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) \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}(\mathrm{I})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I}) \stackrel{\mathrm{HCl}(\mathrm{l})}{\longrightarrow} \mathrm{CH}_{3} \mathrm{COOH}\) \(+(\mathrm{aq})+\mathrm{CH}_{3} \mathrm{OH}(\mathrm{aq})\) (a) (ii), (iii) (b) (ii), (iii), (iv) (c) (i), (ii), (iii) (d) (iv)

Step-by-Step Solution

Answer

Option (a): Reactions (ii) and (iii) involve heterogeneous catalysis.

Step 1: Understanding Heterogeneous Catalysis

In heterogeneous catalysis, the catalyst is in a different phase (solid, liquid, gas) compared to the reactants. We will examine the phase of each reactant and the catalyst in the provided reactions to determine if they involve a heterogeneous catalyst.

Step 2: Analyzing Reaction (i)

Reaction (i) is: \(2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \stackrel{\mathrm{NO}(\mathrm{g})}{\longrightarrow} 2 \mathrm{SO}_{3}(\mathrm{~g})\). Here, both reactants and the catalyst \(\mathrm{NO}\) are in gaseous phase, which indicates that this is a homogeneous catalysis.

Step 3: Analyzing Reaction (ii)

Reaction (ii) is: \(2 \mathrm{SO}_{2}(\mathrm{~g}) \stackrel{\mathrm{Pt}(\mathrm{S})}{\longrightarrow} 2 \mathrm{SO}_{3}(\mathrm{~g})\). Here, the reactant \(\mathrm{SO}_{2}\) is in gaseous phase, while the catalyst \(\mathrm{Pt}(\mathrm{S})\) is solid. The difference in phases means this is a heterogeneous catalysis.

Step 4: Analyzing Reaction (iii)

Reaction (iii) is: \(\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \stackrel{\mathrm{Fe}(\mathrm{S})}{\longrightarrow} 2 \mathrm{NH}_{3}(\mathrm{~g})\). The reactants \(\mathrm{N}_{2}(\mathrm{~g})\) and \(\mathrm{H}_{2}(\mathrm{~g})\) are gases, while the catalyst \(\mathrm{Fe}(\mathrm{S})\) is a solid. This phase difference indicates heterogeneous catalysis.

Step 5: Analyzing Reaction (iv)

Reaction (iv) is: \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \stackrel{\mathrm{HCl}(\mathrm{l})}{\longrightarrow} \mathrm{CH}_{3} \mathrm{COOH}(\mathrm{aq})+\mathrm{CH}_{3} \mathrm{OH}(\mathrm{aq})\). Both the reactants and the catalyst \(\mathrm{HCl}\) are in liquid phase (or dissolved in water making them aqueous), indicating homogeneous catalysis.

Step 6: Identifying Reactions with Heterogeneous Catalysis

From the analysis: Reactions (ii) and (iii) involve heterogenous catalysis because they have catalysts in different phases from the reactants.

Key Concepts

Catalyst Phase

In chemical reactions, the phase of a catalyst plays a crucial role in determining the type of catalysis occurring. A catalyst is a substance that speeds up a reaction without being consumed in the process. The phases are often categorized as solid, liquid, or gas.

A heterogeneous catalyst is in a different phase than the reactants. For example, in Reaction (ii) and (iii) from the exercise, the catalysts are solids, while the reactants are gaseous. This phase distinction means that the catalysts are heterogeneous.

Understanding the phase of catalysts and reactants helps determine the mechanism of the reaction and the efficiency with which the reaction progresses.

Reaction Analysis

Analyzing a reaction requires a detailed look into the elements and compounds involved, the phases of the substances, and the presence of a catalyst. This analysis helps us understand whether a reaction is homogeneous or heterogeneous.

In Reaction (i), the analyzation reveals that the reactants and catalyst are all in the gaseous phase. Reaction (ii), meanwhile, shows a solid catalyst with gaseous reactants, indicating heterogeneous catalysis. Such comparisons are essential for understanding the dynamics and efficiencies of different chemical reactions.

• Identify phases of reactants and catalysts
• Understand the nature of interaction between different phases
• Determine if the catalysis is homogeneous or heterogeneous

Chemical Reactions

A chemical reaction involves the transformation of one or more substances into different substances by breaking and forming bonds. Catalysis is a catalyst-driven process to enhance this transformation. In our examples, several reactions take place in different phase situations.

Catalysts participate in the reaction by providing an alternative reaction pathway with a lower activation energy. In the reactions given, the presence of a catalyst facilitates the conversion of reactants into products more rapidly and efficiently. Understanding these concepts allows us to predict and manipulate reaction pathways to achieve desired outcomes.

This understanding also extends to different industrial applications where specific phase-based catalysis is beneficial for efficient chemical manufacturing.

Gaseous Phase

In the gaseous phase, the molecules of a substance spread far apart, moving freely and randomly. The freedom of movement makes gases highly suitable for reactions because the chances of particle collisions are higher. In Reaction (i), all components are in the gaseous phase, which promotes rapid mixing and interaction.

When the reactants and catalysts are gases, the reaction benefits from homogeneity, meaning the catalyst and reactants can easily intermingle. This is common in catalytic converters in vehicles where gaseous emissions react with gaseous catalysts to reduce pollutants.

Understanding the behavior of substances in the gaseous phase can significantly affect the design and control of industrial gas reactions.

Liquid Phase

The liquid phase is characterized by its ability to flow while maintaining a fixed volume. Substances in this phase are less compressible, and molecules remain closer together than in gases but are not as rigid as in solids. This phase often facilitates reactions where dissolving of substances is needed, as in Reaction (iv).

Homogeneous catalysis in the liquid phase can occur when all the reactants and catalysts are dissolved in the same liquid. This situation allows for better contact and collision between molecules. Liquid phase reactions are common in many chemical and pharmaceutical industries where precise and controlled reactions are desired.

• Ensures effective mixing and solvation
• Facilitates reactions at lower temperatures
• Ideal for reactions involving solvents and aqueous solutions

Understanding liquid-phase dynamics is essential for maximizing the efficiency and yield of various chemical reactions.