The reaction
\( \ce{KClO3 + 6FeSO4 + 3H2SO4 -> KCl + 3Fe2(SO4)3 + 3H2O} \)
apparently seems to be of tenth order, but it is found to be a second order reaction. What does this show?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The reaction takes place in several steps.
The coefficients of the reactants add up to 1 + 6 + 3 = 10, so the reaction looks tenth order.
The chance of more than three molecules colliding and reacting at the same time is very small, so a reaction with ten molecules in its equation cannot take place in one step.
It goes through several steps, and the slowest step decides the order, which is found to be two.
Why the other options are incorrect
Option (1) The reaction is an elementary reaction with a molecularity of two. — Incorrect. An elementary reaction takes place in one step. A reaction with ten molecules in its equation cannot take place in one step, so this is a complex reaction.
Option (2) Ten molecules of the reactants collide at the same time in a single step. — Incorrect. Reactions of molecularity three are already very rare; ten molecules colliding at once is practically impossible.
Option (3) The molecularity of the overall reaction is two, which is the same as its order. — Incorrect. Molecularity applies only to elementary reactions; for a complex reaction it has no meaning. It is the molecularity of the slowest step, not of the reaction as a whole, that matches the order.
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.” … “It is, therefore, evident that complex reactions involving more than three molecules in the stoichiometric equation must take place in more than one step.”
“This reaction which apparently seems to be of tenth order is actually a second order reaction. This shows that this reaction takes place in several steps.”