Question

Which of the following is not a favourable condition for physical adsorption?

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

Answer 1

Answer: (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.
  
• In adsorption, more of gas is adsorbed when pressure is increased as the volume of the gas decreases (By Le–Chateliers’s principle) and the gas can be removed by decreasing pressure. Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature.
  
• Hence, option D is the right answer.