Problem 29 - 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 \(\mathrm{kJ} / \mathrm{mole}\), 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 for an endothermic reaction is more than \( \Delta \mathrm{H} \).

Step 1: Understand the Concept

To solve this problem, we need to recall that an endothermic reaction absorbs heat from the surroundings, which means \( \Delta H > 0 \). The activation energy, denoted as \( E_a \), is the minimum amount of energy required to initiate the reaction.

Step 2: Evaluate the Options

For an endothermic reaction, the energy of the products is higher than the energy of the reactants due to the absorption of heat (\( \Delta H > 0 \)). The activation energy \( E_a \) must always be greater than the energy difference \( \Delta H \) to ensure that the reaction can overcome the required energy barrier and start. Therefore, \( E_a > \Delta H \).

Step 3: Choose the Correct Option

Given the options:- (a) Less than \( \Delta H \)- (b) Zero- (c) More than \( \Delta H \)- (d) Equal to \( \Delta H \)The correct answer is (c) More than \( \Delta H \), because the activation energy must be greater than the heat absorbed during the reaction to initiate the process.

Key Concepts

Endothermic Reactions

Endothermic reactions are a fascinating type of chemical reaction that absorb energy, typically in the form of heat, from their surroundings. This means that as the reaction proceeds, the surroundings become cooler. Here, the energy stored in the chemical bonds of the products is greater than that of the reactants. The key indicator of an endothermic reaction is a positive enthalpy change (\( \Delta H > 0 \)), signifying that energy is gained by the system. For example:

• Photosynthesis in plants, where plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.
• Melting of ice, where ice absorbs heat to convert into water.

Understanding endothermic reactions is crucial, as they underline many natural and industrial processes. They require a continuous input of energy to sustain the process since they do not release energy but absorb it instead.

Enthalpy Change

Enthalpy change, represented as \( \Delta H \), is a fundamental concept in chemistry that measures the heat content change in a system during a reaction. It's crucial for knowing whether a reaction is endothermic or exothermic. To put it simply:

• When \( \Delta H > 0 \), we have an endothermic reaction, meaning the system absorbs heat.
• Conversely, if \( \Delta H < 0 \), the reaction is exothermic and releases heat.

Enthalpy change is usually measured in kilojoules per mole (\( \mathrm{kJ/mol} \)). During an endothermic reaction, the enthalpy change reflects that additional energy is needed because the products have more stored energy than the reactants. Therefore, the system pulls in heat from outside sources to compensate for this difference.

Energy Barrier

The energy barrier in a chemical reaction is a metaphorical wall that reactants must climb over in order to transform into products. This concept is often closely linked with activation energy. It represents the minimum energy that reacting molecules need to overcome for a reaction to proceed.Here's how it relates to endothermic reactions:

• The energy barrier is usually higher than the enthalpy change (\( \Delta H \)) for an endothermic reaction. This ensures there's enough energy to push the reactants to a transition state where they can transform into products.
• Activation energy (\( E_a \)) acts as this barrier. For endothermic reactions, \( E_a \) is more significant than \( \Delta H \) because the system needs not only to absorb heat but also to acquire enough energy to initiate the reaction.

A good way to visualize this is imagining the energy barrier as a hill that molecules must climb in order to roll down into the new valley of the product. Hence, in any reaction, overcoming the energy barrier is a vital step towards progress.