Question

Assuming adsorption to be a spontaneous process, show thermodynamically that it is always an exothermic process.

Answer 1

Assuming adsorption is spontaneous, the Gibbs free energy change is:

ΔG = ΔH − TΔS < 0

Here, ΔG is negative for spontaneity, and T is absolute temperature. Adsorption decreases the entropy of the system because gas molecules in free state become more ordered when adsorbed on the surface, so:

ΔS < 0

Since ΔS is negative, the term −TΔS is positive. For ΔG to be negative, the enthalpy change ΔH must be negative and of greater magnitude to overcome the positive −TΔS term. Therefore,

ΔH < 0

which means adsorption is an exothermic process thermodynamically.