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Molecularity

Determine the number of molecules reacting in steps of a reaction

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Race!

How do the number of "participants" in a reaction affect its rate?

Credit: Phil Roeder

[Figure1]

Consider this: you are participating in the 100 meter run for your school team in a district track meet. Your desired end result is winning the race, but first you are looking for a good start, one that will give you an early lead and excellent ability to accelerate. Remember: this all depends on no one else but yourself.

But what about the 400 meter relay? Sure, you still have to get a good start and acceleration for your part of the run, but now winning also depends on your teammates!

Chemical reactions also work on a similar principle, known as molecularity. Molecularity gives the number of molecules or ions that participate in the rate-determining (slowest) step of a reaction. If a reaction is unimolecular, only a single species is involved in the reactants side of the rate-determining step. If a reaction is bimolecular, the rate depends on two species. Termolecular reactions are mostly improbable.

Creative Applications

1. What is the molecularity of the reaction NO + O2 → NO2 + O2 (assuming it is the rate determining step for a larger reaction)?
2. Determine the rate-determining step for the reaction below. Determine the molecularity from your answer.

1. NO+NON2O2(fast)\begin{align*}\mathrm{{NO}+{NO}\rightarrow{N}_2{O}_2\;{(fast)}} \end{align*}

2. N2O2+H2N2O+H2O(slow)\begin{align*}\mathrm{{N}_2{O}_2+{H}_2\rightarrow{N}_2{O}+{H}_2{O}\;{(slow)}}\end{align*}

3. N2O+H2N2+H2O(fast)\begin{align*}\mathrm{{N}_2{O}+{H}_2\rightarrow{N}_2+{H}_2{O}\;{(fast)}}\end{align*}

3. What is the molecularity of the reaction 2H2 + O2 → 2H2O (assuming it is the rate determining step for a larger reaction)?
4. Research: Is it possible for a molecularity of 4 to exist?

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