Rate law
Rate law is the expression in which reaction rate is given in terms of molar concentration of reactants, with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation. The sum of powers of the concentration of the reactants, in the rate law expression is called the order of that chemical reaction. The number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction, is called molecularity of a reaction.
The order of a reaction
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → can be fractional
The order of a reaction is the sum of the powers of the concentration terms in the experimentally determined rate law. For a reaction with rate law
\( \displaystyle \text{Rate}=k[A]^{x}[B]^{y} \)
the order is x + y. Since x and y are found by experiment, they need not be whole numbers: the order can be zero, a whole number or a fraction. For example, the decomposition of ethanal has a rate law with a power of 3/2:
\( \displaystyle \ce{CH3CHO -> CH4 + CO} \)
\( \displaystyle \text{Rate}=k[\ce{CH3CHO}]^{3/2} \)
\( \displaystyle \text{Order}=\frac{3}{2}=1.5 \)
Why the other options are incorrect
Option (1) varies with temperature — Incorrect. Temperature changes the rate constant k (Arrhenius equation), not the powers in the rate law.
Option (3) depends on stoichiometric coefficients of reactants — Incorrect. The order is found by experiment and often differs from the coefficients; for example, \( \ce{2N2O5} \) → \( \ce{4NO2} \) + \( \ce{O2} \) has Rate = k[N2O5], which is first order. Only for elementary reactions do the two agree.
Option (4) depends on time used to carry out reaction — Incorrect. The order is a fixed feature of the rate law; it does not change as the reaction proceeds.
NCERT (2026-27 edition):
Order of a Reaction
