Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

A catalyst (a) Increases the frequency of collision of reacting species. (b) Alters the reaction mechanism. (c) Decreases the activation energy (d) Increases the average kinetic energy of the reacting molecules.

Step-by-Step Solution

Answer

(b) Alters the reaction mechanism, and (c) Decreases the activation energy.

Step 1: Understanding the Role of a Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. It provides an alternative pathway for the reaction with a lower activation energy.

Step 2: Evaluating Each Option

We will evaluate if the options describe what a catalyst does: (a) Is incorrect because the catalyst doesn't increase collision frequency but makes the collision path easier. (b) Is correct since a catalyst alters the reaction mechanism by providing a different pathway. (c) Is correct because a catalyst decreases the activation energy, allowing more molecules to participate in the reaction at a lower energy. (d) Is incorrect because a catalyst doesn't change the kinetic energy of molecules.

Step 3: Conclusion

Based on the evaluation, options (b) and (c) accurately describe the functions of a catalyst. A catalyst alters the reaction mechanism and decreases the activation energy, facilitating more reactions without increasing molecular kinetic energy or collision frequency.

Key Concepts

Reaction Mechanism

In chemical reactions, the reaction mechanism is a step-by-step sequence of elementary reactions by which an overall chemical change occurs. Understanding the mechanism helps chemists to visualize what happens at each stage of a reaction. Catalysts play an important role in altering these mechanisms.
Catalysts provide a different pathway for the reaction. This alternative pathway often involves breaking a reaction into simpler steps, where each step has lower energy barriers compared to the original mechanism.

• This can involve the formation of temporary intermediate compounds.
• The presence of a catalyst doesn't change the overall stoichiometry of the reaction but changes the journey to reach the products.
• By lowering these energy barriers, catalysts make it easier for the reaction to occur efficiently at lower temperatures.

To help visualize, imagine a complex mountain path that becomes accessible through a tunnel created by a catalyst, enabling a smoother journey.

Activation Energy

Activation energy is the minimum amount of energy required for a reaction to occur. It can be thought of as the energy barrier that must be overcome for reactants to be transformed into products.
The presence of a catalyst reduces this activation energy, effectively lowering the energy hill that must be climbed.

• This effectually increases the number of molecules that have enough energy to react by providing them an easier pathway.
• The catalyst does this by stabilizing the transition state of the reaction, which decreases the energy required to reach this critical threshold.
• Lower activation energy means that even at lower temperatures, more reactant molecules can participate in the reaction.

Imagine trying to jump over a wall; a high wall is challenging to overcome, but a catalyst is like adding a stepping stool, making the jump manageable and less energy-consuming.

Chemical Kinetics

Chemical kinetics is the branch of chemistry that studies the rates of chemical reactions and the factors affecting them. It helps chemists understand how fast a reaction proceeds and what influences this rate.
Key factors that affect chemical kinetics include temperature, concentration, particle size, and the presence of a catalyst.

• Temperature generally increases reaction rates by providing energy to overcome activation energy barriers.
• Higher concentrations of reactants increase the likelihood of collision and reaction.
• Smaller particle sizes provide more surface area for reactions to occur.
• Catalysts specifically influence chemical kinetics by providing an alternative reaction pathway with a lower activation energy, thus speeding up the reaction process without being consumed.

By understanding these aspects, chemists can predict and control the speed of reactions, tailoring them for industrial processes or laboratory experiments.