Spontaneous adsorption of a gas on a solid surface is exothermic process because

Options

A. Entropy decreases during process
B. It is exothermic
C. `T triangle S` is negative
D. It occurs only at high temperature

Answer

To solve the question regarding why the spontaneous adsorption of a gas on a solid surface is an exothermic process, we can break it down into several steps:

### Step-by-Step Solution:

1. **Understanding Spontaneous Adsorption**:
- Adsorption is the process where gas molecules adhere to the surface of a solid. For this process to be spontaneous, it must occur without the need for external energy input.

2. **Gibbs Free Energy (ΔG)**:
- The spontaneity of a process is determined by the change in Gibbs free energy (ΔG). For a process to be spontaneous, ΔG must be negative (ΔG < 0).

3. **Entropy Change (ΔS)**:
- During adsorption, gas molecules transition from a more random (disordered) state in the gas phase to a less random (ordered) state when they adhere to the solid surface. This results in a decrease in entropy (ΔS < 0).

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

5. **Analyzing the Equation**:
- Since ΔS is negative (due to decreased randomness), the term \(TΔS\) will also be negative (as T is always positive).
- Therefore, if ΔG is negative and \(TΔS\) is negative, it implies that ΔH must also be negative for the equation to hold true.

6. **Conclusion**:
- A negative ΔH indicates that the process is exothermic. Thus, the spontaneous adsorption of a gas on a solid surface is an exothermic process because the decrease in entropy (ΔS < 0) leads to a negative ΔG, which in turn necessitates a negative ΔH.

### Final Answer:
The spontaneous adsorption of a gas on a solid surface is an exothermic process because the entropy of the system decreases (ΔS < 0), leading to a negative Gibbs free energy change (ΔG < 0), which indicates that the enthalpy change (ΔH) must also be negative, signifying an exothermic reaction.

---