Problem 2 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Amongst the following statements regarding adsorption, those that are valid are :(1) \(\Delta \mathrm{H}\) becomes less negative as adsorption proceeds. (2) On a given adsorbent, ammonia is adsorbed more than nitrogen gas. (3) On adsorption, the residual force acting along the surface of the adsorbent increases. (4) With increase in temperature, the equilibrium concentration of adsorbate increases. (a) (4) and (1) (b) (2) and (3) (c) (1) and (2) (d) (3) and (4)

Step-by-Step Solution

Answer

Option (c) (1 and 2) is correct.

Step 1: Evaluate Statement 1

Consider the statement: "\(\Delta \mathrm{H}\) becomes less negative as adsorption proceeds." When adsorption occurs, exothermic reactions usually take place initially, leading to a negative \(\Delta \mathrm{H}\). As adsorption proceeds and the adsorbent becomes saturated, \(\Delta \mathrm{H}\) indeed becomes less negative because there's less surface area available, resulting in less heat release. This statement is valid.

Step 2: Evaluate Statement 2

Consider the statement: "On a given adsorbent, ammonia is adsorbed more than nitrogen gas." Ammonia (\(\text{NH}_3\)) is a polar molecule, while nitrogen (\(\text{N}_2\)) is a non-polar molecule. Adsorbent surfaces often have higher affinity for polar substances due to the stronger van der Waals or dipole interactions. Hence, ammonia would typically be adsorbed more than nitrogen gas. This statement is valid.

Step 3: Evaluate Statement 3

Consider the statement: "On adsorption, the residual force acting along the surface of the adsorbent increases." Upon adsorption, surface tension usually decreases because surface area and active sites for bonding decrease. Hence, the residual force on the surface does not increase; it tends to decrease. This statement is not valid.

Step 4: Evaluate Statement 4

Consider the statement: "With increase in temperature, the equilibrium concentration of adsorbate increases." Typically, adsorption is an exothermic process. Raising the temperature usually reduces adsorption because heat is supplied externally, disturbing the exothermic equilibrium and reducing adsorbate concentration. Therefore, this statement is not valid.

Step 5: Select Valid Statements

From the evaluation of steps: Statements 1 and 2 are valid, whereas Statements 3 and 4 are not valid. Therefore, option (c) (1 and 2) is correct.

Key Concepts

Exothermic Reactions

Exothermic reactions are processes that release energy, typically in the form of heat. In chemistry, they are often encountered during adsorption. When a substance adheres to the surface of a solid or liquid, energy is released.
This heat release is due to the formation of new bonds between the surface and the molecules being adsorbed.
During initial adsorption,

• The process is highly exothermic as numerous sites are available on the adsorbent for interaction.
• As these sites are occupied, he heat released or the negative enthalpy change ( \( \Delta \mathrm{H} \) ) becomes less intense or less negative.

This is because fewer molecules bond to the reduced available surface area.
Understanding this concept is crucial in thermodynamics and industrial applications where controlling heat and energy release is imperative.

Polar and Non-polar Molecules

Molecules can be either polar or non-polar, indicating their distribution of electrical charge. Polar molecules have uneven charge distribution, resulting in regions that are more positive or negative, which affects their behavior in adsorption.
Consider ammonia (\( \text{NH}_3 \)) and nitrogen (\( \text{N}_2 \)).

• Ammonia is a polar molecule because of its shape and the electronegativity difference between nitrogen and hydrogen. It results in stronger electrostatic interactions with adsorbent surfaces.
• Nitrogen, however, is non-polar because the bonding electrons are shared equally, leading to no permanent dipoles.

Because adsorbent surfaces typically have a stronger attraction to polar molecules, the process sees molecules like ammonia being adsorbed more readily than non-polar molecules like nitrogen.
This concept is fundamental in chemical separation processes, where selecting the right adsorbent can lead to enhanced efficiency and effectiveness.

Thermodynamics in Chemistry

Thermodynamics provides the framework for understanding energy changes in chemical systems, including adsorption. Adsorption, being often exothermic, aligns with several key thermodynamic principles.
When a gas or liquid molecule adheres to the surface of a solid,

• the system releases energy, frequently resulting in decreased free energy ( \( \Delta \mathrm{G} \)).
• The entropy change (\( \Delta \mathrm{S} \)) can be complex, overall indicating a more ordered state due to the interaction of molecules with the adsorbent surface.

Temperature changes can significantly impact adsorption – typically, an increase in temperature decreases adsorption efficiency for exothermic processes. This is because the supplied heat can shift the equilibrium, opposing the natural release of energy.
A solid grasp of these principles is crucial in fields such as pharmaceuticals, environmental engineering, and material science, where adsorption plays a vital role in synthesis and purification processes.