Question

Why does Le chatelier's principle fail in case of chemisorption. Chemisorption is also and exothermic process but it increases with increase in temperature

Answer 1

Like all chemical reactions, some activation energy is required for chemisorption. As temperature is increased, the molecules of the adsorbate gain energy and become equal to activation energy so that proper bonds are formed with the adsorbent molecules. Therefore, initially, the amount of gas adsorbed increases with rise in temperature. Further increase of temperature will increase the energy of the molecules which have already been adsorbed. This would increase the rate of desorption, thereby decreasing the extent of adsorption. According to Le Chatelier's principle, increase in temperature decreases adsorption and vice versa. During chemisorption, there is a tendency of the formation a compound which requires some actvation energy which means that chemisorption first increases with the increase in temperature till the activated complex is formed.