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Current time:0:00Total duration:5:21

Worked example: Interpreting potential energy curves of diatomic molecules

AP.Chem:
SAP‑3 (EU)
,
SAP‑3.B (LO)
,
SAP‑3.B.1 (EK)
,
SAP‑3.B.2 (EK)

Video transcript

in a previous video we began to think about potential energy as a function of inter nuclear distance for diatomic molecules what do I mean by diatomic molecules well we looked at molecular hydrogen which is just or h2 which is just two hydrogen's covalently bonded to each other and at standard temperature and pressure there they would naturally the distance between the two nuclei would go would be based on where there is the lowest potential energy and if you were to squeeze them together you would have to put energy into the system and have a higher potential energy or if you were to pull them apart you would have to put energy into the system and have a higher potential energy what I want to do in this video is do a little bit of a worked example over here I have three potential energies as a function of inter nuclear distance graphs and what I'm going to tell you is one of these is molecular hydrogen one of these is molecular nitrogen or diatomic nitrogen and two and one of these is diatomic oxygen and what I want you to think about pause this video is which graph is the potential energy as a function of inter nuclear distance for each of these diatomic molecules and I'll give you a hint look at the low point in potential energy the low point in potential energy is what you would typically observe that diatomic molecules in turn nuclear distance to be at standard temperature and pressure and this distance right over here is going to be a function of two things it's going to be a function of how small the atoms actually are how small their radii are so smaller atoms are in gentlemen have a shorter stable internuclear distance but the other thing to think about is the bond order between these atoms and I'll give you a little bit of a hint diatomic hydrogen you just have a single covalent bond for diatomic nitrogen it is a triple bond and for diatomic oxygen it is a double bond so the higher order of the bond that will also bring the two atoms closer together and it also makes it have a higher energy the energy required to separate the atoms remember we talked about it in the previous video this right over here is the bond energy and so with that said pause the video and try to figure it out which of these is the graphs of h2 which is ends do and which is o2 so let's first just think about it in terms of bond energy if you look at it the single bond double bond triple bond here you would expect the highest order bond here to have the highest bond energy and the highest bond energy is this salmon-colored one right over here so just based on that I would say that this is a good candidate for n2 so this one right over here this looks like diatomic nitrogen to me then the next highest bond energy if you look at it carefully it looks like this purple one right over here and so just based on bond order I would say this is a good candidate for Oh two and then the lowest bond energy is this one all right over here and so just based on the bond order here it's just a single covalent bond this looks like a good candidate for diatomic hydrogen let's also think about the radii of these atoms if we get a periodic table of elements here we can see that hydrogen only has one electron in that first shell and so it's going to be the smallest so that makes sense over here that your distance where you have the lowest potential energy is shortest for the diatomic molecule that's made up of the smallest atoms but then when you look at the other two something interesting happens remember your radius for an atom increases as you go down a column but as you go to the right on a row your radius decreases because you're adding more and more electrons to the same shell but the Coulomb forces are increasing between that outermost shell and your nucleus and so if you just look at that trend as you go from nitrogen to oxygen you would actually expect your atomic radius to get a little bit smaller they're right next to each other they might be close but you say okay oxygen you have one extra electron in that same second shell maybe it's going to be a little bit smaller so if you were to base things just on that you'd say all right well the inter nuclear distance for this salmon-colored one is a little bit shorter maybe that one is oxygen and maybe this one is nitrogen but they would be close and I would say in general the bond order would trump things and the bond order because you see this high bond energy that's the biggest giveaway that this is going to be the higher bond order diatomic molecule or n2 they're close an atomic radius but this is what makes all of the difference and we'll take those two nitrogen atoms and squeeze them together just a little bit more even though they might be a little bit bigger and so I feel pretty good with this labeling