Which of the following statements is not correct?

Answer & explanation

Correct answer: option 1

The correct answer is Option (1) → If the concentration of the reactants is increased X times, the rate constant k also increases X times

In the rate law, the rate constant k is only the proportionality constant between the rate and the concentration terms:
\( \displaystyle \text{Rate} = k\,[A]^x\,[B]^y \)
When the concentrations are raised, it is the rate that goes up; k keeps the same value at a given temperature. For a first order reaction, increasing the concentration X times makes the rate X times faster while k stays unchanged. k changes with temperature (Arrhenius equation, \( k = A\,e^{-E_a/RT} \)), not with concentration.

Option (1) If the concentration of the reactants is increased X times, the rate constant k also increases X times — A statement that is not correct. Increasing the concentration increases the rate; k does not depend on concentration.

Option (2) The units of the rate constant depend on the order of the reaction — A correct statement. For order n the unit of k is \( \mathrm{(mol\,L^{-1})^{1-n}\,s^{-1}} \): \( \mathrm{mol\,L^{-1}\,s^{-1}} \) for zero order, \( \mathrm{s^{-1}} \) for first order and \( \mathrm{L\,mol^{-1}\,s^{-1}} \) for second order.

Option (3) The order of a reaction can be determined only by experiment — A correct statement. The powers in the rate law need not match the coefficients in the balanced equation, so the rate law and its order cannot be predicted from the equation and must be found experimentally.

Option (4) The overall rate of a reaction that takes place in several steps is determined by the slowest step — A correct statement. The slowest step is the rate determining step; the whole reaction cannot go faster than it.

NCERT (2026-27 edition):
3.2.2 Rate Expression and Rate Constant – “… Conversely,rates generally increase when reactant concentrations increase. So, rate of a reaction depends upon the concentration of reactants. …

\( \displaystyle \text{Rate} = k\,[A]^x\,[B]^y \qquad (3.4a) \)

\( \displaystyle -\frac{d[R]}{dt} = k\,[A]^x\,[B]^y \qquad (3.4b) \)

… where k is a proportionality constant called rate constant.”

3.4 Temperature Dependence of the Rate of a Reaction – “It has been found that for a chemical reaction with rise in temperature by 10°, the rate constant is nearly doubled. The temperature dependence of the rate of a chemical reaction can be accurately explained by Arrhenius equation (3.18).”