Consider the following statements in reference to the collision theory for chemical reaction. Identify the correct statements.
A + B → Products
(A) Expression for the rate of reaction = PZABe(-Ea/RT)
(B) Collision theory considers the structural aspects of atoms and molecules.
(C) For an effective collision between the reactants, the reactant molecules must possess (i) sufficient kinetic energy called threshold energy and (ii) proper orientation.
(D) e(-Ea/RT) represents the fraction of molecules with energies less than Ea.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (C) only.
Collision theory treats reactant molecules as hard spheres and says a reaction occurs only when molecules collide. Only some collisions are effective; they need two conditions:
Enough energy: the colliding molecules must have at least the threshold energy (activation energy measured above the average energy).
Proper orientation: the molecules must meet in a way that lets the old bonds break and new bonds form. The probability or steric factor P takes care of this.
\( \displaystyle \text{Rate}=P\,Z_{AB}\,e^{-E_a/RT} \)
Here ZAB is the collision frequency and e−Ea/RT is the fraction of molecules with energy equal to or greater than Ea.
(A) Expression for the rate of reaction = PZABe(-Ea/RT) — Correct. This is the collision theory rate expression, with P for orientation and ZAB for the collision frequency.
(B) Collision theory considers the structural aspects of atoms and molecules. — Incorrect. It treats atoms and molecules as hard spheres and ignores their structural aspects; this is its main drawback.
(C) For an effective collision between the reactants, the reactant molecules must possess (i) sufficient kinetic energy called threshold energy and (ii) proper orientation. — Correct. These are the two conditions for an effective collision.
(D) e(-Ea/RT) represents the fraction of molecules with energies less than Ea. — Incorrect. It is the fraction with energies equal to or greater than Ea, the molecules that can react.
NCERT (2026-27 edition):
Collision Theory of Chemical Reactions – “… e–Ea/RT represents the fraction of molecules with energies equal to or greater than Ea. … 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. …”
“… It takes into account the fact that in a collision, molecules must be properly oriented i.e., Rate = PZABe–Ea/RT … Collision theory also has certain drawbacks as it considers atoms/molecules to be hard spheres and ignores their structural aspect.”