Answer to Question #97030 in Physical Chemistry for Abhishek

Question

What are pseudo first order reactions ? Give one example of such a reaction. Also write the expressions for the pseudo first order rate constant.

Answer 1

If the concentration of a reactant remains constant, it can be included in the rate constant, obtaining a pseudo–first order  rate equation. For a typical second-order reaction with rate equation v = k[A][B], if the concentration of reactant B is constant then v = k[A][B] = k'[A], where the pseudo–first-order rate constant k' = k[B].

CH₃COOC₂H₅ + H₂O = CH₃COOH + C₂H₅OH --- water is in large excess (it is a solvent), so it's concentration is included in K, and it is pseudo–first order  rate reaction.