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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 C0balt)
    Uh it appears to be 2.39 to my eyes?
    Oh - Im so sorry - I read that as 2.39 * 10 ^ -3 because of the dm^-3

    That makes so much more sense

    Sorry again - I sometimes read things wrong

    Thanks for your help
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    can anyone please explain the difference between amine and amide?
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    (Original post by Don Pedro K.)
    what the heck o.O 78....on that evil paper? You're sorted lol.
    I'm going for 100% in A level Chemistry man, I'm not too sorted for that lol
    (Original post by the-anonymous-me)
    Oh - Im so sorry - I read that as 2.39 * 10 ^ -3 because of the dm^-3

    That makes so much more sense

    Sorry again - I sometimes read things wrong

    Thanks for your help
    Hahaha no worries, I read things wrong all the time, including that unit 4 exam we sat XD
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    Could someone please explain recrystalisation including why each step is done thank you
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    (Original post by RUNSran)
    can anyone please explain the difference between amine and amide?
    An amide has the -CONH2 functional group whereas amines just have NH2/NH/N
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    (Original post by blueynuey)
    Could someone please explain recrystalisation including why each step is done thank you
    Here goes!
    1) Dissolve impure compound in a minimum amount of hot solvent. It's a minimum amount so that you obtain a saturated solution. If you use an excess some crystals may remain in solution

    2) Hot filtration - hot so you avoid premature formation of crystals and it's filtered to remove insoluble impurities

    3) Cool the filtered solution to ensure maximum amount of solid crystallises

    4) Suction filtration with Büchner funnel to remove any soluble impurities and then wash with distilled water to dry
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    (Original post by Don Pedro K.)
    An amide has the -CONH2 functional group whereas amines just have NH2/NH/N
    oh I see, thanks
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    (Original post by blueynuey)
    Could someone please explain recrystalisation including why each step is done thank you
    Add minimum hot solvent
    minimum so crystals form

    Hot filtration
    to remove insoluble impurities

    Cool the filtrate so crystals can form

    Filter crystals through Buchner funnel then wash on filter paper using cold solvent
    to remove soluble impurities

    Dry in desiccator or oven
    to remove water
    Desiccator is better for substnaces with low melting points or substances that decompose on heating
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    I posted this a while back but can someone please explain solvent extraction??
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    (Original post by C0balt)
    Everyone's fav MCQ
    20cm3 contracted because water condensed as it was carried out at room temp
    10cm3 of the hydrocarbon therefore produced 20cm3 of water. Water has 2 H atoms per molecule so from the hydrocarbon : water ratio you know that a mole of the hydrocarbon produces two moles of water, which means a mole of the hydrocarbon has four moles of H atoms.
    Thankyou!
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    (Original post by ilovecake123)
    oh god won't there be a legacy paper?
    Would seem veeery unlikely.
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    For recrystallisation, why it important that the product is very soluble at high temps and almost insoluble at low temps?
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    (Original post by the-anonymous-me)
    I posted this a while back but can someone please explain solvent extraction??
    1. put the distalle of the impure product into a separating funnel
    2. wash with NaHCO3 to neutralise acid
    3. allow separation to occur
    4. run the organic liquid into a conical flask and place with anhydrous sodium sulphate
    (the drying agent must not react with the organic liquid and should be insoluble)
    5. decant the liquid into a distillation flask
    6. distill to obtain pure product
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    (Original post by Rahatara Sadique)
    Thankyou!
    No problem, gl tomorrow
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    (Original post by Don Pedro K.)
    For recrystallisation, why it important that the product is very soluble at high temps and almost insoluble at low temps?
    Heat the solution until it all dissolves.

    Cool the solution until that which is insoluble at low temperatures will "crash out" of the solution. Hence you can throw away the solution containing all the impurities, and be left with the product (followed by that buchner funnel stuff).
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    (Original post by tayloryeah)
    1. put the distalle of the impure product into a separating funnel
    2. wash with NaHCO3 to neutralise acid
    3. allow separation to occur
    4. run the organic liquid into a conical flask and place with anhydrous sodium sulphate
    (the drying agent must not react with the organic liquid and should be insoluble)
    5. decant the liquid into a distillation flask
    6. distill to obtain pure product
    Thanks - I couldn't find a simple step-by-step explanation anywhere
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    [QUOTE=Wunderbarr;65991421]
    (Original post by RUNSran)
    x /QUOTE]

    I think you can just look at Cr2+ oxidising straight to Cr6+.

    If you look at the oxidation of Cr2+ by MnO4-, then you can formulate some equations with:
    Cr2+ -> Cr6+
    MnO4- -> Mn2+
    then balance these by balancing the electrons and add them together (for now I'm just ignoring all the other junk that actually appear in the equations).

    We know MnO4- goes to Mn2+ and Cr2+ goes to Cr6+:

    MnO4- + 5e- -> Mn2+ + stuff
    Cr2+ -> Cr6+ + 4e-

    Therefore: 4MnO4- + 5Cr2+ -> 4Mn2+ + 5Cr6+
    So mole ratio of MnO4- to Cr2+ is 4:5 or the moles of MnO4- will be 4/5 times the moles of Cr2+.

    You were right with dividing the moles given by 10 to get 2.66x10-4 but then you need to multiply this number by 8/5 to get the moles of MnO4-.

    Why 8/5 instead of the just mentioned 4/5? Because there are twice as many Cr2+ to oxidise.

    Finish it for me baby.
    What do you mean?
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    (Original post by Don Pedro K.)
    What do you mean?
    I just dealt with one Cr2+ but can you see that the original complex ion (top) has two Cr atoms in its formula?
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    (Original post by Wunderbarr)
    I just dealt with one Cr2+ but can you see that the original complex ion (top) has two Cr atoms in its formula?
    Ah of course Thanks! Also, how can you tell when something won't react with 2,4-DNP? I know that carboxylic acids don't, but do acyl chlorides/amides for example? Or is it purely aldehydes and ketones?
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    (Original post by Don Pedro K.)
    Ah of course Thanks! Also, how can you tell when something won't react with 2,4-DNPH? I know that carboxylic acids don't, but do acyl chlorides/amides for example? Or is it purely aldehydes and ketones?
    I'm just going to say yes to aldehydes and ketones xD

    Someone else may be able to clarify because I'm at the point of using the old tactic of hoping some things won't come up tomorrow .
 
 
 
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