00:01
All right, on question 92, we're given a table with six different compounds, which i have drawn out here, and included their heat of vaporizations and their molecular weights.
00:12
And the question asks us to explain really what's happening with the variance in the heat of vaporization.
00:21
With reference to in part a, we're looking at the molar mass, the molecular weight.
00:28
We can think of it as part b.
00:30
Looking at shape.
00:34
In part c, we're looking at polarity.
00:41
And in part d, we're looking at hydrogen bonding.
00:48
So in terms of molecular weight, in general, as we increase the molecular weight with other factors being equal, we usually increase the interim strength of the london dispersion forces.
01:01
And therefore, it's going to be harder to take more energy to separate the liquid.
01:08
Break those intermectar forces and go from a liquid to a gas, which is what we're doing when we're vaporizing here, this delta asia vaporization.
01:15
So the stronger the intermuclear forces, the higher this heat of vaporization.
01:21
But that's with all other things being equal.
01:23
So the two to really want to compare for part a would be these first two.
01:28
They are both just carbon hydrogen chains, straight chains.
01:31
They don't have other atoms in there that can lead to polarity issues.
01:37
So really the only thing involved, the only type of intermeculary forces involved in these first two molecules are london dispersion forces because that they are non -polar molecules.
01:48
So that's the only thing that's going to be holding them together.
01:50
So this is where molecular weight really becomes important, the molar mass.
01:55
As the molar mass increases, going from 44 to 72, our heat of vaporization increases from 19 to 27 .6.
02:05
Because again, the larger this molecule, the greater the molecular weight, the more polarizable it is.
02:12
The more polarizable it is, the stronger the london dispersion forces.
02:16
The stronger the london dispersion forces, the more energy it takes to separate out those molecules from a liquid to a gas, therefore the higher the heat of vaporization.
02:26
In part b, we're looking at shape.
02:31
And shape becomes important in terms of interaction between the molecules.
02:36
Molecules when they're trying to form these intermolecular forces...