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Chemistry help needed

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Wed Mar 10, 2010 7:44 pm
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Ranger Hawk says...



Can someone please explain the concept of boiling points and how the intermolecular forces affect them? Thanks!!
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Wed Mar 10, 2010 8:01 pm
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Blink says...



Yes!

Well, actually, I did the exam on this ages ago and have forgotten most of it. But.

Intermolecular forces are very weak; they require very little energy to break them. Covalently-bonded substances thus have low boiling points: when they are boiled/melted, the strong covalent bonds are not broken, but the intermolecular forces are.

If I'm wrong, someone better should come along and explain. =]
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Fri Mar 12, 2010 4:10 pm
Ranger Hawk says...



Thanks Blink! :D After re-reading that about five times, I think it makes sense...
There are two kinds of folks who sit around thinking about how to kill people:
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I'm the kind that pays better.
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Wed Apr 07, 2010 10:20 pm
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SuperNintendo says...



When something gets hot, it boils at some point in time. Boiling Point.


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Sat Apr 10, 2010 4:23 pm
Ranger Hawk says...



Thank you Bolt. I shall defer to you next time I am in need of chemistry assistance. ;)
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Sun Apr 18, 2010 3:16 am
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Lava says...



Hey Ranger.

Okay, so mostly intermolecular forces are Hydrogen bonds, dipole interactions and London forces (in decreasing order of strength. I hope you know what these are.)

Heating/Boiling basically leads to weakening of inter-molecular bonds. Therefore, higher the intermolecular attraction, the more energy you'll need to supply to break these bonds.

If you need something more detailed, PM me. :)
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Wed Apr 21, 2010 2:16 pm
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Ranger Hawk says...



Thanks Lava! Yep, I've got the forces covered. ;)
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Sat Apr 24, 2010 8:44 am
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Navita says...



Intermolecular forces are the forces of electrostatic attraction between molecules. All intermolecular forces are a result of this attraction between positive and negative particles.

Weak van der Waal's forces are present in discrete molecular substances, where covalent bonds join non-metals to non-metals (or polyatomic ions...). Here, either the molecules are polar or they're non polar.

If molecules are non-polar, then they have no positive or negative end/pole, and therefore will form INSTANTANEOUS dipoles = dipole-induced-dipoles, when the positive end of one molecule temporarily induces a negative pole in the neighbouring molecule, due to an excess or lack of electrons, causing this temporary force of electrostatic attraction to hold the molecules together. This is actually a VERY WEAK van der Waal's force, because the poles are temporary.

If the molecules are polar, then they have a permanent positive and negative end, and this means that the positive end of one will always be oriented next to the same (negative) end of the neighbouring molecule. This is stronger than the above forces, and so are called, simply...weak van der Waal's forces.

If the molecules contain a hydrogen atom, with F, O or N (the three most electronegative elements) then a HYDROGEN-bond forms in addition to the existing weak van der Waal's forces[i]!!! This forms because the highly electronegative elements tend to take away the electron from the H atom so completely that it is left as a naked proton, which is attracted to the highly negative F, O, or N of the [i]neighbouring molecules and so this 'joint custody' of the H atom is called an H-bond. Since here two types of bonds form, these are the strongest intermolecular forces.

So...in order of strength...non-polar van der Waal's forces > polar van der Waal's forces > hydrogen bonds (which, incidnetally, have 10% the strength of a covalent bond :D).

The stronger the intermolecular force of attraction, the greater the energy is required to break these bonds, thus the greater the melting point or the boiling point.

Hope that's helped!