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.
Read more about this topic: Order Of Reaction
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“Play not with paradoxes. That caustic which you handle in order to scorch others may happen to sear your own fingers and make them dead to the quality of things.”
—George Eliot [Mary Ann (or Marian)