Question

State and explain Le Chatelier’s principle with reference to :

1. change in temperature 

2. change in concerntration.

Answer 1

Statement : 

When a system at equilibrium is subjected to a change in any of the factors determining the equilibrium conditions of a system, system will respond in such a way as to minimize the effect of change.

1. Change in temperature :

• Consider the equilibrium reaction, PCl_5(g) ⇌ PCl_3(g)  + Cl_2(g)  + 92.5 kJ
• The forward reaction is exothermic. According to Le Chatelier’s principle an increase in temperature shifts the position of equilibrium to the left.
• The reverse reaction is endothermic. An endothermic reaction consumes heat. Therefore, the equilibrium must shift in the reverse direction to use up the added heat (heat energy converted to chemical energy).
• Thus, an increase in temperature favours formation of PCl₅ while a decrease in temperature favours decomposition of PCl₅.

 2. Change in concentration :

• Consider reversible reaction representing production of ammonia (NH₃). N_2(g) + 3H_2(g) ⇌ 2NH_3(g) + Heat
• According to Le Chatelier’s principle, when H₂ or N₂ is added to equilibrium, the effect of addition of H₂ or N₂ or is reduced by shifting the equilibrium from left to right so that the added N₂ or H₂ is consumed.
• The forward reaction occurs to a large extent than the reverse reaction until the new equilibrium is established. As a result, the yield of NH₃is increases.
• In general, if the concentration of one of the species in equilibrium mixture is increased, the position of equilibrium shifts in the opposite so as to reduce the concentration of this species. However, the equilibrium constant remains unchanged.