welcome to this episode of Real Chemistry I'm Dr. Morris today we are
going to be talking about intermolecular forces which are the forces by which
molecules interact so for example think about this steam floating above the
water you may know that steam is made of water and that each water is made of two
hydrogens and an oxygen and those are bound together by covalent bonds but the
question is how do separate water molecules in the steam interact with
each other well that turns out to be by intermolecular forces and in the case
of water it's something called hydrogen bonding and that's one of our
intermolecular forces and this is really important because it explains why we see
mist forms why we see liquid forms the stronger those intermolecular forces are
the more they want to hang out together and so water turns out have pretty
strong intermolecular forces and when the surface of that water you see there
gets cool what happens is water molecules in the air that are flying
around start to slow down and those attractive forces start to draw them
together into this mist so let's take a look at three different intermolecular
forces and talk about the relative strengths first up is something called
dispersion forces all molecules have dispersion forces what's going on in
dispersion forces is imagine your typical atom right and it's got this
shell of electrons and inside is a positive nucleus well these electrons
can kind of move around right you can sort of think it like about like a jello
or something and so they can sort of shift to the right or shift to the left
and occasionally every so often some of them will shift to the left or right so
if you look down here what this is saying is some of our electrons have
shifted this way making it partially negative and this is one of our
molecules and an adjacent molecule does the same thing where some of the
electrons shift to the right and now temporarily we have a dipole moment
where there's more negative charge over here and more positive charge over there
and that makes them attracted to each other and because all atoms and
molecules have electrons all molecules have dispersion forces so if you're ever
asked to list what intermolecular forces does this molecule have dispersion
forces is always gonna the option sometimes these are called London
dispersion forces now one important thing about dispersion forces is as you
get larger molecules they get stronger the reason is is you get bigger and
bigger clouds of electrons so if you have a huge giant cloud
a little bit of swishing and swashing left and right makes you have a stronger
induced dipole so as things increase in molecular weight they have stronger
dipoles because of those larger electron clouds so you can see that here that as
we go down this list our molecular weight gets heavier and you'll also
notice our boiling point gets higher that means it's taking more energy to
take them from liquid separated on the gas melting point similarly gets higher
that means it's taking more energy to take them from a solid to a separate
liquid that can float around so larger atoms and molecules have stronger
dispersion forces okay so that's our first intermolecular forces dispersion
molecules sorry dispersion forces and all molecules have those then we have
permanent dipoles this is basically due to a polar molecule so if you don't know
how to tell if a molecule is polar then this is a hard thing to talk about so go
check out my video on polar molecules I'll link to it below before continuing
this video in a polar molecule we have a permanent distribution of electrons
that's uneven and in this case we have negative charge on the electron and a
positive charge on carbon because electron because oxygen is more
electronegative it pulls the electrons closer to the oxygen and this gives it a
permanent dipole so and before with dispersion forces it was something that
happened due to the motion of the electrons on a temporary basis this is
always like this oxygen will always have more electrons near it than carbon
oxygen will always be negative and it can line up and be attracted to other
molecules these are stronger than dispersion forces and polar molecules
have dipole moment so if you want to know from all you has a dipole moment
you have to just ask is it polar? and if it is then it has dipole forces. lastly
we have hydrogen bonds this is the strongest class of intermolecular forces
we're going to talk about and here it's basically just a dipole right because
our hydrogens are partially positive and our oxygens are partially negative
but it's a really strong dipole and that's why we pull it out as a separate
category anytime you have hydrogen bonded to fluorine nitrogen or oxygen so
that means directly a hydrogen on a fluorine
directly a hydrogen on a nitrogen or directly hydrogen on oxygen
you have hydrogen bonds and those attract other hydrogen bonds to the
molecules of rabbits so here you see right just like we talked about the
beginning of the video separate water molecules attracted to each other due to
the imbalance in charge that comes from hydrogen bonds so remember three forces
dispersion forces all molecules have them dipole forces polar molecules have
them hydrogen bonds anything with HF HN or HO has them all right now let's
look at some problems that use our intermolecular forces below we have a
series of molecules and ask us what sort of forces do they have so first let's
look at this top guy this guy is called a acetonitrile and we want to know does
it have dispersion forces well what do you think yeah of course it does because
all molecules have dispersion forces so we'll put a check there does it have a
dipole well here we have to decide is it polar it turns out because of this
carbon nitrogen bond the nitrogen is partially negative and the carbon is
partially positive and that would make it polar and it would have dipole forces
again if you're not so clear on that go check out my polarity video so yeah it has
dipole forces does is have hydrogen bonding? Well no where in there is a
hydrogen directly connected to an F and N or an O, So no hydrogen bonding.
