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Determine the number of molecules reacting in steps of a reaction

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

Credit: Phil Roeder
Source: http://www.flickr.com/photos/tabor-roeder/4742291814/sizes/o/in/photolist-8e4u8Y-8e4qTA-8e4ujs-8e1akB-8e19X6-8e4reA-8e1cGZ-8e4rXW-8e4tJu-8e4qif-8e4sTA-8e4rvN-8e4tby-8e1adH-8e1bLB-8e1cYV-8e4ro7-8e1dg2-8e4ttb-9TCxPy-9JRs5b-cd4VLu-8e4rSd-8e4uuA-8e1avH-8e4seE-8e4sqy-9TSJoW-9ToYf3-9ToWUQ-9TSMdw-9Tp2BW-9TPXVP-9STzp8-9TCrch-9ToXyU-9TPWQn-9TCvts-9TCuuS/
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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. \begin{align*}\mathrm{{NO}+{NO}\rightarrow{N}_2{O}_2\;{(fast)}} \end{align*}

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

    3. \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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