Question

What are pseudo-unimolecular reactions and why are they called so? Explain with an example.

What are pseudo-uni molecular reactions?

Answer 1

Pseudo-unimolecular reactions are reactions that involve two reactants but appear to be first-order because one of the reactants is present in large excess and its concentration remains almost constant during the reaction.

They are called “pseudo” (false) unimolecular because although the reaction is actually bimolecular, it behaves like a unimolecular reaction due to the excess of one reactant.

Example:
Hydrolysis of ethyl acetate in excess water:

\[\ce{CH3COOC2H5 + H2O −> CH3COOH + C2H5OH}\]

Rate law:

Rate = k[CH₃COOC₂H₅][H₂O]

Since water is in large excess, [H₂O] is almost constant. So,

Rate = k′[CH₃COOC₂H₅] where k′ = k[H₂O]

Thus, the reaction appears first-order even though it is second-order, and hence it is a pseudo-unimolecular reaction.