Catalysts and Kc

Catalysts have no effect on the value of K_c. They affect both the rates of the forward and reverse reactions by the same amount. Catalysts affect the rate at which equilibrium is reached.A catalyst provides an alternative reaction pathway with a lower activation energy. The same pathway is used in both directions, so the forward and reverse rates increase by the same factor. K_c depends only on temperature, so a catalyst does not change K_c, the equilibrium concentrations or the position of equilibrium, and it does not change the yield. It only reduces the time taken to reach equilibrium.This matters in industry. In the Haber processN₂(g) + 3H₂(g) ⇌ 2NH₃(g)   ΔH = −92 kJ mol^-1the forward reaction is exothermic, so K_c is largest at low temperature, but at low temperature the rate is very slow. A compromise temperature of about 450 °C is used: the yield is lower, but equilibrium is reached quickly. An iron catalyst gives a useful rate without raising the temperature further, so K_c is not reduced any further. The catalyst saves time and energy, but it cannot increase the equilibrium yield.