exp 14 and attractive forces

[robert zellmer]

I had someone ask about one of the questions in the exp 14 pre-lab 
dealing with
attractive forces.  Take a quick look at section 11.2.

 From class the last couple of weeks you should be able to tell whether 
the molecules
are polar or nonpolar.  Something like C5H12 is nonpolar since it has 
only C and H
in the formula and you don't have to worry about what it looks like 
exactly, as
mentioned in lecture.  There are 3 types of attractive forces between 
molecular
substances.

Properties of solids and liquids depend on the AF.  The stronger the AF 
the lower
the vapor pressure and the higher the b.p. and the greater the heat of 
vaporization.

London Dispersion AF.  These tend to be the weakest and increase with the
size of the molecules.  All molecules have these but these are the ONLY AF
between nonpolar molecules.  Bigger molecules have greater LF.

Dipole-Dipole AF.  Polar molecules have DD AF between them in addition
to the LF.  So if you have two molecules of relatively the same size the 
one which
is polar will have stronger AF.

Hydrogen Bonding AF.  Some molecules can have HB which are very strong 
relative
to LF and DD.  This can only occur between molecules when there's a H 
atom bonded
to a N, O or F.  The H atom on a N, O or F can form a HB to the lone 
pairs of
electrons on a N, O or F on another molecule.  This AF is in addition to 
any LF
and DD AF.  So if you have two molecules which are of relatively the 
same size
the one with HB would be expected to have the stronger AF.

Dr. Zellmer