Why must the reacting molecules have a minimum energy for an effectiv…

The correct answer is Option (2) → to facilitate breaking of bonds and bond formation

According to the collision theory, a reaction takes place when reactant molecules collide, but not every collision gives products. In a reaction the bonds of the reactants must be broken and new bonds formed, and this needs energy. Only molecules that collide with at least a minimum kinetic energy, the threshold energy, can rearrange their bonds; the others bounce apart.

So the minimum energy is needed to break the old bonds and form the new ones; proper orientation is a separate condition.

Why the other options are incorrect

• Option (1) minimum energy is required for the particles to collide many times per second — Incorrect. The number of collisions per second (collision frequency) depends on concentration and temperature; molecules collide very often whatever their energy.

• Option (3) the reaction mixture has sufficient energy to give off heat in a reaction — Incorrect. The heat given off depends on the enthalpy change of the reaction; endothermic reactions, which absorb heat, also need the minimum energy.

• Option (4) minimum energy is required to orient the particles correctly — Incorrect. Orientation is a separate condition, taken into account by the probability (steric) factor P; energy does not orient the molecules.

NCERT (2026-27 edition):
Collision Theory of Chemical Reactions – “… 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.”