How do you know what the strongest intermolecular forces within a compound are?

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Anonymous1502
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The question asks: what is the strongest intermolecular force present for each of the following compounds:

1) water - which Im guessing is hydrogen bonds (I learnt this in bio)
2) carbon tetrachloride
3)ammonia
4)co2
5)phosphorus trichloride
6)nitrogen
7) ethane (c2h6)
8)methanol(ch3oh)
9)borane (bh3)
10) hydrogen sulphide
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CaraStudying
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I'd say draw out each of the compounds and see if there's the opportunity for H bonding to take place and to see if there are any dipoles. Generally, H bonding will be the strongest, permanent dipole-dipole attractions will be the next and then induced dipole-dipole attractions last (unless you've got a large number of electrons, then maybe this could be stronger but from an A level point of view it's generally in this order).
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Pigster
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Are you asking how to identify which type of IMF is present? Or which IMF is stronger than which?

If it is the first one: H-bonds require H attached to N,O or F. Then you have to work out whether you have polar molecules or not (i.e. does it have polar bonds and do the dipoles produced by these bonds cancel out - is the molecule symmetrical) if so, it'll be pdd. Otherwise it'll be idd.

If it is the second one: H-bonds > pdd > idd is a good rule of thumb.

Are there other IMF on your spec?
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Anonymous1502
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(Original post by Pigster)
Are you asking how to identify which type of IMF is present? Or which IMF is stronger than which?

If it is the first one: H-bonds require H attached to N,O or F. Then you have to work out whether you have polar molecules or not (i.e. does it have polar bonds and do the dipoles produced by these bonds cancel out - is the molecule symmetrical) if so, it'll be pdd. Otherwise it'll be idd.

If it is the second one: H-bonds > pdd > idd is a good rule of thumb.

Are there other IMF on your spec?
How to identify them.I dont get how to identify if it's permanent dipole or induced dipole?
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Anonymous1502
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(Original post by Pigster)
Are you asking how to identify which type of IMF is present? Or which IMF is stronger than which?

If it is the first one: H-bonds require H attached to N,O or F. Then you have to work out whether you have polar molecules or not (i.e. does it have polar bonds and do the dipoles produced by these bonds cancel out - is the molecule symmetrical) if so, it'll be pdd. Otherwise it'll be idd.

If it is the second one: H-bonds > pdd > idd is a good rule of thumb.

Are there other IMF on your spec?
There is dispersion.London forces are those induced d-d or permanent d-d?I do OCR?
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Anonymous1502
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(Original post by Pigster)
Are you asking how to identify which type of IMF is present? Or which IMF is stronger than which?

If it is the first one: H-bonds require H attached to N,O or F. Then you have to work out whether you have polar molecules or not (i.e. does it have polar bonds and do the dipoles produced by these bonds cancel out - is the molecule symmetrical) if so, it'll be pdd. Otherwise it'll be idd.

If it is the second one: H-bonds > pdd > idd is a good rule of thumb.

Are there other IMF on your spec?
How do you know if a molecule is polar?
I keep hearing the word polar which I know what it means slightly charged or something like that but have no idea how to identify if a molecule has it or not?
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Pigster
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You have polar bonds if the atoms attached to each end are different (due to differences in electronegativity)

Is the molecule symmetrical? For OCR A that means working out whether there are lone pairs on the central atom, i.e. is it bent or pyramidal?

Alternatively, for something like chloromethane, the bonds are (roughly) at 109.5, but the four bonds aren't all the same. Therefore not symmetrical.
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Anonymous1502
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(Original post by Pigster)
You have polar bonds if the atoms attached to each end are different (due to differences in electronegativity)

Is the molecule symmetrical? For OCR A that means working out whether there are lone pairs on the central atom, i.e. is it bent or pyramidal?

Alternatively, for something like chloromethane, the bonds are (roughly) at 109.5, but the four bonds aren't all the same. Therefore not symmetrical.
Would hydrogensulfide be permanent because the H has a 2- charge and S a +1 charge,so the charges dont cancel out?
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Pigster
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S is more electronegative than H. The S will get a d- charge and the H will get a d+ charge.

How many bond pairs and how many lone pairs are there around the central S atom? And therefore what shape is it? And therefore is it symmetrical? And therefore will the dipoles cancel out? (think of the last one is a similar way to resultant forces in physics, think of the polar bonds being forces - will the forces cancel our?)
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Anonymous1502
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(Original post by Pigster)
S is more electronegative than H. The S will get a d- charge and the H will get a d+ charge.

How many bond pairs and how many lone pairs are there around the central S atom? And therefore what shape is it? And therefore is it symmetrical? And therefore will the dipoles cancel out? (think of the last one is a similar way to resultant forces in physics, think of the polar bonds being forces - will the forces cancel our?)
2 bond pair and 2 lone pair?.Pyramidial.Not symmetric?No dipoles won't cancel?
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Pigster
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2bp 2lp, non-linear, asymmetric, dipole do not cancel, therefore pdd.
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