boiling and melting

[Dencology]

hey i wanted to know if boiling point is high does it mean that the melting point is high too or they work against each other?

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[topdent1]

It depends on the branching of the molecule. The more branched the molecule, the higher the melting point (especially if the molecule is symmetrical). The more branching, the lower the boiling point because of dispersion forces.

 

[Dencology]

so if you have a carboxylic acid and a alkane which has the highest melting point. i know that carboxylic acid has the highest boiling point because of the hydrogen bonding but what about melting point?

 

[topdent1]

so if you have a carboxylic acid and a alkane which has the highest melting point. i know that carboxylic acid has the highest boiling point because of the hydrogen bonding but what about melting point?

Thats a good question. I would say that the caboxylic acid has the highest melting point. Remember, when you melt something, you still need to destroy inter and intra molecular forces and hydrogen bond is, like you said, very strong and would require a high temp to rip. It also is polar which also supports my answer. I might be wrong so I want to hear other opinions.

 

[wifi08]

the CA has a much larger MP value because of hydrogen bonding
branching rules: when it comes to MP more branched has higher mp because it fits more comfortably in the crystall lattice shape and doesnt want to leave easily. i.e. must crank up the heat. but for bp due to surface area it will have a lower bp

 

[klutzy1987]

A carboxy acid will even have a higher boiling pointb than almost all alcohols and most hydrogen bond containing compounds. This is due to its ability to form dimers, which are extra strong hydrogen bonds.

 

[excusezmoi]

This post got me thinking so I researched about the properties of carboxylic acid and alkanes...it goes down into that Carboxylic acid will have higher melting point and boiling point than alkanes. Just by looking at propanol and propane for example, if you check the molecular weight ...Propanol would be heavier (also the OH for intermolecular force etc)

However, if we are talking about Colligative properties, for instance when a substance is mixed with a pure substance (like adding salt to water).
The resulting mixture would have higher B.P. and higher Osmotic.P with Lower F.P/M.P and Vapor Pressure.

my mneumonics is high BOP vs low FVP just remembering BOP helps me remember the opposite.

 

[Dencology]

so let me ask you then, between trans-2-butene and 1-pentanol, which has the highest melting point?
now remember that if a molecule is trans, its melting point is higher than it boiling point due to the symmetry. but if a molecule is cis its boiling point is higher because it it has a dipole moment. for example ClCH=CHCl if this is CIS, then its BP is higher than it s MP. but if it is Trans, then its MP is higher than BP. so what is the answr to my first question?

 

[td4azklz]

so let me ask you then, between trans-2-butene and 1-pentanol, which has the highest melting point?
now remember that if a molecule is trans, its melting point is higher than it boiling point due to the symmetry. but if a molecule is cis its boiling point is higher because it it has a dipole moment. for example ClCH=CHCl if this is CIS, then its BP is higher than it s MP. but if it is Trans, then its MP is higher than BP. so what is the answr to my first question?

It would be 1-pentanol.

 

[td4azklz]

It would be 1-pentanol.

Oops, sorry...I didnt read the question properly. It would be trans butene. My bad 😀

 

[klutzy1987]

so let me ask you then, between trans-2-butene and 1-pentanol, which has the highest melting point?
now remember that if a molecule is trans, its melting point is higher than it boiling point due to the symmetry. but if a molecule is cis its boiling point is higher because it it has a dipole moment. for example ClCH=CHCl if this is CIS, then its BP is higher than it s MP. but if it is Trans, then its MP is higher than BP. so what is the answr to my first question?

How is it possible fo somethings melting point to be higher than its boiling pont. What you said makes absolutely no sense.

Pentanol has H bonding so i wuld say that it is going to have the higher melting point.
Stronger intermolecular forces=higher boiling and melting point

Someone correct me if I am wrong.

 

[td4azklz]

How is it possible fo somethings melting point to be higher than its boiling pont. What you said makes absolutely no sense.

Pentanol has H bonding so i wuld say that it is going to have the higher melting point.
Stronger intermolecular forces=higher boiling and melting point

Someone correct me if I am wrong.

That's right. Yeah, that's what i thought too.

 

[Dencology]

yes. you guys are right. the alcohol has the higher mt, bp. thanks for clarifying.

 

[flycd05]

so let me ask you then, between trans-2-butene and 1-pentanol, which has the highest melting point?
now remember that if a molecule is trans, its melting point is higher than it boiling point due to the symmetry. but if a molecule is cis its boiling point is higher because it it has a dipole moment. for example ClCH=CHCl if this is CIS, then its BP is higher than it s MP. but if it is Trans, then its MP is higher than BP. so what is the answr to my first question?

klutzy - what u said seems logical in teh sense that how the MP be higher than BP but what OP mentioned about cis/trans is true too

now im confused - whats right and wrong with the above statement the OP made??

 

[futuredent22]

klutzy - what u said seems logical in teh sense that how the MP be higher than BP but what OP mentioned about cis/trans is true too

now im confused - whats right and wrong with the above statement the OP made??

The OP meant that if you compared a cis vs. trans molecule, the trans would have the higher melting point because of symmetry, but the cis would have the higher boiling point because of dipole-dipole forces.

In response to the question comparing melting points of propanol vs. trans alkene: While the trans does have increased melting point because of its symmetry, I don't see how it can be higher than propanol's melting point, because of H-bonding

 

[td4azklz]

The OP meant that if you compared a cis vs. trans molecule, the trans would have the higher melting point because of symmetry, but the cis would have the higher boiling point because of dipole-dipole forces.

In response to the question comparing melting points of propanol vs. trans alkene: While the trans does have increased melting point because of its symmetry, I don't see how it can be higher than propanol's melting point, because of H-bonding

H-bonding is a stronger interaction, therefore the melting point of propanol would definitely be higher.

 

[nzaads]

according to Kaplan blue book, an increase in branching causes BOTH the melting and boiling points to decrease. they say that MP and BP both follow the same exact rules.
Trans alkenes have higher MP than cis alkenes.
Cis alkenes have higher BP than trans alkenes.

--straight from the book.

 

[futuredent22]

I think I remember seeing that Kaplan and Destroyer disagreed over whether branching increases or decreases melting point.

According to wikipedia: "The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to packing well in the solid phase"

 

[nzaads]

hmm.. well that complicates things! 😕