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) \(2 \mathrm{CO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{~g}) \stackrel{\mathrm{Pt}(\mathrm{S})}{\longrightarrow} 2 \mathrm{CO}_{2}\) (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

(c) The reaction with a solid catalyst Pt and gaseous reactants is an example of heterogeneous catalysis.

Step 1: Understand Heterogeneous Catalysis

Heterogeneous catalysis involves a catalyst that is in a different phase compared to the reactants. This typically means the catalyst is a solid while the reactants are gases or liquids.

Step 2: Analyze Each Option

Go through each reaction option and identify the phases of the reactants and catalysts: (a) The catalyst and reactants are all in gaseous state, NO(g). (b) The catalyst and reactants are all in gaseous state, NO(g). (c) The catalyst is Pt(s), a solid, and reactants are gases, hence different phases. (d) The catalyst and reactants are all in gaseous state, I2(g).

Step 3: Identify Reaction with Different Phases

In heterogeneous catalysis, one must look for the reaction where the catalyst is in a different phase from the reactants. Only option (c) has a solid catalyst (Pt) while the reactants are gases, making it an example of heterogeneous catalysis.

Key Concepts

Catalyst Phases

Catalyst phases are crucial to understanding how chemical reactions can be enhanced. In general, a catalyst speeds up the rate of a chemical reaction without itself being consumed. The most important aspect in the concept of catalyst phases is whether the catalyst is in the same phase (homogeneous) or a different phase (heterogeneous) than the reactants.

• In a homogeneous catalysis, both the catalyst and the reactants are in the same phase. This makes it convenient as they can interact more easily without phase boundaries.
• In contrast, heterogeneous catalysis involves the catalyst being in a different phase from the reactants. This often happens when a solid catalyst interacts with gaseous or liquid reactants, providing unique sites on its surface for the reaction to occur.

This phase difference is essential as it allows for reactions on the catalyst's surface, where different processes, such as adsorption of reactants, can occur effectively.

Solid Catalyst

A solid catalyst often plays a significant role in heterogeneous catalytic reactions. Such catalysts are typically metal or metal oxide materials. Their solid-state structure provides a surface onto which reactants can adsorb, react, and then desorb as product molecules.

• Solid catalysts like platinum ( ext{Pt}), widely used in automotive catalytic converters, are particularly effective due to their stable, solid surface, which accommodates various chemical transformations.
• The structure of solid catalysts provides reactive sites where molecules can undergo transformation. These sites include edges, corners, and defect sites on the catalyst surface.

By interacting through these solid surfaces, chemical bonds of reactants can be weakened or formed during the reaction process. This turns solid catalysts into essential players in the chemical industry, accelerating reaction rates and improving efficiency.

Gas-Phase Reactions

Gas-phase reactions can be greatly influenced by the presence of a solid catalyst, creating an optimal pathway for reactant molecules to transform into products. In these situations, even if all reactants are in the gas phase, the solid catalyst can provide binding sites that stabilize reaction intermediates that would not be possible in the absence of the catalyst.

• The reactants in a gaseous state contact the solid surface, allowing for rapid diffusion and interaction with the catalytic sites.
• When a reaction is catalyzed on solid surfaces, it's often due to the adsorption of gas molecules onto the catalyst surface, where subsequent reactions take place resulting in new molecular formations.

Ultimately, gas-phase reactions facilitated by solid catalysts can offer enhanced control over the rates and selectivities of chemical transformations, making them vital in industrial gas-phase processes like ammonia production or hydrocarbon processing.