Order of Reaction - Pseudo-first Order

Pseudo-first Order

If the concentration of a reactant remains constant (because it is a catalyst or it is in great excess with respect to the other reactants), its concentration can be included in the rate constant, obtaining a pseudo–first-order (or occasionally pseudo–second-order) rate equation. For a typical second-order reaction with rate equation, if the concentration of reactant B is constant then :, where the pseudo–first-order rate constant . The second-order rate equation has been reduced to a pseudo–first-order rate equation, which makes the treatment to obtain an integrated rate equation much easier.

For example, the hydrolysis of sucrose in acid solution is often cited as a first-order reaction with rate . The true rate equation is third-order, ; however, the concentrations of both the catalyst H+ and the solvent H₂O are normally constant, so that the reaction is pseudo–first-order.