Chemical equilibrium, corresponding to the equality of the rates of forward and reverse reactions ( = ) and the minimum value of the Gibbs energy (∆ G р,т = 0), is the most stable state of the system under given conditions and remains unchanged as long as the parameters under which balance has been established.
When conditions change, the equilibrium is disrupted and shifted towards a direct or reverse reaction. The shift in equilibrium is due to the fact that external influences change the speed of two mutually opposite processes to varying degrees. After some time, the system again becomes equilibrium, i.e. it passes from one equilibrium state to another. The new equilibrium is characterized by new equality of the rates of forward and reverse reactions and new equilibrium concentrations of all substances in the system.
The direction of the equilibrium shift in the general case is determined by Le Chatelier’s principle: if an external influence is exerted on a system in a state of stable equilibrium, then the equilibrium shifts towards a process that weakens the effect of the external influence.
A shift in equilibrium can be caused by a change in temperature or concentration (pressure) of one of the reactants.
Temperature is the parameter on which the value of the equilibrium constant depends chemical reaction. The issue of equilibrium shift when temperature changes depending on the conditions of use of the reaction is solved by using the isobar equation (1.90) - =
1. For an isothermal process ∆ r H 0 (t)< 0, в правой части выражения (1.90) R >0, T > 0, therefore the first derivative of the logarithm of the equilibrium constant with respect to temperature is negative< 0, т.е. ln Kp (и сама константа Кр) являются убывающими функциями температуры. При увеличении температуры константа химического равновесия (Кр) уменьшается и что согласно закону действующих масс (2.27), (2.28)соответствует смещению химического равновесия в сторону обратной (эндотермической) реакции. Именно в этом проявляется противодействие системы оказанному воздействию.
2. For an endothermic process ∆ r H 0 (t) > 0, the derivative of the logarithm of the equilibrium constant with respect to temperature is positive (> 0), so ln Kp and Kp are increasing functions of temperature, i.e. in accordance with the law of mass action, as the temperature increases, the equilibrium shifts towards the direct (endothermic reaction). However, we must remember that the speed of both isothermal and endothermic processes increases when the temperature increases, and decreases when the temperature decreases, but the change in speeds is not the same when the temperature changes, therefore, by varying the temperature, it is possible to shift the equilibrium in a given direction. A shift in equilibrium can be caused by a change in the concentration of one of the components: the addition of a substance to the equilibrium system or its removal from the system.
According to Le Chatelier’s principle, when the concentration of one of the reaction participants changes, the equilibrium shifts in the direction that compensates for the change, i.e. with increasing concentration of one of starting materials- V right side, and with increasing concentration, one of the reaction products moves to the left. If gaseous substances participate in a reversible reaction, then when the pressure changes, all their concentrations change equally and simultaneously. The rates of processes also change, and consequently, a shift in chemical equilibrium may occur. So, for example, with an increase in pressure (compared to equilibrium) on the system CaCO 3 (K) CO (k) + CO 2 (g), the rate of the reverse reaction increases = which will lead to a shift in equilibrium in left side. When the pressure on the same system decreases, the rate of the reverse reaction decreases, and the equilibrium shifts to the right. When the pressure on the 2HCl H 2 +Cl 2 system, which is in a state of equilibrium, increases, the equilibrium will not shift, because both speeds will increase equally.
For the system 4HCl + O 2 2Cl 2 + 2H 2 O (g), an increase in pressure will lead to an increase in the rate of the forward reaction and a shift of equilibrium to the right.
And so, in accordance with Le Chatelier’s principle, with increasing pressure, the equilibrium shifts towards the formation of fewer moles of gaseous substances in the gas mixture and, accordingly, towards a decrease in pressure in the system.
Conversely, with an external influence that causes a decrease in pressure, the equilibrium shifts towards the formation of more moles of gaseous substances, which will cause an increase in pressure in the system and will counteract the effect produced.
