1. Reaction Mechanisms
Most chemical reactions occur by a series of steps called the reaction mechanism.
The overall progress of a chemical reaction can be represented at the molecular level by a series of simple elementary steps or elementary reactions.
The sequence of elementary steps that leads to product formation is the reaction mechanism.

2. NO2(g) + CO(g) → NO(g) + CO2(g)
Mechanism is thought to involve the following steps:
NO2(g) + NO2(g) → NO3(g) + NO(g)
NO3(g) + CO(g) → NO2(g) + CO2(g)
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NO3(g) is an intermediate – neither a reactant nor a product but formed and consumed during the reaction.

3. NO2(g) + NO2(g) → NO3(g) + NO(g)
NO3(g) + CO(g) → NO2(g) + CO2(g)
Each of these two steps is called an elementary step, a reaction whose rate can be written from its molecularity.
Molecularity is defined as the number of species that must collide to produce the reaction indicated by that step.
A reaction involving one molecule is called a unimolecular step.
Reactions involving the collisions of two and three species are termed bimolecular and termolecular, respectively.
Note that the rate for an elementary step follows directly from the molecularity of that step.

4. Reaction mechanism = series of elementary steps that must satisfy two requirements:
The sum of the elementary steps must give the overall balanced equation for the reaction.
The mechanism must agree with the experimentally determined rate law.
NO2(g) + NO2(g) → NO3(g) + NO(g)
NO3(g) + CO(g) → NO2(g) + CO2(g)
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NO2(g) + CO(g) → NO(g) + CO2(g)
First requirement met.

5. NO2(g) + NO2(g) → NO3(g) + NO(g) NO3(g) + CO(g) → NO2(g) + CO2(g)
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NO2(g) + CO(g) → NO(g) + CO2(g)
Slow (rate-determining)
Fast
Rate-determining step: multistep reactions often have one step slower than all the others.
Reactants can become products only as fast as they get through this slowest step.
Thus, this step determines the rate of the reaction.
Consider pouring water rapidly through a funnel into a container. The water collects in the container at a rate determined by the size of the funnel opening.
For our reaction, we assume the first step is rate-
determining and the second step is fast.

6. NO2(g) + NO2(g) → NO3(g) + NO(g) NO3(g) + CO(g) → NO2(g) + CO2(g)
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NO2(g) + CO(g) → NO(g) + CO2(g)
Slow (rate-determining)
Fast
Since step 1 is an elementary step, we can write the rate law from its molecularity.
The bimolecular first step has the rate law
Rate = k[NO2]2
This rate law agrees with the experimentally determined rate law.
The mechanism that was assumed meets the two requirements and may be the correct mechanism for the reaction.