D29.5 Method of Initial Rates

If you are given a chemical reaction equation, you cannot tell from the overall equation whether this reaction is elementary or if it occurs in a multi-step manner. This information can often be obtained by experimentally determining the reaction order with respect to each reaction species’ concentration and the rate constant of the reaction.

There are several different methods to do this; the method you use in lab may be different from the method of initial rates we describe below. As the name implies, the method of initial rates involves measuring the initial rate of a reaction for various initial concentrations of various reaction species. When analyzing the data, select two sets of rate data where all concentrations but one are the same and set up a ratio of the two rates and the two rate laws. This will yield an equation that contains only one unknown: the reaction order of the substance whose concentration varies. Repeat this analysis for all relevant reaction species, and the reaction rate law, including the rate constant, can be determined from the experimental data.