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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I often get students in office hours who ask about intermolecular<br>
attractive forces (I abbreviate this as IAF, some people use IMF).<br>
This is in chapter 11 but you need this for ch 13.<br>
<br>
Lets start with London Forces. People use different terms for this<br>
attractive force. You may see London Forces or Dispersion Forces<br>
or London Dispersion Forces. I call them London Forces and use<br>
the abbreviation LF. LF are due to the instantaneous (temporary)<br>
dipole moments created due to the 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>
this first week. <br>
<br>
ALL molecular substances experience LF between them.<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. There can<br>
be intramolecular (w/in the same molecule) HB, particularly<br>
with long chain molecules which have HB parts on different<br>
parts of the chain. The molecules can wrap around and<br>
form HB between different HB groups in the same molecule.<br>
This is common in proteins. We don't discuss this much.<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. It 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>
Go to my "Notes" link and you'll find links there about AF and properties<br>
which depend on them such as b.p., solubility, etc. The 2nd one is about<br>
attractive forces, both in pure substances and solutions (nature of solute<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">and solvent). The 3rd one is about the energy considerations of solution<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">formation (13.1) and solubility effects due to nature of solute and solvent<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">and temperature, which we will cover on Tue (13.3).<br>
<br>
<a href="https://www.asc.ohio-state.edu/zellmer.1/chem1220/notes/Table_13-10_solids_no_lines.pdf"><b>Types of Solids and Their Properties</b></a><br>
<br>
<a href="https://www.asc.ohio-state.edu/zellmer.1/chem1220/notes/ch11_12_13_rev.htm"><b>Ch. 11/12 & 13 - Review of IAF, Solids & Solubility</b></a><br>
<br>
<a href="https://www.asc.ohio-state.edu/zellmer.1/chem1220/notes/ch13_soln_formation.pdf"><b>Ch. 13 - Solution Formation and Solubility Effects</b></a><br>
<br>
Dr. Zellmer <o:p></o:p></span></p>
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