Which of the following is not correct about the order of a reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The order of reaction is always equal to the sum of the stoichiometric coefficient of reactants in the balanced chemical equation of a reaction.
The order of a reaction is the sum of the powers of the concentration terms in the experimentally determined rate law. It need not be equal to the stoichiometric coefficients, which only show the net change; the rate depends on the mechanism. For example,
\( \ce{2N2O5 -> 4NO2 + O2} \), rate = k[N2O5], first order although the coefficient is 2.
Option (1) The order of a reaction is the sum of the powers of molar concentration of the reactants in the rate law expression. — Correct about the order of a reaction. For rate = k[A]x[B]y, order = x + y.
Option (2) The order of reaction is always equal to the sum of the stoichiometric coefficient of reactants in the balanced chemical equation of a reaction. — Not correct about the order of a reaction. The order comes from the rate law, not from the balanced equation; \( \ce{N2O5} \) has a coefficient of 2 but its decomposition is first order.
Option (3) The order of a reaction can be a fractional number. — Correct about the order of a reaction. Order can be 0, a whole number or a fraction; \( \ce{CH3CHO -> CH4 + CO} \) has rate = k[CH3CHO]3/2.
Option (4) Order of a reaction is experimentally determined quantity. — Correct about the order of a reaction. It cannot be read from the balanced equation; it is found from rate measurements.
NCERT (2026-27 edition):
Rate Expression and Rate Constant – …
