Question

Which of the following is not a favourable condition for physical adsorption of gas over a solid surface?

• A. High pressure
• B. Negative $\Delta H$
• C. High critical temperature of gas
• D. High temperature

Answer 1

The correct option is D High temperature

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. Adsorption, therefore, is invariably an exothermic process. In other words, $\Delta H$ of adsorption is always negative.

The amount of gas adsorbed by a solid depends on the nature of gas. In general, easily liquefiable gases (i.e., with higher critical temperatures) are readily adsorbed as van der waals' forces are stronger near the critical temperatures.

Physical adsorption of gas is generally reversible.
$Solid+Gas\rightleftharpoons Gas/Solid+Heat$

In adsorption, more of gas is adsorbed when pressure is increased as the volume of the gas decreases (By Le-Chateliers's principle).
Also, the gas can be desorbed/ removed from surface by decreasing pressure.

Since adsorption process is exothermic, with increase in temperature, the rate of desorption increases. Hence, high temperature is not favourable for physical adsorption.

Hence, option D is the right answer.