Question

Which of the following statements is correct for the spontaneous adsorption of a gas?

• A. $\Delta S$ is negative and therefore, $\Delta H$ should be highly positive
• B. $\Delta S$ is negative and therefore, $\Delta H$ should be highly negative
• C. $\Delta S$ is positive and therefore, $\Delta H$ should be negative
• D. $\Delta S$ is positive and therefore, $\Delta H$ should also be highly positive

Answer 1

Answer: (B)

$\Delta S$ [change in entropy) and $\Delta H$ [change in enthalpy] are related by the equation $ $ $\Delta G$= $\Delta H$ - T $\Delta S$ [Here, $\Delta G$ = change in Gibbs free energy) For adsorption of a gas, $\Delta S$ is negative because randomness decreases. Thus, in order to make $\Delta G$ negative [for spontaneous reaction], $\Delta G$ must be highly negative because reaction is exothermic. Hence, for the adsorption of a gas, if $\Delta S$ is negative, therefore, $\Delta H$ should be highly negative.