Reversible and irreversible reactions. Chemical equilibrium

Chemical reactions are reversible and irreversible.

those. if some reaction A + B = C + D is irreversible, this means that the reverse reaction C + D = A + B does not occur.

i.e., for example, if a certain reaction A + B = C + D is reversible, this means that both the reaction A + B → C + D (direct) and the reaction C + D → A + B (reverse) proceed simultaneously ).

In fact, because both direct and reverse reactions proceed, reagents (starting substances) in the case of reversible reactions can be called both substances on the left side of the equation and substances on the right side of the equation. The same goes for products.

For any reversible reaction, it is possible that the rates of the forward and reverse reactions are equal. Such a state is called state of equilibrium.

In a state of equilibrium, the concentrations of both all reactants and all products are unchanged. The concentrations of products and reactants at equilibrium are called equilibrium concentrations.

Shift in chemical equilibrium under the influence of various factors

Due to such external influences on the system as a change in temperature, pressure or concentration of starting substances or products, the equilibrium of the system may be disturbed. However, after the cessation of this external influence, the system will pass to a new state of equilibrium after some time. Such a transition of a system from one equilibrium state to another equilibrium state is called shift (shift) of chemical equilibrium .

In order to be able to determine how the chemical equilibrium shifts with a particular type of exposure, it is convenient to use the Le Chatelier principle:

If any external influence is exerted on a system in a state of equilibrium, then the direction of the shift in chemical equilibrium will coincide with the direction of the reaction that weakens the effect of the impact.

The influence of temperature on the state of equilibrium

When the temperature changes, the equilibrium of any chemical reaction shifts. This is due to the fact that any reaction has a thermal effect. In this case, the thermal effects of the forward and reverse reactions are always directly opposite. Those. if the forward reaction is exothermic and proceeds with a thermal effect equal to +Q, then the reverse reaction is always endothermic and has a thermal effect equal to -Q.

Thus, in accordance with Le Chatelier's principle, if we increase the temperature of some system that is in a state of equilibrium, then the equilibrium will shift towards the reaction, during which the temperature decreases, i.e. towards an endothermic reaction. And similarly, if we lower the temperature of the system in a state of equilibrium, the equilibrium will shift towards the reaction, as a result of which the temperature will increase, i.e. towards an exothermic reaction.

For example, consider the following reversible reaction and indicate where its equilibrium will shift as the temperature decreases:

As you can see from the equation above, the forward reaction is exothermic, i.e. as a result of its flow, heat is released. Therefore, the reverse reaction will be endothermic, that is, it proceeds with the absorption of heat. According to the condition, the temperature is lowered, therefore, the equilibrium will shift to the right, i.e. towards a direct reaction.

Effect of concentration on chemical equilibrium

An increase in the concentration of reagents in accordance with the Le Chatelier principle should lead to a shift in equilibrium towards the reaction in which the reagents are consumed, i.e. towards a direct reaction.

Conversely, if the concentration of the reactants is lowered, then the equilibrium will shift towards the reaction that results in the formation of the reactants, i.e. side of the reverse reaction (←).

A change in the concentration of reaction products also affects in a similar way. If you increase the concentration of products, the equilibrium will shift towards the reaction, as a result of which the products are consumed, i.e. towards the reverse reaction (←). If, on the contrary, the concentration of products is lowered, then the equilibrium will shift towards the direct reaction (→), in order for the concentration of products to increase.

Effect of pressure on chemical equilibrium

Unlike temperature and concentration, a change in pressure does not affect the equilibrium state of every reaction. In order for a change in pressure to lead to a shift in chemical equilibrium, the sums of the coefficients in front of gaseous substances on the left and right sides of the equation must be different.

Those. from two reactions:

a change in pressure can affect the state of equilibrium only in the case of the second reaction. Since the sum of the coefficients in front of the formulas of gaseous substances in the case of the first equation on the left and right is the same (equal to 2), and in the case of the second equation it is different (4 on the left and 2 on the right).

From this, in particular, it follows that if there are no gaseous substances among both the reactants and the products, then a change in pressure will not affect the current state of equilibrium in any way. For example, pressure will not affect the equilibrium state of the reaction:

If the amount of gaseous substances is different on the left and on the right, then an increase in pressure will lead to a shift in equilibrium towards the reaction, during which the volume of gases decreases, and a decrease in pressure in the direction of the reaction, as a result of which the volume of gases increases.

