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) \(2 \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{l})+\mathrm{MnO}_{2}(\mathrm{~s}) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (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 \(+\mathrm{H}_{2} \mathrm{O} \longrightarrow{\mathrm{H}^{+}}{\longrightarrow}\) glucose \(+\) fructose

Step-by-Step Solution

Answer

(b) uses MnO₂ as a heterogeneous catalyst (solid in liquid reaction).

Step 1: Understanding Catalysts

To find a heterogeneous catalyst, we need to remember that such catalysts exist in a different phase from the reactants. This means that in the chemical equation, the catalyst should have a different physical state from one or more of the other reactants.

Step 2: Evaluate Each Option

Look at each option provided. We need to determine the state of matter for all reactants and catalysts involved in the reactions to find out if the catalyst is in a different phase.

Step 3: Option Analysis

Let's analyze the options: - (a) All substances seem to be in the same phase. No distinct catalyst is shown. - (b) The reaction uses MnO₂(s), a solid, as a catalyst while the reactants (H₂O₂) are in liquid form. Thus, the catalyst and reactants are in different phases. - (c) FeCl₃ does not show a different phase; both reactants and FeCl₃ are in aqueous phase. - (d) The reaction involves hydrogen ions (H⁺) as a catalyst in the same phase as the reactants.

Step 4: Identifying the Heterogeneous Catalyst

From the analysis, option (b) uses MnO₂, a solid catalyst, with liquid reactants. This matches the definition of a heterogeneous catalyst since the catalyst is in a different phase than the reactants.

Key Concepts

Catalysis

Catalysis is a process that speeds up a chemical reaction without being consumed or permanently altered in the process. It involves a catalyst, a substance that provides an alternative pathway with a lower activation energy. This helps reactants convert to products faster without the catalyst going through permanent changes.

There are two main types of catalysis that you should be aware of: homogeneous and heterogeneous catalysis. Homogeneous catalysts exist in the same phase as the reactants, such as both being liquids. Heterogeneous catalysts are in a different phase from the reactants, such as a solid catalyst with gaseous or liquid reactants. The key feature of a good catalyst is that it doesn't take part in the actual chemical change but rather facilitates it by lowering the energy barrier.

Phase of Matter

Matter can exist in different phases or states, primarily solid, liquid, and gas. Knowing the phase of matter is crucial in understanding chemical reactions and catalysis. Each phase has its own properties that affect how substances interact and react.

• Solids have a fixed shape and volume due to tightly packed particles that do not have much freedom to move around.
• Liquids have a fixed volume but take the shape of the container because the particles are less tightly packed and can move around more freely.
• Gases do not have a fixed volume or shape and will expand to fill any container due to widely spaced particles that move independently.

Understanding these differences helps in identifying whether a catalyst is heterogeneous or not. A catalyst in a different phase from the reactants typically means different physical properties that contribute to how catalysis proceeds.

Chemical Reactions

Chemical reactions are processes that involve the transformation of reactants into products. They are influenced by several factors, including temperature, concentration, and the presence of catalysts.

During a chemical reaction, bonds between atoms in the reactants are broken, and new bonds are formed to create the products. The use of catalysts is a common method to enhance the rate of reaction. Catalysts do this by providing an alternative mechanism or pathway with a lower activation energy, allowing more reactant molecules to possess enough energy to react.

• Catalysis doesn't change the final outcome but influences reaction speed.
• Catalysts can be specific and only work for particular reactions.

Understanding the role of catalysts is essential for the efficient design of chemical reactions in industrial processes and laboratory experiments.

MnO2 as Catalyst

MnO₂, or manganese dioxide, is a well-known inorganic compound that acts as a catalyst specifically in heterogeneous catalysis. In the example provided, MnO₂ is a solid catalyst while the reactant, hydrogen peroxide ( H₂O₂ ), is in liquid form. This physical state difference classifies MnO₂ as a heterogeneous catalyst.

The role of MnO₂ in the decomposition of hydrogen peroxide is to lower the activation energy required for the reaction. When used, it speeds up the breakdown of hydrogen peroxide into water and oxygen without itself undergoing a permanent change. It's especially effective because, as a solid, it provides a surface that facilitates the reaction.

• MnO₂ remains unchanged at the end of the reaction, preserving its catalytic properties.
• The solid nature allows it to be easily separated from the liquid reactants after the reaction.

This makes MnO₂ a practical choice for reactions where phase difference is beneficial for the catalytic process.