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The objective
of a kinetics experiment is to determine the rate of a
chemical reaction. To do this, you measure changes in the
concentration of one or more of the reactants or products as
a function of time. In NMR, one measures the rate of the
reaction by mixing precisely known concentrations of the
reactant solutions and recording several proton (or 13C,
31P, etc.) spectra at carefully timed intervals.
More important
than the rate of the reaction is the dependence of the rate
on the concentrations of the reagent and/or reactant. In
order to determine this you have to perform a series of
experiments in which you first vary the concentration of the
reagent solution. Then you perform another series in which
you vary the concentration of the reactant solution. Such
measurements allow you to determine the order of the
reaction. If the rate of a reaction doubles when you double
the reagent concentration, the reaction is said to be first
order with respect to the reagent. If the rate of a reaction
doubles when you double the concentration of the reactant,
the reaction is said to be first order with respect to the
reactant. A reaction that is first order with respect to
both the reagent and the reactant is said to be second order
overall. Such a reaction is described as a bimolecular
reaction because its rate depends upon the concentration of
both species. |
