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    Hey, so I'm currently going into A2 and since energetics was a topic I really struggled with last year I'm going over it again...and I've completely forgotten Hess' Law.

    I keep trying to do equations but I can never get them the right way around - is there one equation i can just use for combustion and formation?

    What I thought could work was this:

    formation:
    ΔH = ΔHproducts - ΔHreactants

    combustion:
    ΔH = ΔHreactants - ΔHproducts

    But I've been told I have to make a new equation every time?

    Also, if the question gives you the enthalpies of formation but asks for the enthalpy of combustion, do you use the formation equation?
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    Remember the equations as they are much quicker to do in exam.
    In cases where you are not sure, draw the cycle, personally if you practice this from now on A2 will get much much easier, especially thermochemistry and some of physical chem and electrochemical cycles.
    I finished A2 this year so I am qualified to say this.
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    So the equations I wrote above are correct? I can just remember those?

    Yeah, I figured I really need to sort this out otherwise I'm gonna get in one hell of a mess later down the line. ¯\_(ツ)_/¯
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    (Original post by brrrigid)
    So the equations I wrote above are correct? I can just remember those?

    Yeah, I figured I really need to sort this out otherwise I'm gonna get in one hell of a mess later down the line. ¯\_(ツ)_/¯
    Those equations are true but I don’t think it can be used in all situations.

    While they will get you through AS level questions without a problem but don’t rely on them in A2

    Cycles work 100% of the time and for any question you can face at A levels
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    I think I understand now - the way my previous teacher taught it was confusing, and I've read through the chemguide page of Hess's Law and it makes a lot more sense now. It was the arrow directions that got me confused.
    Thankyou!
 
 
 
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