Intermediates
If the slow step is second, the predicted rate law will usually involve an intermediate.
It is not possible to test this rate law by varying [intermediate] because the intermediate is consumed as soon as it is formed, and is thus never present in significant concentration.
To test such a pathway, [intermediate] must be expressed in terms of concentrations of stable species (reactants and/or products).This can be done using the equilibrium constant (K) for the step forming the intermediate.
[intermediate] = K[A][B]
Substitution of the expression for the concentration of the intermediate into the predicted rate law above gives a rate law that can be experimentally verified.
rate = k × K[A][B]
The predicted rate constant for the overall reaction of A and B to give C would be the product of the rate constant (k - lower case) for the second step and the equilibrium constant (K - upper case) for the first step.
Possible mechanism
For reaction A + B
C
Step 1: fast equilibrium
A + B
intermediate
Step 2: slow
intermediate C
Predicted overall rate law
rate = k[intermediate]
For reaction A + B
CStep 1: fast equilibrium
A + B
intermediateStep 2: slow
intermediate
CPredicted overall rate law
rate = k[intermediate]
It is not possible to test this rate law by varying [intermediate] because the intermediate is consumed as soon as it is formed, and is thus never present in significant concentration.
To test such a pathway, [intermediate] must be expressed in terms of concentrations of stable species (reactants and/or products).This can be done using the equilibrium constant (K) for the step forming the intermediate.
| K | = | [intermediate] |
| [A][B] |
[intermediate] = K[A][B]
Substitution of the expression for the concentration of the intermediate into the predicted rate law above gives a rate law that can be experimentally verified.
rate = k × K[A][B]
The predicted rate constant for the overall reaction of A and B to give C would be the product of the rate constant (k - lower case) for the second step and the equilibrium constant (K - upper case) for the first step.