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 Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS).

### Step-by-Step Solution:

1. **Understanding Spontaneity**:
- A process is considered spontaneous if it occurs without external intervention. In thermodynamics, this is often determined by the sign of Gibbs free energy (ΔG). For a process to be spontaneous, ΔG must be less than 0 (ΔG < 0).

2. **Gibbs Free Energy Equation**:
- The relationship between Gibbs free energy, enthalpy, and entropy is given by the equation:
\[ ΔG = ΔH - TΔS \]
- Here, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.

3. **Analyzing the Terms**:
- For ΔG to be negative (spontaneous process), we can rearrange the equation:
\[ ΔH < TΔS \]
- This implies that either ΔH must be negative (exothermic process) or ΔS must be positive (increase in disorder).

4. **Entropy Change During Adsorption**:
- Adsorption of vapors involves the transition of gas molecules (which have high entropy due to their randomness) to a surface where they are more ordered. This results in a decrease in entropy (ΔS < 0) because the gas molecules lose their freedom of movement.

5. **Enthalpy Change During Adsorption**:
- The process of adsorption is typically exothermic, meaning that it releases heat. Therefore, ΔH is negative (ΔH < 0).

6. **Conclusion**:
- Since ΔS is negative and ΔH is negative, we can analyze the Gibbs free energy equation:
- The negative ΔH term can dominate the negative TΔS term at lower temperatures, making ΔG negative.
- Therefore, the adsorption of vapors on a clean surface is a spontaneous process primarily because the change in enthalpy (ΔH) is highly negative.

### Final Answer:
The adsorption of vapours on a clean surface is a spontaneous process because the change in enthalpy is highly negative.

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