Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is an example of heterogeneous catalyst? (a) \(2 \mathrm{SO}_{2}+\mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(2 \mathrm{H}_{2} \mathrm{O}_{2}(1)+\mathrm{MnO}_{2}(\mathrm{~s}) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (c) \(2 \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq}) \stackrel{\mathrm{FeCl}_{3}}{\longrightarrow} 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (d) sucrose \(+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{H}^{+}}{\longrightarrow}\) glucose \(+\) fructose

Step-by-Step Solution

Answer

Option (b) is an example of a heterogeneous catalyst.

Step 1: Understanding Heterogeneous Catalysis

Heterogeneous catalysis occurs when the catalyst is in a different phase than the reactants. This typically involves a solid catalyst and reactants in gas or liquid form.

Step 2: Identify Phases in Option (a)

In option (a), the reaction does not specify a catalyst, so it cannot be an example of heterogeneous catalysis.

Step 3: Identify Phases in Option (b)

In (b), \( ext{MnO}_2\) is a solid and \( ext{H}_2 ext{O}_2\) is typically in a liquid state. The solid \( ext{MnO}_2\) acts as a catalyst for the liquid \( ext{H}_2 ext{O}_2\), making this an example of heterogeneous catalysis.

Step 4: Identify Phases in Option (c)

In (c), \( ext{FeCl}_3\) is dissolved to form an aqueous solution, and the reactant \( ext{H}_2 ext{O}_2\) is also in an aqueous solution. Since both the catalyst and the reactants are in the same phase (aqueous), this is not heterogeneous catalysis.

Step 5: Identify Phases in Option (d)

In (d), hydrogen ions ( \( ext{H}^+\)), act as a catalyst in an aqueous solution. As both the catalyst and the reactants are in the aqueous phase, this does not qualify as heterogeneous catalysis.

Step 6: Determine the Answer

From the analysis above, the only option where the catalyst is in a different phase from the reactants is option (b) with \( ext{MnO}_2\). Thus, (b) is the example of heterogeneous catalyst.

Key Concepts

Solid Catalyst

A solid catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. Unlike other types of catalysts that might be in gaseous or liquid form, solid catalysts are usually distinct from the reactants in terms of phase.

A classic example of a solid catalyst is manganese dioxide ( MnO_2 ), which is commonly used in the decomposition of hydrogen peroxide ( H_2O_2 ) into water ( H_2O ) and oxygen ( O_2 ). In this scenario, the solid catalyst provides a surface for the hydrogen peroxide, which is typically in liquid form, to react and decompose more readily.

• The solid catalyst remains unchanged and can be reused.
• It often provides a site where reactants can come together more easily, thus enhancing the reaction rate.
• Catalysts do not alter the equilibrium position of a reaction; they only make it reach equilibrium faster.

Aqueous Solution

An aqueous solution is a solution in which the solvent is water. It is often denoted by the symbol (aq) in chemical equations. This type of solution is prevalent in many natural and artificial processes because water is an excellent solvent for dissolving a wide range of substances.

In the context of catalytic reactions, an aqueous solution can either host the reactants or the catalyst, or both. For instance, ferric chloride ( FeCl_3 ) when dissolved in water forms an aqueous solution. If a reaction occurs within this aqueous environment, such as in some homogeneous catalysis processes, all participants are uniformly dissolved in the water.

• Aqueous solutions are critical in biochemical reactions, such as those occurring in the human body.
• They facilitate ion exchange and transport in a variety of chemical reactions.
• If both the catalyst and reactants are in the aqueous phase, the process might not be a heterogeneous catalysis.

Reaction Phases

In chemistry, the term "reaction phases" refers to the different states of matter present in a reaction, such as solid, liquid, gas, or aqueous. A key feature of heterogeneous catalysis is the presence of different phases between the catalyst and the reactants.

Understanding the phases of the substances involved is crucial in determining the type of catalysis taking place.

• In heterogeneous catalysis, the catalyst is typically in a solid phase, while the reactants are in a liquid or gaseous phase.
• This difference in phases allows the reactants to adsorb onto the surface of the catalyst, facilitating the reaction by providing a dedicated reaction site.
• The reaction phases dictate how a catalyst can interact with reactants and influence the mechanism and efficiency of a reaction.