Stankovic123's chem q's

[zvezda]

totally ignore that question ^^

[zvezda]

Draw structural formulas and state the names of all the straight chain alcohols with the molecular formula C5H12O.
Is this a trick question or something? There's only one of them as far as I know, and it's pentan-1-ol

[zvezda]

Explain why the differences in boiling points between corresponding alcohols and alkanes decrease as the number of carbon atoms increases.
I don't know why. I would've thought that the differences would've remained the same as the only difference between the corresponding molecules is the O-H bond

[Stick]

I'm only relying on 1/2 knowledge to answer this question, but basically the O-H hydrogen bond has the same strength regardless of the molecule size. So, it'd be more difficult to hold together molecules as they get larger and heavier and eventually that bond will basically become negligible.

[zvezda]

I'm only relying on 1/2 knowledge to answer this question, but basically the O-H hydrogen bond has the same strength regardless of the molecule size. So, it'd be more difficult to hold together molecules as they get larger and heavier and eventually that bond will basically become negligible.

If we think about it logically though, there'll always be that O-H bond and it will always contribute to the intermolecular forces that bond alkanols together, no matter how dominant or not it is. Well at least as far as I know, lol.

[teletubbies_95]

I dont know if this is right. But when there is an increase in carbon atoms ,increases the molecular weight , therefore increased surface area for more intermolecular forces to bond (ie. dispersion ,and in this case , OH bonds), therefore increased B.P.

[Stick]

If we think about it logically though, there'll always be that O-H bond and it will always contribute to the intermolecular forces that bond alkanols together, no matter how dominant or not it is. Well at least as far as I know, lol.

Well that's why the difference just becomes smaller and not necessarily non-existent. The alcohol will always hold together slightly better than the alkane, but as the molecule becomes larger and heavier and dispersion forces become greater, the hydrogen bond will not matter as much.

[zvezda]

I understand, but then I don't

[Stick]

Hmm, I'll see if I can give you an analogy.

Just imagine you have sets of blocks. They're all covered in blue tack (dispersion forces) and a few of them have some small magnets on them (hydrogen bonds). If you want to put two small blocks together, the magnets will have a larger effect on the bond relative to the blue tack. When you're joining two larger blocks together, there is a lot more blue tack and the force of those small magnets will be less relative to the blue tack.

Does that make more sense?

[Mao]

If we think about it logically though, there'll always be that O-H bond and it will always contribute to the intermolecular forces that bond alkanols together, no matter how dominant or not it is. Well at least as far as I know, lol.

You are exactly right.

What Stick and Ms smiley faces said are exactly what VCAA and your teacher wants to hear from you. These answers aren't incorrect, but they don't address your question directly.

The real reason is a bit more subtle than what has already been discussed. It's true that OH bonds will still contribute to the intermolecular cohesion of the molecules. It's also true that its effect will become negligible. This is for two reasons.

Firstly, hydroxyl groups interact with each other more strongly than the non-polar hydrocarbon bits. Some might call this hydrogen bonding (though it isn't, but that's a different can of worms). This favourable interaction gives rise to a strong cohesion that is much stronger than the non-polar to non-polar dispersion forces (or Van Der Waals forces). As the molecule gets larger, chances of two hydroxyl groups finding each other becomes smaller, so for the most part, these hydroxyl groups will instead interact with the non-polar groups. This means the extra cohesion from polar-polar attraction diminishes as your molecule gets larger. Therefore, as your molecule gets larger, the strength of cohesion coming from hydroxyl groups will decrease due to the sheer size of the molecule.

Secondly, the temperature difference doesn't scale precisely equal to the 'amount' of cohesion. As you get larger, more flexible molecules, other factors affect the boiling point, and it can be shown that the boiling point rises more slowly for larger molecules. That is, as the molecule gets large, there is diminishing returns for boiling point increases. This means the difference in boiling point will intrinsically become smaller. (for graphs, see figure 1 here as well as this picture, note the curvature of the alkane lines.)

These two factors together means the difference in boiling point between alkane and alkanol becomes smaller as the molecule gets larger.

[Stick]

I stand corrected. Thanks, Mao!

[teletubbies_95]

Thanks Mao and Stick for the great explanations! and stankovic for asking this question !!

[zvezda]

You are exactly right.

What Stick and Ms smiley faces said are exactly what VCAA and your teacher wants to hear from you. These answers aren't incorrect, but they don't address your question directly.

The real reason is a bit more subtle than what has already been discussed. It's true that OH bonds will still contribute to the intermolecular cohesion of the molecules. It's also true that its effect will become negligible. This is for two reasons.

Firstly, hydroxyl groups interact with each other more strongly than the non-polar hydrocarbon bits. Some might call this hydrogen bonding (though it isn't, but that's a different can of worms). This favourable interaction gives rise to a strong cohesion that is much stronger than the non-polar to non-polar dispersion forces (or Van Der Waals forces). As the molecule gets larger, chances of two hydroxyl groups finding each other becomes smaller, so for the most part, these hydroxyl groups will instead interact with the non-polar groups. This means the extra cohesion from polar-polar attraction diminishes as your molecule gets larger. Therefore, as your molecule gets larger, the strength of cohesion coming from hydroxyl groups will decrease due to the sheer size of the molecule.

Secondly, the temperature difference doesn't scale precisely equal to the 'amount' of cohesion. As you get larger, more flexible molecules, other factors affect the boiling point, and it can be shown that the boiling point rises more slowly for larger molecules. That is, as the molecule gets large, there is diminishing returns for boiling point increases. This means the difference in boiling point will intrinsically become smaller. (for graphs, see figure 1 here as well as this picture, note the curvature of the alkane lines.)

These two factors together means the difference in boiling point between alkane and alkanol becomes smaller as the molecule gets larger.

Brilliant, thoroughly answered my question, thanks heaps.
Although, is there a reason why the boiling point rises more slowly for larger molecules?

[zvezda]

Thanks Mao and Stick for the great explanations! and stankovic for asking this question !!

no worries lol

[Mao]

Although, is there a reason why the boiling point rises more slowly for larger molecules?

Yes there is. But it took me like 2 hours of searching and reading. I eventually found a good answer in someone's PhD thesis. The answer involves several more things (such as the concept of 'entropy', which is not introduced in VCE), so I don't think I should invoke that level of theory here.

You asked a pretty damn good question I learned a few things while trying to confirm my explanation.