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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 Wunderbarr)
    D because you can see Sulfur is reduced from +7 to +6 and that means Fe2+ is oxidised to Fe3+ for this to happen.
    Similarly, Iodide ions are being oxidised to Iodine so Fe3+ is reduced to Fe2+ for this to happen.

    A because it's the same idea as adding Hydrogen under a Raney Nickel catalyst to Benzene, except replacing hydrogen with chlorine.
    Why can't it be B, electrophilic addition, though? I'm slightly confused
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    (Original post by imnoteinstein)
    A solution of potassium manganate(VII) was used to determine the concentration ofiron(II) ions in solution by titration in the presence of excess dilute sulfuric acid
    (b) If insufficient acid is added, the titre value is
    A low and a brown precipitate forms.
    B low and a green precipitate forms.
    C high and a brown precipitate forms.
    D high and a green precipitate forms.
    could someone please explain this andd what the role of the acid is in this titration?

    Is the answer supposed to be C?
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    (Original post by dpoojaraa)
    Why can't it be B, electrophilic addition, though? I'm slightly confused
    Benzene doesn't undergo addition very easily at all since this would involve disrupting the delocalised electron system which would require a lot of energy and thus is not thermodynamically feasible !
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    (Original post by dpoojaraa)
    Why can't it be B, electrophilic addition, though? I'm slightly confused
    It would be electrophilic addition in the case of hydrogenation. But if you remember to AS, we need the aid of U.V. light to create radicals which are much more reactive. So this wouldn't work with just ordinary chlorine and benzene as the chlorine simply isn't reactive enough to strip away the aromaticity of benzene.

    (so free radical is the type and addition is the mechanism).

    Much like how hydrogen needs the nickel catalyst to undergo addition with benzene.
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    (Original post by ayvaak)
    Is the answer supposed to be C?
    Okay yeah it is C I was completely wrong xD Can you explain?
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    (Original post by Wunderbarr)
    It would be electrophilic addition in the case of hydrogenation. But if you remember to AS, we need the aid of U.V. light to create radicals which are much more reactive. So this wouldn't work with just ordinary chlorine and benzene as the chlorine simply isn't reactive enough to strip away the aromaticity of benzene.

    (so free radical is the type and addition is the mechanism).

    Much like how hydrogen needs the nickel catalyst to undergo addition with benzene.
    Oh I seee! Thank you
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    I just wanted to take this moment to say a quick brain refresher:

    What happens when you add 3 H2 (g) (Raney Ni catalyst) to Benzene?

    Answer:
    Spoiler:
    Show
    You get Benzane

    Badum TSSSST. :bban:

    (Yes I know it's cyclohexane).
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    (Original post by Wunderbarr)
    It would be electrophilic addition in the case of hydrogenation. But if you remember to AS, we need the aid of U.V. light to create radicals which are much more reactive. So this wouldn't work with just ordinary chlorine and benzene as the chlorine simply isn't reactive enough to strip away the aromaticity of benzene.

    (so free radical is the type and addition is the mechanism).

    Much like how hydrogen needs the nickel catalyst to undergo addition with benzene.
    Was I right about why benzene doesn't typically undergo addition reactions?
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    (Original post by Don Pedro K.)
    Was I right about why benzene doesn't typically undergo addition reactions?
    Yeeees sir, except I don't exactly know much about the thermodynamically feasible part.
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    Now that physics Unit 4 is done my focus is on this haha
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    (Original post by Wunderbarr)
    Yeeees sir, except I don't exactly know much about the thermodynamically feasible part.
    Ah ok haha thanks ! Thermodynamically feasible is probably not the right way to put it...xD
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    So

    Name:  2014 IAL Q4 Q6.png
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    Q4: I know it gets oxidised but why at the negative electrode?

    Q6: Why A's way round rather than C's? Could it be something to do with the stoichiometric ratios?
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    (Original post by Wunderbarr)
    So

    Name:  2014 IAL Q4 Q6.png
Views: 87
Size:  61.5 KB

    Q4: I know it gets oxidised but why at the negative electrode?

    Q6: Why A's way round rather than C's? Could it be something to do with the stoichiometric ratios?
    Yeah I always thought oxidation occurs at the anode (positive electrode)?
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    (Original post by dpoojaraa)
    Oh I seee! Thank you
    I just wanted to go back on what I briefly said about hydrogen and benzene.

    Hydrogen and Benzene do react (electrophilic addition) but very slowly.

    You add the Raney Nickel to speed it up, rather than to allow them to react.

    (said it a bit wrong earlier, sorry)
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    (Original post by Wunderbarr)
    So

    Name:  2014 IAL Q4 Q6.png
Views: 87
Size:  61.5 KB

    Q4: I know it gets oxidised but why at the negative electrode?

    Q6: Why A's way round rather than C's? Could it be something to do with the stoichiometric ratios?
    In general at each electrode we have the equilibrium:

    X^n+ + ne- <-------> X

    The more negative the value of e cell, the further to the left this eq lies so the more electrons there are. When a current is allowed to flow, electrons will move from where there are more of them (the more negative electrode) to where there are less of them (the less negative electrode). This will shift the eq of the more negative one as e- are removed so will become a one way X -------> x^+n and the other way around at the less negative electrode.

    For 6 you need to think about the reaction. How much have you seen about iron 4+ compared to iron 2+
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    (Original post by samb1234)
    In general at each electrode we have the equilibrium:

    X^n+ + ne- <-------> X

    The more negative the value of e cell, the further to the left this eq lies so the more electrons there are. When a current is allowed to flow, electrons will move from where there are more of them (the more negative electrode) to where there are less of them (the less negative electrode). This will shift the eq of the more negative one as e- are removed so will become a one way X -------> x^+n and the other way around at the less negative electrode.

    For 6 you need to think about the reaction. How much have you seen about iron 4+ compared to iron 2+
    Don't know about this " iron 4+ "
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    (Original post by samb1234)
    In general at each electrode we have the equilibrium:

    X^n+ + ne- <-------> X

    The more negative the value of e cell, the further to the left this eq lies so the more electrons there are. When a current is allowed to flow, electrons will move from where there are more of them (the more negative electrode) to where there are less of them (the less negative electrode). This will shift the eq of the more negative one as e- are removed so will become a one way X -------> x^+n and the other way around at the less negative electrode.

    For 6 you need to think about the reaction. How much have you seen about iron 4+ compared to iron 2+
    Does the "oxidation always occur at the anode" rule only apply to electrolysis then?
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    (Original post by Wunderbarr)
    Don't know about this " iron 4+ "
    So if the Fe3+ was oxidised, what oxidation state would the iron have to have, and the same for if it was reduced and which is more common
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    samb1234 I am honestly still confused as to why oxidation occurs at the negative electrode...Is that always the case? I always remember learning that oxidation occurs at the anode and surely the anode is the positive electrode? Or is that not the case in electrochemical cells?
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    (Original post by Don Pedro K.)
    samb1234 I am honestly still confused as to why oxidation occurs at the negative electrode...Is that always the case? I always remember learning that oxidation occurs at the anode and surely the anode is the positive electrode? Or is that not the case in electrochemical cells?
    Yes it's always the case. Chemguide has good info on them (I would happily send my notes, but they're handwritten and would take too long to digitise)
 
 
 
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