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Main content
Current time:0:00Total duration:10:43
AP.Chem:
SAP‑4 (EU)
,
SAP‑4.C (LO)
,
SAP‑4.C.3 (EK)
,
SAP‑4.C.4 (EK)

Video transcript

this video we're going to look at the SP 3 hybridisation present in methane and ethane let's start with methane so that's ch4 if I want to draw a dot structure for methane I would start with carbon and it's four valence electrons and then we put hydrogen around that each hydrogen has one valence electron so we go ahead and draw in our hydrogen's with one valence electron and that gives us the Lewis dot structure alright usually you see it drawn like this with carbon with its four bonds to hydrogen around it like that and in methane all of these bonds are equivalent in terms of things like bond length and energy and so the four valence electrons that carbon brought to the table over here let me go ahead and highlight those four valence electrons those should be equivalent and if we look at the the electron configuration for carbon let's go ahead and do that right now it's 1s two so go ahead and put in two electrons in the 1s orbital 2's to go ahead and put in two electrons in the 2's orbital and then 2p to and so I'm assuming you already know you already know your electron configuration so it would look something like that if we look at those four valence electrons on our on our orbital notation here that would be these four electrons here the valence electrons in the outer shell and if we look at this this implies that carbon would only form two bonds because I have these unpaired electrons right here and everything's of different energies and so what we see from the dot structure and and and experimentally doesn't quite match up with the electron configuration here and so to explain this difference Linus Pauling came up with the idea of hybridization and so the first thing that he said was you could you could go ahead and take out one of these electrons in the 2's and promote it up to the the p orbital here so let me go ahead and show that so we've moved one of the electrons up to the 2p orbitals so we're in the excited state now and now we have the opportunity for carbon to form four bonds however those electrons are not of equivalent energy and so Linus Pauling said let's do let's do something else here let's go ahead and let's go ahead and promote the 2's orbital all right so we're going to take this s orbital and we're going to promote it in energy and we're going to take these P orbitals and and demote them in energy so we're going to lower those P orbitals like that so we have our P orbitals here all right and these had all one electron in each of them but we're going to hybridize them so this is no longer going to be an S orbital it's going to be an sp3 hybrid orbital this is no longer going to be ap orbital it's going to be an sp3 hybrid orbital and same with these so the idea is you're taking some of the s character and some of the P character and you're hybridizing them together into brand new orbitals and since you're taking this from 1s orbital and 3p orbitals right we're doing this using 1s orbital and 3p orbitals we call this sp3 hybridization so this is sp3 hybridization we create four new hybrid orbitals and now we have what we're looking for because now we have four unpaired electrons right so carbon can form four bonds now and they're equal in energy so that's the idea of hybridization all right let's think about let's think about the character or the shape of this new hybrid orbital well we know that an S orbital is shaped like a sphere all right so we're taking one of those s orbitals here so one s orbital and we know that a p orbital is shaped like a dumbbell alright so we're taking three of these P orbitals here so one of these s orbitals and three of these P orbitals and we're going to hybridize them together and when that happens it turns out the the shape of the new hybrid orbital you get has this large frontal lobe here like this and then a smaller back lobe back here like this so we're going to make four of these alright so once again we have four sp3 hybrid orbitals and each one of these hybrid orbitals is going to have an electron in it right so we can see that each one of these sp3 hybrid orbitals has one electron in there like that and and so the final orbital the final hybrid orbitals here contain 25% s character let me go ahead and write this down here so 25% s character and 75% p-character in this new hybrid orbital once again that's because we started out with one s orbital and three P orbitals for our hybridization all right let's go back to let's go back to methane and and let's go ahead and draw on a picture because now we know that this carbon is sp3 hybridized so let's go ahead and draw a picture of that hybridized carbon here alright so we're going to go ahead and draw in our carbon and we know that it has four sp3 hybrid orbitals and once again when we draw the orbitals we're going to ignore the smaller back lobe here so it doesn't confuse us so we go ahead and draw in here's one of our orbitals four carbons that's an sp3 hybrid orbital here's another sp3 hybrid orbital here's another one and then finally a fourth one alright so