Which of the following are pseudo first order reactions?
(A) \( \displaystyle \ce{C12H22O11 + H2O} \xrightarrow{\ce{H^+}} \ce{C6H12O6 + C6H12O6} \)
(B) \( \displaystyle \ce{CH3COOC2H5 + H2O} \xrightarrow{\ce{H^+}} \ce{CH3COOH + C2H5OH} \)
(C) \( \displaystyle \ce{C2H4(g) + H2(g)} \xrightarrow{\ce{H^+}} \ce{C2H6(g)} \)
(D) \( \displaystyle \ce{2NH3(g)} \xrightarrow[\text{Pt catalyst}]{\text{1130K}} \ce{N2(g) + 3H2(g)} \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (B) only
A pseudo first order reaction is a reaction of higher molecularity that behaves as a first order reaction because one reactant is present in large excess. Its concentration hardly changes during the reaction, so it is absorbed into the rate constant. Typical examples are reactions in water where water is the solvent and a reactant:
\( \displaystyle \text{Rate}=k'[\text{substrate}][\ce{H2O}]=k[\text{substrate}]\quad\text{where }k=k'[\ce{H2O}] \)
(A) \( \ce{C12H22O11 + H2O} \xrightarrow{\ce{H^+}} \ce{C6H12O6 + C6H12O6} \) (inversion of cane sugar) — Correct. Water is in large excess, so Rate = k[C12H22O11]; the products are glucose and fructose.
(B) \( \ce{CH3COOC2H5 + H2O} \xrightarrow{\ce{H^+}} \ce{CH3COOH + C2H5OH} \) (acid hydrolysis of ethyl acetate) — Correct. Again water is in large excess, so Rate = k[CH3COOC2H5].
(C) \( \ce{C2H4(g) + H2(g)} \xrightarrow{\ce{H^+}} \ce{C2H6(g)} \) (hydrogenation of ethene) — Incorrect. This is an addition of hydrogen to ethene between two gases. Neither reactant is a solvent present in large excess, so there is nothing to absorb into the rate constant; it is not a pseudo first order reaction.
(D) \( \ce{2NH3(g)} \xrightarrow[\text{Pt catalyst}]{\text{1130K}} \ce{N2(g) + 3H2(g)} \) (decomposition of ammonia on hot platinum) — Incorrect. At high pressure the platinum surface is fully covered with \( \ce{NH3} \) molecules, so the rate does not depend on the concentration: it is a zero order reaction.
Note: It seems that there is a mistake in the original NTA paper, which prints H+ over the arrow for C2H4 + H2 → C2H6 when it should read Ni (or Pt/Pd).
NCERT (2026-27 edition):
Pseudo First Order Reaction
Zero Order Reaction – “… The decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction at high pressure. … In this reaction, platinum metal acts as a catalyst. At high pressure, the metal surface gets saturated with gas molecules. So, a further change in reaction conditions is unable to alter the amount of ammonia on the surface of the catalyst making rate of the reaction independent of its concentration.”