Question

Which one of the following characteristics is not correct for physical adsorption?

• A. Adsorption is spontaneous
• B. Both enthalpy and entropy of adsorption
• C. Adsorption on solid is reversible
• D. Adsorption increases with increase in temperature

Answer 1

Answer: (D)

Adsorption increases with increase in temperature

Physical adsorption decreases with an increase in temperature because an increase in temperature leads to an increase in kinetic energy of molecules and hence, their desorption. Since physical adsorption is an exothermic process, it occurs more readily at lower temperatures and decreases with an increase in temperature (Le-Chatelier's Principle). If the adsorption is a spontaneous phenomenon, it should be exothermic, at least for adsorption from the gaseous phase.

A molecule in the gas phase will have an entropy higher than in the adsorbed state, then the $\Delta S$ should be negative, the only way to have a $\Delta G$ negative (spontaneous) is to have a $\Delta H$ negative (exothermic process). Therefore, the process will be spontaneous $(\Delta G=-ve)$ if the temperature is low.