Le Chatelier's principle has great practical significance. Based on this, it is possible to select conditions for chemical interaction that will ensure the maximum yield of reaction products.
>> Chemistry: Chemical equilibrium and methods of shifting it In reversible processes, the rate of a direct reaction is initially maximum, and then decreases due to the fact that the concentrations of starting substances consumed in the formation of reaction products decrease. On the contrary, the rate of the reverse reaction, minimal at the beginning, increases as the concentration of reaction products increases. Finally, a moment comes when the rates of the forward and reverse reactions become equal.
The state of a chemical reversible process is called chemical equilibrium if the rate of the forward reaction is equal to the rate of the reverse reaction.
Chemical equilibrium is dynamic (mobile), since when it occurs, the reaction does not stop, only the concentrations of the components remain unchanged, that is, per unit time the same amount of reaction products is formed as is converted into the starting substances. At constant temperature and pressure, the equilibrium of a reversible reaction can be maintained indefinitely.
In production, they are most often interested in the preferential occurrence of the direct reaction. For example, in the production of ammonia, sulfur oxide (VI). nitric oxide (II). How to derive a system from a state of equilibrium? How does a change in the external conditions under which this or that reversible chemical process occurs affect it?
Lesson content lesson notes supporting frame lesson presentation acceleration methods interactive technologies Practice tasks and exercises self-test workshops, trainings, cases, quests homework discussion questions rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures, graphics, tables, diagrams, humor, anecdotes, jokes, comics, parables, sayings, crosswords, quotes Add-ons abstracts articles tricks for the curious cribs textbooks basic and additional dictionary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in a textbook, elements of innovation in the lesson, replacing outdated knowledge with new ones Only for teachers perfect lessons calendar plan for a year methodological recommendations discussion programs Integrated LessonsIf external conditions chemical process do not change, then the state of chemical equilibrium can be maintained indefinitely. By changing the reaction conditions (temperature, pressure, concentration) you can achieve displacement or shift in chemical equilibrium in the required direction.
A 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. A shift in 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 equilibrium concentrations of reaction participants.
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 subject to any influence, for example, by changing temperature, pressure or concentrations of reagents, then the equilibrium will shift in the direction of the reaction that weakens the effect .
The effect of changes in concentration on the shift in chemical equilibrium.
According to Le Chatelier's principle An increase in the concentration of any of the reaction participants causes a shift in equilibrium towards the reaction that leads to a decrease in the concentration of this substance.
The influence of concentration on the state of equilibrium is subject to the following rules:
As the concentration of one of the starting substances increases, the rate of the forward reaction increases and the equilibrium shifts towards the formation of reaction products and vice versa;
As the concentration of one of the reaction products increases, 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 lead to a shift of equilibrium to the right, which will lead to the consumption of starting substances and an increase in the concentration of reaction products. A new equilibrium state will be established with new equilibrium concentrations of the starting 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 reaction products.
The influence of pressure changes 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 fewer 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. Stoichiometric coefficients in front of the formulas of gaseous substances indicate that the occurrence of a forward reaction leads to an increase in the number of moles of gases, and the occurrence 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 by, for example, increasing pressure, then the system will react in such a way as to weaken this influence. The pressure may decrease if the equilibrium of a given reaction shifts toward fewer moles of the gaseous substance, and therefore 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 before and after the reaction remains constant, i.e. the volume of the system does not change during the reaction, then a change in pressure equally changes the rates of 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 shift when pressure changes.
The influence of temperature changes 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 process. So in the reaction of ammonia synthesis, 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 forward and reverse reactions change, however, the change in rates does not occur to the same extent. According to the Arrhenius equation in to a greater extent An endothermic reaction, characterized by great value activation energies.
Therefore, to assess the influence of temperature on the direction of shift of 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 chemical equilibrium constant (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 towards the exothermic reaction.
