Once the base (oh-) is added, why is it important to immediately start collecting data to determine the reaction order? What will happen if you let the solution sit around for a while in the cuvette?

Answer 1

It would help if you included the actual experiment, but in the absence of that information, I will guess at what is involved.

In any kinetic experiment, it is advisable to use the initial rate before significant product accumulates and before any reverse reaction can take place. If one lets the reaction mixture sit around, reactants disappear and products appear. When this is the case, you no longer know the actual concentration of reactants because they are no longer the initial concentration that you put in. In summary, you want to measure initial rates when doing a kinetic study to determine the order of the reaction.