Which of the following are examples of pseudo first order (pseudo unimolecular) reactions?
(A) Acid-catalysed hydrolysis of an ester
(B) Inversion of cane sugar
(C) Decomposition of ozone
(D) Decomposition of N2O5
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (B) only
A pseudo first order reaction is really of higher order, but one reactant (usually water) is present in large excess. Its concentration hardly changes, so the rate depends on the other reactant only and the reaction behaves as first order.
(A) Acid-catalysed hydrolysis of an ester — Pseudo first order. \( \ce{CH3COOC2H5 + H2O} \xrightarrow{\ce{H+}} \ce{CH3COOH + C2H5OH} \); water is in large excess, so the rate depends on the ester only.
(B) Inversion of cane sugar — Pseudo first order. \( \ce{C12H22O11 + H2O} \xrightarrow{\ce{H+}} \ce{C6H12O6 + C6H12O6} \) (glucose + fructose); again water is in large excess.
(C) Decomposition of ozone — Not pseudo first order. \( \ce{2O3 -> 3O2} \) has a single reactant; there is no second reactant in large excess.
(D) Decomposition of \( \ce{N2O5} \) — Not pseudo first order. \( \ce{N2O5 -> 2NO2 + 1/2O2} \) has one reactant and is a true first order reaction, rate = k[\( \ce{N2O5} \)].
NCERT (2026-27 edition):
3.3.3 Half-Life of a Reaction – “… Consider the hydrolysis of ethyl acetate which is a chemical reaction between ethyl acetate and water. In reality, it is a second order reaction … But water is taken in large excess for hydrolysis, therefore, concentration of water is not altered much during the reaction. …
\( \displaystyle \ce{CH3COOC2H5 + H2O} \xrightarrow{\ce{H+}} \ce{CH3COOH + C2H5OH} \)
… So, the reaction behaves as first order reaction. Such reactions are called pseudo first order reactions. Inversion of cane sugar is another pseudo first order reaction.”
3.3.2 First Order Reactions – “… Decomposition of N2O5 and N2O are some more examples of first order reactions.”