Question

Which one of the following characteristics is associated with adsorption ?

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> NEET 2016

• A. ΔG and ΔS are negative but ΔH is positive
• B. ΔG, ΔH and ΔS all are negative
• C. ΔG is negative but ΔH and ΔS are positive
• D. Δ G and ΔH are negative but ΔS is positive

Answer 1

The correct option is B ΔG, ΔH and ΔS all are negative

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.
$\therefore$
$\Delta H=-ve$
$\Delta G=\Delta H-T\Delta S$
$\Delta G+T\Delta S=\Delta H$
$\Delta G+T\Delta S=-ve$

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

$\Delta G=\Delta H-T\Delta S$
For adsorption to be spontaneous, $\Delta G$ must be negative.
$\Delta H$ is sufficiently high negative, in an adsorption process which makes $\Delta G$ negative.

Hence, option (a) is correct.