Amongst the following statements regarding adsorption, those that are valid are:
(a) $$\Delta H$$ 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.

We start by recalling that adsorption is an exothermic surface phenomenon. Because the molecules that get adsorbed release energy when they attach to the surface, the enthalpy change of adsorption $$\Delta H$$ is negative at the beginning of the process.

As more and more molecules occupy the fixed number of surface sites, the strongest sites get filled first and the subsequently adsorbed molecules occupy weaker sites. According to the principle that the most stable interactions occur first, the magnitude of the heat evolved gradually falls. Mathematically we write this qualitative idea as

$$\lvert \Delta H\rvert_{\text{initial}} \; > \; \lvert \Delta H\rvert_{\text{later}},$$

or in words, “the numerical value of $$\Delta H$$ becomes less negative as adsorption proceeds.” This directly validates statement (a).

Next, let us compare ammonia gas, $$\mathrm{NH_3},$$ with nitrogen gas, $$\mathrm{N_2}.$$ The ‘ease’ of adsorption for a gas on a given solid generally follows the order of the critical temperature $$T_c,$$ which itself is linked to the strength of intermolecular attractions and polarity. Ammonia is highly polar and has $$T_c = 405\ \text{K},$$ whereas nitrogen is non-polar and has $$T_c = 126\ \text{K}.$$ A higher critical temperature implies stronger van der Waals or dipole forces and therefore a higher tendency to get adsorbed. Hence, for the same adsorbent, ammonia will indeed be adsorbed to a greater extent than nitrogen. So statement (b) is also correct.

Statement (c) says, “On adsorption, the residual force acting along the surface of the adsorbent increases.” The residual force referred to here is nothing but surface tension. When molecules from the gas phase get attached to the surface, they partially satisfy the unbalanced surface forces; consequently, the net residual force or surface tension decreases, not increases. Therefore statement (c) is false.

Finally, statement (d) asserts that “With increase in temperature, the equilibrium concentration of adsorbate increases.” Since adsorption is exothermic, Le Chatelier’s principle tells us that raising the temperature will shift the equilibrium so as to counteract the added heat, i.e., by desorbing molecules. Thus the amount of adsorbate on the surface actually falls when temperature is increased. Hence statement (d) is also false.

Collecting our results: statements (a) and (b) are valid, while (c) and (d) are not.

Hence, the correct answer is Option C.