Choose the correct statement for the collision theory of chemical reactions
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Collision theory considers the atoms or the molecules as hard (solid and elastic) spheres
Collision theory treats the reactant molecules as hard spheres and says that a reaction occurs when they collide. Only some collisions are effective, that is, lead to products. An effective collision needs two things together: the colliding molecules must have energy at least equal to the threshold energy, and they must collide in the proper orientation, so that the bonds to be broken and formed are brought close together.
Why the other options are incorrect
Option (1) Proper orientation of the colliding particles makes them bounce back — Incorrect. It is the other way round: with improper orientation the molecules bounce back without forming products.
Option (2) If the energy of the colliding particles is greater than the threshold energy, the collision is not effective — Incorrect. Energy equal to or greater than the threshold energy is what a collision needs to be effective (given proper orientation).
Option (4) Activation energy is the only criteria for determining an effective collision — Incorrect. Activation energy and proper orientation together decide whether a collision is effective.
NCERT (2026-27 edition):
Collision Theory of Chemical Reactions – “Though Arrhenius equation is applicable under a wide range of circumstances, collision theory, which was developed by Max Trautz and William Lewis in 1916 -18, provides a greater insight into the energetic and mechanistic aspects of 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.”
Collision Theory of Chemical Reactions – “… Thus, in collision theory activation energy and proper orientation of the molecules together determine the criteria for an effective collision and hence the rate of a chemical reaction. Collision theory also has certain drawbacks as it considers atoms/molecules to be hard spheres and ignores their structural aspect.”