The adsorption of vapours on a clean surface is a spontaneous process because

Options

A. change in the entropy of the process is highly positive
B. enthalpy change is highly positive
C. change in entropy is zero
D. change in enthalpy is highly negative

Explanation

To determine why the adsorption of vapors on a clean surface is a spontaneous process, we can analyze the thermodynamic principles involved, particularly focusing on the Gibbs free energy change (ΔG) and the relationship between enthalpy (ΔH) and entropy (ΔS).

### Step-by-Step Solution:

1. **Understanding Spontaneity**:
- A process is spontaneous if the change in Gibbs free energy (ΔG) is less than zero (ΔG < 0). The relationship is given by the equation:
\[ \Delta G = \Delta H - T \Delta S \]
- Here, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.

2. **Analyzing the Entropy Change (ΔS)**:
- When vapors (which are in a gaseous state) are adsorbed onto a surface, the randomness or disorder of the system decreases.
- Gases have higher entropy due to their random molecular motion. When they are adsorbed onto a surface, they lose this freedom, leading to a decrease in entropy.
- Therefore, for the adsorption process, ΔS is negative (ΔS < 0).

3. **Analyzing the Enthalpy Change (ΔH)**:
- The adsorption process is typically exothermic, meaning that it releases heat. This results in a negative change in enthalpy (ΔH < 0).
- A negative ΔH contributes favorably to making ΔG less than zero.

4. **Evaluating the Gibbs Free Energy Equation**:
- Substituting the signs of ΔH and ΔS into the Gibbs free energy equation:
\[ \Delta G = \Delta H - T \Delta S \]
- Since ΔH is negative and ΔS is negative, the term \( -T \Delta S \) becomes positive (because multiplying two negative values results in a positive value).
- Thus, we can say that:
\[ \Delta G = \text{(negative)} - \text{(positive)} \]
- This indicates that ΔG can indeed be less than zero, confirming that the process is spontaneous.

5. **Conclusion**:
- Therefore, the adsorption of vapors on a clean surface is a spontaneous process primarily because the enthalpy change (ΔH) is highly negative, which dominates the negative contribution from the entropy change (ΔS), leading to a negative Gibbs free energy (ΔG < 0).