JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Surface Chemistry

Adsorption binds gas molecules to a solid surface by satisfying the unbalanced attractive forces of the surface atoms, and because new attractive interactions are formed, energy is released and $\Delta H<0$. The adsorbed molecules give up free translation in three dimensions for motion restricted to the surface, so the disorder of the system falls and $\Delta S<0$. The negative enthalpy change is large enough to keep $\Delta G=\Delta H-T\Delta S$ negative at ordinary temperatures, which is why adsorption occurs at all. For the adsorption of a gas on a solid surface, $\Delta H<0$ and $\Delta S<0$.