Q.vi. What are pseudo-first order reactions? Give one example and explain why it is pseudo-first order.

Step 1. Define: a pseudo-first order reaction genuinely involves more than one reactant (so its stoichiometry suggests a higher order) but is experimentally found to obey first-order kinetics.

Step 2. Example: methyl acetate hydrolysis, CH3COOCH3(aq)+H2O(l) -> CH3COOH(aq)+CH3OH(aq), with true rate law rate=k'[CH3COOCH3][H2O] (second order overall).

Step 3. Water is present in huge excess (it is also the solvent), so its concentration is essentially unchanged over the course of the reaction; treating it as a constant, k''=[H2O], the rate law reduces to rate=k'[CH3COOCH3]k''=k[CH3COOCH3], where k=k'k''. …

Large-excess-reactant condition producing apparent first-order kinetics from …

• Claiming the reaction is genuinely first order (rather than 'pseudo') -- the true mechanism still involves both reactants. …