The mathematical expression that relates the rate of a reaction to the concentrations of the reactants is known as the rate expression, rate equation or rate law.
For the reaction \(A + B \rightarrow \text{Products}\), the rate equation is
\[\text{Rate} \propto [A][B]\]
\[\text{Rate} = k[A][B]\]
k is known as the rate constant or specific reaction rate. The units of the rate constant are \((\text{mol})^{1-n}\,(\text{L})^{n-1}\,\text{s}^{-1}\), where n is the order of the reaction.
The rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically, but must be determined experimentally.
Several chemical reactions take place in a sequence of steps, and the overall rate of the reaction is governed by the slowest step, called the rate determining step.
Which of the following statements is false?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → In the acid-catalysed hydrolysis of an ester with water taken in large excess, the concentration of water changes appreciably during the reaction
In the hydrolysis of ethyl acetate, water is taken in large excess. When 0.01 mol of the ester reacts with 10 mol of water, only 0.01 mol of water is used up:
\( \displaystyle \ce{CH3COOC2H5 + H2O} \xrightarrow{\ce{H+}} \ce{CH3COOH + C2H5OH} \)
Water at the start: 10 mol
Water at the end: 9.99 mol
So the concentration of water hardly changes, and only the ester concentration affects the rate. The reaction behaves as a pseudo first order reaction:
\( \displaystyle \text{Rate}=k[\ce{CH3COOC2H5}]^1[\ce{H2O}]^0 \)
Option (1) The molecularity of a reaction cannot be zero or a non-integer — True statement. Molecularity is the number of reacting species that collide in an elementary reaction, so it is a whole number (1, 2 or 3).
Option (2) The half-life (t1/2) of a zero order reaction is directly proportional to the initial concentration of the reactant — True statement. For a zero order reaction \( t_{1/2}=\frac{[\mathrm{R}]_0}{2k} \), so t1/2 ∝ [R]0.
Option (3) In the acid-catalysed hydrolysis of an ester with water taken in large excess, the concentration of water changes appreciably during the reaction — False statement. Water falls only from 10 mol to 9.99 mol, so its concentration is not altered much.
Option (4) The units of the rate constant of a reaction of order n are \((\text{mol})^{1-n}\,(\text{L})^{n-1}\,\text{s}^{-1}\) — True statement. This is the general unit given in the passage; for n = 0, 1 and 2 it gives mol L−1 s−1, s−1 and L mol−1 s−1.
NCERT (2026-27 edition):
3.3.3 Half-Life of a Reaction – “The concentration of water does not get altered much during the course of the reaction. So, the reaction behaves as first order reaction. Such reactions are called pseudo first order reactions.”