Codifier Topics: reversible and irreversible reactions. Chemical balance. Shift in chemical equilibrium under the influence of various factors.
If a reverse reaction is possible, chemical reactions are divided into reversible and irreversible.
Reversible chemical reactions - these are reactions whose products under given conditions can interact with each other.
For example, ammonia synthesis is a reversible reaction:
N2 + 3H2 = 2NH3
The process takes place at high temperature, under pressure and in the presence of a catalyst (iron). Such processes are usually reversible.
Irreversible reactions - these are reactions whose products cannot interact with each other under given conditions.
For example, combustion reactions or reactions that occur with an explosion are most often irreversible. Carbon combustion proceeds irreversibly:
C + O 2 = CO 2
More details about classification chemical reactions can be read.
The likelihood of product interaction depends on the process conditions.
So, if the system open, i.e. exchanges with environment both matter and energy, then chemical reactions in which, for example, gases are formed, will be irreversible.
For example , when calcining solid sodium bicarbonate:
2NaHCO 3 → Na 2 CO 3 + CO 2 + H 2 O
carbon dioxide gas is released and evaporates from the reaction zone. Therefore, this reaction will be irreversible under these conditions.
If we consider closed system , which can't exchange a substance with the environment (for example, a closed box in which the reaction occurs), then carbon dioxide will not be able to escape from the reaction zone, and will interact with water and sodium carbonate, then the reaction will be reversible under these conditions:
2NaHCO 3 ⇔ Na 2 CO 3 + CO 2 + H 2 O
Let's consider reversible reactions. Let the reversible reaction proceed according to the scheme:
aA + bB ⇔ cC + dD
The rate of direct reaction according to the law of mass action is determined by the expression:
v 1 =k 1 ·C A a ·C B b
Feedback speed:
v 2 =k 2 ·C С с ·C D d
Here k 1 And k 2 are the rate constants of the forward and reverse reactions, respectively, C A, C B, C C, C D– concentrations of substances A, B, C and D, respectively.
If at the initial moment of the reaction there are no substances C and D in the system, then particles A and B collide and interact predominantly, and a predominantly direct reaction occurs.
Gradually, the concentration of particles C and D will also begin to increase, therefore, the rate of the reverse reaction will increase. At some point the rate of the forward reaction will be equal to the rate of the reverse reaction. This state is called chemical equilibrium .
Thus, chemical equilibrium is a state of the system in which the rates of forward and reverse reactions are equal .
Since the rates of forward and reverse reactions are equal, the rate of formation of reagents is equal to the rate of their consumption, and the current concentrations of substances do not change
. Such concentrations are called equilibrium
.
Please note that at equilibrium Both forward and reverse reactions occur, that is, the reactants interact with each other, but the products also interact with each other at the same rate. At the same time, external factors can influence displace chemical equilibrium in one direction or another. Therefore, chemical equilibrium is called mobile, or dynamic .
Research in the field of mobile balance began in the 19th century. The works of Henri Le Chatelier laid the foundations of the theory, which he later generalized scientist Karl Brown. The principle of mobile equilibrium, or the Le Chatelier-Brown principle, states:
If a system in a state of equilibrium is influenced by an external factor that changes any of the equilibrium conditions, then processes in the system aimed at compensating for the external influence are intensified.
In other words: When there is an external influence on the system, the equilibrium will shift so as to compensate for this external influence.
This principle, which is very important, works for any equilibrium phenomena (not just chemical reactions). However, we will now consider it in relation to chemical interactions. In the case of chemical reactions, external influences lead to changes in the equilibrium concentrations of substances.
Chemical reactions in a state of equilibrium can be influenced by three main factors - temperature, pressure and concentrations of reactants or products.
1. As is known, chemical reactions are accompanied by a thermal effect. If the direct reaction occurs with the release of heat (exothermic, or +Q), then the reverse reaction occurs with the absorption of heat (endothermic, or -Q), and vice versa. If you raise temperature in the system, the equilibrium will shift so as to compensate for this increase. It is logical that in an exothermic reaction the temperature increase cannot be compensated. Thus, as the temperature increases, the equilibrium in the system shifts towards heat absorption, i.e. towards endothermic reactions (-Q); with decreasing temperature - towards an exothermic reaction (+Q).
2. In the case of equilibrium reactions, when at least one of the substances is in the gas phase, the equilibrium is also significantly affected by a change pressure in the system. As pressure increases, the chemical system tries to compensate for this effect and increases the rate of reaction, in which the amount of gaseous substances decreases. As the pressure decreases, the system increases the rate of reaction, which produces more molecules of gaseous substances. Thus: with an increase in pressure, the equilibrium shifts towards a decrease in the number of gas molecules, and with a decrease in pressure - towards an increase in the number of gas molecules.
