[Polleke]

hallo all,

Its been a while since I had organic, inorganic chemistry and I would like to know how I can simply tell my students how to arranges substances from low to high boilingpoints.

As I remember from my days at school it was something like this:

first you have to see wether its an covalent bond or an ionic bond.
The ionic bonds are stronger, so those elements have higher boiling points then the covalent ones.

Thats the first step, then I would simple tell them that the bigger the difference in EN the better the bounding, thus the higher the boiling or melting point.

Then they have to see wether the covalent bonds have a polarity or not. Meaning that those that have one, have higher melting or boiling points.

But what then? How can I then make a difference between those substances that still need to be placed.

And if you guys know any other rules too, plz let me know.
(maybe about the length of the molecule? how many single bounds or double bounds etc.?)


be advised that it is a very basic chemistry lesson and I am not able to speak about  Van der Waals forces (dispersion forces), hydrogen bonding and other specific things, it has to be very basic.

[nj_bartel]

be advised that it is a very basic chemistry lesson and I am not able to speak about  Van der Waals forces (dispersion forces), hydrogen bonding and other specific things, it has to be very basic.

Those are pretty basic things... Are you sure you're not allowed to talk about them?

[Polleke]

Those are pretty basic things... Are you sure you're not allowed to talk about them?

It really need to be very basic as I am not sure they allready know, maybe I can indeed speak about it.

but I have the impression they do not really know a lot about it since they didnt even seem to remember the difference between ionic bounds and covalent boundings.

[Astrokel]

i kinda agree with nj_bartel. i remember when i first took chemistry 4 years back when i was in grade 9, on the 6 or 7th chapter we have to start learning van der waals forces and of course ionic stronger than covalent due to electrostatic forces of attraction, and we even went on to learn about giant covalent diamond etc..

of course many things were simplfied. ionic not necessary stronger than covalent, NH₄OH is actually NH₃(aq), anomaly in H₂O. so maybe you could tell them, "everything has an exception!" 

[Polleke]

i kinda agree with nj_bartel. i remember when i first took chemistry 4 years back when i was in grade 9, on the 6 or 7th chapter we have to start learning van der waals forces and of course ionic stronger than covalent due to electrostatic forces of attraction, and we even went on to learn about giant covalent diamond etc..

of course many things were simplfied. ionic not necessary stronger than covalent, NH₄OH is actually NH₃(aq), anomaly in H₂O. so maybe you could tell them, "everything has an exception!" 

ok, I agree, there are exceptions etc.. but I really want to give them the basic rules.

[macman104]

I'm sorry, I just don't see them getting very far without those other topics included.  You could easily talk to them about ionic compounds.  However, discussing trends in covalent compounds is going to be difficult.

[Polleke]

I'm sorry, I just don't see them getting very far without those other topics included.  You could easily talk to them about ionic compounds.  However, discussing trends in covalent compounds is going to be difficult.

yeah, I realise that, thats why its so hard to give them a very general view on the problem.

But what if I could use some more "in depth" chemistry, how would you then describe an "easy" method of determing wich substance has a higher boiling point then another one.

[macman104]

If you are comparing like organic molecules.

-Longer alkyl chains increase boiling and melting points, with increasing weight and van der waals interaction (assuming the chain is straight).

-Two hydrocarbons that are isomers of each other (same molecular formula), the one with a less linear shape will have a lower boiling and melting point as van der waals interaction is decreased by a lower surface area.

Those are probably the two biggest for evaluating similar simple organic molecules.

[cliverlong]

<< snip >>

first you have to see wether its an covalent bond or an ionic bond.
The ionic bonds are stronger, so those elements have higher boiling points then the covalent ones.

<< snip >>

Hi,

You have just reminded me of something I hadn't clearly understood and this is a good opportunity to raise it.

It is not correct to write that ionic bonds are stronger than covalent bonds - they are comparable in strength

So the question becomes why do covalently bonded compounds such as hydrocarbons or fats have very much lower melting and boiling points than ionic compounds such as sodium chloride?

The answer is because there are different forces between the molecules from those within the molecules

If we think of alkanes such as methane or butane which are gases, the C-H and C-C covalent bonds within the molecule are very strong. However, these are gases because the forces between the molecules are very weak. When the substance is heated from below its melting point the forces between the molecules diminish but the individual molecules stay intact, the covalent bonds do not break. (The following bit I'm not sure about) heavier hydrocarbons are volatile liquids because there are some intermolecular forces (van der Waals?) to hold the molecules loosely together. In the case of long chain organic molecules such as lipids there are many inter-molecular hydrogen bonds to hold the substance together as a solid. In the case of plastics, there are specific atoms such as sulphur bridges that bind the molecules together as a solid.

Now consider sodium chloride (or any ionic metal salt). In this case it does really mean anything to talk of a sodium chloride molecule. Every sodium ion is bonded ionically to six chloride ions and conversely every chloride ion is bonded ionically to six sodium ions. Every bond is equal in strength through the whole lattice and this causes a high melting point. Compare against the organic molecules where the intramolecular forces are much, much stronger than the intermolecular forces.

Clive