Question

During adsorption, enthalpy and entropy of the system are ……… but $\Delta G$ must be ………….
A. negative, positive
B. negative, negative
C. positive, negative
D. positive, positive

Answer 1

Adsorption is the process which describes the accumulation of a substance at higher concentration on the surface in molecular species. Adsorption requires two components known as adsorbent and absorbent.

Complete answer:
Enthalpy is given by the sum of the internal energy and the product of pressure and volume while entropy is defined as a measure of randomness or disorder of the system. Entropy is a thermodynamic function. Whereas $\Delta G$ defines the change in free energy state which is given by the change in the enthalpy of the system minus the product of temperature and change in the entropy which is given by:
$\Delta G=\Delta H-T\Delta S$
During adsorption process enthalpy and entropy of the system are negative and in this case $\Delta G$ have to be negative so that the process is spontaneous. As we know that adsorption is processed with decrease in the free energy of the system for a spontaneous process. In the case when $\Delta G$ becomes zero then adsorption equilibrium is established which describes that it is an exothermic process in which $\Delta H$ is negative. So we can say that the adhering of gas molecules to the surface lowers the entropy and this results $\Delta S$ to be negative.
Hence we can say that during adsorption, enthalpy and entropy of the system are negative but $\Delta G$ must be negative.

So,option B is the correct answer.

Note:
Adsorption is of two types known by physical adsorption and chemical adsorption. Where physical adsorption is due to weak van der waal forces between adsorbate and adsorbent while chemical adsorption is due to strong chemical bonding between adsorbate and adsorbent.