Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

A catalyst in a chemical reaction (a) decreases rate constant of the reaction (b) reduces enthalpy of the reaction (c) does not affect equilibrium constant of the reaction (d) increases activation energy of the reaction

Step-by-Step Solution

Answer

(c) does not affect equilibrium constant of the reaction

Step 1: Understanding the Role of a Catalyst

In chemistry, a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent change. It helps the reaction proceed faster by providing an alternative reaction pathway with a lower activation energy.

Step 2: Analyzing Each Option

Let's analyze each given option: (a) A catalyst decreases the rate constant of the reaction. This is incorrect. A catalyst actually increases the reaction rate by lowering the activation energy. (b) A catalyst reduces the enthalpy of the reaction. This is incorrect. Enthalpy change is a state function and does not depend on the pathway; thus, a catalyst does not affect it. (c) A catalyst does not affect the equilibrium constant of the reaction. This is correct. A catalyst speeds up both the forward and reverse reactions equally, so the equilibrium position remains unchanged. (d) A catalyst increases the activation energy of the reaction. This is incorrect. A catalyst lowers the activation energy.

Step 3: Concluding the Correct Option

Based on the analysis, option (c) is the correct statement. A catalyst does not change the position or value of the equilibrium constant; it only affects the rate at which equilibrium is achieved.

Key Concepts

Understanding Activation Energy

Activation energy is the energy barrier that reactants must overcome for a chemical reaction to occur. You can think of it as a wall that molecules need to climb over to transform into products. This energy barrier stops the reactants from changing too quickly.

Catalysts offer an alternative path with a lower activation energy, making it easier for the reaction to happen. When the activation energy is lower, more molecules have enough energy to reach the transition state at any given temperature. This is why a reaction with a catalyst can proceed faster.

• Activation energy is crucial as it determines the speed of the reaction.
• A lower activation energy means more frequent successful collisions between reactant molecules.
• Catalysts do not change the reactants or products, just the speed of the transformation.

Understanding this concept is essential for chemical kinetics, the study of reaction rates.

The Role of the Equilibrium Constant

The equilibrium constant, denoted as K, is a value that expresses the ratio of product concentrations to reactant concentrations at equilibrium. Each chemical reaction has its own equilibrium constant. It's determined by the equation:\[K = \frac{[Products]}{[Reactants]}\]At equilibrium, the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products remain constant. A catalyst acts on the reaction without altering this equilibrium constant.

Why? Because a catalyst speeds up both the forward and reverse reactions equally, leaving the ratio of products to reactants – or the equilibrium constant – unchanged.

• Equilibrium constant is key to understanding chemical balance.
• This concept tells us about the position of equilibrium, not the speed to reach it.
• Catalysts do not affect equilibrium constants, only the time taken to reach them.

A good grasp of equilibrium constants allows you to predict the direction of a reaction and assess its completeness.

Chemical Reaction Rate

The rate of a chemical reaction refers to how fast the reactants are converted into products. It's a measure of change over time. Reaction rates can be affected by several factors, including temperature, concentration, and the presence of a catalyst.

With a catalyst, the rate of reaction increases because it lowers the activation energy needed. This means that more reactant molecules have the necessary energy to undergo successful collisions.

• Temperature: Increasing temperature raises reaction rates by energizing molecules to collide more effectively.
• Concentration: Higher concentrations lead to more collisions between reactant particles, accelerating reactions.
• Catalyst: A catalyst specifically lowers activation energy, speeding up both the forward and reverse reactions.

By understanding these factors, you can predict and control reaction rates in practical applications, from industrial synthesis to biochemical processes.