals zero. The concentration of reactants decreases over time, while the concentration of reaction products increases. Therefore, the rate of the forward reaction decreases and the rate of the reverse reaction increases. At a certain point in time, the rates of the forward and reverse reactions become equal:

In all reversible reactions, the rate of the forward reaction decreases, the rate of the reverse reaction increases until both rates become equal and an equilibrium state is established:

υ pr =υ arr

The state of a system in which the rate of the forward reaction is equal to the rate of the reverse reaction is called chemical equilibrium.

In a state of chemical equilibrium, the quantitative ratio between the reacting substances and the reaction products remains constant: how many molecules of the reaction product are formed per unit time, so many of them decompose. However, the state of chemical equilibrium is maintained as long as the reaction conditions remain unchanged: concentration, temperature and pressure.

Quantitatively, the state of chemical equilibrium is described the law of mass action.

At equilibrium, the ratio of the product of the concentrations of the reaction products (in powers of their coefficients) to the product of the concentrations of the reactants (also in the powers of their coefficients) is a constant value, independent of the initial concentrations of substances in the reaction mixture.

This constant is called equilibrium constant - k

So for the reaction: N 2 (G) + 3 H 2 (G) ↔ 2 NH 3 (D) + 92.4 kJ, the equilibrium constant is expressed as follows:

υ 1 =υ 2

υ 1 (direct reaction) = k 1 [ N 2 ][ H 2 ] 3 , where– equilibrium molar concentrations, = mol/l

υ 2 (reverse reaction) = k 2 [ NH 3 ] 2

k 1 [ N 2 ][ H 2 ] 3 = k 2 [ NH 3 ] 2

Kp = k 1 / k 2 = [ NH 3 ] 2 / [ N 2 ][ H 2 ] 3 – equilibrium constant.

Chemical equilibrium depends on concentration, pressure, temperature.

Principledetermines the direction of equilibrium mixing:

If an external influence was exerted on a system that is in equilibrium, then the equilibrium in the system will shift in the opposite direction to this influence.

1) Influence of concentration - if the concentration of the starting substances is increased, then the equilibrium shifts towards the formation of reaction products.

For example,Kp = k 1 / k 2 = [ NH 3 ] 2 / [ N 2 ][ H 2 ] 3

When added to the reaction mixture, for example nitrogen, i.e. the concentration of the reagent increases, the denominator in the expression for K increases, but since K is a constant, the numerator must also increase to fulfill this condition. Thus, the amount of the reaction product increases in the reaction mixture. In this case, we speak of a shift in the chemical equilibrium to the right, towards the product.

Thus, an increase in the concentration of reactants (liquid or gaseous) shifts towards products, i.e. towards a direct reaction. An increase in the concentration of products (liquid or gaseous) shifts the equilibrium towards the reactants, i.e. towards the back reaction.

A change in the mass of a solid does not change the equilibrium position.

2) Temperature effect An increase in temperature shifts the equilibrium towards an endothermic reaction.

a)N 2 (D) + 3H 2 (G) ↔ 2NH 3 (D) + 92.4 kJ (exothermic - heat generation)

As the temperature rises, the equilibrium will shift towards the reaction of ammonia decomposition (←)

b)N 2 (D) +O 2 (G) ↔ 2NO(G) - 180.8 kJ (endothermic - heat absorption)

As the temperature rises, the equilibrium will shift in the direction of the formation reaction NO (→)

3) Influence of pressure (only for gaseous substances) - with increasing pressure, the equilibrium shifts towards the formationi substances occupying less about b eat.

N 2 (D) + 3H 2 (G) ↔ 2NH 3 (G)

1 V - N 2

3 V - H 2

2 V – NH 3

When the pressure rises ( P): before reaction4 V gaseous substances → after reaction2 Vgaseous substances, therefore, the equilibrium shifts to the right ( → )

With an increase in pressure, for example, by 2 times, the volume of gases decreases by the same number of times, and therefore, the concentrations of all gaseous substances will increase by 2 times. Kp = k 1 / k 2 = [ NH 3 ] 2 / [ N 2 ][ H 2 ] 3

In this case, the numerator of the expression for K will increase by 4 times, and the denominator is 16 times, i.e. equality will be broken. To restore it, the concentration must increase ammoniaand decrease concentration nitrogenandwaterkind. The balance will shift to the right.

