General Chemistry

Le-Chatelier’s Principle

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Le-Chatelier’s Principle: It states that if a ‘stress is applied to a system at equilibrium, the system acts in such a way so as to nullify the effect of that stress, as far as possible.

Predictions of Le-Chatelier’s Principle

Le Chatelier’s principle predicts the effect of changes on equilibrium system. It predicts:

1) The effect of change in concentrations

2) The effect of change in pressure

3) The effect of change in temperature

Le-Chatelier’s Principle: Effect of change in concentration

Consider the following reaction

N2O4 (g)              ⇆            2NO2 (g)

Suppose the reaction is at equilibrium. Four possibilities are there to explain the effect.

i) Addition of the reactant i.e., N2O4

This will cause to increase the concentration of N2O4, which will disturb the equilibrium. According to Le Chatelier’s principle, effect of increase in concentration of N2O4 will be nulled by the decrease in the concentration N2O4.

To do so the position of equilibrium will shift to the right that is the reaction will move in the forward direction to. Re-establish equilibrium.

ii) Removal of the reactant i.e., N2O4

. This will cause to decrease the concentration of N204, which will disturb the equilibrium. According to Le Chatelier’s principle, effect of decrease in concentration of N204 will be nulled by the increase in the concentration N204.

To do so the position of equilibrium will shift to the left that is the reaction will move in the backward direction to re-establish equilibrium.

iii) Addition of the product i.e., NO2

This will cause to increase the concentration of NO2, which will disturb the equilibrium. According to Le Chatlier’s principle, effect of increase in concentration of NO2 will be nulled by the decrease in the concentration NO2.

To do so the position of equilibrium will shift to the left that is the reaction will move in the backward direction to re-establish equilibrium.

iv) Removal of the product i.e., NO2

This will cause to decrease the concentration of NO2, which will disturb the equilibrium. According to Le Chatlier’s principle, effect of decrease in concentration of NO2 will be nulled by the increase in the concentration NO2.

To do so the position of equilibrium will shift to the right that is the reaction will move in the forward direction to re-establish equilibrium.

Le-Chatleier’s Principle: Effect of Change in pressure.

A change of pressure on a gaseous system at equilibrium affects the yield of product if that reaction proceeds with a change in the number of moles. In such reactions when pressure is increased, their positions of equilibrium shift in the direction having less number of moles.

Positions of equilibrium shift neither to the right nor to the left when pressure is increased on the reactions, which proceed without any change in the number of moles.

This is because increase in pressure causes to increase the concentration of both the reactant and the products in the same proportions. Note:

A change of pressure on the system containing only liquids and solids has virtually no effect on the position of equilibrium.

(i) Effect of pressure on the decomposition of N2O4.

N2O4 (g)              ⇆            2NO2 (g)

The reaction proceeds with increase in number of moles i.e., (one mole of the reactant converts into two moles of product).

Suppose pressure is increased on this reaction when it is at equilibrium position. This disturbs the equilibrium.

According to Le Chatlier’s principle, the, effect of increased pressure is nullified by the decrease in pressure. Since the pressure is directly proportional to the number of moles, therefore, to decrease the pressure position of equilibrium shift towards the direction having less number of moles.

Hence, by the effect of pressure on decomposition of N204, the reaction occurs in forward direction.

(ii) Effect of pressure on the formation of N204

2NO2 (g)              ⇆           N2O4 (g)

This reaction proceeds with decrease in number of moles i.e., (2 moles of the reactant converts into 1 mole of the product).

Suppose pressure is increased on this reaction when it is at equilibrium position. This disturbs the equilibrium.

According to Le Chatlier’s principle, the effect of increased pressure is nullified by the decrease in pressure.

Since the pressure is directly proportional to the number of moles, therefore, to decrease the pressure position of equilibrium shift towards the direction having less number of moles. Hence, by effect of pressure on formation of N2O4, the reaction occurs in forward direction.

(iii) Effect of pressure on the formation of HI b

H2 (g) + I2 (g)     ⇆           2HI (g)

This reaction proceeds with neither decrease nor increase in number of moles i.e., (2 moles of the reactant converts into 2 moles of the product).

Suppose pressure is increased on this reaction when it is at equilibrium position. This will only increase the concentrations of the reactants and products in the same proportion keeping the equilibrium at the same position.

This means increase in pressure has no effect on the position of equilibrium that is it shifts neither to the right nor to the left when pressure is increased.

Le-Chatelier’s Principle: Effect of change in temperature

Changes in the concentration of reactant and products affect only the position of equilibrium whereas change in temperature not only affects the position of equilibrium but also the equilibrium constant

(a)          Endothermic reactions are favored by the increase in temperature.

In such reactions, the value of Kc increases with the increase in temperature.

(b)          Exothermic reactions are favored by the decrease in temperature.

In such reactions, the value of Kc increases with the decrease in temperature.

 (i) Effect of temperature on the formation of NO2

N2O4 (g) + heat                ⇆            2NO2 (g)

It is an endothermic reaction. Heat is consumed as the N204 is consumed. When heat is added by increasing temperature, it disturbs equilibrium.

According to Le Chatlier’s principle, the effect of increase in temperature will be nullified by consumption of heat.

To do so, the reaction will move in the forward direction to reestablish the equilibrium. In other words, increase in temperature favors the formation of NO2.

Increase in temperature also helps to re-establish equilibrium in less time.

