# Need help with enthaply

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#1
In partc of this question, I am really unsure whether to draw a Hess cycle diagram for it or just simply use the equation to work it out. I thought if you are asked to work out the enthalpy change of combustion, you will always draw a diagram because you will only use the equation if you are given the average bond enthalpy. But for this question you can't really though. Can anyone explain when I need to draw a diagram? Thanks
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4 years ago
#2
(Original post by coconut64)
In partc of this question, I am really unsure whether to draw a Hess cycle diagram for it or just simply use the equation to work it out. I thought if you are asked to work out the enthalpy change of combustion, you will always draw a diagram because you will only use the equation if you are given the average bond enthalpy. But for this question you can't really though. Can anyone explain when I need to draw a diagram? Thanks
You could draw a diagram if you wanted, but you really don't need to for this question. Check out this interactive on Hess law
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#3
(Original post by charco)
You could draw a diagram if you wanted, but you really don't need to for this question. Check out this interactive on Hess law
Hi, thanks for replying. But for part c, it does seem like I have to draw the diagram though because I was given the enthalpy change of formation and if I draw a diagram and go from there I get -4866. Whereas if I just use the equation (sum of reactant -sum of products) I get 4866. Also for the equation , I thought it is only used if you are given the average bond enthalpy( I found out about this just now)....
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4 years ago
#4
(Original post by coconut64)
Hi, thanks for replying. But for part c, it does seem like I have to draw the diagram though because I was given the enthalpy change of formation and if I draw a diagram and go from there I get -4866. Whereas if I just use the equation (sum of reactant -sum of products) I get 4866. Also for the equation , I thought it is only used if you are given the average bond enthalpy( I found out about this just now)....
Firstly, where are you getting -4866 from?

Secondly, you don't have to. Like charco said, you can use the equation or you can draw the Hess Cycle, but both methods work because the Hess Cycle is pretty much drawing out the equation.

(Original post by coconut64)
(sum of reactant -sum of products)

Also for the equation , I thought it is only used if you are given the average bond enthalpy( I found out about this just now)....
That equation is used if you have enthalpies of combustion. What you have are enthalpies of formation so the equation will be Sum of Product - Sum of Reactants.

The equation is used also in mean bond enthalpies but you can still use it in this question, it doesn't matter.
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