So, when the pressure increases, the equilibrium shifts towards a decrease in volume, and when the pressure decreases, it shifts towards an increase in volume.

A change in pressure has practically no effect on the volume of solid and liquid substances, i.e. does not change their concentration. Consequently, the equilibrium of reactions in which gases do not participate is practically independent of pressure.

! Substances that influence the course of a chemical reaction catalysts. But when using a catalyst, the activation energy of both the forward and reverse reactions decreases by the same amount, and therefore the balance does not change.

Solve problems:

No. 1. Initial concentrations of CO and O 2 in the reversible reaction

2CO (g) + O 2 (g) ↔ 2 CO 2 (g)

Equal to 6 and 4 mol/L, respectively. Calculate the equilibrium constant if the concentration of CO 2 at the moment of equilibrium is 2 mol/L.

No. 2. The reaction proceeds according to the equation

2SO 2 (g) + O 2 (g) \u003d 2SO 3 (g) + Q

Indicate where the equilibrium will shift if

a) increase pressure

b) raise the temperature

c) increase the concentration of oxygen

d) the introduction of a catalyst?

The transition of a chemical system from one equilibrium state to another is called shift (shift) of balance. Due to the dynamic nature of chemical equilibrium, it turns out to be sensitive to external conditions and is able to respond to their change.

The direction of displacement of the position of chemical equilibrium as a result of changes in external conditions is determined by the rule, first formulated by the French chemist and metallurgist Henri Louis Le Chatelier in 1884 and named after him Le Chatelier's principle:

If an external influence is exerted on a system in a state of equilibrium, then such an equilibrium shift occurs in the system that weakens this influence.

There are three main parameters, by changing which, it is possible to shift the chemical equilibrium. These are temperature, pressure and concentration. Consider their influence on the example of an equilibrium reaction:

1) Temperature effect. Since for this reaction DH°<0, следовательно, прямая реакция идет с выделением тепла (+Q), а обратная реакция – с поглощением тепла (-Q):

2NO (G) + O 2 (G) 2NO 2 (G)

When the temperature rises, i.e. when additional energy is introduced into the system, the equilibrium shifts towards the reverse endothermic reaction, which consumes this excess energy. When the temperature decreases, on the contrary, the equilibrium shifts in the direction of the reaction that goes with the release of heat so that it compensates for the cooling, i.e. the equilibrium shifts in the direction of the direct reaction.

As the temperature rises, the equilibrium shifts towards an endothermic reaction that proceeds with the absorption of energy.

As the temperature decreases, the equilibrium shifts in the direction of an exothermic reaction that proceeds with the release of energy.

2) Volume effect. With an increase in pressure, the rate of the reaction proceeding with a decrease in volume (DV<0). При понижении давления ускоряется реакция, протекающая с увеличением объема (DV>0).

During the course of the reaction under consideration, 2 moles of gases are formed from 3 moles of gaseous substances:

2NO (G) + O 2 (G) 2NO 2 (G)

3 moles of gas 2 moles of gas

V REF > V PROD

DV = V PROD - V REF<0

Therefore, with an increase in pressure, the equilibrium shifts towards a smaller volume of the system, i.e. reaction products. When the pressure is lowered, the equilibrium shifts towards the initial substances occupying a larger volume.

With increasing pressure, the equilibrium shifts towards the reaction proceeding with the formation of a smaller number of moles of gaseous substances.

As the pressure decreases, the equilibrium shifts in the direction of the reaction proceeding with the formation of more moles of gaseous substances.

3) Influence of concentration. With an increase in concentration, the rate of reaction increases, according to which the introduced substance is consumed. Indeed, when an additional amount of oxygen is introduced into the system, the system "expends" it on the flow of a direct reaction. With a decrease in the concentration of O 2, this disadvantage is compensated by the decomposition of the reaction product (NO 2) into the starting materials.