let's go back up here to this picture because once again we need to we need to show that each of these hybrid orbitals has one valence electron in it so I can go ahead and put in my one valence electron right in each of my hybrid orbitals like that so here's our valence electron if we're talking about methane alright so carbon is bonded to four hydrogen's each hydrogen right hasn't has an unhybridized S orbital and each hydrogen has one electron in that so go ahead and and sketch that in let's go ahead and use blue here all right so here's an unhybridized s orbital i'm go ahead and draw these in so an unhybridized s orbital each one of these unhybridized s orbitals for hydrogen has one valence electron so go ahead and put in those one valence electrons in here like that so same for here and then finally for here and so this is just this is just one picture of the methane molecule so this is hydrogen alright these are all the hydrogen's right here like that all right let's think about let's think about the this bond that we've formed right here so here we have an overlap of orbitals right an overlap of an sp3 hybrid orbital from carbon with an unhybridized s orbital from hydrogen here and so this is a head-on overlap all right so we're sharing we're sharing electrons here in this head-on overlap the head-on overlap in chemistry is called a sigma bonds right so this is a sigma bond a sigma bond here a head-on overlap and this happens three more times in the methane molecule right so here's a head-on overlap here's a head-on overlap and here's a head-on overlap and so we have a total of four sigma bonds in the methane molecule so a single bond here is represented we're instead of saying a single bond now we're saying it's it's also can be called a sigma bond so this head-on overlap let's look at let's look at the ethane molecule now so for ethane go ahead and draw that in so ethane would be c2h6 alright so we have two carbons so let's go ahead and draw those two carbons and then six hydrogen's so we put in our six hydrogen's around there like that alright when we're thinking about hybridization we've just seen with methane that a carbon atom with four single bonds will be sp3 hybridized alright so I go back up to here alright this carbon right here alright for single bonds it's sp3 hybridized we can use that same logic and apply it to ethane here each of the carbons in ethane has four single bonds so each carbon and ethane is sp3 hybridized so and go ahead and put sp3 hybridized here so let's go ahead and draw the picture with the orbitals all right so let's get some more room if each carbon is sp3 hybridized that means each carbon is going to have four sp3 hybrid orbitals so I can go ahead and sketch in one carbon right once again I'm ignoring the back lobe alright one carbon with four sp3 hybrid orbitals and we know the other carbon is also sp3 hybridized so I can sketch in four sp3 hybrid orbitals for this one two so here's my four sp3 hybrid orbitals for this carbon alright in terms of electrons let's go ahead and put in our electrons here so this C there's one electron in this orbital one electron in this orbital and one electron in this orbital one electron from this carbon and then for this other carbon right so there's one electron in this orbital one electron in this orbital one electron this orbital and one electron in this orbital and then we can go ahead and put in our hydrogen's right so we know each hydrogen has an unhybridized orbital with one valence electron so I can go ahead and do that and draw on the rest of our hydrogen's so that's four hydrogen's five hydrogen's and then six hydrogen's like that all right we just said that a sigma bond is a head-on overlap of orbitals all right so here we have a head-on overlap of orbitals the bond between the two carbons and then of course all of these are two so when we count all of those up right that's four five six and seven so there are seven sigma bonds in the ethane molecule so seven sigma bonds here and we can go up here to this dot structure and and look at them again right so here's one two three four five six and then seven seven Sigma bonds let's focus in on the bond between the two carbons now so so this Sigma bond right in here and so that's a sigma bond and there's free rotation about this Sigma bond so if you can imagine if you can imagine rotating around this bond so these carbons can rotate in space and that's going to give different confirmations so you can have different confirmations of the ethane molecule which is in later videos all right so you have free rotation about Sigma bonds I'm going to go ahead and write that because that's pretty important so free rotation about about Sigma bonds and then the last thing I want to point out about the ethane molecule here is the bond length all right so the length between this carbon and this carbon so this bond length in here turns out to be approximately 1.5 4 angstroms so you'll see slightly different values in different textbooks but we'll say it's approximately this value and the reason we wanted we want to know that is we're going to compare this carbon-carbon bond length to the bond length and some other molecules in some later videos