Pay attention! Systems where the number of molecules of reactant gases and products are the same are not affected by pressure! Also, changes in pressure have virtually no effect on the equilibrium in solutions, i.e. on reactions where there are no gases.
3. Also, equilibrium in chemical systems is affected by changes concentrations reactants and products. As the concentration of reactants increases, the system tries to use them up and increases the rate of the forward reaction. As the concentration of reagents decreases, the system tries to produce them, and the rate of the reverse reaction increases. As the concentration of products increases, the system also tries to consume them and increases the rate of the reverse reaction. When the concentration of products decreases, the chemical system increases the rate of their formation, i.e. rate of forward reaction.
If in a chemical system the rate of forward reaction increases right , towards the formation of products And reagent consumption . If the rate of reverse reaction increases, we say that the balance has shifted left , towards food consumption And increasing the concentration of reagents .
For example, in the ammonia synthesis reaction:
N 2 + 3H 2 = 2NH 3 + Q
An increase in pressure leads to an increase in the rate of reaction, in which fewer gas molecules are formed, i.e. direct reaction (the number of molecules of reactant gases is 4, the number of gas molecules in products is 2). As pressure increases, the equilibrium shifts to the right, towards the products. At temperature rise the balance will shift in the opposite direction of the endothermic reaction, i.e. to the left, towards the reagents. An increase in the concentration of nitrogen or hydrogen will shift the equilibrium towards their consumption, i.e. to the right, towards the products.
Catalyst does not affect balance, because accelerates both forward and reverse reactions.
Chemical equilibrium is a state of a system where both reactions - forward and reverse - have the same rates. How is this phenomenon characterized, and what factors influence chemical equilibrium?
Chemical balance. General characteristics
Chemical equilibrium can be understood as a state chemical system, in which the initial amount of substances in the reaction does not change over time.
Chemical equilibrium can be divided into three types:
- true balance– this is an equilibrium characterized by constancy over time in the absence of external influence. If external conditions change, the state of the system also changes, but after the conditions are restored, the state also becomes the same. The state of true equilibrium can be considered from two sides: from the side of reaction products and from the side of starting substances.
- metastable (apparent) equilibrium– this condition occurs when any of the conditions of true equilibrium are not met.
- inhibited (false) balance– this is a state of the system that changes irreversibly when external conditions change.
Shifting equilibrium in chemical reactions
Chemical equilibrium depends on three parameters: temperature, pressure, and concentration of the substance. The French chemist Henri Louis Le Chatelier formulated the principle of dynamic equilibrium in 1884, according to which an equilibrium system, when exposed to external influences, tends to return to a state of equilibrium. That is, under external influence, the equilibrium will shift in such a way that this influence is neutralized.
Rice. 1. Henri Louis Le Chatelier.
The principles formulated by Le Chatelier are also called the principles of “equilibrium shifts in chemical reactions.”
The following factors influence chemical equilibrium:
- temperature. As the temperature increases, the chemical equilibrium shifts towards the absorption of the reaction. If the temperature is lowered, then the equilibrium shifts towards the release of the reaction.
Rice. 2. The effect of temperature changes on chemical equilibrium.
The absorption reaction is called an endothermic reaction, and the release reaction is called exothermic.
- pressure. If the pressure in a chemical reaction increases, then the chemical equilibrium shifts towards the smallest volume of the substance. If the pressure decreases, then the equilibrium shifts towards the largest volume of the substance. This principle applies only to gases, but to solids it doesn't work.
- concentration. If, during a chemical reaction, the concentration of one of the substances is increased, the equilibrium will shift towards the reaction products, and if the concentration is decreased, the equilibrium will shift towards the starting substances.
Rice. 3. The effect of changes in concentration on chemical equilibrium.
The catalyst is not one of the factors influencing the shift in chemical equilibrium.
What have we learned?
In chemical equilibrium, the rates in each pair of reactions are equal. Chemical equilibrium, studied in grade 9, can be divided into three types: true, metastable (apparent), inhibited (false). The thermodynamic theory of chemical equilibrium was first formulated by the scientist Le Chatelier. The equilibrium of the system is influenced by only three factors: pressure, temperature, and concentration of the starting substance.
Chemical equilibrium is maintained as long as the conditions in which the system is located remain unchanged. Changing conditions (concentration of substances, temperature, pressure) causes an imbalance. After some time, the chemical equilibrium is restored, but under new, different from previous conditions. Such a transition of a system from one equilibrium state to another is called displacement(shift) of equilibrium. The direction of displacement obeys Le Chatelier's principle.
