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PODCAST 11 – 12/9/2015 Intermolecular Forces.
Hello and welcome to this podcast from allerytutors.com. My name is Chris Harris and I will be
running through the attractive topic of intermolecular forces. I will go through what we mean by
polarity in a molecule and why this is so important for intermolecular forces. We will look at van der
waals, dipole-dipole and hydrogen bonding and what impact this has on the melting and boiling
points of molecules. As usual, there will be opportunities for you to practice too.
This podcast is designed to go through content required for the new A Level Chemistry specifications
and although most content is the same across all exam boards, you should check your specification
to make sure you are learning the right content at the right level of detail. All specifications can be
fou d o the oard s e site a d ill list e a tl hat ou eed to k o .
Just a quick reminder that I have a set of A Level Chemistry Revision Videos on my YouTube channel
just type in Allery Tutors into a search engine or YouTube and you will find them there, please
remember to subscribe and join a growing community of fans. Follow me on Twitter @allerytutors
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questions on all things Chemistry and I will do my best to answer them. It may be a good idea to
print off the script for this podcast so you can read and listen at the same time. This way you can
scribble all over them with your own notes and thoughts. Visit allerytutors.com to get a hold of
them.
Ok, so I suppose the best place to start is with polarity in molecules. A molecule is polar when it
contains elements with different electronegativity values. Electronegativity is where some atoms can
attract electrons more readily to itself than other atoms. The proper scientific definition is –
The ability for an atom to attract electrons towards itself in a covalent bond
So we know from this definition that this occurs only in molecules that have covalent bonds. It also
important to note that the further up and right we go in the periodic table the more electronegative
the element is (excluding the noble gases at the end). So Fluorine is the most electronegative
element. We can measure the level of electronegativity using the Pauling Scale named after Linus
Pauling who was an American Chemist that developed the scale in 1932. The bigger the number the
more electronegative the element is. So for example Fluorine is the most electronegative element
and has a value of 4.0 where as Carbon is less electronegative and has a value of 2.5.
Using this idea of electronegativity we can now explain polar molecules! When we bond an
electronegative element with a less electronegative atom we get a polarity. The most
electronegative element will pull electrons that are being shared as part of the covalent bond
towards its self. This leaves one element deficient in electrons and one rich in electrons and we
show this by drawing a small δ symbol followed by a negative or positive sign above the element.
Take a look at the script for this podcast at allerytutors.com to see a diagram showing this-
This shows that Cl is more
electronegative than H and so Cl has
a δ –. The electrons in the covalent
bond are being pulled towards the
Cl atom.
© C Harris (AlleryTutors.com)
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Not all molecules are polar e.g. diatomic molecules such as Cl2. The electrons in the covalent bond sit
in the middle and equal distance from each atom. Also symmetrically charged molecules are classed
as non-polar e.g. CO2 is non-polar despite having a carbon and 2 more electronegative oxygen
atoms. This is because CO2 has its positive and negative charges spread evenly across the molecule.
View the script for this podcast at alleytutors.com to see a diagram to illustrate this –
The diagram shows the cloud of
electrons around the atoms. You can
see that the electrons are
distributed evenly so there is no
overall polarity.
Time for you to have a practice. Draw the following molecules and mark on the partial charges for
each atom. H2O and NH3. Which molecule will have the biggest polarity? (Visit allerytutors.com view
the script for this podcast to find out the answer)
ANSWER –
Water would have the greatest polarity as Oxygen is more electronegative than Nitrogen.
Now that we know what a polar molecule is we need to apply this to some meaningful chemistry.
This is where we look at the world of intermolecular forces. An intermolecular force is just a weak
force BETWEEN MOLECULES. It is certainly not a bond and is in fact significantly weaker than a bond.
To put this into a perspective, image you had 2 bar magnets and you placed them next to each
other. Providing there was opposite poles they would stick together but can be pulled apart with a
little energy. This is just like an intermolecular force. Now if we take the same 2 bar magnets and
stick them together with super glue. The magnetic force is still sticking them together but now we
have a much stronger bond holding them together. We need a lot more energy to pull them apart
because of the super glue.
There are 3 types of intermolecular force –
1. Van der Waals
2. Dipole-Dipole
© C Harris (AlleryTutors.com)
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3. Hydrogen Bonding
Van der Waals
Van der Waals is the weakest type of intermolecular force and exists in every atom and molecule
that has electrons in (which is pretty much every everything). They are also known as induced
temporary dipoles or London forces. The word induced dipole gives a little hint about what a van der
waals force is. In a molecule electron cloud moves around it with a near even distribution in non –
polar molecules such as Cl2 or Hydrocarbons. When 2 molecules come near each other the electron
clouds move (due to repulsion) to one side of the molecule. This creates a molecule with a high
density of electrons on one side and low density on the other effectively a mini dipole. This dipole
allows the molecule to attract to a neighbouring molecule. This will only exist until the molecules
drift apart then the cloud of electrons will move back to how they were before, hence a temporary
dipole. You can imagine this like a bowl of water. The water would represent electron clouds. Hold
the bowl flat and the water is distributed evenly in the bowl. Tip the bowl to one side and the water
collects at one side of the bowl leading to a water rich side and a water deficient side. This is just the
same with electrons in a molecule. View the script for this podcast at alleytutors.com to see a
diagram to illustrate this –
The diagram shows the cloud of
electrons around 2 atoms. When
they come close the electron cloud
distorts to produce a dipole in the
atom! Red means lots of electrons
and blue means little electrons. We
have an ATTRACTION where the red
dotted line is!
