Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

A graph of adsorption isobar of chemisorption shows that adsorption 1(a) first decreases with temperature and then increases (b) first increases with temperature and then decreases (c) increases with temperature (d) decreases with temperature

Step-by-Step Solution

Answer

First increases with temperature and then decreases.

Step 1: Understand Adsorption

Adsorption is the process by which solid surfaces attract and hold gas or liquid molecules on their surface. Chemisorption involves a chemical bond formation and is usually specific to the type of gas.

Step 2: Analyze the Temperature Effect in Chemisorption

Chemisorption is commonly an exothermic process, meaning that it releases heat. Initially, with an increase in temperature, the rate of reaction increases as molecules gain kinetic energy, leading to an increase in adsorption rates. However, beyond a certain temperature, the process becomes less favorable due to the exothermic nature, leading to decreased adsorption.

Step 3: Determine the Trend for Adsorption Isobar

Understanding the effect of temperature on chemisorption, the pattern typically observed is that adsorption initially increases with increasing temperature due to enhanced molecular activity, but as the temperature continues to rise, the adsorption decreases because the adsorption process is exothermic and less energetically favorable at high temperatures.

Key Concepts

Chemisorption

Chemisorption involves a process where chemical bonds form between the molecules on the surface of a solid and the adsorbate. This type of adsorption is specific, meaning it will only occur with certain molecules, often depending on the chemical structure and reactivity of the adsorbent and adsorbate.
Unlike physisorption which relies on van der Waals forces, chemisorption requires higher energy as it forms stronger bonds like covalent or ionic bonds.
Given its nature, chemisorption typically affects the material properties of the adsorbent and creates new chemical species upon adsorption.

• It is usually irreversible.
• It involves activation energy.
• It has a saturation point where no more adsorption can occur regardless of increased pressure.
• Highly specific due to the need for bond formation.

Chemisorption is crucial in industrial processes like catalysis, where chemical surface reactions need enhancement. Since this process usually forms one layer of adsorbate, its efficiency depends on surface coverage and the availability of active sites.

Temperature effect on adsorption

The effect of temperature on adsorption, especially in chemisorption, is quite significant. Initially, as the temperature rises, adsorption increases.
This is because molecules receive more kinetic energy, colliding more effectively with the adsorbent surface and potentially overcoming the activation energy barrier needed for chemisorption.
This ends up accelerating the rate of reaction.

• At low temperatures, adsorption is slow because molecular energy is low, reducing reaction rates.
• At moderate temperature increases, the kinetic energy of gas molecules enhances adsorption rate.

However, once the temperature passes a certain threshold, the adsorption begins to decline. This reduction is because the increased kinetic energy tends to desorb the molecules at the surface, reversing the adsorption. This happens as chemisorption is an exothermic process.

Exothermic process

An exothermic process is one that releases energy in the form of heat. Chemisorption is typically exothermic, meaning it gives off heat when adsorption occurs.
The heat released is due to the formation of new chemical bonds between the adsorbed molecules and the surface.
Although this heat boost may initially increase the surface activity, making adsorption favorable, it presents a limitation at higher temperatures.

• Since energy is released, increasing temperature pushes the equilibrium to favor desorption.
• The system will resist further adsorption because additional heat energy makes desorption more likely.

The innate nature of exothermic reactions means that while lower temperatures initially help adsorption, continuing to heat the substance counters the benefits by increasing reverse reactions. In summary, chemisorption being exothermic, starts favorably with rising temperatures but reaches an optimal point beyond which the adsorption process is no longer energetically efficient.