As the concentration of one of the starting substances increases, the equilibrium shifts towards greater consumption of this substance, and the direct reaction intensifies. A decrease in the concentration of the starting substances shifts the equilibrium towards the formation of these substances, as the reverse reaction increases. An increase in temperature shifts the equilibrium towards an endothermic reaction, while a decrease in temperature shifts the equilibrium towards an exothermic reaction. An increase in pressure shifts the equilibrium towards decreasing amounts of gaseous substances, that is, towards smaller volumes occupied by these gases. On the contrary, as pressure decreases, the equilibrium shifts towards increasing amounts of gaseous substances, that is, towards larger volumes formed by gases.
Example 1.
How will an increase in pressure affect the equilibrium state of the following reversible gas reactions:
a) SO 2 + C1 2 =SO 2 CI 2;
b) H 2 + Br 2 = 2НВr.
Solution:
We use Le Chatelier's principle, according to which an increase in pressure in the first case (a) shifts the equilibrium to the right, towards a smaller amount of gaseous substances occupying a smaller volume, which weakens the external influence of the increased pressure. In the second reaction (b), the quantities of gaseous substances, both the starting materials and the reaction products, are equal, as are the volumes they occupy, so pressure has no effect and the equilibrium is not disturbed.
Example 2.
In the reaction of ammonia synthesis (–Q) 3H 2 + N 2 = 2NH 3 + Q, the forward reaction is exothermic, the reverse reaction is endothermic. How should the concentration of reactants, temperature and pressure be changed to increase the yield of ammonia?
Solution:
To shift the balance to the right you need to:
a) increase the concentrations of H 2 and N 2;
b) reduce the concentration (removal from the reaction sphere) of NH 3;
c) lower the temperature;
d) increase the pressure.
Example 3.
The homogeneous reaction between hydrogen chloride and oxygen is reversible:
4HC1 + O 2 = 2C1 2 + 2H 2 O + 116 kJ.
1. What effect will the following have on the equilibrium of the system?
a) increase in pressure;
b) increase in temperature;
c) introduction of a catalyst?
Solution:
a) In accordance with Le Chatelier's principle, an increase in pressure leads to a shift in equilibrium towards the direct reaction.
b) An increase in t° leads to a shift in equilibrium towards the reverse reaction.
c) The introduction of a catalyst does not shift the equilibrium.
2. In what direction will the chemical equilibrium shift if the concentration of reactants is doubled?
Solution:
υ → = k → 0 2 0 2 ; υ 0 ← = k ← 0 2 0 2
After increasing concentrations, the rate of the forward reaction became:
υ → = k → 4 = 32 k → 0 4 0
that is, it increased by 32 times compared to the initial speed. Similarly, the rate of the reverse reaction increases 16 times:
υ ← = k ← 2 2 = 16k ← [H 2 O] 0 2 [C1 2 ] 0 2 .
The increase in the rate of the forward reaction is 2 times greater than the increase in the rate of the reverse reaction: the equilibrium shifts to the right.
Example 4.
IN Which side will the equilibrium of a homogeneous reaction shift:
PCl 5 = PC1 3 + Cl 2 + 92 KJ,
if you increase the temperature by 30 °C, knowing that the temperature coefficient of the forward reaction is 2.5, and the reverse reaction is 3.2?
Solution:
Since the temperature coefficients of the forward and reverse reactions are not equal, increasing the temperature will have different effects on the change in the rates of these reactions. Using Van't Hoff's rule (1.3), we find the rates of forward and reverse reactions when the temperature increases by 30 °C:
υ → (t 2) = υ → (t 1)=υ → (t 1)2.5 0.1 30 = 15.6υ → (t 1);
υ ← (t 2) = υ ← (t 1) =υ → (t 1)3.2 0.1 30 = 32.8υ ← (t 1)
An increase in temperature increased the rate of the forward reaction by 15.6 times, and the reverse reaction by 32.8 times. Consequently, the equilibrium will shift to the left, towards the formation of PCl 5.
Example 5.
How will the rates of forward and reverse reactions change in the isolated system C 2 H 4 + H 2 ⇄ C 2 H 6 and where will the equilibrium shift when the volume of the system increases by 3 times?
Solution:
The initial rates of forward and reverse reactions are as follows:
υ 0 = k 0 0 ; υ 0 = k 0 .