Obviously there are so many molecules in any one space that these interactions happen so
frequently that the overall effect is attractive forces. Van der waals forces are very weak forces, and
molecules with only this type of force have low melting and boiling points as very little energy is
required to overcome the forces. It is important to note that the bigger the molecule or atom (bigger
the atomic or molecular mass) the more van der waals forces we have so the higher the melting and
boiling point. So for example Butane has a higher boiling point then Methane as Butane has a Mr of
58 whereas Methane has an Mr of 16. Be careful if the molecule has other stronger forces such as
dipole-dipole a d h droge o di g the this state e t o t fit. Co pare like ith like!
Have a go at this question. Predict which will have the higher boiling point, Hexane (C6H14) or Octane
(C8H18). Explain your answer. (Visit allerytutors.com view the script for this podcast to find out the
answer)
ANSWER –
Octane will have the higher boiling point. This is because it has a larger mass (Mr) and so it will have
more van der waals forces. This means more energy is needed to overcome the forces and hence its
boiling point will be greater.
© C Harris (AlleryTutors.com)
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Dipole-Dipole
The next strongest intermolecular force is a dipole-dipole interaction but it is also known as a
permanent dipole-dipole. These are molecules that have a dipole that exists permanently and not
just when a molecule moves near another one like in van der waals. This is straight forward as all
you need is a molecule with a polarity as described earlier in the podcast. HCl and CFCs all have
dipole-dipole interactions. Molecules that have these have higher melting and boiling points than
molecules of the same mass that only have van der waals. It is important to note that molecules with
dipole-dipole interactions also have van der waals forces too.
Hydrogen Bonding
The strongest intermolecular force is Hydrogen Bonding. Despite its name it is still a force and not a
bond! Hydrogen bonding occurs between molecules that contain Hydrogen and one of either
Nitrogen, Oxygen or Fluorine. These elements are the most electronegative elements and sit in a
row in the top right hand side of the periodic table. You can remember this as NOF. So if a molecule
does ot o tai h droge a d o e of N, O or F the it o t ha e a h droge bonding! It is as
simple as that. Molecules with hydrogen bonding have higher melting and boiling points than
molecules with the same Mr that have no hydrogen bonding. This is because more energy is
required to overcome the stronger hydrogen bond. E.g. Water has a Mr of only 18 but has a boiling
poi t of
˚C it has h droge o di g et ee the ole ules , hereas Metha e CH4 has a Mr of
16 but has a boiling point of - 6 ˚C! Metha e o l has a der aals for es et ee the
molecules). Again molecules with hydrogen bonding will have Dipole-Dipole and van der waals
interactions too.
It is common for you to be asked to draw molecules that interact with each other via hydrogen
bonding. So for example if we had to show how water molecules interact we would draw 2 water
molecules near each other and include the following –
1. 2 lone pairs of electrons on the oxygen
2. All partial harges o the ato s δ+ o the hydrogens a d δ- on the oxygens)
3. Draw a dotted line (hydrogen bond) from a hydrogen on one molecule to a lone pair of
ele tro s the eigh ouri g ole ule s o ge ato .
This generally obtains a good number of marks in the exam so make sure you include what is asked
for in the exam. View the script for this podcast at alleytutors.com to see a diagram to illustrate this
–
Remember when talking about boiling and melting points of simple covalent molecules you need to
talk about intermolecular forces NOT bonds. Water has a high boiling point because you are
breaking the weak forces not covalent bonds. Think about it. Liquid water has the formula of H2O,
when boiled it turns into steam, this has the formula of H2O as well! We have obviously not broken
any bonds just weakened the forces between the molecules to turn it into steam.
© C Harris (AlleryTutors.com)
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Have a go yourself. Draw a labelled diagram to show the intermolecular forces between Ammonia
(NH3). In your diagram you must show partial charges, any lone pairs of electrons and show at least 2
molecules of ammonia. (Visit allerytutors.com view the script for this podcast to find out the answer)
ANSWER –
δ+
δ+
δ-
δ+
δ+
δ-
δ+
δ+
δ-
δ+
δ+
δ+
On a final note it is important to point out that anything that will hydrogen bond with water is
classed as soluble. Many polar molecules will dissolve in water but large non polar molecules like
h dro ar o s do t dissol e.
If you are still a little stuck then watch my videos on these topi s u der the bonding pla list.
Worked through examples are available on all these mini tutorials.
Well that s the end of this podcast on Intermolecular Forces. I hope this has proved useful and
remember to follow me on Twitter @allerytutors or like me on Facebook for all the pre-release info
on up and coming videos to my YouTube channel and podcasts.
That s all folks! Love Chemistry!
© C Harris (AlleryTutors.com)