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. $\Delta H=0$

Answer 1

The correct option is B $\Delta H=T\Delta S$

• The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.
  
• During adsorption, there is always a decrease in residual forces of the surface, i.e., there is a decrease in surface energy which appears as heat. Hence, adsorption is an exothermic process.
• So the value of $\Delta H$ is -ve
• On the basis of the equation, $\Delta G=\Delta H-T\Delta S$ can be negative if $\Delta H$ has sufficiently high negative value as $\Delta H-T\Delta S$ is positive. Thus, in an adsorption process, which is spontaneous, a combination of these two factors makes $\Delta G$ negative. As the adsorption proceeds, $\Delta H$ becomes less and less negative ultimately $\Delta H$ becomes equal to $T\Delta S$ and $\Delta G$ becomes zero. At this state, equilibrium is attained.
• Thus at equilibrium, $\Delta H=T\Delta S$