Which of the following is not characteristic of chemisorption?

Low enthalpy of adsorption

The correct answer is option 4. Low enthalpy of adsorption.

Let us break down each characteristic of chemisorption and explain them:

1. Highly specific: Chemisorption involves specific chemical interactions between the adsorbate molecules and the surface of the adsorbent. These interactions are highly specific and often involve the formation of chemical bonds between the adsorbate and the surface.

2. Irreversible: Chemisorption is typically irreversible because the bonds formed between the adsorbate and the surface of the adsorbent are strong and require significant energy to break. Once these bonds are formed, they are not easily broken, leading to the irreversibility of chemisorption.

3. Results in unimolecular layer: Chemisorption often leads to the formation of a monolayer of adsorbate molecules on the surface of the adsorbent. This monolayer is usually highly organized and consists of a single layer of adsorbate molecules densely packed on the surface.

4. Low enthalpy of adsorption: Enthalpy of adsorption refers to the heat released or absorbed when molecules of a gas or solute adhere to a surface of a solid. In chemisorption, the formation of chemical bonds between the adsorbate and the surface of the adsorbent typically involves a significant amount of energy being released. This results in a high enthalpy of adsorption rather than a low one. The formation of strong chemical bonds leads to a release of energy, making the enthalpy of chemisorption relatively high.

Therefore, option 4 is not characteristic of chemisorption because chemisorption typically involves a high enthalpy of adsorption due to the formation of chemical bonds between the adsorbate and the surface of the adsorbent.