EntranceWhich side will the equilibrium of the reaction change? Unified State Examination tasks in chemistry test online: Reversible and irreversible chemical reactions
9. Rate of 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 decomposition (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 called a state of chemical equilibrium.
Chemical equilibrium- this is a state of the system in which the rates of forward and reverse reactions become equal. When moving towards chemical equilibrium, the rate of the forward reaction and the concentration of the reactants decrease, while the reverse reaction and the concentration of the products increase.
In a state of chemical equilibrium, as much product is formed per unit time as it is decomposed. 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 (mobile) equilibrium that can respond to external influences.
The transition of an equilibrium system from one equilibrium state to another is called a displacement or shift in equilibrium. In practice, they talk about a shift in equilibrium towards the reaction products (to the right) or towards the starting substances (to the left); a forward reaction is one that occurs from left to right, and a reverse reaction occurs 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 that is in equilibrium leads to a shift in this equilibrium in a direction that weakens the effect of the external influence
Let us formulate the basic rules for shifting equilibrium.
Effect of concentration: when the concentration of a substance increases, the equilibrium shifts towards its consumption, and when it decreases, 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, depending on the hydrogen concentration, will increase. As a result, the balance will shift to the right. As the concentration of H 2 decreases, the rate of the forward reaction will decrease, as a result, the equilibrium of the process will shift to the left.
Effect of temperature: When the temperature increases, the equilibrium shifts towards the endothermic reaction, and when the temperature decreases, it shifts towards the exothermic reaction.
It is important to remember that with increasing temperature, the rate of both exo- and endothermic reactions increases, but the endothermic reaction increases more times, for which E a is always greater. As the temperature decreases, the rate of both reactions decreases, but again by a greater number of times - endothermic. It is convenient to illustrate this with a diagram in which the speed value is proportional to the length of the arrows, and the equilibrium shifts in the direction of the longer arrow.
Effect of pressure: A change in pressure affects the state of equilibrium only when gases are involved in the reaction, and even when the gaseous substance is on only one side 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 (sv) + S (sv) = CuS (sv),
NaOH (solution) + HCl (solution) = NaCl (solution) + H 2 O (l).
When the pressure decreases, the equilibrium shifts towards the formation of a larger chemical amount of gaseous substances, and when it increases, the equilibrium shifts towards the formation of a smaller chemical amount of gaseous substances. If the chemical quantities of gases in both sides of the equation are the same, then pressure does not affect the state of chemical equilibrium:
H 2 (g) + Cl 2 (g) = 2HCl (g).
This 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 n times, the concentration of all substances in equilibrium increases by the same amount (and vice versa).
Effect 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 toward the formation of fewer chemical 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 accelerates both forward and reverse reactions to the same extent. This is due to the fact that the catalyst equally reduces the activation energy of the forward and reverse processes. Then why do they use a catalyst in reversible processes? The fact is that the use of a catalyst in reversible processes promotes the rapid onset of equilibrium, and this increases the efficiency of industrial production.
Specific examples of the influence of various factors on the equilibrium shift are given in Table. 9.1 for the ammonia synthesis reaction that occurs with the release of heat. In other words, the forward reaction is exothermic, and the reverse reaction is endothermic.
Table 9.1
The influence of various factors on the shift in the equilibrium of the ammonia synthesis reaction
| Factor influencing the equilibrium system | Direction of displacement of the equilibrium reaction 3 H 2 + N 2 ⇄ t, p, cat 2 NH 3 + Q |
|---|---|
| Increase in hydrogen concentration, s (H 2) | Equilibrium shifts to the right, the system responds by decreasing c (H 2) |
| Decrease in ammonia concentration, s (NH 3)↓ | Equilibrium shifts to the right, the system responds with an increase in c (NH 3) |
| Increase in ammonia concentration, s (NH 3) | Equilibrium shifts to the left, the system responds by decreasing c (NH 3) |
| Decrease in nitrogen concentration, s (N 2)↓ | Equilibrium shifts to the left, the system responds by increasing c (N 2) |
| Compression (volume decrease, pressure increase) | Equilibrium shifts to the right, towards a decrease in the volume of gases |
| Expansion (increase in volume, decrease in pressure) | Equilibrium shifts to the left, towards increasing gas volume |
| Increased pressure | Equilibrium shifts to the right, towards a smaller volume of gas |
| Decreased pressure | Equilibrium shifts to the left, towards a larger volume of gases |
| Temperature increase | Equilibrium shifts to the left, towards the endothermic reaction |
| Temperature drop | Equilibrium shifts to the right, towards the exothermic reaction |
| Adding a catalyst | The balance does not shift |
Example 9.3. In a state of 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 are respectively 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 out (SO 2) = c equal (SO 2) + c out (SO 2),
c out (O 2) = c equal (O 2) + c out (O 2).
