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 \(\mathrm{kJ} /\) 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 correct answer is (c) more than \( \Delta \mathrm{H} \).

Step 1: Understand the concepts involved

To determine the activation energy for a reaction, it's important to understand that activation energy is the energy barrier that must be overcome for a reaction to proceed. An endothermic reaction absorbs energy, with positive \( \Delta H \). The activation energy must be greater than the enthalpy change \( \Delta H \) for the reaction to proceed.

Step 2: Analyze the given options

We must decide which of the given options (a, b, c, d) aligns with our understanding. Option (a): Activation energy less than \( \Delta H \), is incorrect because it won't be sufficient to reach the energy required to start the reaction. Option (b): Activation energy is zero, means no energy is needed, which is incorrect for an endothermic reaction. Option (c): Activation energy more than \( \Delta H \), correctly implies that extra energy besides \( \Delta H \) is needed to initiate the reaction. Option (d): Activation energy equal to \( \Delta H \) implies just the exact energy for the reaction occurrence, which is unlikely because additional energy generally offsets energy losses.

Step 3: Conclude the correct option

From the analysis, choose option (c) because activation energy must be greater than the energy absorbed \( \Delta H \), ensuring the reaction intermediates reach the transition state and overcome the reaction energy barrier.

Key Concepts

Activation Energy in Reactions

In the realm of chemical reactions, activation energy is a crucial concept. It's the minimum amount of energy that reactant molecules need to collide with one another to successfully form products. This energy essentially acts as a starting boost
to push the reaction forward. Without adequate activation energy, reactants will remain locked in their initial state.In endothermic reactions, which absorb energy from the surroundings, the need for activation energy becomes even more critical. These reactions naturally require energy input to proceed because they absorb rather than release energy. The activation energy in such scenarios must exceed the enthalpy change, \( \Delta H \), to ensure that enough surplus energy is available to undertake the reaction.
This will guarantee the reactants can reach the energetic state necessary to produce products by overcoming the potential energy barrier exemplified by activation energy.

Understanding Enthalpy Change

Enthalpy change, denoted by \( \Delta H \), signifies the total energy change within a system during a reaction. In simple terms, it represents the difference in heat content of reactants and products. In endothermic reactions, \( \Delta H \) is positive because these reactions absorb energy, resulting in products having higher energy than the reactants.
Understanding this concept is vital as it shows how much energy the reaction needs from the surroundings.When analysing reactions, especially endothermic ones, enthalpy change helps indicate whether a reaction consumes or releases energy. However, it should not be confused with
activation energy, as it only covers the energy difference between products and reactants, rather than the energy required to convert reactants into either an intermediate
state or directly into the products.

Energy Barrier and Its Significance

The energy barrier in a reaction refers to the energetic threshold that must be overcome for reactants to be transformed into products. This barrier is precisely what activation energy helps surmount. Think of it as a hill that reactants have to climb over.
Without sufficient energy, reactants can't make it over, and thus, the reaction can't proceed. In endothermic reactions, this barrier is quite significant because
these reactions naturally require input of energy. The energy barrier ensures that only those reactant molecules with enough energy can attain the transition state,
which is the configuration of maximum energy along the reaction path. High energy barriers mean certain reactions are slower because fewer molecules have enough energy to overcome the barrier. Understanding the role of the energy barrier illustrates precisely why even endothermic reactions, which absorb energy, require initial activation energy.
This ensures that reactions progress through the transition state efficiently.