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

The Correct Option is B

To determine the correct statement for the spontaneous adsorption of a gas, we need to consider the thermodynamics involved in the process. Adsorption refers to the process by which atoms, ions, or molecules from a gas adhere to a surface. The spontaneity of an adsorption process can be examined using Gibbs free energy, which is defined by the equation:

\[\Delta G = \Delta H - T \Delta S\]

Where:

• \(\Delta G\) is the change in Gibbs free energy.
• \(\Delta H\) is the change in enthalpy.
• \(T\) is the temperature in Kelvin.
• \(\Delta S\) is the change in entropy.

A process is spontaneous if \(\Delta G\) is negative. The spontaneity depends on the signs and magnitudes of \(\Delta H\) and \(\Delta S\) .

During the adsorption of a gas, the randomness of the molecules decreases as they adhere to the surface, suggesting that \(\Delta S\) is usually negative (the system becomes more ordered).

For \(\Delta G\) to be negative (indicating spontaneity), if \(\Delta S\) is negative, \(\Delta H\) should also be negative and sufficiently large to overcome the positive term \(-T\Delta S\) . This means \(\Delta H\) should be highly negative, leading to a release of heat and making the process exothermic.

Given these conditions, the correct statement is: " \(\Delta S\) is negative and therefore, \(\Delta H\) should be highly negative."