The value of c expended is found using c (SO 3):
x = 0.2 mol/dm3.
c out (SO 2) = 0.6 + 0.2 = 0.8 (mol/dm 3).
y = 0.1 mol/dm3.
c out (O 2) = 0.4 + 0.1 = 0.5 (mol/dm 3).
Answer: 0.8 mol/dm 3 SO 2; 0.5 mol/dm 3 O 2.
When performing exam tasks, the influence of various factors, on the one hand, on the reaction rate, and on the other, on the shift in chemical equilibrium, is often confused.
For a reversible process
with increasing temperature, the rate of both forward and reverse reactions increases; as the temperature decreases, the rate of both forward and reverse reactions decreases;
with increasing pressure, the rates of all reactions occurring with the participation of gases increase, both direct and reverse. As the pressure decreases, the rate of all reactions occurring with the participation of gases, both direct and reverse, decreases;
introducing a catalyst into the system or replacing it with another catalyst does not shift the equilibrium.
Example 9.4. A reversible process occurs, 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 balance to the right:
a) decrease in temperature;
b) increase in pressure;
c) decrease in NH 3 concentration;
d) use of a catalyst;
e) increase in N 2 concentration.
Solution. Factors b), d) and e) increase the rate of ammonia synthesis reaction (as well as increasing temperature, increasing H 2 concentration); shift the balance to the right - a), b), c), e).
Answer: 1) b, d, d; 2) a, b, c, d.
Example 9.5. Below is the energy diagram of a reversible reaction
List all true statements:
a) the reverse reaction proceeds faster than the direct reaction;
b) with increasing temperature, the rate of the reverse reaction increases more times than the direct reaction;
c) a direct reaction occurs with the absorption of heat;
d) the temperature coefficient γ is greater for the reverse reaction.
Solution.
a) The statement is correct, since E arr = 500 − 300 = 200 (kJ) is less than E arr = 500 − 200 = 300 (kJ).
b) The statement is incorrect; the rate of the direct reaction for which E a is greater increases by a greater number of times.
c) The statement is correct, Q pr = 200 − 300 = −100 (kJ).
d) The statement is incorrect, γ is greater for a direct reaction, in which case E a is greater.
Answer: a), c).
Chemical equilibrium and the principles of its displacement (Le Chatelier's principle)
In reversible reactions, under certain conditions, a state of chemical equilibrium may occur. This is a condition 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.
Key points:
1. An external influence on a system that is in a state of equilibrium leads to a shift in this equilibrium in a direction in which the effect of the effect is weakened.
2. When the concentration of one of the reacting substances increases, the equilibrium shifts towards the consumption of this substance; when the concentration decreases, 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 the pressure decreases, the equilibrium shifts towards increasing amounts of gaseous substances, that is, towards increasing pressure. If the reaction proceeds without changing the number of molecules of gaseous substances, then pressure does not affect the equilibrium position in this system.
4. When the temperature increases, the equilibrium shifts towards the endothermic reaction, and when the temperature decreases, towards the exothermic reaction.
For the principles we thank the manual “Beginnings of Chemistry” Kuzmenko N.E., Eremin V.V., Popkov V.A.
Unified State Examination tasks on chemical equilibrium (formerly A21)
Task No. 1.
H2S(g) ↔ H2(g) + S(g) - Q
1. Increased pressure
2. Rising temperature
3. Decreased pressure
Explanation: First, let's consider the reaction: all substances are gases and on the right side there are two molecules of products, and on the left there is only one, the reaction is also endothermic (-Q). Therefore, let us consider the change in pressure and temperature. We need the equilibrium to shift towards the reaction products. If we increase the pressure, then the equilibrium will shift towards decreasing volume, that is, towards the reactants - 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 No. 2.
