Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

A chemical reaction is catalysed by a catalyst \(X\). Hence, X (a) increases activation energy of the reaction (b) does not affect equilibrium constant of the reaction (c) decreases rate constant of the reaction (d) reduces enthalpy of the reaction

Step-by-Step Solution

Answer

The correct answer is (b) does not affect equilibrium constant of the reaction.

Step 1: Understanding the Role of a Catalyst

A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. It works by providing an alternative reaction pathway with a lower activation energy. This means more reactant molecules have sufficient energy to react, increasing the rate of the reaction.

Step 2: Analyzing the Options

Let's analyze each option: (a) Increases activation energy: This is incorrect because a catalyst actually decreases activation energy. (b) Does not affect equilibrium constant: This is correct because a catalyst speeds up both the forward and reverse reactions equally, thus it does not change the equilibrium position. (c) Decreases rate constant: This is incorrect because a catalyst increases the rate constant, hence increasing the reaction rate. (d) Reduces enthalpy: This is incorrect because a catalyst does not change the enthalpy of the reaction; it only affects the activation energy.

Step 3: Selecting the Correct Option

Based on the analysis above, option (b) 'does not affect equilibrium constant of the reaction' is the correct answer. A catalyst only changes the rate of reaction without altering the equilibrium constant or thermodynamic quantities like enthalpy.

Key Concepts

Activation Energy

Activation energy is the minimum amount of energy required for a chemical reaction to occur. It acts like a barrier that reactants must overcome to be converted into products. Higher activation energy means a slower reaction, while lower activation energy facilitates a faster one.

When a catalyst is introduced to a reaction, it provides an alternative pathway with a lower activation energy. This means that more reactant molecules will have enough energy to surpass this barrier, effectively increasing the reaction rate.

• Catalysts do not change the initial or final energy levels of the reaction.
• They only lower the height of the energy barrier that the reactants need to climb.

The beauty of a catalyst lies in its ability to be recovered unchanged after the reaction, ready to be used again.

Equilibrium Constant

The equilibrium constant, denoted as K, is a numerical value that describes the ratio of the concentrations of products to reactants at equilibrium for a reversible chemical reaction. It provides critical insight into the balance of a reaction when it has stabilized.

In the presence of a catalyst, the equilibrium constant remains unaffected. This is because a catalyst speeds up both the forward and reverse reactions equally. Therefore, it does not shift the position of equilibrium, only the time it takes to reach it.

• Equilibrium constant is determined by the temperature and intrinsic nature of the substances involved.
• Catalysis plays no role in altering these factors, ensuring the equilibrium position remains constant.

So, while catalysts make reactions faster, they don't change the inherent balance in a chemical system.

Reaction Rate

Reaction rate refers to the speed at which reactants are converted into products in a chemical reaction. It is influenced by the concentration of reactants, temperature, surface area, and the presence of a catalyst.

When a catalyst is added, it significantly increases the reaction rate by lowering the activation energy. More collisions between reactant molecules become successful, leading to a faster production of products.

• A higher reaction rate implies the reaction reaches equilibrium more quickly.
• Catalysts amplify reaction rates without the need for additional energy input.

In practical applications, the use of catalysts enables more efficient chemical processes, saving time and energy while maintaining the same product yield.

Enthalpy Change

Enthalpy change, often symbolized as ΔH, represents the heat energy change in a reaction at constant pressure. It shows whether a reaction absorbs or releases energy, classifying the reaction as endothermic or exothermic.

Catalysts have no effect on the enthalpy change of a reaction. This is because they do not alter the energy levels of reactants and products, but only provide an easier pathway for the reaction by lowering the activation energy.

• The total energy required or released in a chemical reaction remains the same, with or without a catalyst.
• Catalysts influence kinetic properties, not thermodynamic ones like ΔH.

Understanding that the enthalpy change is a function of the reactants and products helps clarify why catalysts don't impact this energy change.