Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is wrong about chemisorption? (1) it is reversible in nature (2) it is specific in nature (3) it first increases with increase in temperature and then pressure (4) it involves the formation of a compound on the surface of the solid adsorbent (a) 1 only (b) 2 and 3 (c) 1 and 3 (d) 2 and 4

Step-by-Step Solution

Answer

Choice (c) '1 and 3' is incorrect.

Step 1: Understanding Chemisorption

Chemisorption is a type of adsorption where a chemical bond is formed between the adsorbate and the surface of the adsorbent. This is in contrast to physisorption, which involves weaker van der Waals forces and is highly reversible.

Step 2: Analyzing Statement 1

Statement (1) says chemisorption is reversible in nature. Chemisorption involves strong chemical bonding, which makes it mostly irreversible. Therefore, this statement is likely to be false.

Step 3: Analyzing Statement 2

Statement (2) mentions that chemisorption is specific in nature. Chemisorption is indeed specific because it involves the formation of chemical bonds which require specific conditions. Thus, this statement is true.

Step 4: Analyzing Statement 3

Statement (3) asserts that chemisorption first increases with increase in temperature and then pressure. Typically, chemisorption does increase with temperature initially, but the statement's phrase "and then pressure" is a bit odd. However, pressure effects are more significant in physisorption. Chemisorption does not significantly depend on pressure in this context, making this statement likely false.

Step 5: Analyzing Statement 4

Statement (4) asserts that chemisorption involves the formation of a compound on the solid’s surface. This is true, as chemisorption forms a chemical bond on the surface, producing a compound.

Step 6: Determine the Wrong Statement

Since statement 1 is false (irreversible nature) and statement 3 is misleading as it ties temperature increase directly to pressure effects incorrectly, consider choice (c) '1 and 3' as these have potential inaccuracies.

Key Concepts

Adsorption

Adsorption is a process where molecules from a gas, liquid, or dissolved solid adhere to a surface. This typically happens when the molecules make contact with a solid material and stick to its surface. The surface that attracts these molecules is known as the adsorbent, while the captured particles are the adsorbate.

This can occur via two main mechanisms: physisorption and chemisorption. Physisorption is characterized by weak van der Waals forces, making it typically reversible. In contrast, chemisorption involves the formation of stronger chemical bonds, leading to more permanent adsorption that is mostly irreversible. Understanding these different types helps to clarify the specific nature of chemisorption and its irreversibility.

• Physisorption: Weak, reversible, related to van der Waals forces.
• Chemisorption: Strong, often irreversible, involves chemical bond formation.

Recognizing these differences highlights the complexity and specificity of chemisorption.

Bond Formation

In chemisorption, bond formation is a crucial aspect. This type of adsorption involves forming chemical bonds between the adsorbate and the surface of the adsorbent. Unlike physisorption, where interactions are weak, chemisorption results in stronger, more stable interactions.

Bond formation in chemisorption is comparable to a chemical reaction. It often requires specific conditions such as the right temperature and surface characteristics. This specificity can be attributed to the necessity of chemical affinity between the surface and the adsorbate. Essentially, only certain combinations will result in effective bond formation.

This process often results in significant changes to the physical and chemical properties of the surface. New compounds can form when molecules chemically bind to the surface. Recognizing this feature is key to understanding why chemisorption differs from other forms of adsorption. It also explains why certain statements about its reversibility can be problematic, given the permanent nature of the bonds.

Reversibility

Reversibility in the context of adsorption relates to the ease with which adsorbate molecules can be released back into their original phase. Chemisorption is considered mostly irreversible due to the strong chemical bonds formed during the process. This creates a more permanent interaction compared to physisorption.

In physisorption, because bonds are weaker, reversing the process is relatively easy; the molecules can simply desorb from the surface when conditions such as pressure or temperature change. However, for chemisorption, breaking the stronger bonds requires significant energy, and sometimes it may not be entirely reversible.

• Chemisorption: Mostly irreversible, due to strong chemical bonds.
• Physisorption: Generally reversible, due to weak van der Waals forces.

Understanding the degree of reversibility is essential when considering applications of these processes, such as in catalysis where permanent surface modifications might be required.

Temperature Effects on Adsorption

Temperature effects play a significant role in adsorption processes. In the case of chemisorption, as temperature initially increases, the rate of adsorption typically increases.

This happens because higher temperatures provide the necessary activation energy for bond formation.

However, unlike physisorption, for which further temperature increases can often weaken adsorption due to breaking interactions, chemisorption reaches a point beyond which increase will not substantially improve adsorption further. This is because chemisorption relies on specific, strong bonds which once formed, do not easily disassociate with increased temperature.

Pressure, on the other hand, does not notably influence chemisorption as it primarily affects physisorption where gas molecules are involved. In chemisorption, once a chemical bond forms, it remains relatively stable despite changes in external pressure. Recognizing these dynamics helps to distinguish the thermal behavior of chemisorption from that of other adsorption types.