Statement I: The order of a reaction cannot be written from its balanced chemical equation.
Statement II: The rate law of a reaction can provide some clue about the mechanism of the reaction.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both Statement I and Statement II are correct but Statement II is not the correct explanation of Statement I.
The order of a reaction is the sum of the powers of the concentration terms in its rate law, and the rate law is found by experiment.
Statement I: The order of a reaction cannot be written from its balanced chemical equation. — True. The exponents in the rate law are often not the stoichiometric coefficients. For example, in \( \ce{CH3COOC2H5 + H2O -> CH3COOH + C2H5OH} \) the rate law is Rate = k[CH3COOC2H5]1[H2O]0.
Statement II: The rate law of a reaction can provide some clue about the mechanism of the reaction. — True. The reaction of \( \ce{KClO3} \) with \( \ce{FeSO4} \) and \( \ce{H2SO4} \) has ten molecules on the left of its equation but is found to be second order, which shows that it takes place in several steps.
Why Statement II does not explain Statement I?
Statement II works the other way round: it starts from the measured rate law and draws a clue about the steps. It does not give the reason why the order cannot be read from the equation.
Statement I rests on the fact that the order is an experimental quantity: the balanced equation shows only the net change, not the rate.
NCERT (2026-27 edition):
3.2.2 Rate Expression and Rate Constant – “… the exponents of the concentration terms are not the same as their stoichiometric coefficients. Thus, we can say that: Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically but must be determined experimentally.”
3.2.4 Molecularity of a Reaction – “…
\( \displaystyle \ce{KClO3 + 6FeSO4 + 3H2SO4 -> KCl + 3Fe2(SO4)3 + 3H2O} \)
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.”