Concentration Effects on Reaction Rates

Introduction:

The rates of substitution and elimination reactions are dependent on the concentrations of the R-X and nucleophile/base.  An example of a question is: what would happen to the rate of an SN1 reaction if you were to double the concentration of the nucleophile?  

How to:

To answer these types of questions, you first need to know the rate equations for the reaction.  These are dependent on the mechanism.  The k is the rate constant which will differ from reaction to reaction.

        rate of SN1 = k [R-X]

        rate of SN2 = k [R-X] [Nuc]

        rate of E1 = k [R-X] 

        rate of SN2 = k [R-X] [base]

Notice the similarity between the E1 and SN1 rate equations,  also between the E2 and SN2 rate equations.

Examples:

1) Doubling the concentration of the nucleophile in an SN1 reaction will….

(answer) The rate equation of an SN1 reaction is: k [R-X].  Notice that the nucleophile is not in the rate equation.  Therefore, doubling the concentration of the nucleophile will have no effect on the rate of the reaction.

2) Doubling the concentration of the R-X in an SN2 reaction will….

(answer) The rate equation of an SN2 reaction is: k [R-X] [Nuc].  Therefore, doubling the concentration of the nucleophile will double the rate of the reaction.

3) Halving the concentration of the R-X and the base in an E2 reaction will….

(answer) The rate equation of an E2 reaction is: k [R-X] [base].  Therefore, halving the concentration of the R-X and the base will reduce the rate of the reaction by a factor of four.