AP Chemistry multiple choice sample: Boiling points

Consider the molecules represented above and the data in the table below. We have the structure up here for non, the structure for 2, 3, 4-triopentane, which is really hard to say, so I'm going to abbreviate that TFP. Um, and we have this data in the table.

So, nonan and 2, 3, 4-triopentane have almost identical molar masses, 128 versus 126 g per mole, but nonan has a significantly higher boiling point. So we can see that nonan has a boiling point of 151 versus 89 degrees 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 non has more intermolecular forces amongst the molecules compared to TFP?

Answer Choice A says the carbon-fluorine bond is easier to break than the carbon-hydrogen bond. 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.

However, does it explain the boiling point trend? 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 CF bonds, it actually has a lower boiling point. So this observation, which is true, still doesn't explain what we're trying to explain.

So, Choice C says the carbon chains are longer in nonan than they are in 2, 3, 4-triopentane. If we just look at the pictures here of the structures, this is also true. In nonan, we have these 1, 2, 3, 4, 5, 6, 7, 8, 9 carbons. We have nine carbons versus 1, 2, 3, 4, 5 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. As the length of the chain goes up, that actually means that the London dispersion forces—the forces, the intermolecular forces that happen when you get these tiny instantaneous dipoles—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 let's 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 a sample of nonan than they are in 2, 3, 4-triopentane. 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 nonan, further apart would mean the weaker the intermolecular forces. So this would mean nonan 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 the boiling points.

So, the answer is C.