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 about adsorption are correct, we need to analyze each statement based on the principles of physisorption and chemisorption, as well as the thermodynamic aspects of adsorption.

### Step-by-Step Solution:

1. **Understanding Adsorption**:
- Adsorption is the process where molecules (adsorbate) accumulate on the surface of a solid or liquid (adsorbent).
- It can be classified into two types: physisorption (weak van der Waals forces) and chemisorption (strong chemical bonds).

2. **Analyzing the First Statement**:
- **Statement**: "Adsorption always leads to decrease in enthalpy and entropy of the system."
- **Analysis**:
- During adsorption, the system typically releases energy (exothermic process), leading to a decrease in enthalpy (ΔH < 0).
- However, the entropy (ΔS) may decrease because the adsorbate molecules become more ordered at the surface compared to their gaseous state. Thus, this statement is **correct**.

3. **Analyzing the Second Statement**:
- **Statement**: "Adsorption arises due to unsaturation in the enthalpy of valency forces of atoms or molecules on the surface."
- **Analysis**:
- This statement refers to the presence of unoccupied valence orbitals or unsaturated bonds on the surface of the adsorbent, which can attract adsorbate molecules. This is a fundamental reason for adsorption. Thus, this statement is **correct**.

4. **Analyzing the Third Statement**:
- **Statement**: "Adsorption increases with rising temperature."
- **Analysis**:
- Generally, for exothermic processes like adsorption, an increase in temperature tends to decrease the extent of adsorption because the system favors higher entropy states. Therefore, this statement is **incorrect**.

5. **Analyzing the Fourth Statement**:
- **Statement**: "Adsorption decreases the surface energy."
- **Analysis**:
- When adsorption occurs, the surface energy of the adsorbent decreases because the adsorbate molecules occupy surface sites and stabilize the surface. Thus, this statement is **correct**.

### Conclusion:
The correct statements regarding adsorption are:
- Statement 1: Correct
- Statement 2: Correct
- Statement 3: Incorrect
- Statement 4: Correct

### Final Answer:
The correct statements are 1, 2, and 4.

---