With an increase in the concentration of the starting substances or a decrease in the concentration of the products, the equilibrium shifts towards a direct reaction.

With a decrease in the concentration of the starting substances or an increase in the concentration of the products, the equilibrium shifts in the direction of the reverse reaction.

The introduction of a catalyst into the system does not affect the shift in the position of chemical equilibrium, since the catalyst equally increases the rate of both the forward and reverse reactions.

If the external conditions of the chemical process do not change, then the state of chemical equilibrium can be maintained for an arbitrarily long time. By changing the reaction conditions (temperature, pressure, concentration), one can achieve displacement or shift of chemical equilibrium in the required direction.

The shift of equilibrium to the right leads to an increase in the concentration of substances whose formulas are on the right side of the equation. The shift of equilibrium to the left will lead to an increase in the concentration of substances whose formulas are on the left. In this case, the system will move to a new state of equilibrium, characterized by other values ​​of the equilibrium concentrations of the participants in the reaction.

The shift in chemical equilibrium caused by changing conditions obeys the rule formulated in 1884 by the French physicist A. Le Chatelier (Le Chatelier's principle).

Le Chatelier's principle:if a system in a state of chemical equilibrium is affected in any way, for example, by changing the temperature, pressure, or concentrations of reagents, then the equilibrium will shift in the direction of the reaction that weakens the effect .

Influence of concentration change on the shift of chemical equilibrium.

According to Le Chatelier's principle an increase in the concentration of any of the participants in the reaction causes a shift in the equilibrium towards the reaction that leads to a decrease in the concentration of this substance.

The influence of concentration on the state of equilibrium obeys the following rules:

With an increase in the concentration of one of the starting substances, the rate of the direct reaction increases and the equilibrium shifts in the direction of the formation of reaction products and vice versa;

With an increase in the concentration of one of the reaction products, the rate of the reverse reaction increases, which leads to a shift in the equilibrium in the direction of the formation of the starting substances and vice versa.

For example, if in an equilibrium system:

SO 2 (g) + NO 2 (g) SO 3 (g) + NO (g)

increase the concentration of SO 2 or NO 2, then, in accordance with the law of mass action, the rate of the direct reaction will increase. This will shift the equilibrium to the right, which will cause the consumption of the starting materials and an increase in the concentration of the reaction products. A new state of equilibrium will be established with new equilibrium concentrations of the initial substances and reaction products. When the concentration of, for example, one of the reaction products decreases, the system will react in such a way as to increase the concentration of the product. The advantage will be given to the direct reaction, leading to an increase in the concentration of the reaction products.

Influence of pressure change on the shift of chemical equilibrium.

According to Le Chatelier's principle an increase in pressure leads to a shift in equilibrium towards the formation of a smaller amount of gaseous particles, i.e. towards smaller volume.

For example, in a reversible reaction:

2NO 2 (g) 2NO (g) + O 2 (g)

from 2 mol NO 2 2 mol NO and 1 mol O 2 are formed. The stoichiometric coefficients in front of the formulas of gaseous substances indicate that the flow of a direct reaction leads to an increase in the number of moles of gases, and the flow of a reverse reaction, on the contrary, reduces the number of moles of a gaseous substance. If an external influence is exerted on such a system, for example, by increasing pressure, then the system will react in such a way as to weaken this impact. The pressure can decrease if the equilibrium of this reaction shifts towards a smaller number of moles of a gaseous substance, and hence a smaller volume.

On the contrary, an increase in pressure in this system is associated with a shift in equilibrium to the right - towards the decomposition of NO 2, which increases the amount of gaseous matter.

If the number of moles of gaseous substances remains constant before and after the reaction, i.e. the volume of the system does not change during the reaction, then a change in pressure equally changes the rates of the forward and reverse reactions and does not affect the state of chemical equilibrium.

For example, in react:

H 2 (g) + Cl 2 (g) 2HCl (g),

the total number of moles of gaseous substances before and after the reaction remains constant and the pressure in the system does not change. The equilibrium in this system does not change with pressure.

Influence of temperature change on the shift of chemical equilibrium.

