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OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD watch

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    (Original post by RetroSpectro)
    Can someone please elaborate on this statement from the Spec:

    "explain the changes that take place at eachelectrode in a hydrogen–oxygen fuel cell"
    I'm guessing it means in terms of standard electrode potential values and equilibrium
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    (Original post by itsConnor_)
    I don't know what I'm doing with titration curves. Can someone explain?

    e.g. https://gyazo.com/b9b5bf5f7a6fbaf84bbcb3bd78916460
    Deduce whether you are starting with an acid or a base

    If its a strong acid the pH curve will start from a low pH; a weak acid will have a slightly higher pH start but lower than 7

    If its a strong base then the start would be a high pH around 14; a weak base would start at a slightly lower pH but above 7

    Identify whats added and where the pH is expected to drop to
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    (Original post by itsConnor_)
    I don't know what I'm doing with titration curves. Can someone explain?

    e.g. https://gyazo.com/b9b5bf5f7a6fbaf84bbcb3bd78916460
    First thing you wanna do is identify what the strong acid, weak acid, strong base and weak base are
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    (Original post by Serine Soul)
    I'm guessing it means in terms of standard electrode potential values and equilibrium
    As in the equations of H2 splitting and H2O forming from Oxygen, protons and electrons?
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    (Original post by RetroSpectro)
    Deduce whether you are starting with an acid or a base

    If its a strong acid the pH curve will start from a low pH; a weak acid will have a slightly higher pH start but lower than 7

    If its a strong base then the start would be a high pH around 14; a weak base would start at a slightly lower pH but above 7

    Identify whats added and where the pH is expected to drop to
    (Original post by Serine Soul)
    First thing you wanna do is identify what the strong acid, weak acid, strong base and weak base are
    Ok so when you say "if its a strong acid it will start from a low pH" etc. this is the acid that an alkali is ADDED TO and not an acid that is BEING ADDED to an alkali?
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    (Original post by zirak46)
    Hey guys, hope revision is going well.

    Could someone please explaim how to do the following questioms from the June 13 paper?

    The part d) question refers to the table attached.



    Posted from TSR Mobile
    Hi, not too sure if you still need the help.

    So for the question regarding CO2, in order to work out mass you need the number of moles of co2. This can be worked out by dividing the TOTAL energy intake by the enthalpy change of reaction (which is given per mole) and multiplying this by 6 due to the stoichiometry of the equation/reaction. This will give you the number of moles so multiply this by the RFM of co2 (44) to give 3.22*10^17 (g).
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    (Original post by lolo11)
    Hi, not too sure if you still need the help.

    So for the question regarding CO2, in order to work out mass you need the number of moles of co2. This can be worked out by dividing the TOTAL energy intake by the enthalpy change of reaction (which is given per mole) and multiplying this by 6 due to the stoichiometry of the equation/reaction. This will give you the number of moles so multiply this by the RFM of co2 (44) to give 3.22*10^17 (g).
    But why does dividing the total energy intake by enthalpy change give you moles of c6h12o6? Thank you

    Posted from TSR Mobile
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    (Original post by ImNervous)
    But why does dividing the total energy intake by enthalpy change give you moles of c6h12o6? Thank you

    Posted from TSR Mobile
    So the enthalpy change of reaction will give you the total energy change per mole of compound formed (which in this case, as you say, we can say is c6h12o6). The value given to us in the question isn't per mole, it is just the TOTAL energy intake. Therefore, by dividing the two you will get the number of moles of c6h12o6, which is 1/6th of co2 so you multiply this value by 6.

    It might make more sense if you look at it in terms of units:

    Total energy = 'X' KJ
    Enthalpy change (of reaction) = 'Y' KJ mol^-1


    We just want the number of moles so to get this we must do X/Y.

    I might have just over complicated it but hopefully it makes sense.
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    Ca anyone explain how to calculate the gas C?
    I worked it out by writing the other products and seeing what was left over in the reactions giving me N2 but the ms shows a calculation, which I don't understand. Could anyone explain the calculation please?
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    This has been on my mind for ages
    will they give me the mark for this:
    Aldehyde + 2[h] = alcohol + oh-?????????????????????
    Also isomerism question:
    Different groups attatched to the carbons in the double bond
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    (Original post by itsConnor_)
    Ok so when you say "if its a strong acid it will start from a low pH" etc. this is the acid that an alkali is ADDED TO and not an acid that is BEING ADDED to an alkali?
    Yes

    so imagine you have an acid in a beaker, that determines your starting pH, and adding a base to the beaker will raise the pH
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Size:  87.8 KB Help?!? Unsure about overall cell equation. Why dont i switch the negative E value ( Al) equation to the left??
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    (Original post by lolo11)
    So the enthalpy change of reaction will give you the total energy change per mole of compound formed (which in this case, as you say, we can say is c6h12o6). The value given to us in the question isn't per mole, it is just the TOTAL energy intake. Therefore, by dividing the two you will get the number of moles of c6h12o6, which is 1/6th of co2 so you multiply this value by 6.

