Another...
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How do you figure out problems like #7 on the Fall of '07 exam 3b?
7. Rust (Fe₂O₃) can be converted to iron by the following reaction:
2 Fe₂O₃(s) --> 4 Fe(s) + 3 O₂(g)
What is ΔHºreaction for this process? (ΔHfº = -824.2^kJ/_mol for Fe₂O₃.)
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This is an enthalpy of reaction problem, although it might seem like there's not enough info given. The products here are both uncombined elements in their standard states, so their standard enthalpy is zero. That makes the enthalpy of this reaction:
2(824.2^kJ/_mol) + 4(0^kJ/_mol) + 3(0^kJ/_mol) = 1648.4kJ (or ^kJ/_mol, or ^kJ/_{mol rxn}, or ^kJ/_rxn, see the discussion of this below...)