△H rxn

[EMT313]

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When calculating △Hrnx how do we know if we have to do products - reactants, or if we need to do △H bonds broken - △H bonds formed (which would be reactants - products essentially). I am very confused on this and maybe my thinking is completely off. Can anyone clear this up?

 

[justadream]

@EMT313

If you are given stuff relating to bond energies, use bonds broken - bonds formed (which is Reactants - Products).

If you are doing normal enthalpy stuff not relating to bonds, then do products - reactants.

Sorry if this is not the most scientific answer but that's just how I remember it. I'm pretty sure the instance I mentioned above is the only time you ever do Reactants-Products.