Screen grab of interactive animation explaining concept. (Click to enlarge)
Reactions are usually complex events. Bonds are broken, formed, stretched and atoms driven by enthalpy and entropy in their marvelous path. From reactants to products, and vice versa.
Even when a system reaches equilibrium, it is very unusual for compounds to rest peacefully; instead, they will start going back and forth in perpetual motion.
But HOW do reactants transform into products? The kinetic law ( Kinetics module in the Physical Chemistry textbook) gives us really interesting information, “hidden” in the exponents of the reactants’ concentrations.
To understand it, let’s start with an example of a factory (a really oversimplified one, of course!) that produces cars. The whole process is located in three consecutive buildings, and each of them covers one of the following steps:
in the building A, 10 cars/hour are assembled;
in the B one, 8 cars/hour are finished and painted;
in the C one, 12 cars/hour are wheeled and shipped to the customers.
Now, starting our workday with nothing but spare parts, how many cars are produced?
In the first hour, 10 cars are assembled, while in B and C people get some sleep.
In the following hour, while 10 other cars are assembled, only 8 of the first hour can be painted (whilst workers in C are still asleep!).
In the third and following hours, even more cars are assembled and parked outside the gates of B, waiting to be painted; on the other hand, as soon as they come out from B, they enter C (8 cars, each hour) and are immediately completed, faster (12/hour) than they enter.
So, we can easily detect that the process in B is the “weak link” of this chain: it is pointless to increase the productivity of processes in A and C, because the limitation lies in B. B is the step to consider when we are concerned about the global outcome of the process.
In the same manner, a composite reaction (Chapter 10 covers this topic) can consist of many different elementary steps, but only the slowestone will drive the global velocity of the reaction.
The equation of the rate of reaction:
gives us information about the slowest step ONLY, the so-called Rate Determining Step.
This is a powerful tool, indeed, since with this information we know whether to add or to subtract a reactant will influence the global rate of reaction or not. Even if no evidence can “definitely prove” a reaction mechanism, this information is still invaluable for all those who study reaction mechanisms (I’m thinking about you, biochemists!), and need to obtain information that will be used for drug dosage, or for optimization of an industrial-scale reaction.
In all cases, the LAST (slowest) reaction step is the FIRST (most important) of all. Gospel truth.