How to use Hess' Law?

I am confused by the whole triangle system and I've never used it.

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Could someone explain how I would use it to find the enthalpy of combustion of ethene?

The question has given me the following information:
H₂O(l) -285.5
CO₂(g) -393
Ethene C₂H₄(g) +52.0
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This is what I have done, and I'm not sure if it is correct (I didn't use the triangle/re-arranging method):

ΔH_c = sum(ΔH_f of products) - sum(ΔH_f of reactants)
ΔH_c = [2(-393) + 2(-285.5)] - (52.0)
ΔH_c = -1409 kJ mol^-1




The answer is apparently -1009 kJ mol^-1

Hi TARS,

Hess' law is fundamental in your understanding of thermodynamics as it is the basis of all energetics data.

I have prepared an interactive on Hess that you can see here

or if you would like to see the concept explained on video it can be accessed here

[video]https://youtu.be/c66cfA7eq_g[/video]

Thanks for your reply that looks very helpful and impressive! I'll check them out now