Read the Passage carefully and answer the Questions.
Kinetic studies not only help us to determine the rate of a chemical reaction but also describe the conditions by which the reaction rates can be altered. The factors such as concentration, temperature, pressure and catalyst affect the rate of a reaction. The speed of a reaction or the rate of a reaction can be defined as the change in concentration of a reactant or product in unit time.
Rate = \( k [A]^x [B]^y \)
Here k is the rate constant
\( x + y \) gives the overall order of a reaction, whereas x and y represent the order with respect to the reactants A and B, respectively.
The order of a reaction which does not exist is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Pseudo second order reaction
The order of a reaction is the sum of the powers of the concentration terms in the experimentally found rate law. It can be zero, a whole number or a fraction. A reaction that has two reactants but behaves as first order, because one reactant is in large excess, is called pseudo first order; the book describes only this kind of “pseudo” order.
Option (1) Zero order reaction — An order described in the book. The rate does not depend on concentration, as in the decomposition of ammonia on a hot platinum surface at high pressure: \( \text{Rate} = k \).
Option (2) First order reaction — An order described in the book. The rate is proportional to one concentration term, as in the decomposition of \( \ce{N2O5} \) and all radioactive decay.
Option (3) Pseudo first order reaction — An order described in the book. Acid hydrolysis of ethyl acetate in a large amount of water behaves as first order:
\( \displaystyle \ce{CH3COOC2H5 + H2O} \xrightarrow{\ce{H+}} \ce{CH3COOH + C2H5OH} \qquad \text{Rate} = k[\ce{CH3COOC2H5}] \)
Option (4) Pseudo second order reaction — Not an order described in the book. The book treats only the pseudo first order case, so this is the answer expected here.
NCERT (2026-27 edition):
Half-Life of a Reaction – …
