(N/A) Adsorption is always exothermic. This can be explained in two ways:
$(i)$ Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent,which is released as heat. Therefore,adsorption is always exothermic.
$(ii)$ From the thermodynamic perspective,for a process to be spontaneous,the Gibbs free energy change,$\Delta G$,must be negative. The relationship is given by $\Delta G = \Delta H - T\Delta S$. When a gas is adsorbed on a solid surface,its movement is restricted,leading to a decrease in entropy,i.e.,$\Delta S$ is negative. Since $\Delta G$ must be negative and $\Delta S$ is negative,$\Delta H$ must be negative to satisfy the equation. Hence,adsorption is always exothermic.