Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

The role of a catalyst in a reversible reaction is to (a) alter the equilibrium constant of the reaction (b) increase the rate of forward reaction (c) allow the equilibrium to be achieved quickly (d) decrease the rate of backward reaction

Step-by-Step Solution

Answer

(c) allow the equilibrium to be achieved quickly

Step 1: Understanding the Function of a Catalyst

A catalyst provides an alternative pathway with a lower activation energy for a reaction, affecting both forward and backward reactions equally. It does not change the equilibrium position or the equilibrium constant of a reaction.

Step 2: Effect of Catalyst on Reaction Rate

Since a catalyst lowers the activation energy, it increases the rates of both the forward and backward reactions equally, allowing the system to reach equilibrium more quickly.

Step 3: Evaluating Given Options

Option (a) is incorrect because a catalyst does not change the equilibrium constant. Option (b) is incorrect because it doesn't specify that backward reaction rates also increase. Option (d) is incorrect because catalysts do not slow down reactions. Option (c) correctly states that a catalyst allows the equilibrium to be achieved quickly without altering the position of equilibrium.

Key Concepts

Equilibrium Constant

In the study of chemical reactions, the equilibrium constant, denoted as \( K_{eq} \), is a key parameter that quantifies the balance between reactants and products at equilibrium.
Equilibrium is the state where the rate of the forward reaction equals that of the backward reaction. Importantly, a catalyst does not change the equilibrium constant. This constant is solely dependent on the temperature and not on the presence of a catalyst.

This means that while a catalyst can make both reactions faster, it won't affect the concentrations of reactants and products at equilibrium. Thus, the equilibrium constant remains unchanged, as it is only determined by the intrinsic properties of the reaction. So, when working with catalysts, remember that the equilibrium position doesn't shift, but the time taken to reach equilibrium might change.

Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. This energy barrier ensures that only molecules with sufficient energy react.
Catalysts play a pivotal role by lowering this activation energy, providing an alternative pathway for the reaction. This is akin to providing a shortcut, making it easier for both forward and backward reactions to take place.

By lowering the activation energy, catalysts speed up the rate at which equilibrium is reached, but do not favor one direction over the other. Thus, while catalysts are crucial in speeding up reactions, their role does not alter the point of equilibrium, only the rate at which it is achieved.

Reaction Rates

The speed at which chemical reactions occur is termed as reaction rates. Catalysts significantly impact these rates by lowering the activation energy needed for the reactions to proceed.
This acceleration applies to both the forward and the backward reactions in a reversible process.

• As a result, equilibrium is achieved faster than it would naturally without a catalyst.
• It's important to note that catalysts don't just increase the speed of one part of the reaction – both directions are equally affected.

The role of the catalyst is pivotal in processes where time efficiency is crucial, but it does not affect the final balance of reactants and products at equilibrium.

Forward and Backward Reactions

In reversible reactions, two reactions occur simultaneously - the forward reaction and the backward reaction.
The forward reaction involves reactants transforming into products, whereas the backward reaction involves products converting back into reactants. At equilibrium, both reactions occur at equal rates, maintaining a consistent level of reactants and products.

Catalysts influence these reactions by enhancing their rates equally, without biasing one over the other. This means that while the reactions become faster, neither direction becomes favored. Such a balanced approach ensures the equilibrium is attained quickly, leading to efficient chemical processes without altering the equilibrium constant. Understanding this balance is crucial in both academic studies and real-world applications of chemistry.