Determining the Rate Law by Experiment

Both the order of the reactants and the value of the rate constant must be determined through experiment. Finding the rate law on the MCAT is a. relatively simple matter. Consider the hypothetical reaction  

In this case, we will assume that no reverse reaction occurs. We are given the following table with experimental data  

Trial Initial Concentration of A (mol/T) Initial Concentration of B (mol/L) Initial Concentration of C (mol/L) Measured Initial Rate (mol/L s) 

We can find the order of each reactant by comparing the rates between two Trials in which only the concentration of one of the reactants is changed. For instance, comparing Trial 1 to Trial 2, the initial concentration of A is doubled and the concentrations of B and C remain the same. The reaction rate also doubles. Thus the rate of this reaction is directly proportional to the concentration of A. In the rate law, [A] receives an expoiaent of 1, and the reaction is considered first order with respect to reactant A. Comparing Trials 2 and 3 we find that when only the concentration of B is doubled, the reaction rate is quadrupled. This indicates that the rate is proportional to the square of the concentration of B. [B] receives a 2 for its exponent in the rate law and the reaction is second order with respect to B. Comparing Trials 1 and 

confuse the rate constant with the rate of the reaction. They are proportional, but they are NOT identical. 

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