Amongst the following statements regarding adsorption, those that are valid are:

a. 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.

• Adsorption is an exothermic process, as the process of adsorption progresses the number of active sites present on the adsorbent decreases and hence less heat is evolved. This leads to a less negative value of $\mathrm{\Delta H}$. Ammonia has a higher force of attraction on adsorbent due to its polar nature and nitrogen gas is not polar. As the adsorption increases, residual forces acting along the surface of the adsorbent decrease.

Adsorption is an exothermic process, so on increasing temperature, the concentration of adsorbate particles decreases, to shift the equilibrium backward.

Hence, the correct answer is option B) (a) and (b).