Using Large Excess of a Reactant Flooding

For a reaction that can be represented by Eq. (3.12), the general rate law can be written as 

If the concentration of B is made very high with respect to that of A, the concentration of B wiU not change significantly while the concentration of A changes by an amount that can be represented as x. Therefore, the rate law can be represented as 

Many reactions that take place in aqueous solutions or those in which H or OH is a reactant are representative of the conditions just described. For example, in Chapter 1 the reaction of i—(CH3)3CBr with OH in basic solution was described. Under these conditions, the concentration of OH is sufficiently large that the reaction appears to be first order in i—(CH3)3CBr, but is actually a pseudo first-order process. Many hydrolysis reactions appear to be independent of [H2O] only because water is usually present in such a large excess. Of course, not aU reactions can be studied by the method of flooding because a very large excess of a reactant may cause the reaction to take place in a different way. 

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