[ciel]

hi!

My first question is :

Which of these compounds has the highest boiling point?
butanoic acid, ethyl ethanoate, pentanol, hexane.

I thought the anwer is between butanoic acid and pentanol because they both have hydrogen bonds. But then what classification should I use to determine which one has the highest boiling point? I thought the answer is pentanol (it has more C maybe)

2nd question : What is the connection between surface tension, vapour pressure, intermolecular forces, and boiling point?


Thank you.

[jdurg]

I can give you a hint on question one.  What determines the boiling point of something is a combination of the molecular structure, the weight of the molecule, and any electronic attractions amongs the molecules.  So a compound which is highly polar, has a high molecular weight, and is able to stack itself into nice crystal forms will have a higher boiling point than something with a low molecular weight, and inability to hold onto other like molecules, and is completely neutral.

So in your comparison between butanoic acid and pentanol, you have to determine the higher boiling point based on the structure, molecular mass, and polarity.

[ciel]

Thx for the hint but I'm still confused. They both have the same molecular weight. And they both have hydrogen bondings so they are polar. I don't understand about the crystal forms.

All carboxylic acid have hydrogen bondings right? because they have O-H bond.

[jdurg]

I would say most and not 'all'.  This is because if there's enough steric hinderance around the O-H, a charge cannot be felt by other molecules.  So in this case with your two compounds, you might want to first draw out the structures.  Then take a look at which one would be more likely to feel the hydrogen bonding.  When drawing the structures, it's important to draw out the correct angles of the bonds as well.  This way the right answer will REALLY stick out at you since you'll be able to see a major area where some hydrogen bonding will occur.  (Because in this case, H-bonding occurs for both molecules but it's far more pronounced on one of them).  

[ciel]

i think it's butanoic acid ^^

Could you tell me what the relation between surface tension and vapor pressure?
All I know is that non polar molecules has London forces, high vapor pressure, and low boiling point. I don't know about the surface tension.

And formaldehyde (CH2O) is a polar molecule, isn't it? (just want to make sure)

[jdurg]

You are correct!    As you can see when you draw out the molecule, butanoic acid has two areas where hydrogen bonding can occur.  (The H on the -O-H portion can "bond" with the O in the O=C- portion of the other molecule and vice versa).  Meanswhile, the -O-H in the pentanol isn't able to really do that.  As a result, the intermolecular attractions in butanoic acid is slightly greater which results in a higher boiling point.

For your second question, I'm not too sure about it.  I do know that mercury has a VERY high surface tension and a very low vapor pressure.  Water has a high surface tension as well, but it has a decently high vapor pressure.  So I don't think you can really come up with a good correlation.  (Of course, the water and/or mercury could be a rare exception.  I'm just not sure).  One thing I do know is that the surface tension of a liquid is related to how strongly the molecules are attracted to each other.  If the attractions to each other are strong, they'll want to stay together and won't want to 'break' away from each other.  This would also result in a lower vapor pressure because the molecules would want to stay with each other and not fly apart.  So actually, perhaps there is a relation?  Knowing that information, one would think that the higher the surface tension, the lower the vapor pressure.  Water might indeed be the rare exception.  To decide whether this is right or not, just use a CRC Handbook or get some data online via MSDS sheets for the vapor pressure of a liquid and surface tension for the same liquid.  Then you can have plenty of data to try and come up with a correllation.

[ciel]

thx a lot!! ^^

[pizza1512]

Test it....


 :albert: