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 occasionnally 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 (chemistry)
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