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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.

## Want to join the conversation?

• 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?
• they are one step reactions and mostly can be identified when two or more reactants(maximum 3 as 4 is quite unlikely) participate to give a single product. Also in most cases it will certainly be mentioned in the question.
• 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).
• 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??
• I think that this only refers to elementary reactions (reactions with one step and no intermediaries). Most reactions aren’t elementary, so we can only use this with elementary reactions.
• 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?
• Can someone tell me some examples for elementary reactions.?
• Why is it that we can take the coefficients as our exponents in these reactions but not others?
• The reaction happens in one step versus a reaction mechanism involving slow-steps, intermediates, catalysts, etc.
• So for first order reactions the increase in concentration increases the rate of reaction but the half life remains constant?
• That's right.
If a 1 mol/L solution takes 30 min to reach 0.5 mol/L, a 2 mol/L solution will take 30 min to reach 1 mol/L, and another 30 min to reach 0.5 mol/L.
The half-life remains constant even though the concentration changes.