AIIMS 2005 Chemistry Activation Energy MCQ Question

To analyze the question concerning an endothermic reaction with high activation energy for the forward reaction, we must first understand what these terms mean in the context of chemical kinetics.

Key Concepts:

1. Endothermic Reaction: This is a reaction that absorbs energy (usually in the form of heat) from its surroundings. In a thermochemical equation, the enthalpy change $\Delta H$ is positive.

2. Activation Energy ($E_a$): This is the minimum energy required for a reaction to occur. The higher the activation energy, the slower the reaction will be at a given temperature because fewer molecules will possess the necessary energy to overcome this barrier.

3. Reaction Coordinate Diagram: This diagram represents the energy changes during a chemical reaction. The x-axis typically represents the progress of the reaction, while the y-axis represents the energy of the system.

Explanation of the Correct Answer (C):

In the correct diagram (Option C), we expect to see the following characteristics:

• Endothermic Nature: The products should have a higher energy than the reactants, indicating that the reaction absorbs energy. This can be depicted in the diagram where the energy level of the products is higher than that of the reactants.

• High Activation Energy: There should be a significant energy barrier between the reactants and the transition state (the peak of the energy curve). This indicates that a large amount of energy is required for the reactants to reach the transition state, consistent with the notion of high activation energy.

The diagram in Option C likely shows a steep rise to the peak (high activation energy) followed by a drop to a higher energy level of the products, confirming the endothermic nature of the reaction.

Clarification of Incorrect Options:

• Option A (a): This diagram may depict a reaction with either low activation energy or an exothermic process, where the products are lower in energy than the reactants. This contradicts the requirement of high activation energy and endothermicity.

• Option B (b): Similar to Option A, this may show a transition state that is not significantly elevated or may suggest a lower energy product state. Thus, it does not fulfill the criteria of being endothermic or having high activation energy.

• Option D (d): This option might illustrate an exothermic reaction, where the energy drops from reactants to products. Again, this is inconsistent with the defined characteristics of the reaction in question.

Conclusion:

Therefore, the correct answer is Option C because it correctly illustrates an endothermic reaction with a high activation energy. The key features of the reaction coordinate diagram in this option align perfectly with the definitions and characteristics of the specified reaction type.

For further understanding, you may consider the Arrhenius equation, which relates the rate constant $k$ to the activation energy $E_a$:

$k = A e^{-\frac{E_a}{RT}}$

Where:

• $A$ is the pre-exponential factor,
• $R$ is the universal gas constant,
• $T$ is the temperature in Kelvin.

This equation shows that reactions with high activation energy will have lower rate constants at lower temperatures, reinforcing the concept of high activation energy leading to slower reaction rates.