Once an equilibrium is established, no further change is apparent as long as the external conditions remain unchanged.

On changing the external conditions, whether the equilibrium will shift towards the reactants or the products can be predicted by the application of Le-Chatelier’s principle which states that

When a stress (change in concentration, temperature or total pressure) is applied to a system at equilibrium, the system readjusts so as to relieve or offset the stress.

In the synthesis of Ammonia (Haber’s process)

1 vol          3 vol                2 vol

1. Effect of concentration : To increase the yield of NH3, that is to favor the forward reaction, concentration of N2 or preferably H2 should be increased.

2. Effect of pressure : As there is decrease in volume (from 4 to 2), the yield of NH3 increases by increasing pressure.

3. Effect of temperature : The reaction being exothermic, low temperature favors the reaction. However at a low temperature, the rate of formation of NH3 is very slow, hence the reaction is carried out at optimum temperature of 673 - 773 K.

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Index

Chapter 12