Question

Which one of the following is correct for the adsorption of a gas at a given temperature on a solid surface ?

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

Answer 1

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

Adsorption is the accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.
Generally, there is a unbalanced/residual force on the surface of the particle in which adsorption takes place.
During adsorption there is decrease in residual forces or surface energy. Thus, the energy is released, so the heat of adsorption is negative. Hence, adsorption is always exothermic.
$\therefore$
$\Delta H=-ve$

Also freedom of movement of gaseous adsorbate particles is restricted due to adsorption. Hence, entropy will decrease during adsorption.
$\Delta S=-ve$