In each reversible reaction, one of the directions corresponds to an exothermic process, and the other to an endothermic one. So in the ammonia synthesis reaction, the forward reaction is exothermic, and the reverse reaction is endothermic.

N 2 (g) + 3H 2 (g) 2NH 3 (g) + Q (-ΔH).

When the temperature changes, the rates of both the forward and reverse reactions change, however, the change in rates does not occur to the same extent. In accordance with the Arrhenius equation, an endothermic reaction, characterized by a large activation energy, reacts to a change in temperature to a greater extent.

Therefore, in order to estimate the effect of temperature on the direction of the shift in chemical equilibrium, it is necessary to know the thermal effect of the process. It can be determined experimentally, for example, using a calorimeter, or calculated based on G. Hess's law. It should be noted that a change in temperature leads to a change in the value of the constant of chemical equilibrium (K p).

According to Le Chatelier's principle An increase in temperature shifts the equilibrium towards an endothermic reaction. As the temperature decreases, the equilibrium shifts in the direction of the exothermic reaction.

In this way, temperature rise in the ammonia synthesis reaction will lead to a shift in equilibrium towards the endothermic reactions, i.e. to the left. The advantage is obtained by the reverse reaction proceeding with the absorption of heat.

The state of equilibrium for a reversible reaction can last for an indefinitely long time (without outside intervention). But if an external influence is applied to such a system (to change the temperature, pressure or concentration of the final or initial substances), then the state of equilibrium will be disturbed. The rate of one of the reactions will become greater than the rate of the other. Over time, the system will again take an equilibrium state, but the new equilibrium concentrations of the initial and final substances will differ from the initial ones. In this case, one speaks of a shift in the chemical equilibrium in one direction or another.

If, as a result of an external influence, the rate of the forward reaction becomes greater than the rate of the reverse reaction, then this means that the chemical equilibrium has shifted to the right. If, on the contrary, the rate of the reverse reaction becomes greater, this means that the chemical equilibrium has shifted to the left.

When the equilibrium shifts to the right, the equilibrium concentrations of the initial substances decrease and the equilibrium concentrations of the final substances increase in comparison with the initial equilibrium concentrations. Accordingly, the yield of reaction products also increases.

The shift of the chemical equilibrium to the left causes an increase in the equilibrium concentrations of the initial substances and a decrease in the equilibrium concentrations of the final products, the yield of which will decrease in this case.

The direction of the chemical equilibrium shift is determined using the Le Chatelier principle: “If an external effect is exerted on a system that is in a state of chemical equilibrium (change the temperature, pressure, concentration of one or more substances participating in the reaction), then this will lead to an increase in the rate of that reaction, the course of which will compensate (reduce) the impact.

For example, with an increase in the concentration of the starting substances, the rate of the direct reaction increases and the equilibrium shifts to the right. With a decrease in the concentration of the starting substances, on the contrary, the rate of the reverse reaction increases, and the chemical equilibrium shifts to the left.

With an increase in temperature (i.e., when the system is heated), the equilibrium shifts towards the occurrence of an endothermic reaction, and when it decreases (i.e., when the system is cooled), it shifts towards the occurrence of an exothermic reaction. (If the forward reaction is exothermic, then the reverse reaction will necessarily be endothermic, and vice versa).

It should be emphasized that an increase in temperature, as a rule, increases the rate of both the forward and reverse reactions, but the rate of the endothermic reaction increases to a greater extent than the rate of the exothermic reaction. Accordingly, when the system is cooled, the rates of forward and reverse reactions decrease, but also not to the same extent: for an exothermic reaction, it is much less than for an endothermic one.

A change in pressure affects the shift in chemical equilibrium only if two conditions are met:

it is necessary that at least one of the substances participating in the reaction be in a gaseous state, for example:

CaCO 3 (t) CaO (t) + CO 2 (g) - a change in pressure affects the displacement of equilibrium.

