Question

During adsorption which of the following is positive?
$A:\Delta G \\\ B:T\Delta S \\\ C:\Delta G + T\Delta S \\\ D:none{\text{ }}of{\text{ }}these \\\$

Answer 1

Adsorption refers to the deposition of molecular species over the surface. The molecular species getting adsorbed over the surface are called adsorbate while the surface over which adsorption takes place is called adsorbent. Examples of adsorbents include silica gel, clay, metals, colloids, etc.

Complete step by step solution: Adsorption process is always spontaneous and releases free energy (exothermic process) which means that enthalpy i.e. $\Delta H$ of adsorption process must always be negative (< 0). When a gas gets adsorbed, the freedom of migration of its molecules is hindered and this results into the decrease in entropy of the gas after the adsorption process which signifies that entropy i.e. $\Delta S$ is also negative.
Spontaneous adsorption process releases the free energy as they proceed. This free energy involved in the process is most commonly known as Gibbs free energy and is denoted by G in the name of an American scientist i.e. Josiah Gibbs who contributed to thermodynamics in many ways. The determining factors in case of a spontaneous adsorption process are enthalpy ($\Delta H$) as well as entropy ($\Delta S$) changes which occur for the system. Mathematically, change in the Gibbs free energy ($\Delta G$) can be computed with the following expression:

$\Delta G = \Delta H - T\Delta S$

On the basis of the above equation, $\Delta G$ is negative if $\Delta H$ must possess sufficiently high negative value such that -$T\Delta S$ is positive. Thus, in the spontaneous adsorption process, these two factors make the value of $\Delta G$ negative.
Hence, none of the given terms would be positive.

Therefore, the correct answer is Option D i.e. none of these.

Note: Always remember that the temperature in Gibbs free energy expression is the Kelvin temperature, thus, it can only possess a positive value. If $\Delta H$ is negative and $\Delta S$ is positive, $\Delta G$ is negative, indicating that the process is spontaneous at all the temperatures. In other cases if $\Delta H$ is positive and $\Delta S$ is negative, $\Delta G$ is positive, indicating that the process is nonspontaneous.