Most chemical reactions are accelerated by an increase in temperature. The temperature dependence of the rate of a chemical reaction is explained by the Arrhenius equation
\[k = A\,e^{-E_a/RT}\]
where \(A\) is the Arrhenius factor (also called the frequency factor or pre-exponential factor) and \(E_a\) is the activation energy, expressed in J mol−1. A plot of ln \(k\) against \(1/T\) is a straight line with slope \(-E_a/R\). The rate constants \(k_1\) and \(k_2\) of a reaction at temperatures \(T_1\) and \(T_2\) are related as
\[\log\frac{k_2}{k_1} = \frac{E_a}{2.303R}\left[\frac{T_2 - T_1}{T_1T_2}\right]\]
For a chemical reaction, a rise in temperature by 10 K nearly doubles the rate constant, because the fraction of molecules having energy equal to or greater than the activation energy increases.
For a reaction, the activation energies of the forward and the backward reactions are equal. Which of the following is true for this reaction?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Energy is neither absorbed nor released in the reaction.
On a plot of potential energy against reaction coordinate, the reactants (A), the activated complex (C) and the products (B) each sit at a fixed energy. Both the forward and the backward reaction pass through the same activated complex C.
Forward reaction: Ea(forward) = energy of C − energy of A.
Backward reaction: Ea(backward) = energy of C − energy of B.
Equal barriers: Ea(forward) = Ea(backward), so energy of A = energy of B.
Enthalpy change: ΔH = energy of B − energy of A = Ea(forward) − Ea(backward) = 0.
The reactants and products have the same energy, so no energy is absorbed or released in the reaction.
Why the other options are incorrect
Option (1) Energy is released in the reaction. — Incorrect. Energy is released only when the products lie below the reactants (as A and B are drawn in Fig. 3.7); then Ea(backward) is larger than Ea(forward).
Option (2) Energy is absorbed in the reaction. — Incorrect. Energy is absorbed only when the products lie above the reactants; then Ea(forward) is larger than Ea(backward).
Option (4) The activation energy of the reaction is zero. — Incorrect. Equal does not mean zero. The reactants still need energy to form the activated complex C, which lies above both A and B; that energy is the activation energy.
NCERT (2026-27 edition):
3.4 Temperature Dependence of the Rate of a Reaction – “The energy required to form this intermediate, called activated complex (C), is known as activation energy (Ea). Fig. 3.7 is obtained by plotting potential energy vs reaction coordinate. Reaction coordinate represents the profile of energy change when reactants change into products. Some energy is released when the complex decomposes to form products. So, the final enthalpy of the reaction depends upon the nature of reactants and products.”
Fig. 3.7: Diagram showing plot of potential energy vs reaction coordinate