- Rate expressions can be used to help rule out possible reaction mechanisms.
- Adjusting the initial concentrations of reactants can help determine if a rate law is consistent with the true mechanism for a reaction.
Kinetics is very useful because it is a very direct way to gather information about how a reaction proceeds. Let’s take the following overall reaction as an example:
Even if you know that (1) this is a two-step process and (2) that an intermediate is involved, there are many different ways this reaction can proceed. Below are three possibilities
Possibility 1
Possibility 2
Possibility 3
Using the steady state approximation described in Section 3.3, the following rate expressions can be derived for each possibility:
Possibility 1
Possibility 2
Possibility 3
Assuming these are the only three possible mechanisms, how can we use the information gathered from these rate expressions to determine which possibility is correct? Possibility 3 is the only one that has [C] in the rate expression because in every other possibility C only forms after the rate determining step. We can use this to our advantage by adding extra C to the reaction. If the addition of C leads to a decrease in the overall reaction rate, then Possibilities 1 and 2 cannot be correct. If there is no change, then we can rule out Possibility 3.
If a change in C did not lead to a change in rate, we next can look at the rate expressions for Possibilities 1 and 2. The reactant [B] appears in the denominator of Possibility 1, but not Possibility 2. We can exploit this difference by seeing how the rate changes with added [B]. If there is a first-order dependence, then Possibility 2 is correct. Otherwise, Possibility 1 must be correct.
Interactive: