Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following reactions is an example of heterogeneous catalysis? (a) \(2 \mathrm{CO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{~g}) \stackrel{\mathrm{NO}(\mathrm{g})}{\longrightarrow} 2 \mathrm{CO}_{2}\) (b) \(2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \stackrel{\mathrm{NO}(\mathrm{g})}{\longrightarrow} 2 \mathrm{SO}_{3}\) (c) \(2 \mathrm{CO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{~g}) \stackrel{\mathrm{PL}(\mathrm{S})}{\longrightarrow} 2 \mathrm{CO}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}(\mathrm{g}) \stackrel{\mathrm{I}_{2}(\mathrm{~g})}{\longrightarrow} \mathrm{CH}_{4}+\mathrm{CO}\)

Step-by-Step Solution

Answer

The reaction c) is an example of heterogeneous catalysis.

Step 1: Identify Catalyst in Each Reaction

For each given chemical reaction, identify the substance that is serving as the catalyst by looking at the chemical equation's arrow. The catalyst is written above the arrow.

Step 2: Define Heterogeneous vs. Homogeneous Catalysis

Understand the difference between heterogeneous and homogeneous catalysis. In heterogeneous catalysis, the catalyst is in a different phase (solid, liquid, gas) than the reactants. In homogeneous catalysis, the catalyst is in the same phase as the reactants.

Step 3: Analyze Each Reaction

Examine each reaction: - (a) uses NO(g) as a catalyst, which is a gas and thus in the same phase as the reactants (gases) - homogeneous. - (b) uses NO(g) as a catalyst, similar to (a), it's in the same phase as the reactants - homogeneous. - (c) uses Pad+ (PL(s)) as a catalyst, which is a solid while the reactants are gases - heterogeneous. - (d) uses I2(g) as a catalyst, which is a gas and in the same phase as the reactants - homogeneous.

Step 4: Identify Heterogeneous Catalysis Reaction

From the analysis in the previous step, the reaction (c) involving a solid catalyst PL(s) and gaseous reactants fits the definition of heterogeneous catalysis.

Key Concepts

Catalyst Identification

A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. But how do you know which part of a reaction is the catalyst? It's usually written above the arrow in a chemical equation. Identifying the catalyst is the first step in understanding the type of catalysis involved. For instance, in the equation: \(2 \mathrm{CO}(\mathrm{g}) + \mathrm{O}_{2}(\mathrm{g}) \stackrel{\mathrm{NO}(\mathrm{g})}{\longrightarrow} 2 \mathrm{CO}_{2}\), the \(\mathrm{NO}\) gas is the catalyst. It helps the reaction proceed faster without undergoing a permanent change itself. Understanding how to identify the catalyst is essential because it is crucial in determining whether the reaction is homogeneous or heterogeneous catalysis. Knowing what to look for makes it easier to analyze different reactions.

Phases of Matter

The phases of matter—solid, liquid, and gas—play a significant role in determining the type of catalysis occurring in a reaction. Each substance in a chemical reaction can exist in one of these phases.

• Solids are materials with a fixed shape and volume. They do not expand to fill their container.
• Liquids have a definite volume but take the shape of their container.
• Gases have neither a definite shape nor a fixed volume and will expand to fill their container.

When analyzing reactions, it is essential to consider the phases of each component. The key to identifying the nature of catalysis is recognizing whether the catalyst is in the same or different phase compared to the reactants. For example, if a reaction uses \(\mathrm{PL}(\mathrm{s})\) as a catalyst while all reactants are gases, this indicates heterogeneous catalysis as different phases are involved.

Homogeneous Catalysis

Homogeneous catalysis occurs when the catalyst and the reactants are in the same phase. This type of catalysis is particularly advantageous because it often leads to better mixing and allows for a more uniform reaction process. Take the example reaction: \(2 \mathrm{SO}_{2}(\mathrm{g}) + \mathrm{O}_{2}(\mathrm{g}) \stackrel{\mathrm{NO}(\mathrm{g})}{\longrightarrow} 2 \mathrm{SO}_{3}\).In this scenario, \(\mathrm{NO}(\mathrm{g})\) serves as the catalyst. Since it is a gas, just like the other reactants, the reaction is a classic example of homogeneous catalysis.

• Reactions are typically faster due to improved mixing.
• Processing is easier since all reactants and catalysts are in the same phase.

Understanding homogeneous catalysis allows you to predict how reactions will proceed and why having all components in the same phase can be beneficial in chemical manufacturing processes.