Which of the following statement is not correct?

Options

A. Physical adsorption is due to van der Waal’s force
B. Chemical adsorption decreases at high temperature and low pressure
C. Physical adsorption is reversible
D. Heat of adsorption for chemical adsorption is generally greater than that of physical adsorption.

Answer

To determine which statement is not correct regarding adsorption, let's analyze each option step by step.

### Step 1: Analyze Statement 1
**Statement:** Physical adsorption is due to Van der Waals forces.

**Explanation:**
- Physical adsorption (or physisorption) occurs due to weak Van der Waals forces between the adsorbate (the substance being adsorbed) and the adsorbent (the surface on which adsorption occurs).
- This statement is correct as it accurately describes the nature of physical adsorption.

### Step 2: Analyze Statement 2
**Statement:** Chemical adsorption decreases at high temperature and low pressure.

**Explanation:**
- Chemical adsorption (or chemisorption) involves the formation of strong chemical bonds and usually requires a certain amount of activation energy.
- Initially, as temperature increases, the rate of chemisorption can increase because more energy is available for the reaction. However, at very high temperatures, the adsorption can decrease due to desorption effects, especially since chemisorption is often exothermic.
- Therefore, this statement is incorrect as it suggests that chemisorption decreases at high temperature and low pressure, which is not true.

### Step 3: Analyze Statement 3
**Statement:** Physical adsorption is reversible in nature.

**Explanation:**
- Physical adsorption is indeed reversible. The adsorbate can be easily removed from the adsorbent by changing conditions such as temperature or pressure.
- This statement is correct.

### Step 4: Analyze Statement 4
**Statement:** Heat of adsorption for chemical adsorption is generally greater than that of physical adsorption.

**Explanation:**
- The heat of adsorption (enthalpy change) for chemisorption is significantly higher than that for physisorption.
- Typically, the heat of adsorption for chemisorption ranges from 80 to 240 kJ/mol, while for physisorption, it is much lower, around 20 to 40 kJ/mol.
- This statement is also correct.

### Conclusion
Based on the analysis, the statement that is not correct is:

**Answer:** Statement 2: "Chemical adsorption decreases at high temperature and low pressure."

---