Effect of a catalyst on chemical equilibrium

Since a catalyst equally accelerates both the forward and reverse reactions, its presence or absence does not affect to a state of equilibrium.

The only thing that a catalyst can affect is the rate of transition of the system from a non-equilibrium state to an equilibrium state.

The impact of all the above factors on chemical equilibrium is summarized below in a cheat sheet, which at first you can peek at when performing balance tasks. However, she will not be able to use it in the exam, therefore, after analyzing several examples with her help, she should be taught and trained to solve tasks for balance, no longer peeping into her:

Designations: T - temperature, p - pressure, With – concentration, – increase, ↓ – decrease

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 on 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. Temperature increase 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. Increasing temperature 2. Shifts in the direction of 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 the 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.

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.

9. The rate of a chemical reaction. Chemical equilibrium

9.2. Chemical equilibrium and its displacement

Most chemical reactions are reversible, i.e. simultaneously flow both in the direction of the formation of products and in the direction of their decay (from left to right and from right to left).

Examples of reaction equations for reversible processes:

N 2 + 3H 2 ⇄ t °, p, cat 2NH 3

2SO 2 + O 2 ⇄ t °, p, cat 2SO 3

H 2 + I 2 ⇄ t ° 2HI

Reversible reactions are characterized by a special state, which is called the state of chemical equilibrium.

Chemical equilibrium This is the state of the system in which the rates of the forward and reverse reactions become equal. When moving towards chemical equilibrium, the rate of the forward reaction and the concentration of reactants decrease, while the reverse reaction and the concentration of products increase.

In a state of chemical equilibrium, as much product is formed per unit time as it decays. As a result, the concentrations of substances in a state of chemical equilibrium do not change over time. However, this does not mean at all that the equilibrium concentrations or masses (volumes) of all substances are necessarily equal to each other (see Fig. 9.8 and 9.9). Chemical equilibrium is a dynamic (moving) equilibrium that can respond to external influences.

The transition of an equilibrium system from one equilibrium state to another is called displacement or balance shift. In practice, one speaks of a shift in equilibrium towards the products of the reaction (to the right) or towards the starting materials (to the left); A direct reaction is called a reaction proceeding from left to right, and a reverse reaction is called from right to left. The state of equilibrium is shown by two oppositely directed arrows: ⇄.

The principle of shifting equilibrium was formulated by the French scientist Le Chatelier (1884): an external influence on a system in equilibrium leads to a shift in this equilibrium in a direction that weakens the effect of external influence

Let us formulate the basic rules for shifting equilibrium.

Influence of concentration: with an increase in the concentration of a substance, the equilibrium shifts towards its consumption, and with a decrease - towards its formation.

For example, with an increase in the concentration of H 2 in a reversible reaction

H 2 (g) + I 2 (g) ⇄ 2HI (g)

the rate of the forward reaction, which depends on the concentration of hydrogen, will increase. As a result, the equilibrium will shift to the right. With a decrease in the concentration of H 2, the rate of the direct reaction will decrease, as a result, the equilibrium of the process will shift to the left.

Temperature effect: when the temperature rises, the equilibrium shifts towards an endothermic reaction, and when it decreases, it shifts towards an exothermic reaction.

It is important to remember that with an increase in temperature, the rate of both exo- and endothermic reactions increases, but in a greater number of times - the endothermic reaction, for which E a is always greater. With a decrease in temperature, the rate of both reactions decreases, but again, in a greater number of times - endothermic. It is convenient to illustrate what has been said by a diagram in which the value of the speed is proportional to the length of the arrows, and the equilibrium is shifted in the direction of the longer arrow.

Pressure influence: a change in pressure affects the state of equilibrium only when gases are involved in the reaction, and even when the gaseous substance is only in one part of the chemical equation. Examples of reaction equations:

• pressure affects the equilibrium shift:

3H 2 (g) + N 2 (g) ⇄ 2NH 3 (g),

CaO (tv) + CO 2 (g) ⇄ CaCO 3 (tv);

• pressure does not affect the equilibrium shift:

Cu (tv) + S (tv) = CuS (tv),

NaOH (solution) + HCl (solution) = NaCl (solution) + H 2 O (l).

With a decrease in pressure, the equilibrium shifts towards the formation of a larger chemical amount of gaseous substances, and with an increase, towards the formation of a smaller chemical amount of gaseous substances. If the chemical quantities of gases in both parts of the equation are the same, then the pressure does not affect the state of chemical equilibrium:

H 2 (g) + Cl 2 (g) = 2HCl (g).

