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Hess's law.. help!

Hey guys - im so stuck on this hess's law stuff- the enthalpy cycles. if anyone does OCR salters (grr) or you know it well please read on lol.
Ok- well i was looking at the salters textbook for problems for 4.1 and they show a cycle for formation but the thing is the arrows are pointing downards towards the products of combustion?!
Im so confused - i think its a combustion cycle but its appraently a formation cycle?

if anyone has any ideas..
thanks so much :biggrin:

and merry xmas for tommorow!

Reply 1

James G

Erm well I don't do OCR Chemistry, but sometimes the Enthalpy of Formation of something is equivalent to the Enthalpy of Combustion of something else, for example the Enthalpy of Combustion of H2 gas is equivalent to the Enthalpy of Formation of Water, i.e. it's the same reaction but can be described in two ways, both in the Enthalpy change in forming water, and in the enthalpy change in completely combusting H2 gas.

Well I think that's what you're getting at anyway, hope this was helpful!

edit - oh and merry xmas to you too

Reply 2

nataloulou

i've got the textbook here, but i'm not really sure what you're asking. Which diagram are you looking at?

Reply 3

lil_dreamer90

its the diagram on page 63, question 10. thanks for the help so far .. but im still a little confused :s-smilie:

Reply 4

nataloulou

it's just an enthalpy cycle; combustion and formation reactions are both occuring.
10 a) enthalpy change of formation of propane
b) the sum of the enthalpy changes of combustion of three moles of carbon and 4 moles of hydrogen
c) enthalpy change of combustion of propane
d) ΔH1= ΔH2 - ΔH3
e) can't be bothered to substitute numbers

Think that's right. Hope that's helpedxxx