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1. I missed the lesson on Hess's law, and i have some questions asking me to find the enthalpy change in the reaction..
someone help me sort it out please?

it gives..
CH4(g) + 2O2(g) -> CO2(g) + 2H2O)(l)

and tells me
delta Hf CH4= -73 kJmol-1
deltaHf H2O= -285 kJmol-1

2. Adding a third group consisting of C, 2H2 and 2O2, you can then create a cycle with the reactants and products given. An arrow would be pointing from the elements to the reactants, and from the elements to the products. You can then calculate the total enthalpy change of formation of each by adding together those given (ignoring O2, as that doesn't change). Hess' Law states that the enthalpy is the same regardless of route, so, from the cycle you've drawn, you should be able to see the enthalpy change for that reaction is DeltaHf(Products)-DeltaHf(Reactants). Do that, and that should give you your answer.
3. (Original post by Revolution is my Name)
Adding a third group consisting of C, 2H2 and 2O2, you can then create a cycle with the reactants and products given. An arrow would be pointing from the elements to the reactants, and from the elements to the products. You can then calculate the total enthalpy change of formation of each by adding together those given (ignoring O2, as that doesn't change). Hess' Law states that the enthalpy is the same regardless of route, so, from the cycle you've drawn, you should be able to see the enthalpy change for that reaction is DeltaHf(Products)-DeltaHf(Reactants). Do that, and that should give you your answer.

sorry, but i'm confused :P so i've made the cycle... but with the CH4=-74 and H2O=-285, what do i do now? thanks
4. (Original post by lukeluke112)
I missed the lesson on Hess's law, and i have some questions asking me to find the enthalpy change in the reaction..
someone help me sort it out please?

it gives..
CH4(g) + 2O2(g) -> CO2(g) + 2H2O)(l)

and tells me
delta Hf CH4= -73 kJmol-1
deltaHf H2O= -285 kJmol-1

You are missing ΔHf for CO2

you need it to do the calculation...

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