The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

This discussion is now closed.

Check out other Related discussions

Trends and Patterns.. Anybody know?

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
Reply 1
Avatar for BCHL85
BCHL85
12
well, lattice enthalpy is always exothermic anyways, so ... if you got LE of NaCl is +787 kj/mol, it's wrong.
Reply 2
Avatar for jayrox
jayrox
OP
4
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
Reply 3
Avatar for BCHL85
BCHL85
12
just draw the Haber Cycle for the reaction ... then follow the arrows to find LE.
Reply 4
Avatar for jayrox
jayrox
OP
4
BCHL85
just draw the Haber Cycle for the reaction ... then follow the arrows to find LE.


arrows are the same on both diagrams
Reply 5
Avatar for kpg
kpg
11
Lattice enthalpy = Enthalpy of formation - (everything else added together)

I think both are just a variation of that.
Reply 6
Avatar for Revenged
Revenged
15
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
are you sure both were being used to work out the LE? it may have been formation?
Reply 8
Avatar for jayrox
jayrox
OP
4
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
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
Reply 10
Avatar for jayrox
jayrox
OP
4
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
Reply 11
Avatar for lisaf6288
lisaf6288
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!!!!!
Reply 12
Avatar for jayrox
jayrox
OP
4
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 :smile: ive counted 16 mistakes in it so far hehe!

ive sorted my prob out though now hun :biggrin:

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

Related discussions

Latest

Trending

Trending

Articles for you