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

Question

A graph of adsorption isobar of chemisorption shows that adsorption (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

(b) first increases with temperature and then decreases

Step 1: Define Chemisorption

Chemisorption is a type of adsorption where the adsorbate interacts with the surface of the adsorbent through chemical bonds. This process is generally irreversible due to the strength of the bonds formed.

Step 2: Identify the Dependence on Temperature

In chemisorption, increasing temperature generally provides the energy needed to overcome the activation energy barrier for adsorption. Initially, this increase in temperature results in more adsorption as more molecules have enough energy to form chemical bonds.

Step 3: Analyze the Effect of Further Increasing Temperature

After a certain temperature, further increasing the temperature leads to the desorption of adsorbed molecules because the kinetic energy of the adsorbed molecules becomes higher than the binding energy, causing them to break away from the surface.

Step 4: Conclusion Based on Observation

Based on the typical behavior of chemisorption and its adsorption isobar graph, it first increases with temperature and then decreases, following the logic of overcoming activation energies followed by desorption at high temperatures.

Key Concepts

Adsorption Isobar

Adsorption isobars represent the relationship between the amount of gas adsorbed and the pressure at a constant temperature. These graphs help us understand how different factors influence adsorption processes, such as chemisorption. In the context of chemisorption, an adsorption isobar graph is particularly insightful.

Initially, on an adsorption isobar graph for chemisorption, you might observe a rise in adsorption as the temperature is increased. This is because, at the start, more energy is provided to overcome the activation barriers, allowing more molecules to bond to the surface. However, this increase is only up to a point. Following this, with further temperature increase, the graph shows a drop in adsorption. This indicates desorption, as the high kinetic energy from the increased temperature starts breaking the bonds that had formed between the adsorbate and the adsorbent surface.

Temperature Effect on Adsorption

Temperature is a critical factor affecting adsorption processes. For chemisorption, this effect is dual-phased. The initial phase shows that as temperature rises, the rate of adsorption also increases. The reason is that higher temperatures provide the energy required to surpass activation energy barriers.

In chemisorption, when molecules have sufficient energy to bypass these barriers, they effectively form chemical bonds with the surface. But as the temperature continues to rise, the process enters a different phase. Here, too much thermal energy results in the adsorbed molecules gaining excessive kinetic energy, thereby breaking away from the surface, causing desorption. Therefore, if you look at a chemisorption graph, it will depict initially increasing adsorption with temperature, followed by decreased adsorption as temperature increases further.

Activation Energy in Chemisorption

Activation energy is a vital concept in chemisorption, particularly affecting how temperature influences the process. This energy is the minimum energy required for the adsorbate to form chemical bonds with the adsorbent.

• Lower temperatures may not provide sufficient energy to overcome this barrier, resulting in minimal initial chemisorption.
• As temperature increases, enough energy is supplied to overcome activation energy, initiating the bonding process.

However, once the activation energy is surpassed, chemisorption can occur readily until a point where the temperature becomes too high. At this high temperature, despite sufficient activation energy, molecules possess too much energy, resulting in another scenario: desorption, where the molecules detach from the surface. Understanding activation energy helps explain the characteristic shape of adsorption isobars in chemisorption, showing the processes of increased adsorption followed by desorption as temperature changes.