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Current time:0:00Total duration:6:42

Bond length and bond energy

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

Video transcript

if you were to find a pure sample of hydrogen odds are that the individual hydrogen atoms in that sample aren't going to be separate atoms floating around that many of them and if not most of them would have bonded with each other forming what's known as diatomic hydrogen which we would know right as h2 another way to write it is you have each hydrogen in diatomic hydrogen would have bonded to another hydrogen to form a diatomic molecule like this this molecule is only made up of hydrogen but it's two atoms of hydrogen and this makes sense why it's stable because each individual hydrogen has one valence electron if it is neutral so that's one hydrogen there that's another one there and if they could share their variants valence electrons they can both feel like they have a complete outer shell and so this - right over here you can view as a pair of electrons being shared in a covalent bond now we're going to do in this video is think about the distance between the atoms so just as an example imagine two hydrogen's like this so that's one hydrogen atom and that is another hydrogen atom it turns out at standard temperature pressure the distance between the Centers of the atoms that we observe that distance right over there is approximately 74 Pico meters and just as a refresher of how small a Pico meter is a Pico meter is one trillionth of a meter so this is 74 trillions of a meter so we're talking about a very small distance but one interesting question is why is it this distance what would happen if we tried to squeeze them together what would happen if we tried to pull them apart and to think about that I'm gonna make a little bit of a graph that deals with potential energy and distance so in the vertical axis this is going to be potential energy potential energy and I won't give the unit's just yet I'll just think in very broad brush conceptual terms and then we could think about the units in a little bit and then this over here is the distance distance between the Centers of the atoms you could use the distance between the new and let's give this in picometers now potential energy when you think about it's all relative to something else and so let's just arbitrarily say that at a distance of 74 picometers our potential energy is right over here I'm not even going to label this axis yet now what's going to happen to the potential energy if we wanted to pull these two atoms apart well if this is what we typically find the mad this is probably a low point or this is going to be a low point in potential energy so if you make the distances go apart you're going to put energy into it and that makes the potential energy go higher and to think about why that makes might cent why that makes sense imagine a spring right over here if you want to pull it apart if you pull on either sides of a spring you are putting energy in which increases the potential energy because if you let go they're just going to come back - they're going to they're going to accelerate back to each other so as you pull it apart you're adding potential energy to it so as you have further and further distances between the nuclei the potential energy goes up and if you go really far it's going to ask them towards some value and that value is essentially going to be the potential energy if these two atoms were not bonded at all if they to some degree weren't associated with each other if they weren't interacting with each other and so that's actually the point at which most chemists or physicists or scientists would label zero potential energy the energy at which they are infinitely far away from each other and that's what this is asymptoting towards and so let me just draw that line right over here so let's call this zero right over here and actually let me now give you this let's say all of this is in kilojoules per mole now once again if you're pulling them apart as you pull further and further and further apart you're getting closer and closer to these these two atoms not interacting why is that because as you get further and further and further apart the Coulomb forces between them are going to get weaker and weaker and weaker and weaker and so that's why they like to think about that is zero potential energy now what if we think about the other way around what if we want to squeeze these two together well once again if you think about a spring if you imagine a spring like this just as you would have to add energy or increase the potential energy of the spring if you want to pull the spring apart you would also have to do it to squeeze the spring more and so to get these two atoms to be closer than closer and closer together you have to add energy into the system and increase the potential energy and why why are you having to put more energy into it because the more that you squeeze these two things together you're going to have the positive charges of the nuclei repelling each other so you're gonna have to try to overcome that and that puts potential energy into the system and these electrons are starting to really overlap with each other and they will also want to repel each other and so what we've drawn here just is just conceptually is this idea of if you wanted them to really overlap with each other you're going to have a pretty high potential energy and if you're going to have them very separate from each other you're not going to have as high of a potential energy but this is still going to be higher than if you're at this stable point the stable point is stable because that is a minimum point is a low point in this potential energy graph you could view this as just right and it turns out that for diatomic hydrogen this difference between zero and where you will find it at standard temperature and pressure this distance right over here is 432 kilojoules per mole so this is at the point negative 432 kilojoules per mole and so one interesting thing to think about a diagram like this is how much energy would it take to separate these two atoms to completely break this bond what would be the energy of completely pulling them apart and so it would be this energy it would be this energy right over here or 432 kilojoules and that's what people will call the bond energy the energy required to separate the atoms and we'll see in future videos the smaller the individual atoms and the higher the order of the bonds so from a single bond to a double bond to a triple bond the higher order of the bonds the higher of a bond and you're going to be dealing with