Question

Which of the follwing is true in respect of chemical adsorption?
(Here, $\Delta H,\Delta S\text{and}\Delta G$ signifies usual meanings)

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

Answer 1

The correct option is B $\Delta H<0,\Delta S<0,\Delta G<0$

Chemical adsorption always has a negative enthalpy value because it involves chemical bond formation.
$\Delta H<0$
Due to the bond formation, there is a decrease in randomness, so the entropy change of adsorption is negative i.e., $\Delta S<0$
For spontaneous process, the Gibbs free energy must be negative $(\Delta G<0)$. As we know, adsorption is a spontaneous process, hence $\Delta G<0$
Therefore, correct option is B.