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

(c) more than \( \Delta \mathrm{H} \).

Step 1: Understand the Question

The question asks about the minimum value for the activation energy in an endothermic reaction, given the enthalpy change \( \Delta \mathrm{H} \). We need to compare the activation energy against the enthalpy change.

Step 2: Define Key Concepts

In an endothermic reaction, \( \Delta \mathrm{H} \) is positive because the system absorbs heat from the surroundings. Activation energy is the minimum energy required to initiate a chemical reaction.

Step 3: Analyze Relationship Between Activation Energy and \( \Delta \mathrm{H} \)

In an endothermic reaction, the energy of the products is higher than the energy of the reactants and additional energy (i.e., activation energy) is needed to reach the transition state. Therefore, the activation energy is typically greater than the enthalpy change \( \Delta \mathrm{H} \).

Step 4: Choose the Correct Answer

Since the minimum activation energy must surpass the energy needed for the enthalpy change (\( \Delta \mathrm{H} \)) in endothermic reactions, it must be more than \( \Delta \mathrm{H} \). Thus, the correct choice is (c) more than \( \Delta \mathrm{H} \).

Key Concepts

Endothermic Reaction

In simple terms, an endothermic reaction is one that absorbs energy or heat from its surroundings. This can be remembered by breaking down the word "endothermic": "endo" means inside or within, and "thermic" refers to heat.

When a reaction is endothermic, it means the products formed in the reaction have higher energy than the reactants. The system absorbs this energy from the environment, and this is why the surrounding area might get cooler during such a process. An everyday example of an endothermic process is melting ice; it requires heat to turn solid ice into liquid water.

Essentially, if you are asked for the energy dynamics of an endothermic reaction, it’s crucial to recall that the key aspect is the absorption of energy. As a rule of thumb, the enthalpy change ($ $   ), denoted as $  $, is positive because energy is consumed to facilitate the reaction.

Enthalpy Change

Enthalpy change is a concept in chemistry that helps us understand whether a reaction will release or absorb energy. It is denoted by $     $, and represents the total heat content in a system.

This value can either be positive or negative depending on the type of reaction:

• **Positive $$ (Enthalpy gain)**: This occurs in endothermic reactions where energy is absorbed by the system.
• **Negative $$ (Enthalpy loss)**: This is typical in exothermic reactions, where energy is released during the reaction.

Using the concept of enthalpy change helps chemists determine how much energy is involved in a reaction and allows prediction of whether the reaction feels hot or cold to the environment.

Chemical Reaction Mechanisms

A chemical reaction mechanism explores the detailed step-by-step process by which reactants are transformed into products. Understanding these mechanisms is crucial for predicting reaction behavior and controlling it in industrial or laboratory settings.

Every chemical reaction involves the breaking and forming of bonds, represented by the energy changes throughout the reaction's progression. Knowing the mechanism allows scientists to identify the intermediates, transition states, and activation energy - the minimum energy barrier required to drive the reaction forward.

Understanding reaction mechanisms involves analyzing the complex interaction of molecules along the reaction path. This knowledge can lead to the development of catalysts that lower the activation energy, making reactions occur more readily under various conditions. Reaction mechanisms also help elucidate why certain reactions are slow or fast, leading to better control and efficiency in chemical processes.