In a pseudo first order reaction, the rate constant-
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → depends on concentration of reactants present in excess.
A reaction that is really of a higher order but behaves as first order because one reactant is taken in large excess is a pseudo first order reaction. In the acid hydrolysis of ethyl acetate the true rate law is rate = k[CH3COOC2H5][H2O]. Water is present in large excess and its concentration hardly changes, so it is absorbed into the constant:
\( \text{rate} = k'[\ce{CH3COOC2H5}], \quad k' = k[\ce{H2O}] \)
The observed (pseudo first order) rate constant k′ therefore contains the concentration of the reactant present in excess.
Why the other options are incorrect
Option (1) — Incorrect. k′ = k[H2O] changes if the amount of the excess reactant is changed.
Option (2) — Incorrect. The concentration of the reactant present in small quantity appears in the rate, not in the rate constant.
Option (3) — Incorrect. False; like every rate constant, k′ increases with temperature (Arrhenius equation).
NCERT (2026-27 edition):
Half-Life of a Reaction – “… The order of a reaction is sometimes altered by conditions. There are many reactions which obey first order rate law although they are higher order reactions. Consider the hydrolysis of ethyl acetate which is a chemical reaction between ethyl acetate and water. In reality, it is a second order reaction and concentration of both ethyl acetate and water affect the rate of the reaction. But water is taken in large excess for hydrolysis, therefore, concentration of water is not altered much during the reaction. Thus, the rate of reaction is affected by concentration of ethyl acetate only. … So, the reaction behaves as first order reaction. Such reactions are called pseudo first order reactions.”