Question

Which of the following statement is not correct about physisorption ?

• A. It is a reversible process.
• B. Heat of adsorption is low, around $20-40kJmo{l}^{-1}$
• C. It requires activation energy.
• D. It decreases with increase in temperature.

Answer 1

The correct option is C It requires activation energy.

If the accumulation of gas on the surface of a solid occurs on account of weak van der Waals' forces, the adsorption is termed as physical adsorption or physisorption.
Characterstics of physisorption:
1.
Physical adsorption of gas is generally reversible.
$Solid+Gas\rightleftharpoons Gas/Solid+Heat$

Since adsorption process is exothermic, with increase in temperature, the rate of desorption increases ( By Le-Chatelier's principle). Hence, lowering the temperature will favour the rate of physical adsorption.

2. Enthalpy of adsorption:
Physical adsorption is an exothermic process but its enthalpy of
adsorption is very low $\left(\right(20-40\left)kJmo{l}^{-1}\right)$ because the involved van der Waal's force is a weak physical force so the attraction between adsorbate and adsorbent is less.
$\therefore$
Less heat of adsorption is sufficient for the physisorption to occur.
No chemical bonds are formed in physisorption so there is no necessity of activation energy.

So, option C is the wrong statement about physisorption.