Rate Determining Step
The Rate Determining Step is the slowest step of a chemical reaction that determines the speed (rate) of the overall reaction. The rate determining step can be compared to the neck of a funnel. The rate at which water flows through a funnel is limited/ determined by the width of the neck of the funnel and not by the rate at which the water is poured into the funnel. Like the neck of the funnel, the slow step of a reaction determines the rate of a reaction.
The rate determining step is important in deriving the rate equation of a chemical reaction. For example, consider a multi-step reaction,
A + B -> C + D
And assume the elementary steps for this reaction are:
Step 1: Slow A + A -> C + E (with a kinetic constant, k1)
Step 2: Fast E + B -> A + D (with a kinetic constant, k2)
where E is an intermediate product in step 1 and an intermediate reactant in step 2 that will not show up in the overall reaction. This is because when you add steps 1 and 2, intermediate E will cancel out along with the extra reactant A seen in step 1. Note that intermediate reactions do not show up in the overall reaction.
In the case of this hypothetical reaction, if step 1 is the slow step and step 2 is the fast step of the reaction, then the overall reaction rate will depend on step 1; this is because the slow step in a reaction is always the rate-limiting step, which is also called the rate determining step.
Therefore the rate equation will be,
rate = k1 [A] [A] = k1 [A]2
Consider an example of a reaction:
NO2(g) + CO(g) -> NO(g) + CO2(g)
which occurs in two elementary steps:
NO2 + NO2 -> NO + NO3 (slow)
NO3 + CO -> NO2 + CO2 (fast)
Here, we see that for the second step to occur, the first step must occur first, and since the first step is the slowest step, the overall reaction cannot be proceed any faster than the rate of the first elementary step. The first elementary step in this example is therefore the rate-determining step. The rate equation for this reaction is equal to the rate constant of step 1 multiplied by the reactants of that first step. If we denote the rate constant of step 1 to be k1 then the rate of the eqaution will be : rate = k1 [NO2][NO2] = k1[NO2]2
A way to further understand rate-determining steps is to try to apply it to real life events. For example, consider putting together a toy that you just purchased. Let's assume that building a toy purchased from a store requires 4 steps:
In this example, how fast the toy is assembled will be determined by whichever of the four steps takes the longest time to complete. If there was a very long line in the toy store, then step (a) will determine the rate of assembly of the toys If the person driving the car was stuck in traffic or was driving very slowly then step (b) will determine the rate of building the toy.
A reaction between NO and H2 occurs in the following three-step process:
NO + NO -> N2O2 (fast)
(a) What is the rate determining step?
(a) The rate determining step is the second step because it's the slow step.
(b) 2 NO + 2 H2 -> N2 + 2 H2O This balanced reaction can be found by simply adding all 3 reactions together. In adding these reactions, we will be eliminating the intermediates that show up on both the reactant and product side while adding the remaining reactants with the reactants and the products together with the other products.
(c) The intermediates in this reaction are N2O2 and N2O. Both of them show up on both the reactant and product side. Therefore in adding all three elementary reactions together, we will be able to cancel out N2O2 and N2O and as a result, they will not show up in the overall reaction equation.
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