Statement I: The order of a reaction cannot be a fraction.
Statement II: The molecularity of a reaction is a whole number other than zero, and reactions with molecularity three are very rare.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Statement I is incorrect but Statement II is correct.
The order of a reaction is the sum of the powers of the concentration terms in the rate law, found by experiment. The molecularity of an elementary reaction is the number of reacting species that must collide at the same time.
Statement I: The order of a reaction cannot be a fraction. — False. Order is an experimental quantity; it can be zero and even a fraction. For example, for CHCl3 + Cl2 → CCl4 + HCl, Rate = k[CHCl3][Cl2]1/2, so the order is 1 + 1/2 = 3/2.
Statement II: The molecularity of a reaction is a whole number other than zero, and reactions with molecularity three are very rare. — True. Molecularity counts colliding species, so it is 1, 2 or 3 and never zero or a fraction. Three species seldom collide at the same instant, so termolecular reactions are very rare.
NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “The probability that more than three molecules can collide and react simultaneously is very small. Hence, reactions with the molecularity three are very rare and slow to proceed. … (i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.”