Question

At the equilibrium position in the process of adsorption_________
A) $\Delta H>0$
B) $\Delta H=T\Delta S$
C) $\Delta H>T\Delta S$
D)$\Delta H< T\Delta S$

Answer 1

The adsorption is considered to be a surface phenomenon. to make the adsorption process at the equilibrium the Gibbs free energy should be equal to zero. The process of adsorption is an exothermic process so the change in enthalpy is always negative in it.

Complete step-by-step answer:
When the molecules of the substance are being attracted or retained at the liquid surface or the solid surface so that the formation of high concentration of molecules occurs, that process is called adsorption. Adsorbate is the substance which has been adsorbed on the surface and the adsorbent is the substance on which the substance is adsorbed.

Desorption is the reverse process of adsorption in which the particles are tend to be removed from the surface. When the process of adsorption is occuring there is the decrease in the residual forces of the surface that means there is the decrease in the surface energy which appears as the heat. This concludes that it is an exothermic process. So the value of $\Delta H$ will be negative. As we know that the entropy changes during adsorption and adsorption equilibrium.

So when the molecules of the adsorbate are held on the solid adsorbent surface then there will be the decrease in the entropy, so the $\Delta S$ will be negative. As we know the equation $\Delta G=\Delta H-T\Delta S$, here for the adsorption process to occur the $\Delta G$must be negative. We know for the $\Delta G$to be negative we need to have $\Delta S$ also negative.

And the value of $\Delta G$ will be negative if the $\Delta H$ is also negative so for this the $\Delta H>T\Delta S$ should be occurring. On the proceeding of adsorption $\Delta H$ decreases and $T\Delta S$ will be increased till the $\Delta H$ become equal to the $T\Delta S$ so we get $\Delta G$=0.

So the correct answer is option ‘C’.

Note: There is physical adsorption in which the adsorbate and adsorbent have weak Vander Waal forces between them. while in chemical adsorption the strong chemical forces are present between adsorbent and adsorbate. The application of adsorption is purification of water, removal of moisture and air pollution masks.