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Le Chatelier's Principle

Chemical Bonding Assignment / Homework Help
Le Chatelier's Principle: Effect of pressure, temperature and concentration of reactants on chemical equilibrium

Le Chatelier's principle states that if a system in equilibrium is subjected to changes in temperature, pressure or concentration, then the equilibrium shifts in such a way to nullify the effect of the changed condition. This principle helps in obtaining maximum amount of products. Following is the discussion of each of the factors on the state of equilibrium and reactants.


Effect of change of concentration of the reactants:

Let us consider a sample reaction A + B ↔ C + D

In the above reaction, add certain amount of the reactant B after the reaction has come to equilibrium. As per the above stated Le Chatelier's principle, addition of the reactants should shift the equilibrium towards formation of products i.e. forward direction so as to reduce the amount of B. But, this is possible only if more of B reacts with A to form products. Similarly, if some amount of product C or D is added then the equilibrium should shift to the backward direction leading to the consumption of C or D. In general it can be stated that increase in the concentration of the reactants shifts the equilibrium in the forward direction forming more amount of products and increase in the concentration of the products shifts the equilibrium to the backward direction forming reactants.

Consider the reaction between Ferric ions and the sulphocyanide ions.

Fe3+ + 3SCN- ↔ [Fe (CNS)3]2+

In the above reaction, the ferric ions are pale yellow in color, sulphocyanide ions are colorless and they react to form ferrosulphocyanide ions that are reddish brown in color.

Suppose some ferric salts capable of giving Fe3+ ions or potassium sulphocyanide solution capable of giving SCN- ions is added to the equilibrium, then the solution will become darker due to shifting of the equilibrium towards formation of more of [Fe (CNS)3]2+ ions. Thus, the equilibrium has shifted to the forward direction. Similarly, if a small amount of salt capable of giving [Fe (CNS)3]2+ ions is added to the above equilibrium, the darkness of solution decreases. This is due to dissociation of [Fe (CNS)3]2+ into Fe3+ SCN- ions. Thus, equilibrium has shifted to the backward direction favoring the formation of reactants.

Effect of change of temperature on chemical equilibrium:

Temperature effects only those reactions where either heat is absorbed or liberated i.e. endothermic and exothermic reactions respectively. In reactions that are effected by heat, if the forward reaction is endothermic, then the backward reaction will be exothermic and vice versa.

Consider the reaction

N2+3H2 ↔ 2NH3 Δ H = -22.1 k.cal

N2+3H2 ↔ 2NH3 Δ H = +22.1 k.cal

The above reaction of formation of ammonia is exothermic i.e. in the forward direction and the dissociation of ammonia to give nitrogen and hydrogen is endothermic i.e. in the backward direction. Now, according to Le Chateliers principle, increase of temperature should shift the reaction in the direction of absorbing the heat i.e. the backward direction. Similarly, decrease in temperature will shift the equilibrium in the forward direction.

Effect of change of pressure on chemical equilibrium:

The effect of pressure is seen only in gaseous reactions and those that proceed with a change in number of moles of the gases in both the directions. According to Le Chatelier's principle, increase of external pressure shifts the equilibrium in a direction that lowers the pressure. This means that the equilibrium shifts in a direction that forms less number of moles of the gas.

Let us see the influence of pressure on the reaction that result in increase in number of moles in the forward direction.

PCl5 ↔ PCl3 + Cl2

When the pressure is increased, the volume occupied by the system will decrease. This in turn, increases the number of moles present in a unit volume. To reduce this effect, the equilibrium will shift in a direction that decreases the number of moles per unit volume i.e. in this case it favors the backward reaction. Thus, increase in pressure favors the backward reaction and vice versa.

Here is another example that explains the effect of pressure on those reactions that favor a decrease in number of moles in the forward direction.

N2+3H2 ↔ 2NH3

As explained above, increase in pressure increases the number of moles in unit volume. To counteract the effect of pressure, the equilibrium will shift in a direction that favors the reduction in number of moles. In this case increase in pressure favors the forward reaction and vice versa.

From the above discussion, it is clearly understood that there is no effect of pressure on the reactions with same number of moles of reactants and products.

Example of such reactions is:

H2 + I2 ↔ 2HI

In this case two moles of reactants (one mole of hydrogen and one mole of iodine) combine to form two moles of hydrogen iodide.

Effect of catalyst on chemical equilibrium: As discussed in previous sessions, catalyst helps in attaining the equilibrium at a faster rate. Thus, the catalyst speeds up both the forward and backward reactions and helps in attaining the equilibrium quickly, without shifting the equilibrium in one or other direction.

Applications of Le Chatelier's Principle:

When we know the conditions that favor a particular reaction it is possible to obtain the desired results at a faster rate.

As discussed above Le Chatelier's principle explains the effect of pressure, temperature and concentration on the equilibrium state. Here are some applications of the principle.

Synthesis of ammonia:

N2+3H2 ↔ 2NH3

By applying the Le Chatelier's principle it is easily known that synthesis of ammonia is favored by increase in pressure, lowering the temperature and by using excess of reactants Nitrogen and Hydrogen.

Formation of sulphur trioxide

2SO2+O2 ↔ 2SO3

Formation of sulphur trioxide is favored by increase in pressure, low temperature and by using excess of reactants.

Dissociation of phosphorus pentachloride:

PCl5 ↔ PCl3 + Cl2

Dissociation of phosphorus pentachloride is favored by lowering of the pressure, high temperature and by using excess of phosphorus pentachloride.

Solved problems
  • Which of the following is true?
    • Rate of forward reaction increases with the increase in the concentration of the products
    • Rate of forward reaction increases with the increase in the concentration of the reactants
    • Equilibrium shifts in a direction that reduces the extent of pressure.
    • B and c
    Answer: d
  • What happens when increased pressure is applied on a reaction where the number of moles of products is less than the number of reactants?
    • Equilibrium shifts towards the formation of products so as to reduce the extent of applied pressure
    • Equilibrium shifts in the backward direction forming the reactants so as to reduce the extent of products.
    • Equilibrium is not affected by increasing the pressure.
    • Cannot be predicted.
    Answer: a
  • What is the effect of increase of temperature on a reaction at equilibrium which is exothermic in the forward direction and endothermic in backward direction?
    • The equilibrium shifts in the backward direction so as to consume the heat.
    • The equilibrium shits in the forward direction so as to liberate more heat.
    • Equilibrium is not affected by change of temperature.
    • Cannot be predicted.
    Answer: a

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