Question

Which one of the following is not applicable to the phenomenon of adsorption?

• A. $\Delta H>0$
• B. $\Delta G<0$
• C. $\Delta S<0$
• D. $\Delta H<0$

Answer 1

The correct option is A $\Delta H>0$

The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.

• During adsorption, there is always a decrease in residual forces of the surface, i.e., there is the decrease in surface energy which appears as heat. Adsorption, therefore, is invariably an exothermic process. In other words, $\Delta H$ of adsorption is always negative.
  
• When a gas is adsorbed, the freedom of movement of its molecules become restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e., $\Delta S$ is negative.
  
• Adsorption is thus accompanied by the decrease in enthalpy as well as a decrease in entropy of the system. For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, $\Delta G$ must be negative.
  
• Hence, the option A is the right answer.