Chemical equilibrium in the system
SO3(g) + NO(g) ↔ SO2(g) + NO2(g) - Q
will shift towards the formation of reagents when:
1. Increasing NO concentration
2. Increasing SO2 concentration
3. Temperature rises
4. Increased pressure
Explanation: all substances are gases, but the volumes on the right and left sides of the equation are the same, so pressure will not affect the equilibrium in the system. Consider a change in temperature: as the temperature increases, the equilibrium shifts towards the endothermic reaction, precisely towards the reactants. The correct answer is 3.
Task No. 3.
In system
2NO2(g) ↔ N2O4(g) + Q
a shift of balance to the left will contribute
1. Increase in pressure
2. Increase in N2O4 concentration
3. Temperature drop
4. Introduction of catalyst
Explanation: Let us pay attention to the fact that the volumes of gaseous substances on the right and left sides of the equation are not equal, therefore a change in pressure will affect the equilibrium in this system. Namely, with increasing pressure, the equilibrium shifts towards a decrease in the amount of gaseous substances, that is, to the right. This doesn't suit us. The reaction is exothermic, therefore a change in temperature will affect the equilibrium of the system. As the temperature decreases, the equilibrium will shift towards the exothermic reaction, that is, also to the right. As the concentration of N2O4 increases, the equilibrium shifts towards the consumption of this substance, that is, to the left. The correct answer is 2.
Task No. 4.
In reaction
2Fe(s) + 3H2O(g) ↔ 2Fe2O3(s) + 3H2(g) - Q
the equilibrium will shift towards the reaction products when
1. Increased pressure
2. Adding a catalyst
3. Adding iron
4. Adding water
Explanation: the number of molecules in the right and left parts is the same, so a change in pressure will not affect the equilibrium in this system. Let's 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 No. 5.
Chemical equilibrium
H2O(l) + C(t) ↔ H2(g) + CO(g) - Q
will shift towards the formation of products in the case
1. Increased pressure
2. Increase in temperature
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 increases, the equilibrium shifts towards the endothermic reaction, that is, to the right (towards the formation of products). The correct answer is 2.
Task No. 6.
As the pressure increases, the chemical equilibrium will shift towards the products in the system:
1. CH4(g) + 3S(s) ↔ 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: reactions 1 and 4 are not affected by changes in pressure, because not all participating substances are gaseous; in equation 2, the number of molecules on the right and left sides is the same, so pressure will not affect. Equation 3 remains. Let's check: with increasing pressure, the equilibrium should shift towards decreasing amounts 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 No. 7.
Does not affect balance shift
H2(g) + I2(g) ↔ 2HI(g) - Q
1. Increasing pressure and adding catalyst
2. Raising the temperature and adding hydrogen
3. Lowering the temperature and adding hydrogen iodide
4. Adding iodine and adding hydrogen
Explanation: in the right and left parts the amounts of gaseous substances are the same, so a change in pressure will not affect the equilibrium in the system, and adding a catalyst will also not affect it, because as soon as we add a catalyst, the direct reaction will accelerate, and then immediately the reverse and equilibrium in the system will be restored . The correct answer is 1.
Task No. 8.
To shift the equilibrium in a reaction to the right
2NO(g) + O2(g) ↔ 2NO2(g); ΔH°<0
required
1. Introduction of catalyst
2. Lowering the temperature
3. Lower pressure
4. Decreased oxygen concentration
Explanation: a decrease in oxygen concentration will lead to a shift in equilibrium towards the reactants (to the left). A decrease in pressure will shift the equilibrium towards a decrease in the amount of gaseous substances, that is, to the right. The correct answer is 3.
Task No. 9.
Product yield in an exothermic reaction
2NO(g) + O2(g) ↔ 2NO2(g)
with a simultaneous increase in temperature and decrease in pressure
1. Increase
2. Will decrease
3. Will not change
4. First it will increase, then it will decrease
Explanation: when the temperature increases, the equilibrium shifts towards the endothermic reaction, that is, towards the products, and when the pressure decreases, the equilibrium shifts towards an increase in the amounts of gaseous substances, that is, also to the left. Therefore, the product yield will decrease. The correct answer is 2.
