Question

During the adsorption of gas on a solid

• A. ΔG < O, ΔH < O, ΔS > O
• B. ΔG < O, ΔH < O, ΔS < O
• C. ΔG > O, ΔH < O, ΔS < O
• D. ΔG < O, ΔH > O, ΔS < O

Answer 1

The Correct Option is B

During adsorption, the gas molecules are attracted to the surface of the solid, which results in the formation of a bond between the gas and the solid. Adsorption is an exothermic process (releases heat), which implies that the change in enthalpy \( \Delta H \) is negative. The entropy \( \Delta S \) of the system decreases because the gas molecules become more ordered as they are adsorbed onto the solid surface, resulting in a decrease in randomness. The Gibbs free energy change \( \Delta G \) for the adsorption process is negative, indicating that the process is spontaneous. This can be explained by the relation: \[ \Delta G = \Delta H - T\Delta S \] Since both \( \Delta H < 0 \) and \( \Delta S < 0 \), the process can still be spontaneous if the enthalpy change is sufficiently negative to overcome the negative entropy change.

The correct option is (B): ΔG < O, ΔH < O, ΔS < O

Answer 2

In the process of adsorption of a gas on a solid, several thermodynamic properties are involved:

1. Enthalpy Change ($\Delta H$): 
  - The adsorption process is exothermic, which means that energy is released when the gas molecules are adsorbed onto the surface of the solid. As a result, the enthalpy change is negative: \(\Delta H < 0\).
  - This exothermic nature is due to the intermolecular forces (such as Van der Waals forces) between the gas molecules and the solid surface that are stronger than the interactions among the gas molecules themselves.

2. Entropy Change ($\Delta S$): 
  - Entropy, which measures the disorder or randomness of a system, typically decreases during adsorption. This is because gas molecules become more ordered when they adhere to the surface of the solid, resulting in a negative change in entropy: \(\Delta S < 0\).
  - The gas molecules, initially in a more disordered, free state, become localized at the solid surface, reducing the randomness or freedom of movement of the gas particles.

3. Gibbs Free Energy Change ($\Delta G$):
  - The spontaneity of the adsorption process is determined by the Gibbs free energy change, given by the equation:
  \(\Delta G = \Delta H - T\Delta S\)
  - Since $\Delta H$ is negative (exothermic) and $\Delta S$ is negative (decrease in disorder), the term \(- T\Delta S\) is positive, but the overall value of $\Delta G$ remains negative for the adsorption process to be spontaneous at lower temperatures. Thus, $\Delta G < 0$ for spontaneous adsorption.

Therefore, the correct answer is \(\Delta G < 0\), \(\Delta H < 0\), \(\Delta S < 0\).