The following overall reaction can be represented in an energy diagram.
2 NO2 (g) + F2 (g) → 2 NO2F (g)
The accepted mechanism for the reaction is:
NO2 (g) + F2 (g) → NO2F (g) + F (g) (slow step)
F (g) + NO2 (g) → NO2F (g) (fast step)
Below shows an energy diagram for this 2-step reaction. The blue shaded area represents the slow first step while the peach shaded area represents the fast second step.
Below is the labeled energy diagram for this multi-step reaction. There are two transition states; one for each step. The intermediate, the F (g), is between the two transition states on the diagram. The first step is the slow step because the activation energy is high, and the second step is a fast step. The second step also has a smaller activation energy. The rate law is written from the slow step; rate = k [NO2][F2].
The first step is endothermic while the second step is exothermic. The reaction overall is exothermic. The rate constant k is smaller for the first step and larger for the second step. In a mechanism, only unimolecular and bimolecular elementary steps are considered reasonable.
Exercises
Exercise 1. Consider the following energy diagram to answer the questions.
b) Which is the rate determining step?
c) Which is the fastest step?
d) Which step has a higher rate constant, k?
e) Is the overall reaction exothermic or endothermic?
f) How many intermediates, if any?
g) Indicate if each step is endothermic or exothermic.
h) For which step is the rate constant, k, the smallest?
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