Adsorpton of gases on solid surface is generally exothermic because :

Options

A. enthalpy is positive
B. entropy decreases
C. entropy increases
D. free energy increases

Answer

To understand why the adsorption of gases on solid surfaces is generally exothermic, we can break down the reasoning into clear steps:

### Step-by-Step Solution:

1. **Definition of Adsorption**:
- Adsorption is the process by which gas molecules adhere to the surface of a solid. This is different from absorption, where a substance is taken up into the volume of another.

2. **Change in Entropy (ΔS)**:
- When gas molecules are adsorbed onto a solid surface, they lose their freedom of movement. In the gaseous state, molecules are free to move in three dimensions, which corresponds to a higher entropy (disorder).
- Upon adsorption, the gas molecules become localized on the solid surface, leading to a decrease in their freedom of movement and thus a decrease in entropy (ΔS < 0).

3. **Thermodynamic Consideration**:
- The Gibbs free energy change (ΔG) for a process is given by the equation:
\[ ΔG = ΔH - TΔS \]
- For a process to be spontaneous, ΔG must be negative. In the case of adsorption, since ΔS is negative (entropy decreases), the term \( -TΔS \) becomes positive.

4. **Exothermic Nature (ΔH)**:
- To compensate for the negative contribution from \( -TΔS \), the enthalpy change (ΔH) must be negative (exothermic) to ensure that ΔG remains negative.
- This means that energy is released when gas molecules are adsorbed onto the solid surface.

5. **Conclusion**:
- Therefore, the adsorption of gases on solid surfaces is generally exothermic because the process involves a decrease in entropy (ΔS < 0), and the system compensates for this by releasing energy (ΔH < 0).

### Final Answer:
The adsorption of gases on solid surfaces is generally exothermic because the entropy of the system decreases (ΔS < 0) when gas molecules are adsorbed onto the solid.

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