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 characteristic is not associated with physical adsorption (physisorption), we need to analyze the properties of physisorption. Here’s a step-by-step solution:

### Step 1: Understand Physisorption
Physisorption is a type of adsorption where the adsorbate (the substance being adsorbed) adheres to the surface of the adsorbent (the material that adsorbs) through weak van der Waals forces. This process does not involve any chemical bonding.

**Hint:** Remember that physisorption involves weak forces and is a physical process, not a chemical one.

### Step 2: Analyze Characteristics of Physisorption
1. **Enthalpy of Adsorption**: In physisorption, the enthalpy change is low because the interactions are weak. This is a characteristic of physisorption.

**Hint:** Low energy interactions indicate weak forces at play.

2. **Nature of Forces**: Physisorption is due to weak intermolecular forces, such as van der Waals forces. This is also a characteristic of physisorption.

**Hint:** Think about the types of forces involved in physical interactions.

3. **Layer Formation**: Physisorption can form multiple layers of adsorbate on the adsorbent. This means that it does not form just a single layer (unilayer) but can accommodate several layers.

**Hint:** Consider how many layers can form in physical adsorption compared to chemical adsorption.

4. **Activation Energy**: Physisorption requires very little to no activation energy, making it a spontaneous process.

**Hint:** Reflect on how much energy is needed for physical processes versus chemical reactions.

### Step 3: Identify the Incorrect Characteristic
From the analysis above, the statement that physisorption "forms a unilayer at the adsorbent" is incorrect. Physisorption can lead to the formation of multiple layers of adsorbate.

**Conclusion:** The characteristic that is not true for physisorption is that it forms a unilayer at the adsorbent.

### Final Answer
The characteristic that is not associated with physical adsorption is: **It forms a unilayer at the adsorbent.**