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enthalpy change question

Okay so if there's one topic i'm confused on in chemistry it's the maths part of enthalpy change. I just dont know when to use what equation; when do i use the hess cycle, when do i work out the bond enthalpies, and sometimes they give us a question that has nothing to do with what we've been taught and we just have to use a logical approachIMG_3907.jpg
(edited 6 years ago)
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Avatar for chocochip_
chocochip_
OP
17
sorry for the mini rant in the beginning, what my question actually is; how do i work out this question? do i use a hess cycle?
IMG_3907.jpg78.0KB
Original post by noor.m
sorry for the mini rant in the beginning, what my question actually is; how do i work out this question? do i use a hess cycle?


Yes use a hess cycle and its formation so arrows go up from constituent elements
Reply 3
Avatar for chocochip_
chocochip_
OP
17
Original post by glad-he-ate-her
Yes use a hess cycle and its formation so arrows go up from constituent elements


Original post by tajtsracc
They gave you enthalpy of formation values so that's a hint.


I honestly just think it's my maths because for some reason i'm getting around 1000, pray for me
Original post by noor.m
I honestly just think it's my maths because for some reason i'm getting around 1000, pray for me


did you remember carbon and nitrogen are zero and ensure all your values have the right sign and times them by 2 or 3 etc
Original post by tajtsracc
Ignore all that cycle crap, use ∆H reaction = Σ ∆fH products - Σ ∆fH reactants

[(-394) + (4 x -242)] - [2 x -366] = -630kJmol-1


That literally stems from the cycle and if she`s been taught it that way there is no good confusing her this close to exams.
The answer is -630. If you need the working i can show you.
Original post by tajtsracc
I understand that but it clearly isn't helping OP lol


You do have a point!
Reply 8
Avatar for chocochip_
chocochip_
OP
17
[QUOTE="tajtsracc;71327146"]Ignore all that cycle crap, use ∆H reaction = Σ ∆fH products - Σ ∆fH reactants

[(-394) + (4 x -242)] - [2 x -366] = -630kJmol-1

Carbon and nitrogen are in their standard states hence by definition

Original post by glad-he-ate-her
That literally stems from the cycle and if she`s been taught it that way there is no good confusing her this close to exams.

That makes so much sense thank you :-). Just one more question, my chem teacher said that when doing the hess cycle the signs are swapped so whether it's + or - if it's going the opposite direction of the arrow, if i just input the numbers like that and don't change the signs surely i'll get it wrong?

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