Problem 133 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following reactions heterogenous catalysis is involved? (i) \(2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \stackrel{\mathrm{No}(\mathrm{g})}{\longrightarrow} 2 \mathrm{SO}_{3}(\mathrm{~g})\) (ii) \(2 \mathrm{SO}_{2}(\mathrm{~g}) \stackrel{\mathrm{Pt}(\mathrm{S})}{\longrightarrow} 2 \mathrm{SO}_{3}(\mathrm{~g})\) (iii) \(\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \stackrel{\mathrm{Fe}(\mathrm{S})}{\longrightarrow} 2 \mathrm{NH}_{3}(\mathrm{~g})\) (iv) \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}(\mathrm{I})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I}) \stackrel{\mathrm{HCl}(\mathrm{l})}{\longrightarrow} \mathrm{CH}_{3} \mathrm{COOH}\) \(+(\) aq \()+\mathrm{CH}_{3} \mathrm{OH}(\mathrm{aq})\) (a) (ii), (iii) (b) (ii), (iii), (iv) (c) (i), (ii), (iii) (d) (iv)

Step-by-Step Solution

Answer

Option (a) (ii), (iii) involve heterogeneous catalysis.

Step 1: Understand the Reaction Types

Heterogeneous catalysis involves a catalyst in a different phase than the reactants. Begin by identifying the state (solid, liquid, gas) of the reactants and catalysts in each reaction.

Step 2: Analyze Reaction (i)

In reaction (i), both the reactants (SO2 and O2) and the catalyst (NO) are in the gas phase. This is homogeneous catalysis, not heterogeneous.

Step 3: Analyze Reaction (ii)

In reaction (ii), SO2 gas is converted to SO3 gas using a platinum (Pt) catalyst, which is solid. Because the catalyst is in a different phase than the reactants, this is an example of heterogeneous catalysis.

Step 4: Analyze Reaction (iii)

In reaction (iii), gaseous N2 and H2 are converted to NH3 gas using a solid iron (Fe) catalyst. Since the catalyst is solid and the reactants are gases, this is heterogeneous catalysis.

Step 5: Analyze Reaction (iv)

In reaction (iv), both reactants (CH3COOCH3 and H2O) and the catalyst (HCl) are in the liquid phase. Therefore, this involves homogeneous catalysis, not heterogeneous.

Step 6: Determine the Correct Choice

Reactions (ii) and (iii) demonstrate heterogeneous catalysis, where a solid catalyst is used with gaseous reactants. Therefore, the correct answer is option (a), (ii) and (iii).

Key Concepts

Reaction Types

Chemical reactions often require an extra boost to proceed more efficiently, which is where catalysts come in. There are two primary types of catalysis: **homogeneous** and **heterogeneous**. Homogeneous catalysis occurs when the catalyst is in the same phase as the reactants. This usually means all substances are in either the liquid or gas phase.
On the other hand, heterogeneous catalysis involves catalysts and reactants in different phases, which often means a solid catalyst interacts with gaseous reactants. This allows for an easier separation of the catalyst after the reaction. Understanding these fundamentals helps in predicting the nature of the chemical reactions in different processes.

Catalyst Phases

Catalysts can exist in different phases: solid, liquid, or gas. When the catalyst is in a distinct phase from the reactants, it is termed heterogeneous. This presents advantages, such as the ability to easily separate and recover the catalyst after the reaction.

• **Solid catalysts** are often metals or metal oxides used in industries for their robust nature.
• **Liquid catalysts** can offer uniform interaction with liquid reactants, though separation can be challenging.
• **Gas-phase catalysts** are used less frequently due to difficulty in recovery.

Solid catalysts are particularly valued in industrial applications, such as the Haber process, for their durability and effectiveness in high-temperature reactions.

Chemical Reactions

Chemical reactions are processes where substances, called reactants, are transformed into different substances, known as products. The role of a catalyst is to speed up these reactions without being consumed in the process. For instance, during the synthesis of ammonia in the Haber process, a solid iron catalyst provides a surface for the gaseous nitrogen and hydrogen to react more efficiently.
Reaction conditions, such as temperature and pressure, alongside catalyst choice, greatly influence the rate of a chemical reaction. This makes the study and application of catalysts integral in optimizing chemical processes in both laboratory and industrial settings.

Homogeneous Catalysis

Homogeneous catalysis plays a vital role in reactions where the catalyst and reactants share the same phase. Such compatibility allows uniform distribution of the catalyst throughout the reaction medium, enhancing the reaction rate.
An example of homogeneous catalysis can be seen when hydrochloric acid (HCl) catalyzes ester hydrolysis. Here, both the reactants and the catalyst are in the liquid phase, allowing consistent interaction. Despite the advantages like uniform mixing, challenges exist in separating catalysts post-reaction. This contrasts with heterogeneous catalysis, where different phases make such separations more feasible.