Effect of temperature on the formation of N204

2NO2 (g)              ⇆           N2O4 (g) + Heat

It is an exothermic reaction. Heat is produced as one of the product. When heat is removed by decreasing temperature, it disturbs equilibrium.

According to Le Chatlier’s principle, the effect of decrease in temperature will be nullified by evolution of heat.

To do so, the reaction will move in the forward direction to re-establish the equilibrium. In other words, decrease in temperature favors the formation of N204.

However, at low temperature much time is required to establish equilibrium.

Le-Chatelier’s principle: Effect of Catalyst on Equilibrium Constant

In most of the reversible reactions, the equilibrium is not always reached within a suitable short time. So, an appropriate catalyst is added.

A catalyst does not affect the equilibrium position of the reaction.

It increases the rates of both forward and backward reactions and this reduces the time to attain the state of equilibrium.

Synthesis of Ammonia by Haber’s Process industrially by applying Le Chatelier’s principle.

Production of Ammonia:

The principle of chemical equilibrium can predict the effect of change in concentration, temperature and pressure, therefore, Haber process applies it to select the favorable conditions necessary to get maximum amount of NH3 during its industrial production.

Effect of change in concentration on the production of NH3

N2 (g) + 3H2 (g)                 ⇆            2NH3 (g)

Addition of anyone of the reactant at equilibrium state will cause to increase the concentration of that reactant which will disturb equilibrium.

According to Le Chatlier’s principle, the effect of increase in the concentration of added reactant will be nullified by the decrease in the concentration of the added reactant.

To do so the position of equilibrium will shift to the right i.e., the reaction will move in the forward direction to re-establish equilibrium.

In this way, maximum amount of NH3 is obtained by increasing the concentration of anyone of the reactants.

Effect of pressure on the production of ammonia

N2 (g) + 3H2 (g)                 ⇆            2NH3 (g)

This reaction proceeds with decrease in number of moles i.e., (4 moles of the reactant converts into 2 mole of the product). When pressure is increased on this reaction when it is at equilibrium position, this disturbs the equilibrium.

According to Le Chatlier’s principle, the “effect of increased pressure is nullified by the decrease in pressure.

Since the pressure is directly proportional to the number of moles, therefore, to increase the pressure position of equilibrium shift towards the direction having less number of moles. Hence more ammonia is formed.

This means that pressure should be increased to get maximum amount of ammonia. The reaction is carried at 200-600 atm pressure.

Effect of temperature on the production of ammonia

N2 (g) + 3H2 (g)                 ⇆            2NH3 (g) + Heat

It is an exothermic reaction. Heat is produced as one of the product. When heat is removed by decreasing temperature, it disturbs equilibrium.

According to Le Chatlier’s principle, the effect of decrease in temperature will be nulled by evolution of heat.

To do so, the position of equilibrium will move to the right that is the reaction will move in the forward direction to re-establish the equilibrium.

In other words, decrease in temperature favors the position of equilibrium. There is a drawback of decreasing the temperature that is it decreases the speed of the reaction.

For example, speed becomes virtually zero at room temperature, therefore, temperature cannot be decreased so much just to gain favor in the position of equilibrium. To get maximum amount of NH3 in less time the reaction is carried at 400°C-500°C.

Le-Chatelier is principle Production of Sulphur Trioxide

The principle of chemical equilibrium can predict the effect of change in concentration, temperature and pressure, therefore, it is applied to select the favorable conditions necessary to get maximum amount of SO3 required in the manufacturing of – H2SO4 by Contact process.

Effect of Change in Concentration on the Production of SO3

2SO2 (g) + O2 (g)                              ⇆            2SO3 (g)

Addition of anyone of the reactant at equilibrium state will cause to increase the concentration of that reactant which will disturb equilibrium.

According to Le Chatlier’s principle, the effect of increase in the concentration of added reactant will be annulled by the decrease in the concentration of the added reactant.

To do so, the position of equilibrium will shift to the right i.e., the reaction will move in the forward direction reestablish equilibrium.

In this way, maximum amount of SO3 is obtained by increasing the concentration of anyone of the reactants.

Effect of pressure on the production of SO3

2SO2 (g) + O2 (g)                              ⇆            2SO3 (g)

This reaction proceeds with decrease in number of moles i.e., (3 moles of the reactant converts into 2 moles of the product). When pressure is increased on this reaction, it is at equilibrium position. This disturbs the equilibrium.

According to Le Chatlier’s principle, the effect of increased pressure is annulled by the decrease in pressure.

Since the pressure is directly proportional to the number of moles, therefore, to increase the pressure position of equilibrium shift towards the direction having less number of moles.

Hence more SO3 is formed. This means that pressure should be increased to get maximum amount of SO3. The reaction is carried at 1.5-1.7 atm pressure.

Effect of temperature on the production of SO3

2SO2 (g) + O2 (g)                              ⇆            2SO3 (g) + Heat

It is an exothermic reaction. Heat is produced as one of the product. When heat is removed by decreasing temperature, it disturbs equilibrium.

According to Le Chatlier’s principle, the effect of decrease in temperature will be annulled by evolution of heat.

To do so, the position of equilibrium will move to the right that is the reaction will move in the forward direction to re-establish the equilibrium.

In other words, decrease in temperature favors the position of equilibrium.

There is a drawback of decreasing the temperature that is it decreases the speed of the reaction.

Therefore, temperature cannot be decreased so much just to gain favor in the position of equilibrium. To get maximum amount of SO3′ in less time the reaction is carried at 400°C-450°C.

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