An increase in the volume of the system causes a decrease in the concentrations of reactants by 3 times, hence the change in the rate of forward and reverse reactions will be as follows:
υ 0 = k = 1/9υ 0
υ = k = 1/3υ 0
The decrease in the rates of forward and reverse reactions is not the same: the rate of the reverse reaction is 3 times (1/3: 1/9 = 3) higher than the rate of the reverse reaction, therefore the equilibrium will shift to the left, to the side where the system occupies a larger volume, that is, towards the formation of C 2 H 4 and H 2.
If a system is in a state of equilibrium, then it will remain in it as long as external conditions remain constant. If the conditions change, the system will go out of equilibrium - the rates of the forward and reverse processes will change unequally - a reaction will occur. The most important are cases of imbalance due to changes in the concentration of any of the substances involved in the equilibrium, pressure or temperature.
Let's consider each of these cases.
Disturbance of equilibrium due to a change in the concentration of any of the substances participating in the reaction. Let hydrogen, hydrogen iodide and iodine vapor be in equilibrium with each other at a certain temperature and pressure. Let us introduce an additional amount of hydrogen into the system. According to the law of mass action, an increase in the concentration of hydrogen will entail an increase in the rate of the forward reaction - the HI synthesis reaction, while the rate of the reverse reaction will not change. The reaction will now proceed faster in the forward direction than in the reverse direction. As a result of this, the concentrations of hydrogen and iodine vapor will decrease, which will slow down the forward reaction, and the concentration of HI will increase, which will accelerate the reverse reaction. After some time, the rates of the forward and reverse reactions will become equal again, and a new equilibrium will be established. But at the same time, the concentration of HI will now be higher than it was before adding , and the concentration will be lower.
The process of changing concentrations caused by an imbalance is called a displacement or equilibrium shift. If at the same time there is an increase in the concentrations of substances on the right side of the equation (and, of course, at the same time a decrease in the concentrations of substances on the left), then they say that the equilibrium shifts to the right, i.e., in the direction of the direct reaction; when the concentrations change in the opposite direction, they speak of a shift in equilibrium to the left - in the direction of the reverse reaction. In the example considered, the equilibrium has shifted to the right. At the same time, the substance, the increase in concentration of which caused an imbalance, entered into a reaction - its concentration decreased.
Thus, with an increase in the concentration of any of the substances participating in the equilibrium, the equilibrium shifts towards the consumption of this substance; When the concentration of any of the substances decreases, the equilibrium shifts towards the formation of this substance.
Disturbance of equilibrium due to changes in pressure (by decreasing or increasing the volume of the system). When gases are involved in a reaction, equilibrium may be disrupted when the volume of the system changes.
Consider the effect of pressure on the reaction between nitrogen monoxide and oxygen:
Let a mixture of gases be in chemical equilibrium at a certain temperature and pressure. Without changing the temperature, we increase the pressure so that the volume of the system decreases by 2 times. At the first moment, the partial pressures and concentrations of all gases will double, but at the same time the ratio between the rates of forward and reverse reactions will change - the equilibrium will be disrupted.
In fact, before the pressure increased, the gas concentrations had equilibrium values , and , and the rates of the forward and reverse reactions were the same and were determined by the equations:
At the first moment after compression, the gas concentrations will double compared to their initial values and will be equal to , and , respectively. In this case, the rates of forward and reverse reactions will be determined by the equations:
Thus, as a result of increasing pressure, the rate of the forward reaction increased 8 times, and the reverse reaction only 4 times. The equilibrium in the system will be disrupted - the forward reaction will prevail over the reverse one. After the speeds become equal, equilibrium will be established again, but the quantity in the system will increase, and the equilibrium will shift to the right.
It is easy to see that the unequal change in the rates of forward and reverse reactions is due to the fact that on the left and right sides of the equation of the reaction under consideration the number of gas molecules is different: one oxygen molecule and two nitrogen monoxide molecules (three gas molecules in total) are converted into two gas molecules - nitrogen dioxide. The pressure of a gas is the result of its molecules hitting the walls of the container; other things being equal, the higher the number of molecules contained in a given volume of gas, the higher the gas pressure. Therefore, a reaction that occurs with an increase in the number of gas molecules leads to an increase in pressure, and a reaction that occurs with a decrease in the number of gas molecules leads to a decrease in pressure.
With this in mind, the conclusion about the effect of pressure on chemical equilibrium can be formulated as follows:
When the pressure increases by compressing the system, the equilibrium shifts towards a decrease in the number of gas molecules, i.e. towards a decrease in pressure; when the pressure decreases, the equilibrium shifts towards an increase in the number of gas molecules, i.e. towards an increase in pressure.
