Which of the following is not a characteristic of chemisorption 

Options

A. It is irrversible
B. it is specific
C. it is multi layer phenomenon
D. heat of adsorption is about 40 - 400 KJ

Answer

To determine which of the following is not a characteristic of chemisorption, we first need to understand what chemisorption is and its key characteristics.

### Step-by-Step Solution:

1. **Definition of Chemisorption**:
- Chemisorption is a type of adsorption where a chemical bond is formed between the adsorbate (the substance being adsorbed) and the adsorbent (the surface on which adsorption occurs).

2. **Characteristics of Chemisorption**:
- **Irreversibility**: Chemisorption is generally irreversible. Once a chemical bond is formed, it requires a significant amount of energy to break it, making the process not easily reversible.
- **Specificity**: Chemisorption is specific to certain sites on the adsorbent surface. It occurs at specific points where the chemical interaction is favorable.
- **Single Layer Phenomenon**: Chemisorption typically results in the formation of a single layer of adsorbate on the adsorbent surface. This is because once a site is occupied by an adsorbate, it cannot be occupied by another.
- **Heat of Adsorption**: The heat of adsorption for chemisorption is relatively high, typically ranging from 40 to 400 kJ/mol, due to the formation of strong chemical bonds.

3. **Identifying the Incorrect Characteristic**:
- Among the characteristics listed, the statement that chemisorption is a "multi-layer phenomenon" is incorrect. Chemisorption is characterized by the formation of a single layer of adsorbate.

### Conclusion:
The characteristic that is not true for chemisorption is that it is a **multi-layer phenomenon**.