    It might make more sense if you look at it in terms of units:

    Total energy = 'X' KJ
    Enthalpy change (of reaction) = 'Y' KJ mol^-1


    We just want the number of moles so to get this we must do X/Y.

    I might have just over complicated it but hopefully it makes sense.
    Actually that was perfect. Thank you very much.
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    (Original post by AqsaMx)
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    Ca anyone explain how to calculate the gas C?
    I worked it out by writing the other products and seeing what was left over in the reactions giving me N2 but the ms shows a calculation, which I don't understand. Could anyone explain the calculation please?
    It's based on the equation given in the data sheet that 1 mole of a gas occupies 24dm^3 at room temperature, and I think you have 1.17 g of the gas right? So 1.17x 24 to give the volume it would occupy, as 1g is 24 dm^3 so 1.17g is 28.08 so N2
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    I understand how KF has a more negative/exothermic lattice enthalpy and hydration enthalpy than RbF. But how does that explain why the enthalpy of solution for RbF is more exothermic than for KF. The mark scheme didn't really answer my question either. Can someone please help me?
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    (Original post by KB_97)
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    I understand how KF has a more negative/exothermic lattice enthalpy and hydration enthalpy than RbF. But how does that explain why the enthalpy of solution for RbF is more exothermic than for KF. The mark scheme didn't really answer my question either. Can someone please help me?
    what does the markscheme say?

    I think ∆Hsol works differently.

    ∆Hsol= ∆Hhyd- ∆Hlatt

    we can assume ∆Hlatt of RbF > ∆Hlatt of KF.
    K has larger charge density than Rb (surely!) So it should have a more exothermic lattice enthalpy.

    I can only assume that Rb has a greater hydration value due to its larger ARadius.

    which outweights its LE. So if you do the math, two very endothermic hydration and lattice enthalpy values give you a -ve sol value.
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    (Original post by KB_97)
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    I understand how KF has a more negative/exothermic lattice enthalpy and hydration enthalpy than RbF. But how does that explain why the enthalpy of solution for RbF is more exothermic than for KF. The mark scheme didn't really answer my question either. Can someone please help me?
    Enthalpy change of solution= (enthalpy change of hydration of cation + enthalpy change of hydration of anion) - lattice enthalpy

    Enthalpy change of hydration of cation K+ will be more exothermic than the value of the cation Rb+ (as potassium ions have a higher charge density so are better able to attract and form bonds with water molecules). Enthalpy change of hydration of anion I- will be the same for both ionic compounds. Lattice enthalpy will be more negative for KI due to stronger electrostatic attraction between oppositely charged ions. Therefore in the equation you will be obtain a more positive number for enthalpy change of solution for KI.

    Hopefully this makes sense and coincides with the mark scheme. If there is a mistake some where in there do correct me 😊
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    (Original post by Ss0)
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Size:  87.8 KB Help?!? Unsure about overall cell equation. Why dont i switch the negative E value ( Al) equation to the left??
    In the question it tells you that the reactions at each electrode are shown below.,so you don't need to switch the aluminium one around, its already been switched around for you.
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    (Original post by Ali1998HRM)
    Enthalpy change of solution= (enthalpy change of hydration of cation + enthalpy change of hydration of anion) - lattice enthalpy

    Enthalpy change of hydration of cation K+ will be more exothermic than the value of the cation Rb+ (as potassium ions have a higher charge density so are better able to attract and form bonds with water molecules). Enthalpy change of hydration of anion I- will be the same for both ionic compounds. Lattice enthalpy will be more negative for KI due to stronger electrostatic attraction between oppositely charged ions. Therefore in the equation you will be obtain a more positive number for enthalpy change of solution for KI.

    Hopefully this makes sense and coincides with the mark scheme. If there is a mistake some where in there do correct me 😊

    yeah pretty much, I thought of it like that too! I hope we're right
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    (Original post by cr7alwayz)
    In the question it tells you that the reactions at each electrode are shown below.,so you don't need to switch the aluminium one around, its already been switched around for you.
    why haven't they given an indication of the +- value on 2.71? How do we know if its -2.71 or + 2.71?

    I mean, obvs you assume its +2.71 bc they wouldve stated otherwise no?
 
 
 
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