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    (Original post by LearningMath)
    Pretty stupid question if they didnt give electronegativity values or ask you to memorize anything...
    No its not. In strength of intermolecular forces,
    hydrogen bonds > permanent dipole-permanent dipole > van der waal (instantaneous dipole-induced dipole)

    You should know that, really. Then you just work out which it is. Its not hydrogen bonding, because that requires either nitrogen oxygen or fluorine. It can be dipole, because as you should know, sulfur is more electronegative than hydrogen (not by as much as say fluorine, but it is) and so there is a permanent dipole on the molecule. Van der waals also occurs, but they're not as strong as PD-PD forces.

    And, what are exams about if it doesn't involve remembering things?
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    (Original post by MrChem)
    No its not. In strength of intermolecular forces,
    hydrogen bonds > permanent dipole-permanent dipole > van der waal (instantaneous dipole-induced dipole)

    You should know that, really. Then you just work out which it is. Its not hydrogen bonding, because that requires either nitrogen oxygen or fluorine. It can be dipole, because as you should know, sulfur is more electronegative than hydrogen (not by as much as say fluorine, but it is) and so there is a permanent dipole on the molecule. Van der waals also occurs, but they're not as strong as PD-PD forces.

    And, what are exams about if it doesn't involve remembering things?
    More electronegative than hydrogen =/= dipole, hence i said electronegativity values should've been given. By your logic, poly(ethene) has permanent dipole-pd bonds between chains. Your 'working' is just going on the qualitative fact that electronegativities increase towards fluorine and that sulfur is quite close to fluorine.

    And your last sentence is the wrong way around?
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    (Original post by LearningMath)
    More electronegative than hydrogen =/= dipole, hence i said electronegativity values should've been given. By your logic, poly(ethene) has permanent dipole-pd bonds between chains. Your 'working' is just going on the qualitative fact that electronegativities increase towards fluorine and that sulfur is quite close to fluorine.
    Actually, the fact that sulpur is more electronegative than hydrogen in H2S does mean that the molecule is polar, not ver polar but none the less polar.

    However, the issue of comparison between V der W and dipole -dipole is not as clear cut as some posters seem to think.

    For example:

    hydrogen bonds > permanent dipole-permanent dipole > van der waal

    This is just a 'rule of thumb' which one applies in the case of very small molecules. As the RMM increases so do the van der Waals forces.

    Iodine is a solid with no permanent dipole -dipole forces whereas water is a liquid. Hence the forces between iodine molecules are stronger than hydrogen bonding.

    There is no logic in this statement as the variables are not controlled, it's not a fair comparison. Clearly the reason is the larger RMM of iodine.

    Hydrogen sulphide is relatively small and it's van der Waals forces are correspondingly small. But it's a gas at RTP and so is HCl, which is clearly polar.

    HCl bp -85.1 ºC
    H2S bp -60.3 ºC
    O2 bp -183 ºC

    So, even though they have the same RMM (approx, 36.5, 34), and hence the same V der W forces, the hydrogen sulphide has a higher bp.

    It seems clear then that there is a degree of dipole dipole interaction from H2S, indeed possibly a degree of very weak hydrogen type bonding, as only this could explain it's relatively higher bp than HCl.

    Compare both with oxygen, which has to only have van der Waals and the effect of polarity becomes more obvious.
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    (Original post by charco)
    Actually, the fact that sulpur is more electronegative than hydrogen in H2S does mean that the molecule is polar, not ver polar but none the less polar.
    I dont dispute that. But this is a-level chemistry, we also ignore the fact that the C-H bond is polar. Therefore, more electronegative than hydrogen =/= dipole!
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    (Original post by LearningMath)
    I dont dispute that. But this is a-level chemistry, we also ignore the fact that the C-H bond is polar. Therefore, more electronegative than hydrogen =/= dipole!
    I didn't say that being more electronegative hydrogen means there will be a dipole. But with H2S we know:
    Its NOT hydrogen bonding as sulfur doesn't do hydrogen bonding.
    It could be permanent dipole-permanent dipole because sulfur is more electronegative than hydrogen, and this causes a dipole.
    It could be van der waals forces.

    The strongest type it could be is PD-PD. Yes?

    And actually, technically, if one atom is more electronegative than another then it does imply the electrons will be distributed unevenly and that the bond will be polar (even if only very slightly) doesn't it?
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    (Original post by LearningMath)
    I dont dispute that. But this is a-level chemistry, we also ignore the fact that the C-H bond is polar. Therefore, more electronegative than hydrogen =/= dipole!
    You shoiuld read the rest of the post...
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    (Original post by charco)
    You shoiuld read the rest of the post...
    I did, it's interesting but not relevant to working out the answer, i dont tend to memorize bp...
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    (Original post by LearningMath)
    I did, it's interesting but not relevant to working out the answer, i dont tend to memorize bp...
    It is, however, relevant to the fact that the correct answer is permanent dipole -dipole attraction. :yes:
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    (Original post by LearningMath)
    I did, it's interesting but not relevant to working out the answer, i dont tend to memorize bp...
    You don't need to be able to memorise boiling points or anything like that. But you need to be able to spot, describe and explain the trends.
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    (Original post by MrChem)
    You don't need to be able to memorise boiling points or anything like that. But you need to be able to spot, describe and explain the trends.
    Please stop patronizing me lmao. I give up on this thread...
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    (Original post by LearningMath)
    Please stop patronizing me lmao. I give up on this thread...
    Sorry I don't mean to - but you're talking about it as if the only way to answer these questions is by memory. Whereas once you've learnt the concept it can be applied to all sorts of questions.

    see ya.
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    Of course it's permanent dipole-dipole. You can tell by the positions on the periodic tables that the EN values are sufficient to bring about this type of bonding. To have only London forces (vdW) requires the EN values to be fairly close to one another. Hydrogen bonding is an exceptionally strong version of a permanent dipole-dipole force of attraction which forms when an atom of O, N or F is DIRECTLY bonded with an atom of hydrogen. For example there is hydrogen bonding present in ethanol (R-O-H) and in carboxylic acids (R-C(=O)-O-H) but not in aldehydes (R-C(=O)-H) or ketones (R-C(=O)-R') because the oxygen isn't directly attached to any hydrogen atoms.

    Hope I helped.
 
 
 
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