Question

For Adsorption phenomenon
A) \[\Delta H = + ve,\Delta S = - ve\]
B) \[\Delta H = - ve,\Delta S = + ve\]
C) \[\Delta H = - ve,\Delta S = - ve\]
D) \[\Delta H = + ve,\Delta S = + ve\]

Answer 1

Hint:The process of adsorption involves the accumulation of the substance on the surface of the other substance. Surface concentration is much higher than bulk concentration. Adsorbate is the substance adsorbed and the adsorbent is the substance that provides surface for the adsorption.

Complete answer:
Gases such as hydrogen, nitrogen or oxygen gas are adsorbed on the surface of catalysts such as platinum, palladium and nickel. We can classify adsorption into physical adsorption and chemical adsorption.

i)Physical adsorption : It is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption.
ii) Chemical adsorption : It involves the transfer, exchange, or sharing of electrons between adsorbates and adsorbents (atoms or molecules), and the adsorption of adsorbates on adsorbents is due to the formation of chemical bonds between them.

Due to adsorption, the residual forces on the surface of the adsorbent decreases. Due to this, the surface energy of the adsorbent decreases. Hence, adsorption is always exothermic with negative value of the enthalpy change.
For a spontaneous process, the Gibbs free energy change is negative.
\[\Delta G = - ve\]
Adsorption is a spontaneous process.
So for adsorption \[\Delta G = - ve\]
But, \[\Delta G = \Delta H - T\Delta S\]
Hence, for adsorption, \[\Delta H - T\Delta S = - ve\]

From this, you can say that \[\Delta S\] can be either positive or negative. But in reality, \[\Delta S\] is negative during adsorption. This is because, during adsorption, the randomness or disorder decreases as the adsorbate and adsorbent are confined over a surface and cannot move freely in space. Hence, for adsorption phenomenon \[\Delta H = - ve,\Delta S = - ve\]

Hence, the correct answer is the option (C).

Note:
i) Adsorption is the surface phenomenon whereas absorption is the bulk phenomenon.
ii) Adsorption is a spontaneous process whereas absorption is a non-spontaneous process.
iii) Adsorption is a non uniform process. Absorption is a uniform process.