Which of the following is not a characteristic of chemisorption ?

Options

A. lrreversible nautre
B. `Delta ` H is the order of 500 J
C. Specific in nature
D. Increases with increase of surface area

Answer

To determine which of the following is not a characteristic of chemisorption, let's analyze the characteristics of chemisorption step by step.

### Step 1: Definition of Chemisorption
Chemisorption is the process where an absorbate (the substance being adsorbed) forms a chemical bond with the surface of an adsorbent (the material on which adsorption occurs). This interaction is typically stronger than physisorption (physical adsorption).

**Hint:** Remember that chemisorption involves a chemical reaction between the absorbate and the adsorbent.

### Step 2: Characteristics of Chemisorption
1. **Irreversible in Nature**: Chemisorption is generally irreversible because once the chemical bond is formed, it does not easily break. This characteristic is true for chemisorption.

**Hint:** Think about what happens to a chemical bond once it is formed; it usually requires significant energy to break.

2. **High Enthalpy of Adsorption**: The enthalpy change (ΔH) for chemisorption is typically high, generally ranging from 80 to 240 kJ/mol. If a value like 500 kJ/mol is given, it is not a characteristic of chemisorption.

**Hint:** Recall the typical energy values associated with chemisorption; they are significant but within a specific range.

3. **Specific in Nature**: Chemisorption is specific because it depends on the nature of the adsorbate and adsorbent. Different gases will interact differently with various surfaces.

**Hint:** Consider how different materials interact uniquely with specific gases.

4. **Increases with Surface Area**: The rate of chemisorption increases with an increase in the surface area of the adsorbent. More surface area allows for more active sites for adsorption.

**Hint:** Think about how a larger surface area provides more opportunities for interaction.

### Step 3: Conclusion
From the analysis above, the characteristic that is not true for chemisorption is the statement regarding the enthalpy of adsorption being 500 kJ/mol. This value is too high and does not align with the known characteristics of chemisorption.

**Final Answer:** The characteristic that is not true for chemisorption is the high enthalpy value of 500 kJ/mol.