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) \(2 \mathrm{SO}_{2}(\mathrm{~g}) \stackrel{\mathrm{P}(\mathrm{S})}{\longrightarrow} 2 \mathrm{SO}_{3}(\mathrm{~g})\) (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) \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}(\mathrm{I})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I}) \stackrel{\mathrm{HCl}(\mathrm{l})}{\longrightarrow} \mathrm{CH}_{3} \mathrm{COOH}\) \(+(a q)+C H_{3} O H(a q)\) (a) (ii), (iii) (b) (ii), (iii), (iv) (c) (i), (ii), (iii) (d) (iv)

Short answer

The correct answer is (a) (ii), (iii).

Step-by-step solution

Understand Heterogeneous Catalysis

Heterogeneous catalysis occurs when the catalyst is in a different phase than the reactants. Commonly, solid catalysts are used with gaseous or liquid reactants.

Analyze Reaction (i)

The reaction \(2 \mathrm{SO}_{2}(mathrm{~g})+\mathrm{O}_{2}(mathrm{~g}) \stackrel{\mathrm{No}(mathrm{g})}{\longrightarrow} 2 \mathrm{SO}_{3}(mathrm{~g})\) has a gaseous catalyst \(\mathrm{NO}(\mathrm{g})\). Since the reactants are also gases, this is an example of homogeneous catalysis, not heterogeneous.

Analyze Reaction (ii)

In the reaction \(2 \mathrm{SO}_{2}(mathrm{~g}) \stackrel{\mathrm{P}(mathrm{S})}{\longrightarrow} 2 \mathrm{SO}_{3}(mathrm{~g})\), the reactant is a gas and the catalyst is a solid (\(\mathrm{P}(\mathrm{S})\)), indicating heterogeneous catalysis.

Analyze Reaction (iii)

The reaction \(\mathrm{N}_{2}(mathrm{~g})+3 \mathrm{H}_{2}(mathrm{~g}) \stackrel{\mathrm{Fe}(\mathrm{S})}{\longrightarrow} 2 \mathrm{NH}_{3}(mathrm{~g})\) uses a solid catalyst \(\mathrm{Fe}(\mathrm{S})\) with gaseous reactants; this fits the criterion for heterogeneous catalysis.

Analyze Reaction (iv)

The reaction \(\mathrm{CH}_{3}\mathrm{COOCH}_{3}(\mathrm{I})+\mathrm{H}_{2}\mathrm{O}(\mathrm{I}) \stackrel{\mathrm{HCl}(\mathrm{l})}{\longrightarrow} \mathrm{CH}_{3}\mathrm{COOH}(aq)+\mathrm{CH}_{3}\mathrm{OH}(aq)\) involves reactants and catalyst all in liquid phase, indicating homogeneous catalysis.

Determine the Correct Answer

From the analysis, reactions (ii) and (iii) involve heterogeneous catalysis. This corresponds to answer choice (a) (ii), (iii).

Key concepts

Phase Difference in Catalysis

In chemical reactions, catalysts play a crucial role in speeding up processes without being consumed in the reaction. However, not all catalysts operate in the same phase as the reactants. This leads to two major types of catalysis: homogeneous and heterogeneous catalysis. Heterogeneous catalysis occurs when the catalyst is in a different phase than the reactants.
For instance, consider a reaction where gaseous reactants interact with a solid catalyst. Here, the catalyst and reactants are present in different phases, hence it's a phase difference. This contrast between the phases is fundamental as it allows the solid surface of the catalyst to facilitate the reaction by providing a site for the reaction to take place.
One classic example is the Haber process, where nitrogen and hydrogen gases react over solid iron to form ammonia. In this reaction, the solid catalyst provides a surface on which the gases can adsorb, align, and react, underscoring the importance of phase differences in catalysis.

Gaseous and Solid Phases in Reactions

Understanding the interaction between gaseous and solid phases in reactions is essential to grasp heterogeneous catalysis. In many industrial processes, gaseous reactants are used with solid catalysts.
The surface of the solid catalyst is where the reaction primarily occurs. In this setting, molecules from the gas phase collide and stick to the surface of the solid. This process is known as adsorption. Here, the molecules are more concentrated and in close contact, allowing the reaction to proceed faster than it would in the gas phase alone.
For example, in the reaction of nitrogen with hydrogen over an iron catalyst, the gas molecules are adsorbed on the iron's surface. This adsorption reduces the energy required for the reaction, leading to the efficient formation of ammonia. Essentially, the solid phase of the catalyst provides a platform that fosters increased reaction rates for gaseous reactants.

Catalysis in Chemical Reactions

Catalysts are indispensable in the world of chemical reactions. They enable reactions to proceed at a much faster rate and often under milder conditions than would otherwise be possible. Catalysts operate by lowering the activation energy barrier, providing a viable pathway for the reaction.
In heterogeneous catalysis, the solid catalyst surface offers active sites that interact with reactant molecules. This interaction facilitates the breaking and forming of chemical bonds. By offering these active sites, the catalyst not only speeds up the reaction but may also influence the reaction pathway, leading to different products or better yields.
Let's consider the conversion of sulfur dioxide (SO8) to sulfur trioxide (SO9) using a solid phosphorus catalyst. The catalyst remains in the solid state, while sulfur dioxide and oxygen react in the gaseous phase. The solid catalyst enables the reaction to happen more efficiently by interacting with gaseous molecules on its surface, showcasing the pivotal role of phase differences in chemical catalysis.