When to add or when to subtract enthalpy,cell potential, etc?

[altogether]

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This might be a stupid q but I don't see a pattern of when to add or when to subtract enthalpies or cell potentials.

For ex: Calculate enthalphy of this rxn: graphite -> diamond.
given heat of combustion. In this case you would set the two combustion rxns so they equal graphite -> diamond. Then ADD the two enthalpies of combustion.

But isn't there an eqaution like... heat of rxn = heat of formation(products) - heat of formation (reactants)??

What's the difference?

For cell potential: E cell = E cathode - E anode, all the time?

 

[Rabolisk]

I'm not sure what you mean by "setting the two combustion rxns so they equal graphite -> diamond". But heat of rxn = heat of formation (products) = heat of formation (reactants) is true.

E cell = E cathode - E anode is also true.