What has been said is easy to understand, given that the effect of a change in pressure is similar to the effect of a change in concentration: with an increase in pressure by n times, the concentration of all substances in equilibrium increases by the same amount (and vice versa).

Influence of the volume of the reaction system: a change in the volume of the reaction system is associated with a change in pressure and affects only the equilibrium state of reactions involving gaseous substances. A decrease in volume means an increase in pressure and shifts the equilibrium towards the formation of a smaller chemical amount of gases. An increase in the volume of the system leads to a decrease in pressure and a shift in equilibrium towards the formation of a larger chemical amount of gaseous substances.

The introduction of a catalyst into an equilibrium system or a change in its nature does not shift the equilibrium (does not increase the yield of the product), since the catalyst equally accelerates both the forward and reverse reactions. This is due to the fact that the catalyst equally reduces the activation energy of the direct and reverse processes. Then why use a catalyst in reversible processes? The fact is that the use of a catalyst in reversible processes contributes to the rapid onset of equilibrium, and this increases the efficiency of industrial production.

Specific examples of the influence of various factors on the shift in equilibrium are given in Table. 9.1 for the ammonia synthesis reaction proceeding with the release of heat. In other words, the forward reaction is exothermic and the reverse reaction is endothermic.

Table 9.1

Effect of Various Factors on the Equilibrium Shift in the Ammonia Synthesis Reaction

Example 9.3. In process equilibrium

2SO 2 (g) + O 2 (g) ⇄ 2SO 3 (g)

the concentrations of substances (mol / dm 3) SO 2, O 2 and SO 3, respectively, are 0.6, 0.4 and 0.2. Find the initial concentrations of SO 2 and O 2 (the initial concentration of SO 3 is zero).

Solution. During the reaction, SO 2 and O 2 are consumed, therefore

c ref (SO 2) \u003d c equal (SO 2) + c waste (SO 2),

c ref (O ​​2) = c equals (O 2) + c out (O 2).

The value of c is found from c (SO 3):

x \u003d 0.2 mol / dm 3.

c ref (SO 2) \u003d 0.6 + 0.2 \u003d 0.8 (mol / dm 3).

y \u003d 0.1 mol / dm 3.

c ref (O ​​2) \u003d 0.4 + 0.1 \u003d 0.5 (mol / dm 3).

Answer: 0.8 mol / dm 3 SO 2; 0.5 mol/dm 3 O 2 .

When performing examination tasks, the influence of various factors is often confused, on the one hand, on the reaction rate, and on the other hand, on the shift in chemical equilibrium.

For a reversible process

as the temperature rises, the rate of both the forward and reverse reactions increases; as the temperature decreases, the rate of both the forward and reverse reactions decreases;

with increasing pressure, the rates of all reactions occurring with the participation of gases increase, both direct and reverse. With a decrease in pressure, the rate of all reactions occurring with the participation of gases decreases, both direct and reverse;

the introduction of a catalyst into the system or its replacement with another catalyst does not shift the equilibrium.

Example 9.4. A reversible process takes place, described by the equation

N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) + Q

Consider which factors: 1) increase the rate of synthesis of the ammonia reaction; 2) shift the equilibrium to the right:

a) lowering the temperature;

b) increase in pressure;

c) decrease in the concentration of NH 3;

d) the use of a catalyst;

e) increase in N 2 concentration.

Solution. Factors b), d) and e) increase the reaction rate of ammonia synthesis (as well as an increase in temperature, an increase in the concentration of H 2); shift the equilibrium to the right - a), b), c), e).

Answer: 1) b, d, e; 2) a, b, c, e.

Example 9.5. Below is an energy diagram of a reversible reaction

List all true statements:

a) the reverse reaction proceeds faster than the forward one;

b) with increasing temperature, the rate of the reverse reaction increases by a greater number of times than the direct reaction;

c) the direct reaction proceeds with the absorption of heat;

d) the value of the temperature coefficient γ is greater for the reverse reaction.

Solution.

a) The statement is correct, since E a rev = 500 - 300 = 200 (kJ) is less than E a pr = 500 - 200 = 300 (kJ).

b) The statement is incorrect, the rate of the direct reaction increases by a greater number of times, for which E a is greater.

c) The statement is correct, Q pr \u003d 200 - 300 \u003d -100 (kJ).

d) The statement is incorrect, γ is greater for a direct reaction, in the case of which E a is greater.

Answer: a), c).

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 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 reverse 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 with 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, at the same time the pressure in the system is 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.