Which is not a characteristic of physical adsorption ?

Options

A. Enthaipy of adsorption is low
B. It is due to weak intermolecular van der Waal forces
C. It forms unimolecular layer on adsorbent
D. It require very small activation energy for the adsorption.

Answer

To determine which statement is not a characteristic of physical adsorption, we need to analyze the properties of physical adsorption (physisorption) and compare them with the given options. Here is a step-by-step solution:

### Step 1: Understand Physical Adsorption
Physical adsorption, also known as physisorption, involves the adhesion of molecules (adsorbate) onto a surface (adsorbent) through weak intermolecular forces, specifically Van der Waals forces.

**Hint:** Remember that physisorption is characterized by weak forces and is generally reversible.

### Step 2: Identify Characteristics of Physical Adsorption
1. **Low Enthalpy of Adsorption:** The enthalpy change during physical adsorption is typically low, around 20-40 kJ/mol, due to the weak Van der Waals forces involved.

**Hint:** Enthalpy is a measure of energy change; lower values indicate weaker interactions.

2. **Multilayer Formation:** Physical adsorption can lead to the formation of multiple layers of adsorbate on the adsorbent surface because the weak forces allow for additional layers to form easily.

**Hint:** Think about how weak interactions allow for more than one layer to stack up.

3. **Requires Low Activation Energy:** Physical adsorption requires very little activation energy for the process to occur, as the forces involved are weak.

**Hint:** Low activation energy means that the process can happen easily and quickly.

4. **Unimolecular Layer Formation:** This characteristic is typically associated with chemical adsorption (chemisorption), where strong chemical bonds lead to the formation of a single layer of adsorbate.

**Hint:** Unimolecular layer formation is a key feature of chemisorption, not physisorption.

### Step 3: Analyze the Options
Given the characteristics of physical adsorption, we can analyze the statements:
- **Enthalpy of adsorption is low:** True for physical adsorption.
- **It is due to weak intermolecular Van der Waals forces:** True for physical adsorption.
- **It forms unimolecular layer on adsorbent:** **Not true** for physical adsorption; it forms multilayers.
- **It requires very small activation energy for the adsorption:** True for physical adsorption.

### Conclusion
The statement that is **not a characteristic of physical adsorption** is: **"It forms unimolecular layer on adsorbent."**

### Final Answer
The correct answer is: **It forms unimolecular layer on adsorbent.**

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