Hess's Cycle - Enthalpy Change of Combustion and Formation

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yeyhelloxoxoxo
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Hey guys, I need some help with this! Really confused as to how to work out the Hess's Cycle
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victoria98
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Well it depends on the question... Do you have any examples of questions you're struggling with?
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cindy_obasa
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(Original post by victoria98)
Well it depends on the question... Do you have any examples of questions you're struggling with?
Combustion and Formation calculations - the arrow diagrams
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victoria98
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(Original post by cindy_obasa)
Combustion and Formation calculations - the arrow diagrams
You don't actually need to learn how to draw the Hess' cycles (though I personally think it's better to do so) you can just remember that if you are given the enthalpies of combustion the enthalpy change is given by reactants - products (CRaP) and if you are given the enthalpies of formation then you do the opposite of that -> products - reactants.

If you do want to learn how to draw the actual Hess' cycles then whenever you're given enthalpies of formation draw arrows from elements (at the bottom) UP to the chemical equation. When you are given enthalpies of combustion draw arrows pointing DOWN towards the combustion products. Then you go around from the top '-' if you're going against the arrow and '+' if you're going with the arrow. I'm really bad at explaining things (especially when I can't draw a diagram!) so I'm sorry if you don't understand what I'm trying to say. The way I used to do it is work out the answer using a Hess' cycle and then check it using the relevant equation (both the cycle and equations ALWAYS work if done correctly so it's quite a nice way of ensuring you get those marks). If you have any other questions do ask, and I'll try to explain.
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