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Questions
How is the rate determining step helpful in postulating the mechanism of a reaction? Explain with an example.
How is the mechanism of a reaction postulated? Illustrate your answer with an example.
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Solution
The rate-determining step (RDS) is the slowest step in a multi-step reaction and controls the overall reaction rate.
The rate law of the overall reaction is determined by the RDS.
By comparing the experimental rate law with the reactants involved in the RDS, chemists can propose or verify the reaction mechanism.
The molecularity of the RDS must match the order of reaction obtained experimentally; otherwise, the proposed mechanism is incorrect.
Example:
\[\ce{2NO + O2 -> 2NO2}\]
Rate = k[NO]2[O2]1
\[\ce{2NO + O2 ->[slow] 2NO2}\]
Molecularity = 3 (two NO + one O2 molecules).
Matches the reaction order 2 + 1 = 3 from the rate law.
The rate-determining step reveals the number and nature of reactants involved in the slowest step, helping to deduce a mechanism that is consistent with both the stoichiometry and the experimentally observed kinetics.
