Question

The extent of physisorption of an adsorbate increases with

• A. increase in temperature
• B. decrease in surface area of the adsorbent
• C. decrease in pressure of adsorbate
• D. decrease in temperature

Hint:

In physisorption, also known as physical adsorption, the adsorbate is held to the adsorbent by weak forces like van der Waals forces of attraction. 

Answer 1

The Correct Option is D

In physisorption, also known as physical adsorption, the adsorbate is held to the adsorbent by weak forces like van der Waals forces of attraction. 

The extent of physisorption, or the amount of adsorbate adsorbed onto the surface, increases with a decrease in temperature. 

• This is because as the temperature decreases, the kinetic energy of the adsorbate molecules also decreases, allowing them to more easily adsorb onto the surface of the adsorbent.
• In contrast, when the temperature increases, the kinetic energy of the adsorbate molecules increases, making it more difficult for them to adsorb onto the surface of the adsorbent. 
• This leads to a decrease in the extent of physisorption.

Therefore, the extent of physisorption increases with a decrease in temperature due to the decrease in kinetic energy of the adsorbate molecules, which allows them to more easily adsorb onto the surface of the adsorbent.

Hence, option D is the correct answer.

Answer 2

The correct option is (D): decrease in temperature

Physisorption is an exothermic process. So, the rate of physisorption rises with a fall in temperature. On the rising of temperature, the molecules get desorbed easily as in physisorption, particles are held to the surface by weak van der Waals’ force of attraction.

Characteristics of Physisorption

Physisorption is due to the van der Waals forces.

• It is reversible in nature.
• It has low adsorption enthalpy, i.e., 20 to 40 kJ/mol.
• Physisorption falls with a rise in temperature.
• Activation energy is very low in physisorption.

For adsorption, a low temperature is favourable. On an adsorbent surface under high pressure, it results in multimolecular layers.

Hence, Extent of physisorption of gas does increase with decrease in temperature.

Answer 3

The correct option is (D): decrease in temperature

When gas accumulates on a solid surface due to weak van der Waals forces, the adsorption is known as physical adsorption or physisorption.

Some of the characteristics of Physisorption are -

• It occurs due to van der Waals’ forces.
• In nature, it is not specific.
• It is reversible.
• It depends on the nature of the gas. Easily liquefiable gases are readily adsorbed.
• The enthalpy of adsorption is low (20−40 kJ mol⁻¹).

A low temperature is ideal for adsorption. The process decreases with an increase in temperature. No noticeable activation energy is needed. It is dependent on the surface area that increases with an increase in surface area.

It results in multimolecular layers on the surface of the adsorbent under high pressure.

Therefore, The extent of physisorption of an adsorbate increases with a decrease in temperature.