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.
Match List-I with List-II.
| List-I | List-II |
|---|---|
| (A) Collision frequency, Z | (I) Kinetic energy of the maximum fraction of molecules |
| (B) Probability or steric factor, P | (II) Unstable intermediate that exists for a very short time and then breaks up to form the products |
| (C) Most probable kinetic energy | (III) Number of collisions per second per unit volume of the reaction mixture |
| (D) Activated complex | (IV) Takes into account that the colliding molecules must be properly oriented |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A-III, B-IV, C-I, D-II
These terms describe how often molecules collide, how they must collide, how their kinetic energies are spread and what they form on collision. In the collision theory, the rate of a bimolecular reaction is Rate = P ZAB e−Ea/RT.
(A) Collision frequency, Z — This matches (III) Number of collisions per second per unit volume of the reaction mixture. It counts how often the molecules collide, without asking whether the collisions succeed.
(B) Probability or steric factor, P — This matches (IV) Takes into account that the colliding molecules must be properly oriented. A collision with improper orientation makes the molecules bounce back without forming products.
(C) Most probable kinetic energy — This matches (I) Kinetic energy of the maximum fraction of molecules. It is the energy at the peak of the distribution curve, where the fraction of molecules is largest.
(D) Activated complex — This matches (II) Unstable intermediate that exists for a very short time and then breaks up to form the products. In \( \ce{H2 + I2 -> 2HI} \), it forms when a molecule of H2 and a molecule of I2 collide.
NCERT (2026-27 edition):
3.5 Collision Theory of Chemical Reactions – “…The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z).”
“To account for effective collisions, another factor P, called the probability or steric factor is introduced. It takes into account the fact that in a collision, molecules must be properly oriented i.e.,”
\( \displaystyle \text{Rate} = P Z_{AB}\, e^{-E_a/RT} \)
3.4 Temperature Dependence of the Rate of a Reaction – “According to Arrhenius, this reaction can take place only when a molecule of hydrogen and a molecule of iodine collide to form an unstable intermediate… It exists for a very short time and then breaks up to form two molecules of hydrogen iodide. The energy required to form this intermediate, called activated complex (C), is known as activation energy (Ea).”
“…The peak of the curve corresponds to the most probable kinetic energy, i.e., kinetic energy of maximum fraction of molecules.”