Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

The activation energy for a simple chemical reaction \(\mathrm{A} \longrightarrow \mathrm{B}\) is \(\mathrm{E}\) in forward direction. The activation energy for reverse reaction (a) can be less than or more than \(\mathrm{E}\) (b) is always double of \(\mathrm{E}\) (c) is negative of \(\mathrm{E}_{4}\) (d) is always less than \(\mathrm{E}_{\mathrm{a}}\)

Step-by-Step Solution

Answer

(a) can be less than or more than \(E\) is correct.

Step 1: Understand Activation Energy

Activation energy (\(E_{a}\)) is the minimum energy required to initiate a chemical reaction. For the forward reaction \(\mathrm{A} \rightarrow \mathrm{B}\), the activation energy is given as \(E\). This is the energy needed to convert reactant \(A\) to product \(B\).

Step 2: Consider Reverse Reaction

For the reverse reaction \(\mathrm{B} \rightarrow \mathrm{A}\), the activation energy is the energy required to revert the product \(B\) back to reactant \(A\). This energy is not fixed and depends on the overall energy profile of the reaction.

Step 3: Analyze Energy Profile Perspective

The forward and reverse activation energies depend on the change in enthalpy (\(\Delta H\)) of the reaction. If \(\Delta H\) is positive, then the forward activation energy is higher than the reverse. If \(\Delta H\) is negative, the reverse activation energy is higher. Thus, the reverse activation energy can be less than, more than, or equal to \(E\).

Step 4: Evaluate Given Options

(a) suggests the reverse activation energy can be less than or more than \(E\), which aligns with our analysis. (b) implies the reverse energy is always double, which is incorrect. (c) suggests a negative relation, incorrect in context. (d) says always less than \(E\), which isn't universally true.

Key Concepts

Chemical Reaction

In science, a chemical reaction involves the transformation of reactants into products through the breaking and forming of chemical bonds. This process can involve either the release or absorption of energy. By understanding the basic components and energy changes in chemical reactions, we can predict how and why reactions occur.

A reaction needs sufficient energy to occur. This is known as the activation energy. It's the minimum energy required to convert reactants into a transition state before forming products. For instance, in the reaction between A and B, activation energy serves as the barrier that must be overcome for the reaction to proceed.

Factors like temperature, concentration, and catalysts affect how easily a reaction can occur. The higher the temperature or concentration, the more likely molecules have enough energy to surpass this energy barrier. Catalysts, while not consumed in the reaction, lower the activation energy making it easier for reactions to take place.

Reverse Reaction

A reverse reaction occurs when the products of a chemical reaction can transform back into the original reactants. Under certain conditions, reactions can achieve a state of equilibrium where the rates of the forward and reverse reactions are equal. This means the concentrations of reactants and products remain constant over time.

Understanding the reverse reaction's activation energy is crucial. It is not fixed and varies according to the energy profile of the reaction. If the reverse reaction has a higher activation energy than the forward reaction, it is less likely to occur as frequently. Conversely, if the reverse activation energy is lower, it could be energetically favorable and occur more often.

Reversible reactions are common in chemical processes like synthesis and decomposition. By analyzing the activation energies, we can understand which direction a reaction is more likely to proceed and under what conditions it happens faster.

Enthalpy Change

Enthalpy change (") is an essential concept in understanding chemical reactions. It represents the overall heat exchange during a chemical reaction at constant pressure. The enthalpy change (") is the difference in enthalpy between products and reactants, indicating whether a reaction is exothermic (releases heat) or endothermic (absorbs heat).

• If " is positive, the reaction absorbs energy, making it endothermic. These reactions have products with higher energy than reactants.
• If " is negative, the reaction releases energy and is exothermic, with products having lower energy than the reactants.

The enthalpy change affects the activation energies of the forward and reverse reactions. In exothermic reactions, the reverse activation energy tends to be higher than the forward activation energy. For endothermic reactions, it's the other way around.

Understanding " helps in predicting the feasibility and spontaneity of chemical reactions, providing insight into how likely a reaction is to occur naturally under given conditions. It also has practical applications, like designing industrial processes and understanding metabolic pathways in biology.