Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

For an endothermic reaction, where \(\Delta \mathrm{H}\) represents the enthalpy of the reaction in \(\mathrm{kJ} / \mathrm{mol}\), the minimum value for the energy of activation will be (a) less than \(\Delta \mathrm{H}\) (b) zero (c) more than \(\Delta \mathrm{H}\) (d) equal to \(\Delta \mathrm{H}\).

Step-by-Step Solution

Answer

The minimum energy of activation is more than \( \Delta H \) (Option c).

Step 1: Understand the Definitions

An endothermic reaction is a chemical reaction that absorbs energy from its surroundings, usually in the form of heat. In such reactions, the change in enthalpy \( \Delta H \) is positive because energy is absorbed. The energy of activation (or activation energy) is the minimum energy required to start the reaction.

Step 2: Analyze the Relationship Between Activation Energy and Enthalpy

For an endothermic reaction, the energy of activation must be greater than the energy needed to form the activated complex from the reactants. Since energy is absorbed, the activation energy includes both the energy needed to reach the \( \Delta H \) level and additional energy to overcome the energy barrier.

Step 3: Compare Activation Energy with \( \Delta H \)

In the energy profile of an endothermic reaction, the peak representing the activation energy is always higher than the energy change \( \Delta H \). This is because \( \Delta H \) alone represents the net energy change, while activation energy accounts for the energy needed to reach and surpass this state.

Step 4: Conclude on the Minimum Value

Therefore, the minimum value for the energy of activation must be greater than \( \Delta H \) as it needs to include not only the enthalpy change but also the energy to overcome the inherent barrier to reaction.

Key Concepts

Understanding Enthalpy Change

In the context of endothermic reactions, understanding enthalpy change is crucial. Enthalpy change, denoted as \( \Delta H \), is a thermodynamic quantity that represents the total heat content in a system. It is essentially the difference in energy between the products and the reactants. For endothermic reactions, the enthalpy change is positive.
This means the products store more energy than the reactants.
This additional energy is typically absorbed from the surroundings, which explains why such reactions can cause a decrease in temperature in their immediate environment. Because energy is absorbed, understanding \( \Delta H \) helps in predicting the energy requirement of a reaction. Simpler terms allow us to view \( \Delta H \) as the amount of heat energy that must be supplied to convert reactants into products, which is essential when considering whether an endothermic reaction would naturally occur at given conditions.

Energy of Activation: A Closer Look

The energy of activation, sometimes referred to as activation energy, is a fundamental concept in understanding chemical reactions. It is defined as the minimum energy barrier that must be overcome for a chemical reaction to proceed.
This energy is necessary for the reactants to transform into products.
For an endothermic reaction specifically, the activation energy is always greater than the enthalpy change \( \Delta H \). This is because the energy of activation includes both the energy necessary to reach the \( \Delta H \) level and additional energy to push beyond this point.

• Activation energy ensures that reactant molecules can break the necessary bonds to start the reaction.
• It also ensures new bonds can form to produce the final products.

The concept of activation energy is crucial when considering how temperature and catalysts can affect reaction rates. By lowering the activation energy, a catalyst can speed up a reaction, allowing it to proceed under lower temperatures than would be possible otherwise.

Exploring Chemical Reaction Mechanisms

Chemical reaction mechanisms provide a detailed description of the step-by-step process that occurs during a chemical reaction. These mechanisms unearth the sequence of elementary steps, or stages, that lead to the final product.
Understanding these details is significant because not all reactions happen in a single step.
For endothermic reactions, these mechanisms often involve multiple transitions where activation energy plays a pivotal role. Reaction mechanisms offer insight into how a reaction proceeds and the intermediates that might be formed along the way.

• Each step in a mechanism has its own activation energy.
• These steps can involve the breaking and forming of different chemical bonds.

By examining the mechanism, scientists can manipulate specific steps to optimize the reaction. For instance, if one step in the sequence is particularly slow, altering reaction conditions or adding a catalyst could improve overall efficiency. Understanding reaction mechanisms is thus essential for advancing chemical processes and developing new technologies.