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8.CHEMICAL EQUILIBRIUM eLearn.Punjab

The equilibrium constant expression for above reaction can be written as

Kc = [BiOCl][HCl]2
[BiCl3 ][H 2O]

Aqueous solution of BiCl3 is cloudy, because of hydrolysis and formation of BiOCl. If a small
amount of HCl is added to this solution, it will disturb the equilibrium and force the system to move
in such a way so that effect of addition of HCl is minimized. The reaction will move in the backward
direction to restore the equilibrium again and a clear solution will be obtained. However, if water
is added to the above solution the system will move in the forward direction and the solution will
again become cloudy. The shifting of reaction to forward and backward direction by disturbing the
concentration is just according to Le-Chatelier’s principle.
So, in general, we conclude that addition of a substance among the reactants, or the removal
of a substance among the products at equilibrium stage disturbs the equilibrium position and
reaction is shifted to forward direction. Similarly, the addition of a substance among the products or
the removal of a substance among the reactants will derive the equilibrium towards the backward
direction. Removing one of the products formed can therefore increase the yield of a reversible
reaction. The value of K however remains constant. This concept is extensively applied in common
ion effect and follows the Le-Chatelier’s principle.

(b) Effect of Change in Pressure or Volume

The change in pressure or volume are important only for the reversible gaseous reactions
where the number of moles of reactants and products are not equal. Le-Chatelier’s principle
plays an important role, to predict the position and direction of the reaction. Take the example of
formation of SO3 gas from SO2 gas and O2 gas.

2SO2 (g) + O2 (g)  2SO3(g)

This gas phase reaction proceeds with the decrease in the number of moles and hence
decreases in volume at equilibrium stage. When the reaction approaches the equilibrium stage,
the volume of the equilibrium mixture is less than the volume of reactants taken initially. If one
decreases the volume further at equilibrium stage, the reaction is disturbed. It will move to the
forward direction to minimize the effect of disturbance. It establishes a new equilibrium position
while Kc remains constant. The reverse happens when the volume is increased or pressure is
decreased at equilibrium stage.

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