Task No. 10.
Increasing the yield of methanol in the reaction
CO + 2H2 ↔ CH3OH + Q
promotes
1. Increase in temperature
2. Introduction of catalyst
3. Introduction of inhibitor
4. Increased pressure
Explanation: with increasing pressure, the equilibrium shifts towards the endothermic reaction, that is, towards the reactants. An increase in pressure shifts the equilibrium towards decreasing amounts of gaseous substances, that is, towards the formation of methanol. The correct answer is 4.
Tasks for independent solution (answers below)
1. In the system
CO(g) + H2O(g) ↔ CO2(g) + H2(g) + Q
a shift in chemical equilibrium towards reaction products will be facilitated by
1. Reducing pressure
2. Increase in temperature
3. Increase in carbon monoxide concentration
4. Increase in hydrogen concentration
2. In which system, when pressure increases, does the equilibrium shift towards the reaction products?
1. 2СО2(g) ↔ 2СО2(g) + O2(g)
2. C2H4(g) ↔ C2H2(g) + H2(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 when
1. Increased pressure
2. Rising temperature
3. Decreased pressure
4. Using a catalyst
4. Chemical equilibrium in the system
C2H5OH + CH3COOH ↔ CH3COOC2H5 + H2O + Q
shifts towards the reaction products when
1. Adding water
2. Reducing the concentration of acetic acid
3. Increasing ether concentration
4. When removing ester
5. Chemical equilibrium in the system
2NO(g) + O2(g) ↔ 2NO2(g) + Q
shifts towards the formation of the reaction product when
1. Increased pressure
2. Rising temperature
3. Decreased pressure
4. Application of catalyst
6. Chemical equilibrium in the system
CO2(g) + C(s) ↔ 2СО(g) - Q
will shift towards the reaction products when
1. Increased pressure
2. Lowering the temperature
3. Increasing CO concentration
4. Temperature rises
7. Changes in pressure 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 substances?
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
С4Н10(g) ↔ С4Н6(g) + 2Н2(g) - Q
will shift towards the reaction products when
1. Increase in temperature
2. Lowering the temperature
3. Using a catalyst
4. Reducing butane concentration
10. On the state of chemical equilibrium in the system
H2(g) + I2(g) ↔ 2HI(g) -Q
does not affect
1. Increase in pressure
2. Increasing iodine concentration
3. Increase in temperature
4. Reduce temperature
2016 assignments
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 Shift of chemical equilibrium
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
B) CaCO3(s) ↔ CaO(s) + CO2(g) - Q 3. There is no shift in equilibrium
D) Fe3O4(s) + 4CO(g) ↔ 3Fe(s) + 4CO2(g) + Q
2. Establish a correspondence between external influences on the system:
CO2(g) + C(s) ↔ 2СО(g) - Q
and a shift in chemical equilibrium.
A. Increase in CO concentration 1. Shifts towards the direct reaction
B. Decrease in pressure 3. No shift in equilibrium occurs
3. Establish a correspondence between external influences on the system
HCOOH(l) + C5H5OH(l) ↔ HCOOC2H5(l) + H2O(l) + Q
External influence Shift in chemical equilibrium
A. Addition of HCOOH 1. Shifts towards the direct reaction
B. Dilution with water 3. No shift in equilibrium occurs
D. Increase 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 Shift in chemical equilibrium
A. Decrease in pressure 1. Shifts towards the forward reaction
B. Increase in temperature 2. Shifts towards the reverse reaction
B. Increase in NO2 temperature 3. No equilibrium shift occurs
D. Addition of O2
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 Shift in chemical equilibrium
A. Decrease in temperature 1. Shift towards direct reaction
B. Increase in pressure 2. Shifts towards the reverse reaction
B. Increase in concentration in ammonia 3. No shift in equilibrium occurs
D. Removal of water vapor
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 Shift in chemical equilibrium
A. Increase in temperature 1. Shifts towards a direct reaction
B. Increase in pressure 2. Shifts towards the reverse reaction
B. Use of a catalyst 3. There is no shift in equilibrium
D. Removal of water vapor
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 Shift in chemical equilibrium
A. Increase in hydrogen concentration 1. Shifts towards a direct reaction
B. Increase in temperature 2. Shifts towards the reverse reaction
B. Increase in pressure 3. No shift in equilibrium occurs
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 the chemical equilibrium towards a direct reaction.
