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Boiling point comparison: AP Chemistry multiple choice

Comparing the boiling point of nonane and 2,3,4-trifluoropentane. A sample multiple choice problem from the 2014 AP course description.

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  • blobby green style avatar for user thegarrettjohnson21
    Why wouldn't the 2,3,4 - trifluoropentane molecule have a larger boiling point than the nonane molecule? The TFP molecule has the electronegative fluorine which should create a dipole and hydrogen bond with other TFP molecules. I understand that nonane has an increased surface area, but shouldn't the hydrogen bonds be stronger than the dispersion forces?
    (14 votes)
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  • aqualine ultimate style avatar for user QUIDES
    This could also be explained by the fact that the number of hydrogen bonds that nonane can form is significantly higher than the number of H bonds that TFP can form, right? this causes intermolecular forces of attraction to go up.
    (0 votes)
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    • duskpin ultimate style avatar for user rosafiarose
      Nonane can't form hydrogen bonds. Hydrogen bonds form when a H is bonded to an N, O or F atom, and has a resulting partial positive charge. This allows the hydrogen to be attracted to other electronegative atoms, forming a hydrogen bond. Since nonane doesn't contain any N, O or F atoms, it can't have hydrogen bonds.
      (25 votes)
  • hopper jumping style avatar for user Yuya Fujikawa
    This question is not about this video, I'm sorry. But I found something a bit strange about the trend of BP for H-X (where X is halide). It makes sense that H-F has the highest BP as F is the most electronegative element, so H-F is very polar, causing high IMF. What I found strange, is that H-Cl is NOT the next character who has second highest BP, even though Chlorine is the next most electronegative element. So what other factors come in play other than polarity when thinking about BP trend?
    (5 votes)
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    • mr pants purple style avatar for user Ryan W
      HF's high boiling point is due to hydrogen bonding which none of the other HX molecules show.

      The general trend is that BP increase as molecules get heavier due to more electrons and hence stronger London dispersion forces. It's HF that bucks the trend.
      (9 votes)
  • mr pink red style avatar for user Maryam Anwaar
    Why boiling point of ccl4 is higher than that of hf even though hf has hydrogen bonding and ccl4 is non polar
    (2 votes)
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    • mr pants purple style avatar for user Ryan W
      Consider how many more electrons CCl4 has compared to HF. It’s really important to consider the strength of dispersion forces when there’s such a big difference in the number of electrons.

      Also note that N and F don’t form as strong hydrogen bond networks as O does.
      (1 vote)
  • blobby green style avatar for user Soumith Reddy
    Which bonded molecules have high melting points
    (1 vote)
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    • piceratops ultimate style avatar for user RowanH
      By bonded, do you mean that they contain covalent bonds, as opposed to being ionic?
      Melting and boiling points are affected by the forces between molecules. The larger the attractive forces between the molecules, the more energy needed to overcome them so that molecules can move past each other freely or evaporate (melt or evaporate).
      Longer chained molecules can interact more with each other, so they tend to have higher melting and boiling points. For example propane is a gas whereas nonane is a liquid at room temperature. Parrafin wax, which contains longer hydrocarbons, is solid. This is because there can be greater London dispersion forces between longer molecules.
      Other things which affect the strength of intermolecular forces are how polar molecules are, and if hydrogen bonds are present. For example, even though there water is a really small molecule, the strength of hydrogen bonds between molecules keeps them together, so it is a liquid. H2S, which doesn't form hydrogen bonds, is a gas.
      (2 votes)

Video transcript

- [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.