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PROFESSOR: Hi, I'm Jocelyn, and today we're going to go
over Fall 2009 Exam 2, Problem number 5.
As always, we're going to read the problem first. Which
compound do you expect to have the higher boiling point: HF
or ammonia?
Justify your choice with an explanation, using narrative
or cartoons or both that makes reference to the operative
chemical bonding.
So first, we are trying to figure out which has a greater
temperature of vaporization, or boiling point; and we're
looking at HF and ammonia.
Now when we're talking about boiling points, we're talking
about breaking a certain type of bond, and that type of bond
is intermolecular bonds.
We're not going to be breaking the actual molecular bonds
bonding hydrogen to fluorine or nitrogen, we're breaking
the bonds in between molecules of HF or ammonia, and so what
types of bond do we have to look at?
And Professor Sadoway, in class, went over a couple of
different types of intermolecular bonds.
And I'm going to write them down in order
of increasing strengths.
So the weakest intermolecular bonds we
have are Van der Waals.
The next is induced dipole-dipole.
Next we have dipole-dipole, and finally a special type of
dipole-dipole is the hydrogen-bond, which is the
strongest. So here we're increasing strength.
Van der Waals is an induced dipole-dipole.
That means that although your atom or molecule does not have
a net dipole, the electrons can shift, and at some point
will cause a difference in polarity between different
sides of the molecule.
Induced dipole-dipole is two different molecules
interacting: one that has no net dipole and one that does.
Dipole-dipole: pretty self-explanatory.
You have two molecules that have dipoles, and they are
interacting.
And then hydrogen-bond is when you have hydrogen bonded with
x, where x is nitrogen, oxygen, and fluorine.
We call that a hydrogen-bond, because these are really,
really strong dipole-dipole interactions and cause much
different behaviors in molecules, and so we, kind of,
separate it out at the subset.
So now that we've reviewed intermolecular bonds, let's go
back to the problem.
So we were asked to figure out the difference, or which one
has a higher vapor pressure, or temperature of boiling, so
that's a relative thing.
And as we now know that we need to look at intermolecular
bonds, a good place to start is with the Lewis structure,
because that will tell us if we have dipoles or not, and
then we can see what the strongest intermolecular bond
is that we need to break to boil these molecules.
So starting with HF, we have hydrogen bonded to fluorine.
We know that fluorine has a much higher electronegativity
than hydrogen, so we're going to have a net dipole.
Well, we have a polarized bond and a net
dipole towards fluorine.
This causes there to be a partial positive charge on the
hydrogen and a partial negative
charge on the fluorine.
So what is our strongest intermolecular bond that we
have for HF?
Going back to our list of bonds, we see that we have a
dipole, so we might first think about dipole-dipole.
However, we need to remember about hydrogen bonds, and here
in HF, we have hydrogen bonded to fluorine, and so that would
be our strongest interaction.
So let's write that down to keep track.
Now for ammonia, we have nitrogen bonded to three
hydrogens, and again we have a difference in
electronegativity, and so each of these bonds is polar.
And because of the asymmetric geometry, we will have a net
dipole, and each of these have a partially --
sorry, the hydrogens have a partially positive charge,
right, because they are less electronegative, and the
nitrogen has a partial negative charge.
So, which is our highest or strongest intermolecular bond
that we have here.
Again, we have a net dipole, and so we have dipole-dipole
interactions, but because hydrogen is bonded to one of
the special three atoms, we have hydrogen bonding, and
that is the strongest. So, that is what we care about.
So now, we're, kind of, at a roadblock here; both have
hydrogen bonding, which is the strongest intermolecular bond.
So how do we decide which one is actually going to be
stronger than the other, and therefore, have the higher
boiling point.
Well even though they're both hydrogen bonds, they will have
difference in strength, and that's going to depend on the
partial positive and partial negative charges, because all
of these interactions are due to coulombic attraction.
In fluorine, we know that fluorine is the most
electronegative element and has a higher electronegativity
than nitrogen, so just to keep track of our thought process
here: this will cause the partial positive on hydrogen
to be larger in the HF than in the ammonia, and so from this
we can say that the hydrogen bonding in HF will be stronger
than the hydrogen bonding in ammonia, and therefore, the t
evaporation of HF will be greater than the t--
the boiling point of ammonia.
So now we're going to move on to Part B.