Reaction equation Changing system parameters
A. H2(g) + F2(g) ↔ 2HF(g) + Q 1. Increase in temperature and hydrogen concentration
B. H2(g) + I2(s) ↔ 2HI(g) -Q 2. Decrease in temperature and hydrogen concentration
B. CO(g) + H2O(g) ↔ CO2(g) + H2(g) + Q 3. Increasing temperature and decreasing hydrogen concentration
D. C4H10(g) ↔ C4H6(g) + 2H2(g) -Q 4. Decrease in temperature and increase in hydrogen concentration
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 chemical equilibrium shift
A. 2HI(g) ↔ H2(g) + I2(s) 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 shift in equilibrium
G. 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. Increase in temperature and pressure
B. N2O4(l) ↔ 2NO2(g) -Q 2. Decrease in temperature and pressure
B. CO2(g) + C(s) ↔ 2CO(g) + Q 3. Increase in temperature and decrease in pressure
D. 4HCl(g) + O2(g) ↔ 2H2O(g) + 2Cl2(g) + Q 4. Decrease in temperature and increase in pressure
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 assignments, 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 state of chemical equilibrium is disrupted by various external influences on the system: heating and cooling, pressure changes, addition and removal of individual substances or solvent. As a result, the equality of the rates of forward and reverse reactions is violated and a certain shift in the state of the system occurs.
A shift in chemical equilibrium is a process that occurs in an equilibrium system as a result of an external influence.
A shift in equilibrium leads to the establishment of a new state of equilibrium in the system, characterized by changed concentrations of substances.
Example 10.6. In what direction will the equilibrium of the reaction shift when oxygen is added?
Solution. When oxygen is added, its concentration increases, and hence the speed in the forward direction. The balance will shift to the right. This increases the proportion of conversion of S0 2 to S0 3.
The displacement of equilibrium under any influence obeys Le Chatelier's principle (1884).
An external influence on a system in a state of equilibrium causes a process leading to a decrease in the result of the influence.
When deciding a specific question about the direction of the equilibrium shift, one should clearly understand the essence of the effect produced and its result. For example, a change in concentration cannot be considered as an effect on the system. Substances can be introduced or removed into the system (ego effects), resulting in a change in concentrations. The application of Le Chatelier's principle to the practically important reaction for the production of ammonia is shown in table. 10.1. The first two columns indicate the impact on the system and the result of the impact. Arrows T and >1 indicate an increase or decrease in the corresponding characteristic. The “System Response” column indicates changes that are opposite to the effect of the impact. These changes are associated with the occurrence of a direct or reverse reaction in the system. Some difficulties arise in understanding the influence of pressure on the state of equilibrium. The pressure of a gas mixture, according to the equation of gas state, depends on temperature and volume for a given amount of substance, but a system as such, having a certain volume and temperature, can respond to changes in pressure only by changing the total amount of substance as a result of the reaction. A corollary follows from Le Chatelier’s principle: with increasing pressure, the equilibrium shifts in the direction of decreasing the sum of stoichiometric coefficients for substances in the gaseous state.
Table 10.1
Application of Le Chatelier's principle using the example of the reaction N2 + 3Н2 2NH3, ArH° =-92 kJ/mol
In reversible heterogeneous reactions, a shift in equilibrium is associated with changes in the concentrations of gaseous and dissolved substances. A change in the mass of a solid does not affect the equilibrium position in the system.
Shifting chemical equilibrium is widely used when carrying out reactions in laboratories and in technological processes. In this case, we are not talking about achieving balance, but shifting it one by one. The process is planned from the very beginning so that the established equilibrium is optimal from the point of view of saving the most valuable reagents. Production costs decrease as product yield increases. It depends on temperature and pressure conditions. Using the example of the reaction for producing ammonia, the principle of the approach to choosing process conditions is shown (the signs “+” and “-” symbolize the desired or undesirable nature of the influence on the final result).
