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Hess's law - confused!

http://gyazo.com/e6775985537da9b0466c92595e0132f6

No idea how the answer is -49.5, I got 198

I set up the Hess's law cycle like this: http://gyazo.com/e3c1e041c0d1109680cf8907caade4ed
sorry that's supposed to be -288 x 6

could anyone help me on what I've done wrong?

Thanks!
(edited 11 years ago)
Reply 1
Avatar for rabia2
rabia2
6 x -288= -1728
-1728--1530=-198
-198/4 = -49.5 because there is 4 moles of ammonia, so you must divide by 4!

Tricky hess's law question!
You've calculated it for the formation of 4 moles of ammonia :smile: You need only divide your answer by 4 to get the correct answer (and correct the sign) :yep:
Reply 3
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Secret.
OP
14
Original post by rabia2
6 x -288= -1728
-1728--1530=-198
-198/4 = -49.5 because there is 4 moles of ammonia, so you must divide by 4!

Tricky hess's law question!


Original post by EierVonSatan
You've calculated it for the formation of 4 moles of ammonia :smile: You need only divide your answer by 4 to get the correct answer (and correct the sign) :yep:



Aah! I thought you only do that if they ask you for the N-H bond in bond enthalpies? So would I do the same for e.g. PCl5 if asked for the enthalpy of formation of PCl5?

Thanks!
Enthalpy is always per mole of something (from your definitions), that can be substance or bonds :yep:
Reply 5
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Secret.
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14
Original post by EierVonSatan
Enthalpy is always per mole of something (from your definitions), that can be substance or bonds :yep:


Aah thank you very much! I didn't know that:biggrin:

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