Question

Assertion :For adsorption $\Delta G,\Delta S$ and $\Delta H$ all have negative values. Reason: Adsorption is a spontaneous process accompanied by decrease in randomness.

• A. If both (A) and (R) are true and (R) is the correct explanation of (A).
• B. If both (A) and (R) are true, but (R) is not the correct explanation of (A).
• C. If (A) is correct, but (R) is incorrect.
• D. If (A) and (R) are both incorrect.

Answer 1

Answer: (A)

To understand this let’s briefly look at entropy.

Entropy is the measure of the randomness of a system. If the randomness of the system increases then the value of entropy increases (ΔS > 0) and if the randomness of the system decreases then the value of entropy decreases (ΔS < 0).

Let’s say that a gas is being adsorbed on the solid. Now, in the gas phase the randomness of the molecules is high. When the gas molecules are adsorbed on the surface of the solid, the movement of these molecules becomes restricted. Thus, there is decrease in the randomness hence decrease in entropy (ΔS < 0)

Now let’s have a look at the impact this has on thermodynamics.

We have a popular thermodynamic relation for ΔG which is given by:

ΔG= ΔH-TΔS

For a spontaneous process, the Gibbs Free Energy decreases (ΔG < 0).

We know that adsorption is a spontaneous process and hence ΔG < 0.

As the value of ΔS < 0 (as discussed above), the value of -TΔS term on the R.H.S becomes positive.

To compensate this, the value of ΔH has to be negative (ΔH < 0).

Thus, negative value of enthalpy change (ΔH < 0) implies that the process of adsorption must be exothermic.