From the data presented it follows that in the production of ammonia it is desirable to use high pressure and find the most active catalysts. Temperature has a positive effect from a technological and economic point of view on the reaction rate and a negative effect on the yield of ammonia. Therefore, it is necessary to choose the optimal temperature, which ultimately ensures the minimum cost of producing the product.
Studying the parameters of a system, including starting materials and reaction products, makes it possible to find out which factors shift the chemical equilibrium and lead to the desired changes. Industrial technologies are based on the conclusions of Le Chatelier, Brown and other scientists about methods of carrying out reversible reactions, which make it possible to carry out processes that previously seemed impossible and obtain economic benefits.
Variety of chemical processes
Based on the characteristics of the thermal effect, many reactions are classified as exo- or endothermic. The first come with the formation of heat, for example, the oxidation of carbon, the hydration of concentrated sulfuric acid. The second type of change is associated with the absorption of thermal energy. Examples of endothermic reactions: decomposition of calcium carbonate with the formation of slaked lime and carbon dioxide, formation of hydrogen and carbon during the thermal decomposition of methane. In the equations of exo- and endothermic processes, it is necessary to indicate the thermal effect. The redistribution of electrons between the atoms of the reacting substances occurs in redox reactions. Four types of chemical processes are distinguished according to the characteristics of the reagents and products:
To characterize the processes, the completeness of the interaction of the reacting compounds is important. This feature underlies the division of reactions into reversible and irreversible.
Reversibility of reactions
Reversible processes make up the majority of chemical phenomena. The formation of final products from reactants is a direct reaction. In the reverse case, the starting substances are obtained from the products of their decomposition or synthesis. In the reacting mixture, a chemical equilibrium arises in which the same number of compounds is obtained as the original molecules decompose. In reversible processes, instead of the “=” sign between reactants and products, the symbols “↔” or “⇌” are used. The arrows may be unequal in length, which is due to the dominance of one of the reactions. In chemical equations, you can indicate the aggregate characteristics of substances (g - gases, g - liquids, t - solids). Scientifically based methods of influencing reversible processes are of great practical importance. Thus, the production of ammonia became profitable after creating conditions that shifted the equilibrium towards the formation of the target product: 3H 2 (g) + N 2 (g) ⇌ 2NH 3 (g). Irreversible phenomena lead to the appearance of an insoluble or slightly soluble compound and the formation of a gas that leaves the reaction sphere. Such processes include ion exchange and the breakdown of substances.
Chemical equilibrium and conditions for its displacement
The characteristics of the forward and reverse processes are influenced by several factors. One of them is time. The concentration of the substance taken for the reaction gradually decreases, and the final compound increases. The forward reaction is getting slower and slower, while the reverse process is gaining speed. At a certain interval, two opposing processes occur synchronously. Interactions between substances occur, but concentrations do not change. The reason is the dynamic chemical equilibrium established in the system. Its preservation or change depends on:
- temperature conditions;
- concentrations of compounds;
- pressure (for gases).
Chemical equilibrium shift
In 1884, the outstanding scientist from France A.L. Le Chatelier proposed a description of ways to remove a system from a state of dynamic equilibrium. The method is based on the principle of leveling the effects of external factors. Le Chatelier noticed that processes arise in the reacting mixture that compensate for the influence of extraneous forces. The principle formulated by the French researcher states that a change in conditions in a state of equilibrium favors the occurrence of a reaction that weakens external influences. The equilibrium shift obeys this rule; it is observed when the composition, temperature conditions and pressure change. Technologies based on the findings of scientists are used in industry. Many chemical processes that were considered practically impossible are carried out using methods of shifting the equilibrium.
Effect of concentration
A shift in equilibrium occurs if certain components are removed from the interaction zone or additional portions of the substance are introduced. Removing products from the reaction mixture usually causes an increase in the rate of their formation; adding substances, on the contrary, leads to their preferential decomposition. In the esterification process, sulfuric acid is used for dehydration. When it is introduced into the reaction sphere, the yield of methyl acetate increases: CH 3 COOH + CH 3 OH ↔ CH 3 COOCH 3 + H 2 O. If you add oxygen that interacts with sulfur dioxide, the chemical equilibrium shifts towards the direct reaction of the formation of sulfur trioxide. Oxygen binds into SO 3 molecules, its concentration decreases, which is consistent with Le Chatelier's rule for reversible processes.
