Question

Which one of the following is not true with regard to physisorption?

• A. It arises because of van der Waals' forces
• B. It is not specific in nature
• C. High activation energy is needed
• D. It depends on the nature of gas
• E. Enthalpy of adsorption is low(20-40kj mol^-1)

Answer 1

The Correct Option is C

Physisorption is the process of adsorption where gas molecules are held on the surface of a solid by van der Waals forces. It is a physical process that does not involve any chemical bonding, and it occurs at low temperatures. The key characteristics of physisorption are:

- It arises due to van der Waals' forces (Option A).
- It is non-specific, meaning it is not dependent on the type of adsorbate (Option B).
- It depends on the nature of the gas, as gases with larger molecules or higher polarizability are more likely to adsorb (Option D).
- The enthalpy of adsorption is low (usually between 20 - 40 kJ mol\(^{-1}\)) (Option E).

However, physisorption does not require high activation energy (Option C). This is because the forces involved are weak, and the adsorption process is spontaneous at low temperatures without needing a significant energy barrier.

Thus, the incorrect statement is that high activation energy is needed for physisorption.

The correct option is (C) : High activation energy is needed

Answer 2

Physisorption refers to the process where a gas is adsorbed onto a solid surface primarily due to weak van der Waals forces, which are non-specific in nature. This type of adsorption typically occurs at low temperatures and requires little energy. It is characterized by a low enthalpy of adsorption (20-40 kJ/mol) and is reversible.  

Let's evaluate the options:

• It arises because of van der Waals' forces: True. Physisorption occurs due to weak van der Waals forces.
• It is not specific in nature: True. Physisorption is non-specific as it involves weak interactions.
• High activation energy is needed: False. Physisorption does not require high activation energy, as it is a relatively simple process driven by weak forces.
• It depends on the nature of gas: True. While physisorption is non-specific, the extent of adsorption can still depend on the properties of the gas, such as molecular size or polarizability.
• Enthalpy of adsorption is low (20-40 kJ/mol): True. The enthalpy of physisorption is relatively low.

Therefore, the statement that is not true with regard to physisorption is: High activation energy is needed.