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I know Ch 11 isn't on the final. Once again though, you need to
understand<br>
this material if you're taking 1220.<br>
<br>
I often get students in office hours who ask what are London Forces.
<br>
This is in chapter 11. People use different terms for this
attractive force.
<br>
You may see London Forces or Dispersion Forces or London Dispersion
Forces.
<br>
I call them London Forces and use the abbreviation LF. LF are due
to
<br>
the instantaneous (temporary) dipole moments created due to the
<br>
motion of the electrons.
<br>
<br>
There's also Dipole-Dipole attractive forces, which I abbreviate as
DD.
<br>
These occur between polar molecules (molecules with permanent dipole
<br>
moments).
<br>
<br>
I've had a number of people ask about these and hydrogen bonding.<br>
<br>
ALL molecular substances experience LF.
<br>
<br>
Non-polar substances have ONLY LF.
<br>
<br>
Polar molecules experience both LF and DD between them.
<br>
<br>
How about Hydrogen bonding (HB)? This AF is in addition to
<br>
LF and DD so a molecule which can form HB experiences
<br>
LF, DD and HB. That means the molecules of the substance
<br>
are attracted to each other through LF, DD and HB.
<br>
<br>
What is a HB and when can it form?
<br>
<br>
HB occurs between molecules which have a H atom directly
<br>
attached (covalently bonded) to a N, O or F atom and there
<br>
are lone pairs of electrons on a N, O or F. This AF occurs
<br>
between a H attached to N, O or F and the lone pairs on
<br>
one of those three atoms in another molecule.
<br>
<br>
H atoms attached to other atoms (such as C, S, P, Cl, etc.)
<br>
do not form H bonds. The lone pair electrons on other atoms
<br>
(such as S, P, Cl, etc.) do not participate in HB.
<br>
<br>
So HF, H2O, NH3, CH3OH, CH3NH2, (CH3)2NH are examples
<br>
of molecules which can form H bonds as pure substances
<br>
(between same molecules) and in solution (to the solvent
<br>
molecules - ch 13). The HB involving N is the weakest because
<br>
it is the least electronegative of the three atoms (N, O, F).
<br>
<br>
By the way, if a molecule can form HB does that mean it will always
<br>
have stronger AF than molecules which can not? NO! Large molecules
<br>
will have strong LF and overall those LF may be stronger than the
<br>
HB AF between a molecule which can form HB.
<br>
<br>
For instance, candle wax (paraffin) is essentially comprised of very
<br>
large nonpolar hydrocarbon molecules. This means the AF present
<br>
between the molecules are LF. Candles are solids at room temp.
<br>
<br>
Polar H2O has LF, DD and HB. However, it is a liquid at room temp.
<br>
This means the AF between H2O molecules are weaker than those
<br>
between the molecules in candles. The hydrocarbon molecules (mostly
<br>
nonpolar) in a candle have such large LF that the AF are stronger
than
<br>
the LF, DD and HB between H2O molecules.
<br>
<br>
LF increase rapidly with the size of the molecule and large
molecules
<br>
with only LF can have very strong AF overall. Even in large
molecules
<br>
which have polar groups and can form HB the overriding AF is often
<br>
the large LF due to the molecule's large size.
<br>
<br>
Take for instance the example of the long-chain alcohol,
<br>
<br>
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-OH .
<br>
<br>
While this molecule does have a polar end, which can also form HB,
the
<br>
nonpolar part of the molecule is so large the LF are the most impt
<br>
AF when thinking about the properties of this molecule, such as
<br>
boiling point or solubility. This molecule behaves more like a
nonpolar
<br>
molecule, especially when you are considering it's solubility, and
is more
<br>
soluble in hexane, C6H14 (nonpolar) than in water (polar).
<br>
<br>
Finally, you may see in my notes the acronym, IAF. This stands for
<br>
INTERmolecular Attractive Forces. This is the attractive force
BETWEEN
<br>
molecules (such as LF, DD and HB) rather than within a single
molecule
<br>
(the covalent bonds holding the atoms together, which are often
referred
<br>
to as INTRAmolecular).
<br>
<br>
The IAF between molecules, which are responsible for properties such
<br>
as b.p., m.p., heat of vaporization, viscosity, etc., are much
weaker than
<br>
the attractive forces between the atoms in a molecule (the covalent
bonds).
<br>
It takes a lot less energy to separate two water molecules when
water
<br>
boils (breaking the LF, DD and HB) than it does to decompose the
water
<br>
molecule into H2 and O2.
<br>
<br>
Also, I have a link at my "Notes" link which reviews AF and
properties<br>
of the four types of solids and also contains information about AF
and<br>
solubility for different types of substances. If you're taking 1220
you<br>
might want to review ch 11 and at least the parts of ch 12 I covered
and<br>
look at the following link before starting 1220.<br>
<br>
(<a
href="http://cbc-wb01x.chemistry.ohio-state.edu/%7Erzellmer/chem1210/notes/ch11_12_13_rev.htm"><b>Ch.
11, 12 & 13 - Review of IAF, Solids & Solubility</b></a><br>
<br>
Dr. Zellmer
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