Reaction Mechanisms
The mechanism of a reaction is the actual series of steps through which a chemical reaction occurs. Consider the reaction below:
Overall Reaction: A2 + 2B → 2AB
This equation seems to imply some sort of encounter between two molecules or atoms of B and one molecule of A2 to form two molecules of AB. But suppose instead that the reaction actually takes place in two steps.
| Step 1: | A2 + B → A2B | (Slow) |
| Step 2: | A2B + B → 2AB | (Fast) |
Note that these two steps add up to the overall (net) reaction. A2B, which does not appear in the overall reaction because it is neither a reactant nor a product, is called an intermediate. Reaction intermediates, unlike activated complexes, are real molecules that exist at least for a while. Nonetheless, they may still be difficult to detect.
The slowest step in a proposed mechanism is called the rate-determining step (or the rate-limiting step), so called because as the bottleneck in the progression of the reaction, it determines the rate by imposing an upper limit on how fast it goes. In the reaction mechanism given above, for example, the first step, which has been described as slow, is the rate-determining step; the overall reaction simply cannot occur any faster than this step, in the same way that in a family outing, no matter how fast the other members are, everyone will still have to wait for the slowest one to be ready before the family can set off.
In the discussion on the potential energy diagram above, we have limited ourselves to considering reactions occurring in a single step. For one that involves several steps, the graph will go through a series of “hilltops and valleys”: Each step will involve a transition state. The low points between these maxima are intermediates. The activation energy of each step is the difference in energy between the transition state and the “valley” immediately before it (corresponding to either reactants or intermediates). The step with the highest activation energy is the rate-determining or rate-limiting step. The two plots below show the potential energy diagrams for a two-step reaction. In one, the first step is the rate-determining step while in the other, the second step is rate determining.
