Amongst the following statements regarding adsorption, those that are valid are
(a) ΔH becomes less negative as adsorption proceeds
(b) On a given adsorbent, ammonia is adsorbed more than nitrogen gas
(c) On adsorption, the residual force acting along the surface of the adsorbent increases
(d) With increase in temperature, the equilibrium concentration of adsorbate increases

Solution

Chemistry ma’am · 2 steps

Adsorption uses up free surface forces and gives out heat; test each statement against that one picture.

1. The picture. Surface particles of an adsorbent have unbalanced, residual attractive forces. Adsorption satisfies these forces and releases energy, so it is exothermic and Δ H is negative.

   Statement	What really happens	Valid?
   (a) Δ H becomes less negative	As sites fill, fewer residual forces remain, so less heat is released per mole adsorbed	Yes
   (b) NH₃ adsorbed more than N₂	NH₃ has a much higher critical temperature (about 405 K vs 126 K), so it liquefies and adsorbs more easily	Yes
   (c) Residual force increases	Adsorbate molecules use up the residual forces, so they decrease	No
   (d) Adsorbate concentration rises with temperature	Adsorption is exothermic, so by Le Chatelier's principle, heating shifts the equilibrium back and adsorption decreases	No

2. So. Only statements (a) and (b) are valid, which is option (b).

The idea to keep. Adsorption is exothermic and uses up surface forces. So heat lowers it, residual forces fall, and gases that liquefy more easily (higher critical temperature) adsorb more.

Answer from the NTA final answer key. Solution by PrepWiser.