professor Dave here, let's talk about intermolecular forces
what is happening when a liquid boils? why do different liquids boil at different temperatures?
to answer these questions we have to learn about intermolecular forces. these
are the electrostatic interactions between molecules. so atoms within a
molecule make covalent and ionic bonds with each other, but molecules also
participate in interactions with other molecules. let's look at the different
types. first we have ion-ion interactions. larger ionic solids are held together by
these networks of ionic bonds which are the strongest intermolecular force
because they involve formal charges. after that we have ion-dipole
interactions, so first we must understand what a dipole is. the covalent bonds in a
water molecule are polar because oxygen is more electronegative than hydrogen
and will pull the electrons in the bond towards itself. because of the bent shape
of the molecule when we combine these vectors we see water has an overall
dipole, or a side of the molecule with some electronic access and a side with
electron deficiency. dipoles can make electrostatic interactions because
the partially negative side is attracted to positive charges and the partially
positive side is attracted to negative charges. so when sodium chloride
dissolves in water, the sodium ions make ion-dipole interactions with the
negative side of water's dipole and the chloride ions make ion-dipole
interactions with the positive side of water's dipole. each ion can make several
of these interactions which store a lot of energy, which is why sodium chloride
will dissociate in water in the first place. next we have dipole-dipole
interactions. as you can guess, this is when dipoles interact with each other, as
with pure water. when in liquid form water molecules will move in such a way
so as to always be making electrostatic interactions between the negative end
of one dipole
and the positive end of another dipole. in this case these dipole-dipole
interactions qualify for a special title: hydrogen bonds. this is when dipoles
generated by N-H, O-H, or F-H bonds interact with each other.
these are just especially strong dipole-dipole interactions. they are especially
strong because these are the most electronegative elements, so they will
create the most strongly polarized bonds resulting in a very strong dipole and
therefore very strong dipole-dipole interactions. we can almost think of
partial charges as some fraction of a formal charge, so the greater the partial
charge the stronger the interaction, though never quite as strong as
interactions between formally charged particles. lastly we have the van der
Waals, or the London dispersion force. these names refer to the same force and
are completely interchangeable so I will arbitrarily refer to them as van der
Waals forces. this is the consolation prize of the intermolecular forces
because any substance can do it. only ions make ion-ion interactions and only
covalent molecules with a dipole can make dipole-dipole interactions but
absolutely anything can do van der Waals. for example take a look at helium. helium
is a noble gas and due to a full valence shell it does not make bonds with other
atoms, so a sample of helium is just a bunch of helium atoms. well the electron
cloud around a helium atom will at any time be slightly lopsided or skewed
towards one direction. this will result in something called a momentary dipole
this means one side of the atom is ever so slightly partially negative and the
other side is slightly partially positive. this is much weaker than a
formal dipole but it still exists and can be measured. if a momentary dipole
approaches another atom, it can generate an induced dipole, meaning the slight
partial negativity repels this electron density over to the other side of the
atom so it will also have a slight dipole, and then there can be a momentary
dipole-induced dipole interaction that is the van der Waals force. this is a
weak and fleeting attraction but this is all that monoatomic species and
nonpolar covalent compounds can do, and for very large molecules like some
hydrocarbons the force can become significant. so the ion-ion force is
strongest because it involves interactions between formally charged
particles. ion-dipole is next because it involves a formal charge and a partial
charge, then dipole-dipole which is between partial charges, and van der
Waals which is between tiny induced dipoles. to see how intermolecular forces
dictate phase change let's do a thought experiment. first recall that a solid's
particles are rigidly packed and not moving. a liquid's particles are moving but
they are still close together and interacting. gaseous particles are moving
and they are far away from each other so compared to liquids they basically don't
interact. so let's pretend we have three substances: helium, water, and sodium
chloride. we will place them at zero kelvin or absolute zero which is the
lowest temperature possible, a complete absence of heat energy, where there is no
energy available for motion. here everything, even helium, is a solid. in
order to go from the solid to liquid to the gas phase heat energy has to go into
the sample and overwhelm the intermolecular forces that are occurring.
in a liquid there is some energy stored in electrostatic interactions and
whatever amount that is, that is precisely the amount of heat energy that
has to be provided to liberate the molecules into the gas phase where they
are not interacting and not storing energy, because nature will not tend to
go to a higher energy spontaneously
so the energy stored in these interactions has to be provided in some
other way. this means that the stronger the forces between molecules, the more heat
energy we will have to provide to melt and boil the sample. so let's take our
three substances slowly raise the temperature and see what happens
helium, as it is only participating in incredibly weak van der Waals forces
needs only a minuscule amount of heat energy to disrupt these weak
interactions. that's why helium will melt and boil at barely one degree above
absolute zero. water on the other hand is participating in strong dipole-dipole
interactions called hydrogen bonds, there is a significant amount of energy stored
in these interactions, so we will need considerable heat energy to overcome
them. water melts and boils at 273 and 373 Kelvin respectively.
lastly, sodium chloride is making extremely strong ion-ion interactions so
it will take a huge amount of energy to melt and boil this solid. it melts at
1074 Kelvin, which means there's a lot of energy stored in the ion-ion forces. we
can use this information to decide which of a given set of compounds might have
the highest boiling point. when we ask this question we are really asking: which
compound is generating the strongest intermolecular forces? the stronger they
are, the more heat energy we will need to pull the molecules apart and put them in
the gas phase, so they will boil at higher temperatures. we need to be able
to look at a molecule and decide what kind of interactions it will make. if it is
a covalent compound with non polar bonds, it can only do van der Waals. if it
is a covalent compound with polar bonds, then we must look at the geometry to see
if there is an overall dipole. for example, water has a dipole.
carbon dioxide does not, because even though carbon-oxygen bonds are polar, the
direction of these polar bonds causes them to cancel each other out and the
molecule is nonpolar overall. similarly compare BF3 with NH3. again, the molecular
geometry determines that this is nonpolar because the vectors precisely
cancel each other out, but with ammonia, all the bonds point somewhat towards one
direction so ammonia has a dipole. CS4 is nonpolar again because of geometry but
CH3F has just one polar bond so that has a dipole. if a molecule has a dipole
it can do
dipole-dipole interactions. and lastly, formally charged ions participate in ion
ion interactions. see which compounds can do what and you will be in business
let's check comprehension
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