Question:

The addition of a catalyst during a chemical reaction alters which of the following quantities ?

Updated On: Apr 20, 2025

Internal energy
Enthalpy
Activation energy
Entropy

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The Correct Option is C

Solution and Explanation

The Role of a Catalyst in a Chemical Reaction

Explanation:

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It does so by lowering the activation energy required for the reaction to occur.

How Does a Catalyst Work?

In a chemical reaction, the activation energy (\( E_a \)) is the energy required to initiate the reaction. A catalyst lowers the activation energy by providing an alternative reaction pathway with a lower activation energy. This makes it easier for reactants to be converted into products.

Key Concept: The catalyst creates a short intermediate path with a lower activation energy, thus decreasing the threshold for the reaction to occur.

Energy Profile Diagram

The presence of a catalyst can be shown in an energy profile diagram:

\(E_a^{\text{uncatalyzed}} > E_a^{\text{catalyzed}}\)

In this diagram, the activation energy for the catalyzed reaction (\( E_a^{\text{catalyzed}} \)) is lower than for the uncatalyzed reaction (\( E_a^{\text{uncatalyzed}} \)), allowing the reaction to proceed more quickly.

Conclusion: A catalyst lowers the activation energy by creating a different reaction pathway, leading to an increase in the reaction rate without being consumed in the process.

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Concepts Used:

Chemical Kinetics

Chemical kinetics is the description of the rate of a chemical reaction. This is the rate at which the reactants are transformed into products. This may take place by abiotic or by biological systems, such as microbial metabolism.

Rate of a Chemical Reaction:

The speed of a reaction or the rate of a reaction can be defined as the change in concentration of a reactant or product in unit time. To be more specific, it can be expressed in terms of: (i) the rate of decrease in the concentration of any one of the reactants, or (ii) the rate of increase in concentration of any one of the products. Consider a hypothetical reaction, assuming that the volume of the system remains constant. R → P

Read More: Chemical Kinetics MCQ

Factors Affecting The Reaction Rate:

The concentration of Reactants - According to collision theory, which is discussed later, reactant molecules collide with each other to form products.
Nature of the Reactants - The reaction rate also depends on the types of substances that are reacting.
Physical State of Reactants - The physical state of a reactant whether it is solid, liquid, or gas can greatly affect the rate of change.
Surface Area of Reactants - When two or more reactants are in the same phase of fluid, their particles collide more often than when either or both are in the solid phase or when they are in a heterogeneous mixture. In a heterogeneous medium, the collision between the particles occurs at an interface between phases. Compared to the homogeneous case, the number of collisions between reactants per unit time is significantly reduced, and so is the reaction rate.
Temperature - If the temperature is increased, the number of collisions between reactant molecules per second. Increases, thereby increasing the rate of the reaction.
Effect Of Solvent - The nature of the solvent also depends on the reaction rate of the solute particles.
Catalyst - Catalysts alter the rate of the reaction by changing the reaction mechanism.