Thus, temperature increase in the ammonia synthesis reaction will lead to a shift in equilibrium towards endothermic reactions, i.e. to the left. The advantage is given to the reverse reaction, which occurs with the absorption of heat.
Chemical reactions can be reversible or 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) occur simultaneously ).
Essentially, because Both direct and reverse reactions occur; in the case of reversible reactions, both the substances on the left side of the equation and the substances on the right side of the equation can be called reagents (starting substances). The same goes for products.
For any reversible reaction, a situation is possible when the rates of the forward and reverse reactions are equal. This condition is called state of balance.
At equilibrium, the concentrations of both all reactants and all products are constant. The concentrations of products and reactants at equilibrium are called equilibrium concentrations.
Shift in chemical equilibrium under the influence of various factors
Due to external influences on the system, such as changes in temperature, pressure or concentration of starting substances or products, the equilibrium of the system may be disrupted. However, after the cessation of this external influence, the system will, after some time, move to a new state of equilibrium. Such a transition of a system from one equilibrium state to another equilibrium state is called displacement (shift) of chemical equilibrium .
In order to be able to determine how the chemical equilibrium for one type of influence or another, it is convenient to use Le Chatelier’s 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 influence.
The influence of temperature on the state of equilibrium
When temperature changes, the equilibrium of any chemical reaction shifts. This is due to the fact that any reaction has a thermal effect. Moreover, 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 a certain 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 can be seen from the equation above, the forward reaction is exothermic, i.e. As a result of its occurrence, heat is released. Consequently, the reverse reaction will be endothermic, that is, it occurs with the absorption of heat. According to the condition, the temperature is reduced, therefore, the equilibrium will shift to the right, i.e. towards direct reaction.
Effect of concentration on chemical equilibrium
An increase in the concentration of reagents in accordance with Le Chatelier’s principle should lead to a shift in equilibrium towards the reaction as a result of which the reagents are consumed, i.e. towards direct reaction.
And vice versa, if the concentration of the reactants is reduced, then the equilibrium will shift towards the reaction as a result of which the reactants are formed, i.e. side of the reverse reaction (←).
A change in the concentration of reaction products also has a similar effect. If the concentration of products is increased, 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 reduced, then the equilibrium will shift towards the direct reaction (→), so that the concentration of products increases.
Effect of pressure on chemical equilibrium
Unlike temperature and concentration, changes in pressure do 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 for gaseous substances on the left and right sides of the equation must be different.
Those. of two reactions:
a change in pressure can affect the equilibrium state 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 here, in particular, it follows that if there are no gaseous substances among both reactants and 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, on the left and right, the amount of gaseous substances differs, 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 will lead to a shift in the equilibrium, as a result of which the volume of gases increases.
Effect of a catalyst on chemical equilibrium
Since a catalyst equally accelerates both forward and reverse reactions, its presence or absence has no effect to a state of equilibrium.
The only thing a catalyst can affect is the rate of transition of the system from a nonequilibrium state to an equilibrium one.
The impact of all the above factors on chemical equilibrium is summarized below in a cheat sheet, which you can initially look at when performing equilibrium tasks. However, it will not be possible to use it in the exam, therefore, after analyzing several examples with its help, you should learn it and practice solving equilibrium problems without looking at it:
Designations: T - temperature, p - pressure, With – concentration, – increase, ↓ – decrease
|
T |
T - equilibrium shifts towards the endothermic reaction |
| ↓T - equilibrium shifts towards the exothermic reaction | |
|
p |
p - equilibrium shifts towards the reaction with a smaller sum of coefficients in front of gaseous substances |
| ↓p - the equilibrium shifts towards the reaction with a larger amount coefficients for gaseous substances | |
|
c |
c (reagent) – the equilibrium shifts towards the direct reaction (to the right) |
| ↓c (reagent) – the equilibrium shifts towards the reverse reaction (to the left) | |
| c (product) – equilibrium shifts towards the reverse reaction (to the left) | |
| ↓c (product) – the equilibrium shifts towards the direct reaction (to the right) | |
| Doesn't affect balance!!! |
1. Among all known reactions, a distinction is made between reversible and irreversible reactions. When studying ion exchange reactions, the conditions under which they proceed to completion were listed. ().
