Question

The difference between heat of adsorption of ortho and para hydrogen is
A.$0.4kJmo{{l}^{-1}}$
B.$0.8kJmo{{l}^{-1}}$
C. Zero
D. None of these

Answer 1

Hint: The heat of adsorption is the amount of heat released during the adsorption process. When molecular dihydrogen is adsorbed on an aerosol particle, an interconversion between ortho-hydrogen and para-hydrogen occurs through magnetic interaction with the surface material.

Complete Step by Step Answer:
Generally, molecular dihydrogen exists in nuclear spin isomers of ortho and para hydrogens according to the total nuclear spin. In ortho-hydrogen, the two hydrogen atoms of the dihydrogen molecule have parallel nuclear spins that result in an overall spin $1$. Similarly, for para-hydrogen, the two hydrogen atoms of the dihydrogen molecule have an anti-parallel nuclear spins that result in an overall spin zero.

The process by which molecular hydrogen is adsorbed on the metal surface is known as physisorption. Though the interconversion between ortho and para hydrogen is very slow in an isolated state this interconversion rate is promoted in a physisorption process via interaction with surfaces of not only magnetic but also diamagnetic materials.
In physisorption adsorbate molecules are held to the surface of an adsorbent by weak van der Waals forces. Therefore there is found a low enthalpy of adsorption or heat of adsorption values which is about $20-40kJ/mol$and the difference between the heat of adsorption of ortho-hydrogen and para-hydrogen is $0.4kJmo{{l}^{-1}}$.
Thus, option (A) is correct.

Note: Physisorption is an exothermic process. Physisorption occurs readily at low temperatures and its rate decreases with increased temperature. The process of adsorption of a gas by a solid surface is reversible and does not require any activation energy for driving this process.