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    In born-haber cycles, does it matter of the order?
    e.g. if say atomisation of Cl2 is before the ionisation energies instead of after? The mark scheme put it after but I thought you would just make sure all the reactants were atomised before you do any I.E. but does it really matter?

    Also, when calculating deltaG (free energy) do you always get a positive temperture (K)? Because, in one reaction in a past paper, delta H was -897 or something and then they wanted you to calculate the temperature when delta G=0 (feasible) but they completely ignored the negative delta H and just used 897 so the temperature came out postitive otherwise it would've been negative...? (Q4.b June O3)

    Thank you thank you
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    (Original post by Asphyxiating)
    In born-haber cycles, does it matter of the order?
    e.g. if say atomisation of Cl2 is before the ionisation energies instead of after? The mark scheme put it after but I thought you would just make sure all the reactants were atomised before you do any I.E. but does it really matter?

    Also, when calculating deltaG (free energy) do you always get a positive temperture (K)? Because, in one reaction in a past paper, delta H was -897 or something and then they wanted you to calculate the temperature when delta G=0 (feasible) but they completely ignored the negative delta H and just used 897 so the temperature came out postitive otherwise it would've been negative...?

    Thank you thank you

    just remember, from the bottom up (ie, your solid ionic compound)

    (F)ive Formation (exothermic, arrow down form ions in their standard states to ionic solid)
    (SA)martians Sublimation, (solid ion to gaseous ion) or Atomisation
    (A)te Atomisation (ie breaking covalent bond of Cl2 Br2 I2 etc)
    (I)ndigenous Ionisation, !st, 2nd etc
    (E)skimos Electron affinity (exo)
    (L)ollies Lattice enthalpy, down to soild ionic compound!!

    I dont know about the other thing, the temp can be -ve tho?? and that would be more likely to make the reaction not feasible!!, they can also ask u to work out when the reaction stops being feasible, which is also deltaG=0!!!
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    (Original post by Emma18)
    just remember, from the bottom up (ie, your solid ionic compound)

    (F)ive Formation (exothermic, arrow down form ions in their standard states to ionic solid)
    (SA)martians Sublimation, (solid ion to gaseous ion) or Atomisation
    (A)te Atomisation (ie breaking covalent bond of Cl2 Br2 I2 etc)
    (I)ndigenous Ionisation, !st, 2nd etc
    (E)skimos Electron affinity (exo)
    (L)ollies Lattice enthalpy, down to soild ionic compound!!

    I dont know about the other thing, the temp can be -ve tho?? and that would be more likely to make the reaction not feasible!!, they can also ask u to work out when the reaction stops being feasible, which is also deltaG=0!!!
    It was AQA Q4.b) June 03 for the delta G question. I'm so confused :confused: thanks for that nice little rhyme thing!
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    (Original post by Asphyxiating)
    It was AQA Q4.b) June 03 for the delta G question. I'm so confused :confused: thanks for that nice little rhyme thing!
    yeh dont worry about the -ve sign. i got it for one paper..they just ignored it.
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    If you could put a bit more about that past paper question I could have a go at it....

    Cheers
    selkie
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    (Original post by selkie222)
    If you could put a bit more about that past paper question I could have a go at it....

    Cheers
    selkie
    heres one:

    http://www.aqa.org.uk/qual/gceasa/qp...W-QP-Jun03.pdf

    question 4.

    thanks..
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    (Original post by selkie222)
    If you could put a bit more about that past paper question I could have a go at it....

    Cheers
    selkie
    Basically in the info for the Born-Haber Cycle enthalpy of formation of the molecule was -897.

    When they came to do the G = ^H - T^S, instead of using -897 for the ^H value, they used +897.
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    Um... there is a reason, not very major though.

    Enthalpy of formation of BaCl2 is -859

    However, the reaction

    BaCl2(s) --> Ba(s) + Cl2(g)
    is actually the reverse of the reaction for formation
    (for formation it goes like Ba(s) + Cl2(g)-->BaCl2(s))

    Therefore you reverse the sign, and the enthalpy for the reaction as they put it is +859

    Hope this helps.

    selkie
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    oh right..yeh i get it. thanks..
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    (Original post by selkie222)
    Um... there is a reason, not very major though.

    Enthalpy of formation of BaCl2 is -859

    However, the reaction

    BaCl2(s) --> Ba(s) + Cl2(g)
    is actually the reverse of the reaction for formation
    (for formation it goes like Ba(s) + Cl2(g)-->BaCl2(s))

    Therefore you reverse the sign, and the enthalpy for the reaction as they put it is +859

    Hope this helps.

    selkie
    oh thank you so you can have a negative T? They never seem to have a negative T though like when they describe if delta H is negative and TdeltaS is positive then delta G is always negative no matter what T value is but what if T is negative then TdeltaS is negative so then delta G = (-deltaH) + TdeltaS ??? Thanks
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    (Original post by Asphyxiating)
    oh thank you so you can have a negative T? They never seem to have a negative T though like when they describe if delta H is negative and TdeltaS is positive then delta G is always negative no matter what T value is but what if T is negative then TdeltaS is negative so then delta G = (-deltaH) + TdeltaS ??? Thanks
    Not really possible to have negative T--- remember T is in Kelvin. Zero T is already quite impossible, since it means that something has zero kinetic energy... and something with negative kinetic energy probably goes into the quantum mechanics realm.

    selkie
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    (Original post by selkie222)
    Not really possible to have negative T--- remember T is in Kelvin. Zero T is already quite impossible, since it means that something has zero kinetic energy... and something with negative kinetic energy probably goes into the quantum mechanics realm.

    selkie
    Cool, thanks
 
 
 
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