There are also known reactions that, under given conditions, do not proceed to completion. 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 form in an aqueous solution. This is explained by the fact that sulfurous acid fragile, and a reverse reaction occurs, i.e. decomposition into sulfur oxide and water. Consequently, this reaction does not go to completion because two reactions occur simultaneously - straight(between sulfur oxide and water) and reverse(decomposition of sulfurous acid). SO 2 +H 2 O↔ H 2 SO 3 .
Chemical reactions occurring 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 speed reverse reaction ( υ arr.) is equal to zero. The concentration of reactants decreases over time, and 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 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 a state of equilibrium is established:
υ pr =υ arr.
The state of the 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 reactants and 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.
The state of chemical equilibrium is described quantitatively law of mass action.
At equilibrium, the ratio of the product of concentrations of reaction products (in powers of their coefficients) to the product of concentrations of reactants (also in 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 (G) + 92.4 kJ the equilibrium constant is expressed as follows:
υ 1 =υ 2
v 1 (direct reaction) = k 1 [ N 2 ][ H 2 ] 3 , where– equilibrium molar concentrations, = mol/l
υ 2 (backlash) = k 2 [ N.H. 3 ] 2
k 1 [ N 2 ][ H 2 ] 3 = k 2 [ N.H. 3 ] 2
Kp = k 1 / k 2 = [ N.H. 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 is exerted on a system that is in equilibrium, then the equilibrium in the system will shift in the direction opposite to this influence.
1) Effect of concentration – if the concentration of the starting substances is increased, the equilibrium shifts towards the formation of reaction products.
For example,Kp = k 1 / k 2 = [ N.H. 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, then to fulfill this condition the numerator must also increase. Thus, the amount of reaction product in the reaction mixture increases. In this case, they speak of a shift in chemical equilibrium to the right, towards the product.
Thus, an increase in the concentration of reactants (liquid or gaseous) shifts towards the products, i.e. towards direct reaction. An increase in the concentration of products (liquid or gaseous) shifts the equilibrium towards the reactants, i.e. towards the opposite reaction.
Changing the mass of a solid does not change the equilibrium position.
2) Effect of temperature – an increase in temperature shifts the equilibrium towards an endothermic reaction.
A)N 2 (G) + 3H 2 (D) ↔ 2N.H. 3 (G) + 92.4 kJ (exothermic - heat release)
As the temperature increases, the equilibrium will shift towards the ammonia decomposition reaction (←)
b)N 2 (G) +O 2 (D) ↔ 2NO(G) – 180.8 kJ (endothermic - heat absorption)
As the temperature increases, the equilibrium will shift towards the formation reaction NO (→)
3) Influence of pressure (only for gaseous substances) – with increasing pressure, the equilibrium shifts towards the formationI substances occupying less o I eat.
N 2 (G) + 3H 2 (D) ↔ 2N.H. 3 (G)
1 V - N 2
3 V - H 2
2 V – N.H. 3
With increasing pressure ( P): before reaction4 V gaseous substances → after the reaction2 Vgaseous substances, therefore, the equilibrium shifts to the right ( → )
When the pressure increases, for example, by 2 times, the volume of gases decreases by the same amount, and therefore, the concentrations of all gaseous substances will increase by 2 times. Kp = k 1 / k 2 = [ N.H. 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 violated. To restore it, the concentration must increase ammoniaand concentrations decrease 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, towards an increase in volume.
A change in pressure has virtually 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.
! The course of a chemical reaction is influenced by substances - 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 shift.
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) = 2SO 3 (g) + Q
Indicate where the equilibrium will shift if
a) increase the pressure
b) increase the temperature
c) increase the oxygen concentration
d) introduction of a catalyst?