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Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread Watch

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    (Original post by ayvaak)
    But the complex ion is seen to be colourless not coloured? And I thought that the colour seen is complementary to the frequency of wave being absorbed, So it should only be colourless as a complex ion if it absorbs those frequencies in the visible region?
    So what is white light? Well white light is a mixture of differing wave lengths of light within the visible range of the EM spectrum, which is around 400-800nm ish. If my compound absorbs light at a wavelength of say 500nm, in the visible spectrim, then the light that reaches me now no longer has/has less of the 500nm wavelength, meaning that it appears coloured. If it absorbs light outside the visible range, i still have all the wavelengths present which make up white light and hence the thing appears colourless
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    ayvaak which paper was this from?
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    (Original post by samb1234)
    So what is white light? Well white light is a mixture of differing wave lengths of light within the visible range of the EM spectrum, which is around 400-800nm ish. If my compound absorbs light at a wavelength of say 500nm, in the visible spectrim, then the light that reaches me now no longer has/has less of the 500nm wavelength, meaning that it appears coloured. If it absorbs light outside the visible range, i still have all the wavelengths present which make up white light and hence the thing appears colourless
    Yeah this makes sense; so you're saying that the complex ion allowed all of the white light to be transmitted so no colour is observed? And instead wavelengths in the UV region or X-ray region for instance were absorbed? (I know that's probably not the case but you know)
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    (Original post by Don Pedro K.)
    Yeah this makes sense; so you're saying that the complex ion allowed all of the white light to be transmitted so no colour is observed? And instead wavelengths in the UV region or X-ray region for instance were absorbed? (I know that's probably not the case but you know)
    yes, it allowed all light in the visible range and absorbed some UV, Xray is probably a bit too far due to E=hbaromega
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    (Original post by samb1234)
    yes, it allowed all light in the visible range and absorbed some UV, Xray is probably a bit too far due to E=hbaromega
    Alright, awesome haha
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    (Original post by Don Pedro K.)
    I'm pretty sure the colour observed is comprised of the wavelengths of visible light transimitted (e.g. those not absorbed by the complex ion).
    Sorry i didnt see the previous messages my phone is playing up. Thank you it makes a lot more sense
    Now the second part of my post makes sense but does anyone have an idea as to why chemically F- causes the d-d energy gap to be large? I understand that greater the attraction the greater the split but why?

    @samb1234

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    (Original post by ayvaak)
    That i agree with but Sambs answer is confusing me a little. Sorry to be a pain but could you try explaining it in a different way. Im not sure what im missing in my theory @samb1234

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    So the colour you see is comprised of the wavelengths of light that are not absorbed yes? So if the thing is colourless, that must mean that none of the wavelengths of light that we can see (the visible parts of the spectrum) has been absorbed, as otherwise we would not have a certain wavelength which would cause us to see it as being coloured. Does that make sense?
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    (Original post by samb1234)
    So the colour you see is comprised of the wavelengths of light that are not absorbed yes? So if the thing is colourless, that must mean that none of the wavelengths of light that we can see (the visible parts of the spectrum) has been absorbed, as otherwise we would not have a certain wavelength which would cause us to see it as being coloured. Does that make sense?
    Yep it makes sense now. Sorry my phone keeps updating posts at snail pace. Do you have any idea as to why the F- ion causes an increase in energy gap. Im assuming its due to the greater strength of interaction but chemically why is this?

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    (Original post by ayvaak)
    Yep it makes sense now. Sorry my phone keeps updating posts at snail pace. Do you have any idea as to why the F- ion causes an increase in energy gap. Im assuming its due to the greater strength of interaction but chemically why is this?

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    You get repulsion between the electrons in the ligand and the orbital, meaning that the energy levels of two of the d orbitals is increased (as to put electrons into those orbitals have to overcome the repulsion with the electrons in the ligand). Fluorine has a strong electric field so as such there is a relatively large repulsion and hence the gap is larger than something like chlorine
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    Finally finished notes and now onto the papers, any in particular that are worth doing?
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    (Original post by Don Pedro K.)
    ayvaak which paper was this from?
    June 2014 unit 5

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    (Original post by samb1234)
    Finally finished notes and now onto the papers, any in particular that are worth doing?
    June 2014 unit 5 had some tricky questions that would be worth doing

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    (Original post by samb1234)
    Finally finished notes and now onto the papers, any in particular that are worth doing?
    Mate your knowledge is unreal haha XD! In terms of papers, I think the IAL papers are harder than our ones for some reason!
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    (Original post by ayvaak)
    June 2014 unit 5 had some tricky questions that would be worth doing

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    Thanks I think i may have done the unit 4 one a while ago (i did 2015 as my mock, then a couple of others when i finished unit 4 notes which was quite a while ago) but ill take a look before the exam definitely
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    (Original post by Don Pedro K.)
    Mate your knowledge is unreal haha XD! In terms of papers, I think the IAL papers are harder than our ones for some reason!
    Haha thanks and yeah I agree for most things IAL is harder, so ill make sure ive done them all before next tuesday
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    (Original post by samb1234)
    You get repulsion between the electrons in the ligand and the orbital, meaning that the energy levels of two of the d orbitals is increased (as to put electrons into those orbitals have to overcome the repulsion with the electrons in the ligand). Fluorine has a strong electric field so as such there is a relatively large repulsion and hence the gap is larger than something like chlorine
    But for example why wouldn't hydrated Fe2+ not be colourless as H2O's electric fields are stronger than F-?

    And ligands form dative covalent bonds in subshells that aren't occupied by the central ion's valent electrons but would we be expected to predict with orbitals are being occupied for these bonds between ligand and central ion?

    Thanks in advance
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    (Original post by ayvaak)
    But for example why wouldn't hydrated Fe2+ not be colourless as H2O's electric fields are stronger than F-?

    And ligands form dative covalent bonds in subshells that aren't occupied by the central ion's valent electrons but would we be expected to predict with orbitals are being occupied for these bonds between ligand and central ion?

    Thanks in advance
    Why would h2os be stronger - it is a neutral molecule whereas fluorine is negatively charged. And no to your second q
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    (Original post by samb1234)
    Why would h2os be stronger - it is a neutral molecule whereas fluorine is negatively charged. And no to your second q
    The chemrevise page on origin of TM colours states water molecules cause greater splitting?

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    (Original post by ayvaak)
    The chemrevise page on origin of TM colours states water molecules cause greater splitting?

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    That's strange...I agree with samb1234 fluoride ions have a wholly negative charge whereas water molecules are neutral overall. So I would think the transition metal ion's d orbital electrons experience greater repulsion with the electrons in the fluoride ion...??
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    (Original post by Don Pedro K.)
    That's strange...I agree with samb1234 fluoride ions have a wholly negative charge whereas water molecules are neutral overall. So I would think the transition metal ion's d orbital electrons experience greater repulsion with the electrons in the fluoride ion...??


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