Which of the following expressions gives the threshold energy of a reaction?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Threshold energy = Activation energy + Energy possessed by the reacting species
The threshold energy is the minimum energy colliding molecules must have for an effective collision. The reacting molecules already possess some energy; the activation energy is the extra energy they must gain on top of it. So the two add up:
\( \displaystyle \text{Threshold energy}=E_a+\text{energy possessed by the reacting species} \)
Why the other options are incorrect
Option (2) Threshold energy = Activation energy − Energy possessed by the reacting species — Incorrect. The activation energy is added to the energy the molecules already have, not taken away from it; this would make the threshold smaller than Ea.
Option (3) Threshold energy = Energy possessed by the reacting species − Activation energy — Incorrect. This would put the threshold below the energy the molecules already have, so they would need no activation energy at all.
Option (4) Threshold energy = Activation energy + Energy of the products — Incorrect. The threshold must be reached by the reacting species before any product is formed; the energy of the products plays no part in it.
NCERT (2026-27 edition):
3.5 Collision Theory of Chemical Reactions – “… 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.”
“* Threshold energy = Activation Energy + energy possessed by reacting species.”