- [Voiceover] Consider the molecules represented above
and the data in the table below.
We have the structure up here for nonane,
the structure for 2,3,4-trifluoropentane,
which is really hard to say
so I'm gonna abbreviate that, TFP.
And we have this data in the table.
So nonane, and 2,3,4-trifluoropentane
have almost identical molar masses,
so 128 versus 126 grams per mole,
but nonane has a significantly higher boiling point.
So we can see that nonane has a boiling point of 151,
versus 89 Celsius for our TFP.
Which of the following statements
best helps explain this observation?
Before we look at our answer choices,
let's think really fast about what it means
to have a higher or lower boiling point.
The boiling point tells us how much energy
we have to add to break the intermolecular bonds
between all of our molecules.
So a higher boiling point means that
you have more intermolecular forces
to overcome.
So what we're really asking here is,
which of these answer choices explains
why nonane has more intermolecular forces
amongst the molecules, compared to TFP?
So, answer choice A says, the carbon-fluorine bond
is easier to break than the carbon-hydrogen bond.
And we know this is a wrong answer,
because this has nothing to do
with intermolecular forces.
When something boils, you're not actually
breaking any of the covalent bonds,
so that doesn't explain anything about the boiling point.
Answer choice B says that the carbon-fluorine bond
is more polar than the carbon-hydrogen bond.
So we can see that TFP does have
these carbon-fluorine bonds,
and we know that a carbon-hydrogen bond isn't
all that polar, and fluorine is pretty electronegative,
so this is true.
This statement by itself is true.
So this is true,
but does it explain the boiling point trend?
And the answer there is, it actually doesn't.
So we're saying that if the carbon-fluorine bond
is more polar than these bonds here,
we're saying that
if TFP
has more polar bonds,
that would normally suggest it has
stronger intermolecular forces,
which would mean you would predict it
to have a higher boiling point.
And so, that's the opposite of what
we're actually seeing here.
Even though our TFP has
more polar C-F bonds,
it actually has a lower boiling point,
so this observation, which is true,
still doesn't explain
what we're trying to explain. (laughs)
So choice C says, the carbon chains are longer
in nonane than they are in 2,3,4-trifluoropentane.
So if we just look at the pictures here
of the structures, this is also true.
In nonane we have these one, two, three, four, five,
six, seven, eight, nine carbons.
We have nine carbons versus one, two, three,
four, five carbons.
So, how could we link this to the boiling point?
We know that even though the molar mass here
is the same, the length of the chain
is actually related to
the London dispersion forces.
So as the length of the chain goes up,
that actually means that the London dispersion forces,
so the intermolecular forces that happen
when you get these tiny instantaneous dipoles,
those forces also go up.
So, we're saying, okay, this has a longer chain.
Therefore, it will have more London dispersion forces
because these molecules are better able to interact
with each other with their instantaneous dipoles,
and that means these forces go up
and the boiling point should go up,
and that's what we're trying to explain.
So C is the correct answer, but lets' look at D anyway,
just to make sure we didn't make any bad decisions.
So, just checking.
D says the carbon chains are further apart
in that sample of nonane than they are
in 2,3,4-trifluoropentane.
Well, we don't actually know if this is true or not,
but let's see, if this statement were true,
would it lead to the boiling point trend we're seeing?
If the carbon chains are further apart in nonane,
further apart would mean, the further apart they are,
the weaker the intermolecular forces.
So this would mean, nonane has weaker
intermolecular forces, and that would suggest
it would have a lower boiling point.
So a lower boiling point.
And again, this is not what we're trying to explain.
We know it has a higher boiling point,
so this also doesn't explain,
that doesn't explain the boiling points.
So the answer is C.