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    hey guys,
    I'm now solving my chemistry past paper AS and i've found some difficulty in this multiple choice question:
    How many of the following molecules will absorb IR radiation?
    A two
    B three
    C four
    D five

    the molecules are:

    H2O N2 CH4 CO2 O2

    i thought it would be only water since it's the only polar molecule here, but there's no option for one molecule..any help will be appreciated..thanks!
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    They all do don't they?

    You get IR spectroscopy from the stretching/bending/vibration of bonds so I assume that as long as something has a bond then it will absorb IR radiation.

    I'd go for D.

    Not entirley sure though.
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    I would say it's three - water because it's polar, and the greenhouse gases too (hence global warming) - so that's carbon dioxide and methane.

    Might be wrong, there.
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    the mark scheme's answer is B :/
    i'm totally confused lol
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    oh tena ur right..thanks for the explanation
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    so it's not only polar molecules that absorb IR radiation right? 'cause what i know is only molecules that can change their polarity are able to absorb IR radiation..
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    (Original post by HorseRider)
    oh tena ur right..thanks for the explanation
    Welcome

    (Original post by HorseRider)
    so it's not only polar molecules that absorb IR radiation right? 'cause what i know is only molecules that can change their polarity are able to absorb IR radiation..
    As far as I can remember, the C-H bonds and C-O bonds vibrate in accordance with the frequency of the IR radiation that is emitted from the Earth, so they absorb it.

    (Again, not sure if that's entirely correct... I have a god-awful teacher for Chem so all of this is what I've been able to pick up.)
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    (Original post by HorseRider)
    so it's not only polar molecules that absorb IR radiation right? 'cause what i know is only molecules that can change their polarity are able to absorb IR radiation..
    Symetrical diatomic molecules don't absorb IR. Just checked my notes.

    As long as they arent linear and diatomic like O2 and N2, then they will absorb IR. They don't have to be polar.
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    (Original post by Ollie901)
    Symetrical diatomic molecules don't absorb IR. Just checked my notes.

    As long as they arent linear and diatomic like O2 and N2, then they will absorb IR. They don't have to be polar.
    The condition for absorption is that any change in vibrational level (stretching, bending etc) produces a change in polarity.

    Thus carbon dioxide is not polar BUT if the O=C=O bond angle changes (bending) it becomes polar. This allows it to absorb IR radiation.

    There is nothing that a diatomic molecule such as O2, or N2 can do to change their polarity (as they don't have any)
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    Symetrical diatomic molecules don't absorb IR. Just checked my notes.

    Yes, because symmetrical molecules are not polar, so that's why only polar molecules can absorb IR radiation..i guess

    thanks all for the help, i appreciate it.
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    (Original post by HorseRider)
    Symetrical diatomic molecules don't absorb IR. Just checked my notes.

    Yes, because symmetrical molecules are not polar, so that's why only polar molecules can absorb IR radiation..i guess

    thanks all for the help, i appreciate it.
    No, that's not correct - read my post immediately above yours.
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    oh ok now I get it thanks.
    one more question, in Chemistry AS edexcel, can questions from unit 1 exams come in unit 2 exams? because I've done unit 1 in January and now I'm studying for unit 2, and when solving the old syllabus exams I find questions from unit 1 in unit 2..I'm really confused..
    thanks in advance!
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    (Original post by HorseRider)
    oh ok now I get it thanks.
    one more question, in Chemistry AS edexcel, can questions from unit 1 exams come in unit 2 exams? because I've done unit 1 in January and now I'm studying for unit 2, and when solving the old syllabus exams I find questions from unit 1 in unit 2..I'm really confused..
    thanks in advance!
    A lot of the stuff in Unit 2 builds on the stuff we did in Unit 1, so that might be why.
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    Right the molecules that absorb IR are greenhouse gases and these molecules HAVE to have different atoms within them, so that their polarities can repeatedly change while absorbing the IR.

    So : C02, H20 ( Water Vapour), and CH4 are the greenhouse gases and that is why it is B.

    O2 and N2 cannot absorb IR as their bond contains the same atoms and so they have no overall polarity and so cannot absorb IR as they cannot change their polarity and make the bond vibrate( As no polarity in bond!).

    Hopefully that helps,

    Quote me for any further questions
 
 
 
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