How many of the following statements about the collision theory of chemical reactions are correct?
(A) Collision theory is based on the kinetic theory of gases.
(B) In collision theory, the reactant molecules are assumed to be hard spheres.
(C) In collision theory, a collision between molecules having sufficient kinetic energy forms products even when the molecules are not properly oriented.
(D) The expression Rate = ZABe−Ea/RT predicts the rate constants fairly accurately for reactions that involve atomic species or simple molecules.
(E) Collision theory takes into account the structural aspect of atoms and molecules.
(F) The number of collisions per second per unit volume of the reaction mixture is known as the collision frequency.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4
In collision theory, a reaction takes place when reactant molecules collide. A collision gives products only when the molecules have enough energy and the proper orientation.
Correct:
Based on the kinetic theory of gases (A)
Reactant molecules taken as hard spheres (B)
Rate = ZABe−Ea/RT fairly accurate for atomic species or simple molecules (D); large deviations appear only for complex molecules
Collisions per second per unit volume = collision frequency (F)
Not correct:
Products form without proper orientation (C) — with improper orientation the molecules bounce back and no products are formed.
Takes into account the structural aspect (E) — a drawback of the theory: it treats atoms and molecules as hard spheres and ignores their structure.
Correct statements: (A), (B), (D) and (F) = 4.
NCERT (2026-27 edition):
3.5 Collision Theory of Chemical Reactions – “…It is based on kinetic theory of gases. According to this theory, the reactant molecules are assumed to be hard spheres and reaction is postulated to occur when molecules collide with each other. The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z).”
“Equation (3.23) predicts the value of rate constants fairly accurately for the reactions that involve atomic species or simple molecules but for complex molecules significant deviations are observed. The reason could be that all collisions do not lead to the formation of products. The collisions in which molecules collide with sufficient kinetic energy (called threshold energy) and proper orientation, so as to facilitate breaking of bonds between reacting species and formation of new bonds to form products are called as effective collisions.”
\( \displaystyle \text{Rate} = Z_{AB}\, e^{-E_a/RT} \qquad (3.23) \)
“Collision theory also has certain drawbacks as it considers atoms/ molecules to be hard spheres and ignores their structural aspect.”