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Homework Help: Heat of Formation and Heat of Reaction Difference

  1. Oct 15, 2011 #1
    1. The problem statement, all variables and given/known data
    Take for example the problem given below:

    C + O2 → CO2 ΔH°f = -393.5 kJ/mol
    2CO + O2 → 2CO2 ΔH°rxn = -566.0 kJ/mol

    To determine the ΔH°f of CO from the reaction of C graphite and O2 given those equations, one would solve it as: -393.5 kJ/mol + 1/2(566.0)= -110.5 kJ/mol

    I realize that ΔH°rxn must be flipped since the reaction must be flipped.

    However, why is it expressed as ΔH°rxn rather than ΔH°f? Is there a difference between the two?
  2. jcsd
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