Question

Assertion: ${N_2}$ is adsorbed on an iron surface at 83K which decreases rapidly with increase in temperature and at room temperature, practically no adsorption of ${N_2}$ occurs. The adsorption, however, becomes significant at 773K.
Reason: At lower temperature, adsorption of ${N_2}$ molecules on iron surface is physisorption and at high temperatures, ${N_2}$ shows chemisorption of N atoms on iron surface.
A.Both assertion and reason are correct and Reason is the correct explanation for Assertion.
B.Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C.Assertion is correct but Reason is incorrect.
D.Assertion is incorrect but Reason is correct.

Answer 1

We know that the phenomenon of attracting and retaining the molecules of a substance present on the liquid surface (or) solid surface leading to higher concentration of the molecules present on the surface is called adsorption. Some of the causes of adsorption are unbalanced forces of attraction or free valencies that are present at the surface of liquid (or) surface of solid, have the property to attract and retain the gas molecules or a dissolved substance on to their surfaces with which they come in contact.

Complete step by step answer:
We have to know that adsorption is a surface phenomenon. In the process of adsorption, the substance is only recollected on the surface and does not go into the bulk (or) solid interior (or) liquid interior. The concentration of the adsorbed molecule is always higher at the free phase. Adsorption is fast in the beginning and it gets slow down near the equilibrium.
There are two kinds of adsorption: physical adsorption and chemical adsorption.
We have to understand that adsorption takes place with the release of heat. Based on Le-Chatelier’s principle there would be a decrease in magnitude of adsorption with increase in temperature.
Physisorption takes place at lower temperature and decreases with increase in temperature, whereas chemisorption takes place at higher temperature.
At 83K, the adsorption of nitrogen takes place on the surface of iron. The adsorption decreases fastly when there is an increase in temperature and no adsorption of nitrogen happens at room temperature. This reason is because at lower temperatures, nitrogen molecules on the surface of iron undergo physisorption and chemisorption of nitrogen on the surface of iron takes place at high temperature.
From the explanations regarding the effect of temperature on physisorption and chemisorption, we can say that both assertion and reason is correct. Reason is the correct explanation for the assertion.

So, the correct answer is Option A.

Note: We know that physisorption is an exothermic process. At low temperatures, the process of physisorption occurs. Chemisorption takes place at all temperatures, the adsorption increases with increase in temperature. Force of attraction in physisorption is Van Der Waals force and force in attraction in chemisorption is chemical bond forces. Physisorption is reversible and chemisorption is irreversible.