Which of the following is not a characteristic of chemisorption?

Options

A. It is irreversible.
B. It is specific.
C. It is multilayer phenomenon.
D. Heat of adsorption is about `-400KJ` .

Answer

To determine which of the following is not a characteristic of chemisorption, let’s analyze each option based on the properties of chemisorption:

1. **Irreversible**: Chemisorption is characterized by the formation of strong chemical bonds between the adsorbate and the adsorbent. Once these bonds are formed, they are difficult to break, making chemisorption an irreversible process. Therefore, this statement is true regarding chemisorption.

2. **Specific**: Chemisorption is specific in nature because it involves the formation of chemical bonds, which typically occurs between specific types of adsorbates and adsorbents. This means that not all substances will chemisorb onto a given surface, making this statement true as well.

3. **Multilayer**: Chemisorption generally results in the formation of a monolayer (unilayer) on the surface of the adsorbent. This is because the second layer of adsorbate particles does not form strong chemical bonds with the adsorbent. Therefore, this statement is false regarding chemisorption, as it does not form multilayers.

4. **Heat of adsorption is about -400 kJ**: The heat of adsorption for chemisorption is indeed high, typically around -40 to -400 kJ/mol, indicating that a significant amount of energy is released when the chemical bonds are formed. Thus, this statement is true.

Based on the analysis, the option that is not a characteristic of chemisorption is:

**Answer**: Option 3 - It is multilayer.