- Most reactions have what is called a reaction mechanism. This shows the series of reactions that occur in a sequence of steps
- The slowest of which controls the rate of the reaction; this is known as the rate-determining step.
- The reactants that are involved in the rate-determining step are present in the rate equation
- Any step that occurs after the rate-determining step will not affect the rate of the reaction; hence the reactants involved will not appear in the rate equation.
The reaction between 2-chloro-2-methylpropane and KOH react as follows:
(CH3)CCl+ OH- ⇨ (CH3)COH+ Cl-
The orders of the reactants are as follows:
- [(CH3)CCl] – First Order
- [OH-] – Zero Order
This means that the rate equation is: rate = k[(CH3)CCl]
Therefore only (CH3)CCl can be present in the rate-determining step. There must be at least two steps in the mechanism as OH- is not present in the first step. An example of a mechanism could be:
(rate-determining step: (CH3)CCl changes to Cl- and (CH3)C+; then second step is Cl- and (CH3)C+ react to form final products of (CH3)COH and Cl- )
Given the reaction:
2NO +O2 ⇨ 2NO2
And the rate equation:
It shows you the rate-determining step has 2 moles of NO present due to the power of two and without O2; thus a mechanism could be:
2NO ⇨ N2O2
N2O2+ O2 ⇨ 2NO2
CO + NO2 → CO2 + NO
If this reaction occurred in a single step, its rate law would be:
rate=k [NO2] [CO]
However, experiments show that the rate equation is:
the experimental rate law is second-order, suggesting that the reaction rate is determined by a step in which two NO2 molecules react, and therefore the CO molecule must enter at another, faster step. A possible mechanism that explains the rate equation is:
2NO2→NO3+NO (slow step, rate-determining)
NO3+CO→NO2+CO2 (fast step)
Since the first step is the slowest, and the entire reaction must wait for it, it is the rate-determining step. We can picture the rate-determining step to be like the narrowest point in an hourglass; it is the “bottleneck” point of the reaction that determines how quickly reactants can become products.
If the first step in a mechanism is rate-determining, it is easy to find the rate law for the overall expression from the mechanism. If the second or a later step is rate-determining, determining the rate law is slightly more complicated.
Drafted by Eunice (Chemistry)