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# Elementary reactions

An elementary reaction is a reaction that occurs in a single step. The rate law for an elementary reaction can be derived from the coefficients of the reactants in the balanced equation. For example, the rate law for the elementary reaction 2A + B → products is rate = k[A]²[B]. Created by Jay.

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• On previous videos, I thought you couldn't write the rate law with the coefficient. Why can you do it in this case? •   The individual steps in a mechanism are elementary reactions. For such reactions, the exponent in the rate law is the coefficient.
Many reactions consist of a number of elementary steps. Since we can't "see" what the steps are, we don't know what exponents to use.
We have to do experiments to determine the rate law, and then we can make guesses about the elementary steps.
• How do we know these reactions, or any reactions in general, are elementary? • how do we know if your reaction is an elementary one step or not? • You need to run experiments in order to determine if a reaction is elementary. It's reasonably easy to experimentally prove that a reaction is not elementary, but it's harder to prove that it is elementary. We can predict the rate law for an elementary reaction easily, so we know that if a reaction doesn't match this predicted rate law experimentally, it must not be elementary. Also, if we see any experimental evidence of an intermediate (e.g. we find some chemical in our reaction that isn't our reactant or product), then we also know it must occur over at least two steps. We also know that elementary reactions involving 3 reactants are rare and those involving >3 are all but impossible. So, if you have >3 reactants, you can assume the reaction is not elementary.

We can support that a reaction is elementary if:
-The elementary rate law matches the rate law we measure experimentally
-We have no evidence of intermediates
-There are 3 or fewer (usually 1 or 2) reactants
-Computer-based calculations help us determine a reasonable 1-step process

Of course if you're not the one in the lab doing the experiments, you'll just have to trust what the question tells you. You'll have to be told if a reaction is elementary or not. Or, you'll have to be told the experimental data so that you can compare for yourself whether the elementary rate law and experimental rate law match (=it could be an elementary reaction) or don't (=it must not be an elementary reaction).
• I found this video quite confusing in the context of the AP/College Chem course.

1. I don’t recall there being a formal introduction to the concept of an elementary one-step reaction before this. Even if there had been, a quick review here and emphasis on how critical it is to the remainder of this video would have been helpful to me.

2. For each example where the stoichiometric coefficients are used to build the rate laws directly, it would be good to emphasize (repetitively) that doing so is valid only for elementary one-step reactions, and not generally valid.

3. The examples discussed covered only the coefficients/exponents 1 and 2. It’s unclear in the video if the concept is generalizable to any coefficient/exponent. Seeing something like x A + y B -> products yields R = [A]^x + [B]^y would have been super handy.

Thank you. • Can someone tell me some examples for elementary reactions.? • My homework just said the exponents in rate law are not determined by the coefficients in a chemical equation, but this video says they are?? • Why is it that we can take the coefficients as our exponents in these reactions but not others? • So for first order reactions the increase in concentration increases the rate of reaction but the half life remains constant?
(1 vote) • Ok, I still dont see why we can now just use the stoichiometric coefficients as the exponents for some reason when making these rate laws.

For any other rxn that is not elementary, we have to use experimental data. And then the exponent for each reactant/product's concentration is the order for the corresponding reacantant/product. But now we can just magically ignore all of this?  