Which of the following is not a characteristic of chemisorption?

Options

A. Adsorption is specific
B. Heat of adsorption is of the order of 200 KJ `"mol"^(-1)`
C. Adsorption is irreversible.
D. Adsorption may be multimolecular layers.

Explanation

To solve the question "Which of the following is not a characteristic of chemisorption?", we need to analyze each statement provided in the options and determine which one does not align with the known characteristics of chemisorption.

### Step-by-Step Solution:

1. **Understanding Chemisorption**:
- Chemisorption refers to the process where a gas or liquid molecule adheres to a solid surface through the formation of chemical bonds. This process is characterized by its specificity, high energy changes, and typically results in a single layer of adsorbate.

2. **Evaluating the Statements**:
- **Statement 1**: "Adsorption is specific."
- **Analysis**: This is true. Chemisorption is a specific process because it involves the formation of strong chemical bonds between the adsorbate and the adsorbent.

- **Statement 2**: "Heat of chemisorption is of the order of 200 kJ/mol."
- **Analysis**: This is also true. The heat of adsorption for chemisorption typically ranges from 20 to 400 kJ/mol, and 200 kJ/mol falls within this range.

- **Statement 3**: "Adsorption is irreversible."
- **Analysis**: This statement is true as well. Chemisorption is generally considered irreversible because the chemical bonds formed are strong and do not easily break, making the process one-way.

- **Statement 4**: "Adsorption may be multi-molecular layer."
- **Analysis**: This statement is incorrect. Chemisorption typically results in a unimolecular layer because it involves the formation of strong chemical bonds. In contrast, physisorption can lead to multi-molecular layers due to weaker van der Waals forces.

3. **Conclusion**:
- The statement that is not a characteristic of chemisorption is **Statement 4**: "Adsorption may be multi-molecular layer."

### Final Answer:
The characteristic that is not associated with chemisorption is: **Adsorption may be multi-molecular layer.**

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