right let's go to the second molecule HF hydrogen fluoride does it have
dispersion forces once again all molecules do so yeah of course it is
does it have dipole forces well it is polar hydrogen it is partially positive
and fluorine is partially negative forms the most electronegative element and so
yeah it has dipole does it have hydrogen bonding well here again the answer is
yes because hydrogen is directly bonded to fluorine so it basically has like a
super strong dipole moment and this is gonna tend to make it more attracted to
other molecules like it then the weaker intermolecular forces alright lastly we
have ethane just carbon and hydrogen carbon hydrogen have electronegativities
that are similar enough that they're considered nonpolar and that's gonna be
important first of all does it have dispersion forces well yes all molecules
have dispersion forces does it have a dipole moment well no there's no polar
bonds there and if there's no polar bonds there it can't have a dipole so it
doesn't have a dipole. Does it have hydrogen bonding?
well it does have a bunch of hydrogen there but they're not bonded to carbon I'm
sorry they're not bonded to fluorine oxygen or nitrogen so there's no
hydrogen bond so that's how you can go sort of through the process of picking
out the molecule with the different type of intermolecular forces which one here
would have the strongest intermolecular forces
well you'd expect it to be HF because it has dispersion dipole and hydrogen
bonding and second place you'd expect acetonitrile this top guy so that gets
second and then this one gets third place so this is the strongest than the
second than the third and this is always the order hydrogen bonding tends to be
the strongest then dipole dipole then dispersion these aren't hard and fast
rules the rules of thumb because of course if you got a really really really
really big molecule then maybe your dispersion forces get really really
strong so it's just as a general rule of thumb because hydrogen bonding
dipole-dipole dispersion one more question we're going to answer here were
asked to list the following molecules in terms of increasing intermolecular
forces and then it asks which has the highest boiling point so first let's
decide which one has the strongest intermolecular forces as you look
through these you'll notice that the only one with hydrogen in it is water
and we've already talked a lot about how that has hydrogen bonds so water of
course has hydrogen bonds so that's gonna be our strongest now let's look
for ones with dipole well you might look and you might say okay well maybe carbon
dioxide has a dipole and maybe CO has a dipole but actually carbon dioxide if
you remember is symmetric where it has an oxygen on each side so this is
actually nonpolar and that means it doesn't have a dipole dipole so water
has the strongest intermolecular forces and then actually carbon monoxide does
so carbon dioxide is going to be polar as carbon and oxygen bound directly to
each other all right what is next well now we have these three to choose from
and remember that when they all just have dispersion forces like these do
none of them have dipole-dipole or hydrogen bonds we go molecular weight
the bigger the molecular weight the stronger the intermolecular forces
CO2 here has the biggest molecular weight of those three then neon then
helium so this goes an increasing order of intermolecular forces the main way
you do that is you decide what has hydrogen bonding that is the strongest
well has dipole that's the second strongest what has dispersion that's the
weakest and then amongst the dispersion you can order in terms of molecular
weight you can also order the polar bonds or the hydrogen bonding in terms
of how polar those molecules tend to be alright so if that's in terms of
increasing intermolecular forces and water has the strongest intermolecular
forces which one has the highest boiling point? well it has to be water so that
has the highest BP that's because remember intermolecular forces or how
much molecules want to stick to each other
well that water has the strongest forces and it's the hardest to pull apart that
means it's gonna take the most energy the highest temperature to boil it so
that's just an introduction to intermolecular forces and I hope you
found this useful there is much more to know about intermolecular forces but if
you have questions that I might be able to clarify please ask them below as
always you can subscribe or like thanks for watching