all right let's kind of do a question that combines some of the concept that we talked about before identify the intermolecular forces presence in the following substance and then use that information to predict which one would have the highest boiling point out of the bunch so let's look at molecule a so it's just an alkane the other one is a methanol right so it would have a structure that would look like this so that's that's ch 3 ch3 so it looked like this the next one would be methanol so it's just our ch3 is here so the property that's this lewis structure and lastly we have ethanol which would look something like this okay so now these are our three different molecules what kind of intermolecular forces are present on each of these molecules well we have dispersion on all of them right so it's automatic what else well can we have a dipole-dipole interaction is the molecule polar are they polar well hopefully you can see that this molecule is nonpolar so there's no dispersion forces and if you remember alcohol are considered polar molecule so you would always have dipole-dipole interaction because these are polar alright what's next and again this one is nonpolar so so that's why there's no dipole dipole interaction what's next can this molecule have a hydrogen bond and hopefully you can see that if i were to have another o and h here i can have that interaction and the same thing with this right i can have an o h and then another ethane molecule i can have that interaction and that is hydrogen clamp between oxygen two oxygen so yes we can have hydrogen bond here and so is this one so in this case the hydrogen there is clamp between oxygen excuse me between two oxygens right so hydrogen bond is fun so that's also a hydrogen bond do we have an ionic interaction in this case and hopefully you can see that this molecule is polar but it's not ionic so we don't have any ionic interaction so so we now have kind of listed all the possible intramolecular forces i can tell this is going to have the lowest boiling point now the question is we have methanol and ethanol they have the same kinds of intermolecular forces so which one is going to have a higher boiling point but remember the highest boiling point would be the molecule that interacts the strongest so we've already gone through the polarity which is basically listing off the intermolecular forces so i'm going to go now with the size remember the bigger the size of the molecule the higher the boiling point and the reason why the bigger the molecule is going to be is because the bigger the molecule the more dispersion forces that you can have on the molecule versus the smaller one because smaller molecules simply doesn't have as much electron to fluctuate so really i'm looking at which one of these two is bigger so this one is bigger here because the ethanol is bigger the dispersion is going to be a little bit bigger compared to this version of the methanol compound and therefore the boiling point is going to be highest of the ethanol one right so first what i did there let me kind of follow this up again i look at all the intermolecular forces that's possible and after i figure out which are all the intermolecular fossils that's possible i can then consider the size of the molecule and whatever is smaller is going to have a less dispersion forces a weaker dispersion force and whichever is bigger is going to have a slightly bigger dispersion contraction and therefore would i would predict that that particular molecule would have the highest boiling point out of the bunch