8.
For an endothermic reaction, where 𝛥𝐻 represents the enthalpy of the reaction in kJ/mol,
the minimum value for the energy of activation will be:
a. Less than 𝛥𝐻
b. Zero
c. More than 𝛥𝐻
d. Equal to 𝛥𝐻

The correct answer is c. More than 𝛥𝐻. For endothermic reactions, the activation energy (EaEa) must exceed the positive enthalpy change ($\Delta H>0$) to reach the transition state from lower-energy reactants to higher-energy products.​

Option Analysis

• a. Less than 𝛥𝐻: Incorrect, as this applies to exothermic reactions where products have lower energy, allowing Ea<∣ΔH∣Ea<∣ΔH∣.​

• b. Zero: Incorrect, since all reactions require some minimum energy barrier for effective collisions; zero would imply spontaneous reaction without activation.​

• c. More than 𝛥𝐻: Correct. The relationship is Ea=Ea′+ΔHEa=Ea′+ΔH, where Ea′Ea′ (reverse activation energy) is always positive, so Ea>ΔHEa>ΔH. This is the minimum possible value.​

• d. Equal to 𝛥𝐻: Incorrect, as equality would require zero reverse activation energy, which violates the energy barrier principle.​

Energy Profile

In endothermic reactions, reactants sit at lower potential energy than products. The activation energy is the uphill climb to the transition state peak, always steeper than $\Delta H$ (the net rise). Catalysts lower EaEa but keep it above $\Delta H$.​

Introduction to Endothermic Reaction Activation Energy

Endothermic reactions absorb heat, with positive $\Delta H$, making products higher in energy than reactants. The endothermic reaction activation energy minimum value is always greater than $\Delta H$, a key concept for CSIR NET Life Sciences and chemistry exams. This ensures molecules overcome the energy barrier to form unstable products.​

Why Activation Energy Exceeds ΔH

The potential energy diagram shows reactants at baseline, rising to a transition state, then to higher-energy products. Mathematically, forward Ea=Ea= reverse Ea′+ΔHEa′+ΔH. Since Ea′>0Ea′>0, the minimum forward Ea>ΔHEa>ΔH. This differs from exothermic cases where Ea<∣ΔH∣Ea<∣ΔH∣.​

• Forward path requires extra energy beyond $\Delta H$ for bond breaking.

• Reverse (exothermic) path has lower barrier.

• Catalysts reduce both EaEa values equally, preserving Ea>ΔHEa>ΔH.

Exam Relevance for CSIR NET

CSIR NET questions test this via multiple-choice on reaction profiles. Practice drawing diagrams: label EaEa, $\Delta H$, and compare options. Common trap: confusing with exothermic minimum EaEa.​

Key Takeaways

• Minimum value: More than $\Delta H$.​

• Visualize via reaction coordinate graphs.

• Applies to biochemistry like photosynthesis (endothermic).​