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# Elementary reactions

AP.Chem:
TRA‑4 (EU)
,
TRA‑4.A (LO)
,
TRA‑4.A.1 (EK)
,
TRA‑4.A.2 (EK)

## Video transcript

let's say we have a simple elementary reaction where we have only one reactant a turning into our products we can classify this reaction according to its molecularity which refers to the number of participating molecules so if we think about one molecule of a giving us our products this would be a uni molecular reaction we have only one molecule so we call this a uni and molecular reaction next let's think about writing the rate law so we know 1 we're writing rate laws we write the rate of our reaction is equal to the rate constant K times the concentration of our reactants and here we have only one reactant so we say times the concentration of a for the exponent we can actually take the coefficient in our balanced equation and take the coefficient and turn that into the exponent so we have a 1 here for our coefficient so we make that a 1 right here so you can only do this for an elementary one-step reaction you can't do this for an overall equation with the detailed mechanism like we'll see in the next video but for these elementary reactions you can do this so the rate of our reaction is equal to the rate constant K times the concentration of a to the first power so for this uni molecule ER reaction its first order in a next let's look at another reaction a one molecule of a plus one molecule of B gives us our products here we have two molecules two participating molecules so this is a by molecular reaction so right by molecular here so we can think about these two molecules colliding in space so molecule a is going to collide with molecule B to give us our products and so it makes sense that the rate of the formation of our products depends on how frequently a and B collide and that depends on the concentration of a and B if you increase the concentration of a and B you increase the frequency of collisions and therefore you increase the overall rate of your reaction so when you write your rate law here so the rate of our react is equal to the rate constant K times the concentration of a times the concentration of a and since this is an elementary one-step reaction we can take the coefficient and turn that into our exponent so times the concentration of a to the first power times the concentration of B and once again we can take our coefficient which is a1 and turn that into our exponent and so now we have the rate law for this by molecular reaction let's look at another biomolecular reaction and this this time we have two molecules of a reacting to give us our products so we could say that this is a plus a gives us our products or we could say this is two a gives us our products so either one if we stick with the first version we have a 1 for our coefficient here and a 1 for our coefficient here and so we write the rate law for this by molecular reaction the rate is equal to the rate constant K times the concentration of a and we look at our coefficient here which is a 1 so we make that to the first power and then times the times the concentration of a again and once again we look at our coefficient and we turn that into our exponent and so that of course would become this would just be the rate is equal to the rate constant K times the concentration of a this would be to the second power right a to the first times a to the first is equal to a squared or we could have looked at ours at our other version of writing it right a to a and once again our coefficient our coefficient would become our exponent so this is another example of a biomolecular reaction finally let's look at a reaction where we have three participating molecules so one a plus 1 B plus 1 C gives us our products so one molecule of A plus one molecule of B plus one molecule of C there are three participating molecules here so we call this a termolecular reaction and this would be for this to occur in one step alright these would all have to collide at the same time so if we had a eeee and see they would all have to collide right at this point in space at the same time and this is rare if you think about it trying to get three molecules to collide at once it's pretty difficult to do so these termolecular reactions are rare but we can write the rate law so the rate of our reaction is equal to the rate constant K times the concentration of a and we have a coefficient of one here so this is to the first power times the concentration of B and once again this would be to the first power and times the concentration of C and this would also be to the first power so for these elementary rate laws for these elementary reactions we can take the coefficients and turn them into the exponents in our rate laws but once again in this next video you'll see that we can't do that we can't look at an overall balanced equation with a detailed mechanism and just take the exponents and figure out the rate law the rate law needs to be determined experimentally