Introduces the concept of covalent bonding with macro-molecules and simple covalent molecules. Next, it covers inter-molecular attraction but explaining how temporary dipoles form Finally, heating and cooling curves together with an explanation for how energy is absorbed or given out during boiling or freezing

1. BIG AND SMALL – COVALENT MOLECULES
When 2 or moreatoms are covalently bonded to form molecules, they move as one and the
molecule is considered one unit all by itself.
The energy of a typical single covalent bond is ~80 kilocalories per mole (kcal/mol).
However, this bond energy can vary from ~50 kcal/mol to ~110 kcal/mol depending on the
elements involved. Once formed, covalent bonds rarely break spontaneously.
This is due to simple energetic considerations; the thermal energy of a molecule at room
temperature (298 K) is only ~0.6 kcal/mol, much lower than the energy required to break a
covalent bond.
Molecules come in different sizes depending on the extensiveness of the number of covalent
bonds formed per atom. A molecule formed completely of carbon would have a very large
size, while that formed of hydrogen and oxygen has a formula of just H2O.
The smallest level is the diatomic molecule which is the
elemental form of group 7 atoms as they just need to form
just one covalent bond to attain octet electronic
configuration.
Very large molecules are called macro-molecules and they
can contain over 1000 atoms and more.
Look at how many water molecules can fit in the space of ONE diamond molecule.
There is very little intermolecular forces for diamond or for the sand on the seashore, because
one molecule is so big… but it is very significant for simple covalent compounds! 
Temporary dipoles
An molecule has an electron cloud combining the electrons from the
atoms that make up the molecule
As electrons are very mobile, the majority of them may move to one
end of the cloud at an instant, and cause a temporary negative charge to form at that end.

2. While at the other end which has very few electrons, a slightly positive charge will be the
result.
This is very temporary and electrons move very very fast and… the polarity may be reversed
very quickly
The uneven distribution of electrons can occur in atoms of noble gases even, such
as helium!
Because the end are slightly charged, consider what happens here
The positive end repels electrons from a nearby molecule to the opposite end and induces a
temporary dipole to form on it!
And even if the electrons move to the other end, the attraction is still maintained!
And this doesn’t just occur between 2 molecules, remember the image of water molecules all
bunched up earlier?
A large number of molecules can be held together by intermolecular forces 
Molecular size and strength of van der waal forces
Let us look at the boiling point of the noble gases
helium -269°C
neon -246°C
argon -186°C
krypton -152°C
xenon -108°C
radon -62°C

3. boiling point increase  means stronger intermolecular attraction down group
strong intermolecular forces  because down the group, the number of electrons increase,
and the size of the atom becomes bigger.
More electrons  means that the temporary dipoles are
stronger
Larger size of atom  more surface area to interact with other
molecules
Melting and boiling points
Melting and boiling points represent the temperature at which a change of state occurs.
Heat, once absorbed as energy, contributes to the overall internal energy of the object. One
form of this internal energy is kinetic energy; the particles begin to move faster, resulting in a
greater kinetic energy.
This more vigorous motion of particles is reflected by a temperature increase. The reverse
logic applies as well. Energy, once released as heat, results in a decrease in the overall
internal energy of the object.
1) At temperatures lower than its melting point if a solid is heated, its KE increases
2) Just at the melting point, heat supplied is used to overcome the attractive forces between
particles, therefore temperature remains the same.
3) After melting is completed, the additional heat increases the KE of the particles.

4. From the graph above, explain what the state of matter which the compound being heated is
in between time D and E.
It is in both liquid and gaseous phase
It is only after point E that all of the compound has been converted to gaseous state
Flat line indicate that temperature is constant
As the compound is melting between point D and E. energy supplied is used to break bonds
only
Additional stuff
The big energy change when water freezes is in the potential energy of
interactions between the water molecules. In the ice, the molecules arrange to
touch in a way that lowers this energy. In the liquid, the arrangement is less
regular and the energy is not lowered as much.
Freezing is a change in the ordering, or structure of the molecules. An ice crystal
has less spatial symmetry (specific crystal axes are defined in space) than water
(every direction is as good as every other direction). There is an energy
associated with this transition -- 80 calories per gram of ice are needed to melt
ice at 0C at ordinary pressure, and 80 calories per gram of water are given off
during the freezing process.
http://van.physics.illinois.edu/qa/listing.php?id=1730