Temperature change
Processes that involve the absorption or release of heat are endothermic and exothermic. To shift the equilibrium, heating or heat removal from the reacting mixture is used. An increase in temperature is accompanied by an increase in the rate of endothermic phenomena, in which additional energy is absorbed. Cooling leads to the advantage of exothermic processes that occur with the release of heat. When carbon dioxide interacts with coal, heating is accompanied by an increase in the concentration of monoxide, and cooling leads to the predominant formation of soot: CO 2 (g) + C (t) ↔ 2CO (g).
Effect of pressure
Pressure changes are an important factor for reacting mixtures involving gaseous compounds. You should also pay attention to the difference in volumes of the starting and resulting substances. A decrease in pressure leads to a preferential occurrence of phenomena in which the total volume of all components increases. An increase in pressure directs the process towards a decrease in the volume of the entire system. This pattern is observed in the reaction of ammonia formation: 0.5N 2 (g) + 1.5 N 2 (g) ⇌ NH 3 (g). A change in pressure will not affect the chemical equilibrium in those reactions that occur at a constant volume.
Optimal conditions for the chemical process
Creating conditions for a shift in equilibrium largely determines the development of modern chemical technologies. The practical use of scientific theory contributes to obtaining optimal production results. The most striking example is the production of ammonia: 0.5N 2 (g) + 1.5 N 2 (g) ⇌ NH 3 (g). An increase in the content of N 2 and H 2 molecules in the system is favorable for the synthesis of complex substances from simple ones. The reaction is accompanied by the release of heat, so a decrease in temperature will cause an increase in the concentration of NH 3. The volume of the initial components is greater than the target product. An increase in pressure will ensure an increase in the yield of NH 3.
Under production conditions, the optimal ratio of all parameters (temperature, concentration, pressure) is selected. In addition, the contact area between the reagents is of great importance. In solid heterogeneous systems, an increase in surface area leads to an increase in the reaction rate. Catalysts increase the rate of forward and reverse reactions. The use of substances with such properties does not lead to a shift in chemical equilibrium, but accelerates its onset.
In order to more completely convert starting substances into products, there is a need to shift the equilibrium towards the direct reaction. This can be achieved by changing the conditions of the reaction. By changing the conditions (concentration, temperature, and for gases also pressure), it is possible to transfer the system from one equilibrium state to another that meets the new conditions.
The chemical equilibrium shifts because changing conditions affect the rates of forward and reverse reactions differently. After some time, these speeds are compared again, and a state of equilibrium occurs that meets the new conditions. A change in the equilibrium concentrations of reacting substances caused by a change in any condition is called displacement , orshift in equilibrium .
If, when conditions change, the concentration of formed substances increases, i.e. substances whose formulas are on the right side of the equation, then we speak of a shift of equilibrium to the right. If a change in conditions entails an increase in the concentrations of the starting substances, the formulas of which are on the left side of the equation, then this is considered as a shift of equilibrium to the left.
The shift in chemical equilibrium with changing conditions obeys a rule known as Le Chatelier-Brown principle:
If you make any impact on a chemical reaction that is in a state of chemical equilibrium (change temperature, pressure, concentrations of substances), then the rate of that reaction (direct or reverse), the occurrence of which will lead to a weakening of this effect, will increase.
It should be noted that the Le Chatelier-Brown principle is applicable not only to chemical reactions, but also to many processes that are not of a purely chemical nature: evaporation, condensation, melting, crystallization, etc.
The influence of temperature changes on the shift of chemical equilibrium. Determined by the sign of the thermal effect. It can be found experimentally or calculated based on Hess's law. The larger it is, the stronger the influence of temperature. If it is close to zero, then the temperature change has virtually no effect on the equilibrium.
According to the Le Chatelier-Brown principle, as the temperature increases, the equilibrium shifts towards the endothermic reaction (i.e. its speed increases). As the temperature decreases, the equilibrium shifts in the direction of an exothermic reaction that releases heat (i.e., its speed increases).
