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 for an endothermic reaction is more than \( \Delta H \), so the answer is (c).

Step 1: Understand Endothermic Reactions

An endothermic reaction is a type of chemical reaction that absorbs energy from the surroundings. This means that the enthalpy change \( \Delta H \) is positive because energy is required to convert the reactants into products. Thus, \( \Delta H > 0 \).

Step 2: Understand Activation Energy

Activation energy is the minimum amount of energy needed to start a chemical reaction. For a reaction to occur, the energy barrier must be overcome. This barrier is represented by the activation energy.

Step 3: Analyzing the Relationship Between Activation Energy and Enthalpy

In an endothermic reaction, not only must the energy to change the enthalpy (\( \Delta H \)) be provided, but also the energy to overcome the initial activation barrier. Therefore, the activation energy must be greater than the enthalpy change \( \Delta H \) to ensure that the reactants can absorb enough energy to proceed to products.

Step 4: Determine the Correct Option

From the analysis above, the minimum energy of activation for an endothermic reaction must be more than the enthalpy change \( \Delta H \). Thus, the correct option is (c) more than \( \Delta H \).

Key Concepts

Enthalpy Change (ΔH)

In a chemical reaction, enthalpy change, denoted as \( \Delta H \), represents the amount of heat absorbed or released during the reaction under constant pressure. It plays a critical role in determining whether a reaction is endothermic or exothermic. When the enthalpy change is positive \((\Delta H > 0)\), it indicates that energy is absorbed from the surroundings, characterizing an endothermic reaction. Such reactions require energy input to proceed, as they store more energy in the products than was present in the reactants.

• The magnitude of \( \Delta H \) in an endothermic reaction signifies the energy requirement to convert reactants to products.
• It influences the overall energy profile of the reaction and provides crucial insights into the energy flow.
• Understanding \( \Delta H \) helps predict the heat exchange between a system and its surroundings.

By analyzing the enthalpy change, chemists can determine if additional energy, like heating, is necessary to facilitate the reaction process.

Activation Energy

Activation energy is the minimum energy threshold that the reactants must overcome to engage in a chemical reaction. It serves as the barrier between the reactants and products, ensuring that only molecules with sufficient energy go over the energy hill to reach the activated complex and eventually form products.

• Activation energy protects the reactants from reaction under normal conditions, maintaining stability until the energy threshold is exceeded.
• In an endothermic reaction, the activation energy is particularly crucial because it is greater than the enthalpy change \((\Delta H)\). This ensures that reactants receive enough energy to not just reach but surpass the necessary transformation threshold.
• Practically, this means for a reaction to proceed at noticeable rates, conditions such as temperature need to be adjusted to provide enough kinetic energy, allowing a larger fraction of molecules to exceed the activation energy.

Understanding activation energy helps in controlling reaction speeds and aids in designing catalysts that reduce this energy, thus facilitating easier and faster reactions.

Chemical Reaction Dynamics

Chemical reaction dynamics involves the study of how reactions proceed from reactants to products, including their speed and the pathway taken. The dynamics are influenced by factors such as enthalpy change, activation energy, and the energy intermediates that occur along the reaction coordinate.

• Each reaction involves navigating a potential energy surface dictated by the energy of reactants, intermediates, and products.
• The reaction dynamics focus on crucial points like the transition state, where the energy is at its peak (activation energy), and subsequent steps leading to products.
• In an endothermic reaction, understanding the dynamics helps to predict energy needs and the influence of variables like temperature, pressure, and catalysts.

By exploring chemical reaction dynamics, chemists uncover the microscopic events and factors determining the feasibility and efficiency of a reaction. Insights from these studies contribute to innovations in industrial processes, pharmaceuticals, and renewable energy technologies.