According to the collision theory which one of the following statements is correct?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The colliding molecules should have proper orientation and at least threshold energy for the reaction to occur.
According to the collision theory, molecules must collide to react, but not every collision gives products. Only effective collisions lead to reaction, and a collision is effective when two conditions are met together:
the colliding molecules have kinetic energy at least equal to the threshold energy, so that old bonds can break;
they collide with proper orientation, so that the atoms that must form new bonds come into contact.
This is why the rate is written with a steric factor P and the energy fraction:
\( \displaystyle \text{Rate} = P\,Z_{AB}\,e^{-E_a/RT} \)
So the correct statement is that the molecules need proper orientation and at least the threshold energy.
Why the other options are incorrect
Option (2) The colliding molecules should have proper orientation and energy less than a threshold energy for the reaction to occur. — Incorrect. With energy less than the threshold energy, the molecules bounce apart; bonds cannot break.
Option (3) If the colliding molecules have proper orientation then the reaction will always occur. — Incorrect. Proper orientation alone is not enough; the energy condition must also be met.
Option (4) “The rate of the reaction is exactly equal to the number of collisions taking place per unit time” — Incorrect. Only the small fraction of collisions that are effective give products, so the rate is much smaller than the number of collisions per unit time.
NCERT (2026-27 edition):
Collision Theory of Chemical Reactions – “… 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.”