Pseudo First Order Reaction

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ncchemist | eNotes Employee

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A pseudo first order reaction is a reaction that is truly second order but can be approximated to be first order under special circumstances.  For example, a second order reaction of the type A + B --> C is dependant on the concentrations of both A and B.  But monitoring the concentration of two different reactants at the same time is very difficult.  If one of the reactants is used in a very large excess then that concentration would hardly change at all and could be considered to be essentially a constant.  If [B]>>>[A], then the [B] would be essentially unchanged and the reaction could be considered pseudo first order with respect to [A] as a mathematical simplification.  An example of this is the hydrolysis of an ester to a carboxylic acid using water.  The reaction is truly second order and the rate depends on the concentration of the ester and water used.  But since water is often used as a solvent, then its concentration is in a huge excess compared to the ester and can be considered a constant.  Thus the reaction is pseudo first order for the ester.