Question

Which one of the following characteristics is associated with adsorption?
A. $\Delta G$ is negative but $\Delta H$ and $\Delta S$ are positive
B. $\Delta G$ , $\Delta H$ and $\Delta S$ all are negative
C. $\Delta G$ and $\Delta H$ are negative but $\Delta S$ is positive
D. $\Delta G$ and $\Delta S$ are negative but $\Delta H$ is positive

Answer 1

To answer this question we need to know whether the adsorption reaction is exothermic or not, spontaneous or not and reactants are random or not. This will help in knowing the values of enthalpy, entropy and Gibbs energy value.

Step by step answer: Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process is a surface phenomenon.
Adsorption is a consequence of surface energy. The nature of bonding between the atoms is due to physisorption which is characteristic of weak van der Waals force or chemisorption which is the characteristic of covalent bonding. It may also occur due to electrostatic attraction.
$\Delta G$ is commonly called as Gibbs free energy is a thermodynamic potential that can be used to calculate the maximum of reversible work that may be performed by a thermodynamic system at a constant temperature and pressure. It can be calculated as given below,
$\Delta G = \Delta H - T\Delta S$
Here, $\Delta G$ is the maximum amount of non-expansion work that can be extracted from a thermodynamically closed system. Also, if $\Delta G$ is positive means that the reaction is nonspontaneous and it needs an external input of energy to take place. Negative value means it is a spontaneous reaction and energy will be released in the process and there is no need for an external energy source.
$\Delta H$ represents the enthalpy of the system. It is the sum of the system's internal energy and the product of its pressure and volume. Positive value of enthalpy means the reaction is endothermic as energy is required in breaking bonds. If its value is negative means reaction is exothermic as energy is released in making bonds.
$\Delta S$ is called entropy and it is the measurement of randomness or disorder from reactants to products. If its value is positive this means that the disorder of the universe is increasing from reactants to products and if it is negative then it means that disorder is decreasing.
Now, as adsorption is a spontaneous process, therefore the change in free energy is negative i.e. $\Delta G$ is negative. Since adsorption is an exothermic process therefore energy will be released and enthalpy or $\Delta H$ would be negative. As the molecules will stick to the surface of the solid, their movement will decrease which means their randomness decreases. So, entropy also decreases and $\Delta S$ is negative. Therefore, all three values are negative.

So answer is option B.

Note: For a reaction, $\Delta G$ depends on whether the reaction is spontaneous or not, $\Delta H$ depends on whether the reaction is exothermic or endothermic and $\Delta S$ depends on randomness of the reactants and products.