In the case when the reaction proceeds without changing the number of gas molecules, the equilibrium is not disturbed during compression or expansion of the system. For example, in the system
equilibrium is not disturbed when volume changes; the HI output is independent of pressure.
Disequilibrium due to temperature changes. The equilibrium of the vast majority of chemical reactions shifts with temperature changes. The factor that determines the direction of the equilibrium shift is the sign of the thermal effect of the reaction. It can be shown that when the temperature increases, the equilibrium shifts in the direction of the endothermic reaction, and when it decreases, in the direction of the exothermic reaction.
Thus, ammonia synthesis is an exothermic reaction
Therefore, as the temperature increases, the equilibrium in the system shifts to the left - towards the decomposition of ammonia, since this process occurs with the absorption of heat.
Conversely, the synthesis of nitric oxide (II) is an endothermic reaction:
Therefore, as the temperature increases, the equilibrium in the system shifts to the right - towards the formation.
The patterns that appear in the considered examples of chemical imbalance are special cases general principle, which determines the influence of various factors on equilibrium systems. This principle, known as Le Chatelier's principle, when applied to chemical equilibria, can be formulated as follows:
If any impact is exerted on a system that is in equilibrium, then as a result of the processes occurring in it, the equilibrium will shift in such a direction that the impact will decrease.
Indeed, when one of the substances participating in the reaction is introduced into the system, the equilibrium shifts towards the consumption of this substance. “When the pressure increases, it shifts so that the pressure in the system decreases; when the temperature increases, the equilibrium shifts towards the endothermic reaction - the temperature in the system drops.
Le Chatelier's principle applies not only to chemical, but also to various physicochemical equilibria. A shift in equilibrium when the conditions of processes such as boiling, crystallization, and dissolution change occurs in accordance with Le Chatelier’s principle.
The state in which the rate of the reverse reaction becomes equal to the rate of the forward reaction is called chemical equilibrium.
This condition is quantitatively characterized equilibrium constant. For a reversible reaction we can write it like this:
Where, in accordance with the law of mass action, is the rate of direct reaction v 1 and reverse v 2 will look like this:
v 1 = k 1 [A] m [B] n,
v 2 = k 2 [C] p [D] q .
At the moment of achievement chemical equilibrium the rates of forward and reverse reactions become the same:
k 1 [A] m [B] n = k 2 [C] p [D] q ,
K = k 1 /k 2 =([C] p [D] q)/([A] m [B] n),
Where TO- equilibrium constant showing the ratio of forward and reverse reactions.
Those concentrations that stop at equilibrium are called equilibrium concentrations. It should be remembered that the values of the degrees m, n, p, q equal to the stoichiometric coefficients in the equilibrium reaction. Numerical value equilibrium constants determine the yield of a reaction. At K>>1 the yield of products is high, and TO<<1 - very small.
Reaction output- the ratio of the amount of product actually obtained to the amount that would have been obtained if this reaction had proceeded to completion (expressed as a percentage).
Chemical equilibrium cannot be maintained indefinitely. In fact, changes in temperature, pressure or concentration of reactants can shift the equilibrium in one direction or another.
Changes occurring in the system as a result of external influences are determined by the principle of moving equilibrium - Le Chatelier's principle:
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.
Those. the ratio between the rates of forward and reverse reactions changes.
The principle applies not only to chemical, but also to physical processes, such as melting, boiling, etc.
Change in concentration.
As the concentration of one of the reactants increases, the equilibrium shifts towards the consumption of this substance.
As the concentration of iron or sulfur increases, the equilibrium will shift towards the consumption of this substance, i.e. to the right.
The influence of pressure on chemical equilibrium.
Only taken into account in gas phases!
As pressure increases, the equilibrium shifts towards decreasing amounts of gaseous substances. If the reaction proceeds without changing the amounts of gaseous substances, then pressure does not affect the equilibrium in any way.
N 2 (d) + 3H 2 (G)2 N.H. 3 (G),
There are 4 moles of gaseous reactants on the left, 2 on the right, so as the pressure increases, the equilibrium will shift to the right.
N 2 (d)+O 2 (g) = 2NO(G),
There are 2 moles of gaseous substances on the left and on the right, so pressure does not affect the equilibrium.
The influence of temperature on chemical equilibrium.
When the temperature changes, both the forward and reverse reactions change, but to varying degrees.
As the temperature increases, the equilibrium shifts towards the endothermic reaction.
N 2
(d) + 3H 2
(G) 2
N.H. 3
(d) +Q,
This reaction proceeds with the release of heat (exothermic), so an increase in temperature will shift the equilibrium towards the starting products (reverse reaction).