Adsorption is the tendency of accumulation of molecular species at the surface of solid or liquid. Depending upon the nature of bonds or forces of attraction between adsorbate and adsorbent. It is classofied between physisorption and chemisorption.
Which of the following statements are correct? (More than one correct)

Options

A. Adsorption always leads to a decrease in enthalpy and entropy of the system.
B. Adsorption arises due to unsaturation in the enthalpy of valency force of atoms or molecules on the surface.
C. Adsorption increases with rise in temperature.
D. Adsorption decreases the surface energy.

Answer

To determine which statements regarding adsorption are correct, we will analyze each statement based on the principles of surface chemistry.

### Step-by-Step Solution:

1. **Understanding Adsorption**:
- Adsorption is the process where molecular species accumulate at the surface of a solid or liquid. It can be classified into two types: physisorption (physical adsorption) and chemisorption (chemical adsorption).

2. **Analyzing Statement A**:
- **Statement A**: "Adsorption always leads to a decrease in enthalpy and entropy of a system."
- **Analysis**:
- Adsorption is generally an exothermic process, meaning it releases heat, which results in a decrease in enthalpy (ΔH < 0).
- However, the entropy (ΔS) of the system typically decreases because the adsorbate molecules become more ordered when they adhere to the surface, leading to a decrease in randomness. Thus, ΔS < 0.
- **Conclusion**: Statement A is **correct**.

3. **Analyzing Statement B**:
- **Statement B**: "Adsorption arises due to unsaturation in the enthalpy of valency force of atoms or molecules on the surface."
- **Analysis**:
- The statement correctly identifies that the residual forces (unbalanced forces) at the surface of the adsorbent lead to adsorption. These forces create a tendency for molecules to adhere to the surface to satisfy their valency.
- **Conclusion**: Statement B is **correct**.

4. **Analyzing Statement C**:
- **Statement C**: "Adsorption increases with rising temperature."
- **Analysis**:
- For physisorption, as temperature increases, the kinetic energy of the molecules increases, which generally leads to a decrease in adsorption because the molecules are more likely to escape from the surface.
- For chemisorption, although there may be an initial increase in adsorption with temperature, ultimately, both types of adsorption typically decrease with rising temperature.
- **Conclusion**: Statement C is **incorrect**.

5. **Analyzing Statement D**:
- **Statement D**: "Adsorption decreases the surface energy."
- **Analysis**:
- When adsorption occurs, the unbalanced forces at the surface are balanced by the adsorbate, leading to a decrease in surface energy. This is a thermodynamically favorable process.
- **Conclusion**: Statement D is **correct**.

### Final Conclusion:
The correct statements regarding adsorption are **A, B, and D**.