For example, in the case of the process N 2 O 4 2NO 2 – 56.84 kJ
the direct reaction occurs with the absorption of heat and is endothermic; the reverse reaction proceeds with the release of heat and is exothermic. An increase in temperature will lead to an increase in the rate of the endothermic reaction and the equilibrium will shift to the right, i.e. the decomposition of N 2 O 4 will accelerate (Vdirect, Vrev.↓). A decrease in temperature will lead to an increase in the rate of the exothermic reaction and the equilibrium will shift to the left, i.e. the formation of N 2 O 4 will accelerate (Vdirect ↓, Vrev.).
The effect of changes in concentration (partial pressure) on the shift in chemical equilibrium. The introduction of additional quantities of any of the reacting substances into an equilibrium system (reaction) accelerates the reaction in which it is consumed. Thus, an increase in the concentration of starting substances shifts the equilibrium towards the formation of reaction products. An increase in the concentration of reaction products shifts the equilibrium towards the formation of starting substances. The degree of equilibrium shift for a given amount of reagent depends on the stoichiometric coefficients. In the case of an equilibrium system
CO + H 2 O steam CO 2 + H 2
the equilibrium can be shifted to the right by increasing the concentration of CO or H 2 O (water vapor); a decrease in the concentration of CO 2 or H 2 also leads to a shift of equilibrium to the right. With an increase in the concentration of CO 2 or H 2, as well as with a decrease in the concentration of CO or H 2 O, the equilibrium shifts to the left. For heterogeneous equilibrium, changing the concentrations of solid phases does not affect the shift in equilibrium.
The influence of pressure changes on the shift of chemical equilibrium. According to the Le Chatelier-Brown principle, an increase in pressure shifts the equilibrium towards the reaction that leads to a decrease in the total number of molecules in gas mixture, and, consequently, to a decrease in pressure in the system. On the contrary, when the pressure decreases, the equilibrium shifts towards a reaction accompanied by an increase in the total number of gas molecules, which entails an increase in pressure in the system. So, the process equation
3H 2 + N 2 2NH 3
shows that from one molecule of nitrogen and three molecules of hydrogen two molecules of ammonia are formed. Due to a decrease in the number of molecules, an increase in pressure causes a shift in the equilibrium of the reaction to the right - towards the formation of ammonia, which is accompanied by a decrease in pressure in the system. On the contrary, a decrease in pressure in the system leads to a shift in equilibrium to the left - towards the decomposition of ammonia, which entails an increase in pressure in the system.
In cases where, as a result of a reaction, the number of molecules of gaseous substances remains constant, when the pressure changes, the rates of the forward and reverse reactions change equally, and therefore the equilibrium does not shift. Such reactions include, for example:
CO + H 2 O steam CO 2 + H 2 N 2 + O 2 2NO
The Le Chatelier-Brown principle is of great practical importance. It makes it possible to find conditions that provide the maximum yield of the desired substance. The technology for the production of the most important chemical products is based on the application of the Le Chatelier-Brown principle and on calculations arising from the law of mass action.
Example 1. What measures can be taken to increase the yield of the reaction product N 2 + 3H 2 2NH 3, N 
=
-92,4
.
Solution
According to the conditions of the problem, it is required to shift the equilibrium towards the direct reaction, therefore it follows:
increase the concentrations of nitrogen and hydrogen, that is, constantly introduce fresh portions of reagents into the system;
reduce ammonia concentration, i.e. remove it from the reaction space;
lower the temperature (however, so that N 2 can be activated), since the direct reaction is exothermic;
increase the pressure (decrease the volume), because in the forward direction the number of moles of gaseous substances decreases (2 moles of gas are formed from 4 moles of gas).
Example 2. How will the equilibrium oxygen concentration change if in the system 2Csolv + O 2 2CO at a constant temperature the CO concentration is increased by 3 times?
Solution
Let us write the expression for the equilibrium constant of this heterogeneous process 
. According to the conditions of the problem 
. Since the equilibrium constant does not depend on the concentrations of the reactants, the equality must be satisfied
or 
.
Thus, with an increase in CO concentration by 3 times, the equilibrium concentration of oxygen should increase by 9 times.