For reaction pathways that involve more than one elementary step, the experimental rate law for the overall process depends on the rate law for the slowest step.
Thus postulated pathways also have postulated relative rates of the various steps.
Since each step in a postulated pathway is an elementary reaction with its own transition state and activation energy, the rate law for that step has the concentrations of the reactants raised to the power of their coefficient in the balanced equation for the step.
Consider the two postulated pathways for
CH3Br + OH–
CH3OH + Br–
Mechanism 1
step 1: CH3Br+CH3 + Br – slow
step 2: H3C+ + OH– CH3–OH fast
Predicts rate = k[CH3Br]
Mechanism 2
CH3Br + :OH– → CH3OH + :Br–
Predicts rate = k[CH3Br][OH–]
Thus the two mechanisms can be distinguished by comparing the experimental and predicted rate laws. If they are the same, that mechanism (or any other that has the same predicted rate law) could be operating.
The slowest step has the highest activation energy.
Because the rate of this step determines the overall rate of the reaction, this step is known as the rate-determining step.
Because the rate of this step determines the overall rate of the reaction, this step is known as the rate-determining step.
Thus postulated pathways also have postulated relative rates of the various steps.
Since each step in a postulated pathway is an elementary reaction with its own transition state and activation energy, the rate law for that step has the concentrations of the reactants raised to the power of their coefficient in the balanced equation for the step.
Consider the two postulated pathways for
CH3Br + OH–
CH3OH + Br–Mechanism 1
step 1: CH3Br
+CH3 + Br – slowstep 2: H3C+ + OH–
CH3–OH fastPredicts rate = k[CH3Br]
Mechanism 2
CH3Br + :OH– → CH3OH + :Br–
Predicts rate = k[CH3Br][OH–]
Thus the two mechanisms can be distinguished by comparing the experimental and predicted rate laws. If they are the same, that mechanism (or any other that has the same predicted rate law) could be operating.