An alternative method which can be used with some reactions involves measuring the initial rate of a reaction. The object is to make a measurement before the concentrations of most species have a chance to change very much. That approximately eliminates the effect of dwindling reactant concentrations. Either the rate must be measured very quickly or else experimental conditions must be manipulated to simulate constant concentrations.

We are going to confine our look at rate laws to the method of initial rates.

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So with enough data of the right sort it is possible to find a predictive equation for the rate of a reaction at a constant temperature. One of the things that rate laws reveal is that what is happening in chemical reactions is not as simple as it might seem from the balanced equation.