3) Chemical bonding (2016)

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6 thoughts on “3) Chemical bonding (2016)

  1. I would like to ask a question on a past year question for S08/Q1/d.

    The ms only suggested that ethoxyethane cannot form H-bonds with another ethoxyethane and H2O forms hydrogen bonds with another H2O.

    But as I draw chemical diagram and it shows that H2O forms H-bonds with ethoxyethane but not stated in MS.

    What im wondering is eventhough ethoxyethane forms H-bonds with H2O but still partially dissolved and why this answer not included in MS?

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    • I presume you mean paper 2.
      Ethoxyethane (usually called ‘ether’) is very commonly used as a solvent for a separation technique called “solvent extraction” of aqueous solutions. To be useful in that role, it has to be immiscible in water – to form distinct separate liquid layers, which can be physically separated by draining off the bottom layer in a separating funnel.

      From this fact alone we can be fairly sure that between H2O and ether, any hydrogen bonding is either extremely small or non-existent, from which we actually make the general statement – there isn’t any hydrogen bonding between them.

      There is a permanent dipole in ether however, so the interaction between it and water is actually dipole-dipole.

      The water loses its “hydrogen bond” label as that term is only used when water bonds to a molecule which can hydrogen bond in return. Well, that’s a convenient way to think about it anyway.

      For hydrogen bonding,
      EACH molecule in the bond must have BOTH
      a) an hydrogen atom bonded to an N,O or F
      b) a lone pair on those N,O of F atoms.
      Ethoxyethane (‘ether’) doesn’t have any of those.

      When drawing the bonding, the delta+ H atom in the O-H bond of water ‘goes to’ the delta- O of ether (usually shown using a dotted line). It looks pretty much the same as how you depict hydrogen bonds, but actually (as mentioned) you are illustrating a dipole-dipole interaction.

      To be honest, I used to think molecules like ether (and propanone and ethanal etc) could hydrogen bond with water too, but actually, it turns out they are all categorised as dipole-dipole interactions (the dipole in propanone and ethanal is quite large enabling them to mix completely with water in all proportions).

      Alcohols (aliphatic alcohols) are different. They CAN do hydrogen bonding, as they, like water, have a H bonded to an O. But as the (hydrocarbon)carbon chain gets longer, the “dissolving power” of the hydrogen bonding in the molecule as a whole, diminishes in response to the increasingly “hydrophobic” nature of the molecule.

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      • Thank you for explaining this question i had.Tried asking my lecturer but she said just follows the MS then change topics immedietely.
        Honestly your explanation is very clear and straight to the point.

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      • Glad to have helped.
        Maybe I shouldn’t second guess your lecturer, but perhaps they are themselves unsure of this point and maybe a bit embarrassed to admit or entertain discussion of it – in case their (possible)ignorance of it gets ‘exposed’. Maybe they are ‘new’ to teaching this stuff? I myself get asked some really difficult questions at times and sometimes have no answer – but now I usually do go away and look up an answer to the questions later (if I don’t forget first that is!). On the odd occasion I’m asked a more simple question in class, and it can happen that put on the spot my mind goes blank (but not that often), or I make an error in class that I aren’t able to spot and get stuck – having to ask the students for help! (this is more frequent!, LOL)! Saved by the students! 🙂
        We teachers/lecturers are humans too. Give your teacher a chance, – they may be red hot in explaining some really tricky thing but a bit weaker in other things 🙂
        If they are quite new, then they need time to develop teaching skills and get into tune with what CIE like to emphasise.
        In my experience, it took quite a few years to get a strong grasp on things.
        I understand that’s not much compensation to you – who will eventually have to face an important A-level exam later, but you are doing the right thing… drawing upon other sources of info when necessary. Millions of students in the past have been through this and their industrious attitude, like yours, has seen them through.

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  2. And I was wondering when will you post 2016 syllabus because the lecterur I got doesn’t give notes in ppt and generally a bad lecturer.

    I almost completed ch5 but only understand a bit.

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  3. I am also wondering when I’m going to post content. I have been extremely busy and simply never had the time to push the “pure” chemistry content of this blog. One reason is that because my students (generation W – whatsapp) seem largely to ignore this blog, (they are partially frustrated by the slow internet speeds that could be a LOT better in our college), hence it’s not ‘time or cost-effective’ for me to develop this blog in terms of chemistry – yet. If they started using it more, then I’ll respond in turn.

    I’ve been focussing on the electronics stuff because I’m quite new to “teaching” it, and I REALLY do need to lay down some reference / foundational material for that.

    I wouldn’t be so hard on your teacher. I’m sure they ‘teach’ in that way for a very good reason, reasons you are probably not aware of. Some teachers don’t believe in handing things things out (like ppt soft or hard copy, or printed notes) as they think (and have seen) that some students become lazy and complacent thinking they can ‘switch off’ in class because they have the notes. Active (participative) learning – as you are doing now !! (thank your teacher!). Plus you should have a book – todays books are very very good indeed (although horribly expensive in Malaysia).

    I recommend to you Chemguide (http://www.chemguide.co.uk) and Doc Brown’s website (you’ll have to Google for that URL). You can also get great info aimed at A-level standard on YouTube.

    Do speak to your teacher outside class hours, you may see a better side of them an I’m sure they will be very happy to help.

    You can also ask any question here. I’m sure many others will also benefit from it.

    Goodnight.

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