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    On the born haber cycle for NaCl in the revision guide they have taken away all the values to get the lattice enthaply

    (-411) - (+107) - (+496) - (+122) - (-349) = 787 kj mol-1

    but on the june 2002 paper there is a question asking to calculate the Lattice enthaply for CsCl! and on the mark scheme to get the LE value

    they have added them together (+76) + (+122) + (+376) + (-349) = -668 Kj mol-1

    the book says take them away.. anybody have an explanation for this? maybe something to with exo/endo changes? please explain if you have! thanks Jay
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    well, lattice enthalpy is always exothermic anyways, so ... if you got LE of NaCl is +787 kj/mol, it's wrong.
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    (Original post by BCHL85)
    well, lattice enthalpy is always exothermic anyways, so ... if you got LE of NaCl is +787 kj/mol, it's wrong.
    i forgot the - sign! -787
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    just draw the Haber Cycle for the reaction ... then follow the arrows to find LE.
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    (Original post by BCHL85)
    just draw the Haber Cycle for the reaction ... then follow the arrows to find LE.
    arrows are the same on both diagrams
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    Lattice enthalpy = Enthalpy of formation - (everything else added together)

    I think both are just a variation of that.
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    (Original post by BCHL85)
    well, lattice enthalpy is always exothermic anyways, so ... if you got LE of NaCl is +787 kj/mol, it's wrong.
    not true... lattice enthalpy of formation is -ve whereas lattice enthaply of dissociation is +ve
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    are you sure both were being used to work out the LE? it may have been formation?
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    (Original post by CogitoErgoSum)
    are you sure both were being used to work out the LE? it may have been formation?
    thats what i thought.. but it says lattice enthaply in both :confused:

    have a look at the june 2002 mark scheme and the example in the revision guide
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    No sorry, I don't get it either! Just stick to what you know i think. I did it differently in the paper and my teacher still marked it right because I got the right answer. To follow the right path it is -E-C-D-B+F, which is the same as F-E-C-D-B which is -443 - -349 -122 -376 -76 = -668
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    (Original post by Chocolatefudge)
    No sorry, I don't get it either! Just stick to what you know i think. I did it differently in the paper and my teacher still marked it right because I got the right answer. To follow the right path it is -E-C-D-B+F, which is the same as F-E-C-D-B which is -443 - -349 -122 -376 -76 = -668
    yea the way i thought actually works! not been taught this too well so i couldnt recognise OCR's shortcut on the mark scheme! but i had it right from the start!

    thanks for help anyway every1
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    (Original post by jayrox)
    yea the way i thought actually works! not been taught this too well so i couldnt recognise OCR's shortcut on the mark scheme! but i had it right from the start!

    thanks for help anyway every1
    yo! read ur PM's. im gonna figure ur prob out, do u have the 'Revise A2 Chemistry' for OCR A. theres a pic of a skeletal hand on the front, (rather weird but anyway..!) cos if u do, there ALWAYS messing calculations and simple facts up, our school is looking for diff books 4 next year cos they're getting so annoyed with them!!!!!
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    (Original post by lisaf6288)
    yo! read ur PM's. im gonna figure ur prob out, do u have the 'Revise A2 Chemistry' for OCR A. theres a pic of a skeletal hand on the front, (rather weird but anyway..!) cos if u do, there ALWAYS messing calculations and simple facts up, our school is looking for diff books 4 next year cos they're getting so annoyed with them!!!!!
    i got it yea ive counted 16 mistakes in it so far hehe!

    ive sorted my prob out though now hun

    but mistakes are so common in that! equations are the worst ;/
 
 
 
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