CH 3 COOH (l.) + C 2 H 5 OH (l.) CH 3 COOS 2 H 5 (l.) + H 2 O (l.) - a change in pressure does not affect the shift in chemical equilibrium, because none of the starting or end substances is in a gaseous state;

if several substances are in the gaseous state, it is necessary that the number of gas molecules on the left side of the equation for such a reaction is not equal to the number of gas molecules on the right side of the equation, for example:

2SO 2 (g) + O 2 (g) 2SO 3 (g) - pressure change affects the equilibrium shift

I 2 (g) + Н 2 (g) 2НI (g) - pressure change does not affect the equilibrium shift

When these two conditions are met, an increase in pressure leads to a shift in the equilibrium towards the reaction, the course of which reduces the number of gas molecules in the system. In our example (catalytic combustion of SO 2), this will be a direct reaction.

A decrease in pressure, on the contrary, shifts the equilibrium in the direction of the reaction proceeding with the formation of a larger number of gas molecules. In our example, this will be the reverse reaction.

An increase in pressure causes a decrease in the volume of the system, and hence an increase in the molar concentrations of gaseous substances. As a result, the rate of forward and reverse reactions increases, but not to the same extent. Lowering the same pressure in a similar way leads to a decrease in the rates of forward and reverse reactions. But at the same time, the reaction rate, towards which the equilibrium shifts, decreases to a lesser extent.

The catalyst does not affect the equilibrium shift, because it speeds up (or slows down) both the forward and backward reactions equally. In its presence, the chemical equilibrium is only more quickly (or more slowly) established.

If the system is affected by several factors at the same time, then each of them acts independently of the others. For example, in the synthesis of ammonia

N 2 (gas) + 3H 2 (gas) 2NH 3 (gas)

the reaction is carried out under heating and in the presence of a catalyst to increase its rate. But at the same time, the effect of temperature leads to the fact that the reaction equilibrium is shifted to the left, towards the reverse endothermic reaction. This causes a decrease in the output of NH 3 . In order to compensate for this undesirable effect of temperature and increase the ammonia yield, the pressure in the system is simultaneously increased, which shifts the reaction equilibrium to the right, i.e. towards the formation of a smaller number of gas molecules.

At the same time, the most optimal conditions for the reaction (temperature, pressure) are selected empirically, under which it would proceed at a sufficiently high rate and give an economically viable yield of the final product.

Le Chatelier's principle is similarly used in the chemical industry in the production of a large number of different substances of great importance for the national economy.

Le Chatelier's principle is applicable not only to reversible chemical reactions, but also to various other equilibrium processes: physical, physicochemical, biological.

The body of an adult is characterized by the relative constancy of many parameters, including various biochemical indicators, including the concentration of biologically active substances. However, such a state cannot be called equilibrium, because it does not apply to open systems.

The human body, like any living system, constantly exchanges various substances with the environment: it consumes food and releases the products of their oxidation and decay. Therefore, the body is characterized steady state, defined as the constancy of its parameters at a constant rate of exchange of matter and energy with the environment. In the first approximation, the stationary state can be considered as a series of equilibrium states interconnected by relaxation processes. In a state of equilibrium, the concentrations of substances participating in the reaction are maintained by replenishing the initial products from the outside and removing the final products to the outside. Changing their content in the body does not lead, in contrast to closed systems, to a new thermodynamic equilibrium. The system returns to its original state. Thus, the relative dynamic constancy of the composition and properties of the internal environment of the body is maintained, which determines the stability of its physiological functions. This property of a living system is called differently homeostasis.

In the course of the life of an organism in a stationary state, in contrast to a closed equilibrium system, there is an increase in entropy. However, along with this, the reverse process simultaneously proceeds - a decrease in entropy due to the consumption of nutrients with a low entropy value from the environment (for example, high-molecular compounds - proteins, polysaccharides, carbohydrates, etc.) and the release of decay products into the environment. According to the position of I.R. Prigozhin, the total production of entropy for an organism in a stationary state tends to a minimum.

A great contribution to the development of nonequilibrium thermodynamics was made by I. R. Prigozhy, winner of the Nobel Prize in 1977, who stated that “in any non-equilibrium system, there are local areas that are in equilibrium. In classical thermodynamics, equilibrium refers to the whole system, and in non-equilibrium - only to its individual parts.

It has been established that entropy in such systems increases during the period of embryogenesis, during the processes of regeneration and the growth of malignant neoplasms.