It's elementary!

Question from email...
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Hello Dr. Bodwin,

I am going over the practice sheet you handed us in class on Thursday and I am stuck on question #10. Where it asks why certain reactions cannot be elementary? And explain why? I'm wondering how to figure that out! If you could clarify that would be awesome!! 
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Elementary reactions or elementary steps describe the individual collisional events that occur in each step of a reaction. When a chemical reaction occurs, the reacting species have to interact with each other (Collision Theory). If we think about those interactions or "collisions", there is a pretty low probability that a collision will take place that is properly oriented and at the correct energy if that collision involved more than two components. From a Gen Chem perspective, this means that we can pretty safely say that an elementary reaction cannot have more than 2 reacting particles.

Because elementary reactions describe molecular level collisions, we can also say that the elementary reaction must be reversible, meaning that if we reverse the reactions (exchange products and reactants), it must also be elementary. So an elementary reaction should have no more than two reactants, but it should also have no more than two products.

Reaction mechanisms and elementary reactions can have a lot of subtle little twists and turns that make them fascinating to study and we are treating elementary reactions very simply at this point. As you explore more an more chemistry, you will find that reaction mechanisms can be a fascinating puzzle to work through. Enjoy!