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Homework Help: Using Hess's Law to calculate ΔH°rxn

  1. Oct 10, 2013 #1
    1. The problem statement, all variables and given/known data
    Calculate ΔH°rxn in kJ/mol for:

    C3H8(g) + 4 CO2(g) + 4 H2(g) → C7H16(g) + 4 O2(g)

    from the following given values of ΔH°rxn:

    C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) ΔH°rxn= -2,010 kJ/mol

    C7H16(g) + 11 O2(g) → 7 CO2(g) + 8 H2O(g) ΔH°rxn= -4,426 kJ/mol

    H2(g) + ½ O2(g) → H2O(g) ΔH°rxn= -241.8 kJ/mol

    2. Relevant equations


    3. The attempt at a solution

    I swapped the middle to give +4,426 kJ/mol. Then I tried multiplying the middle by 1/2 and the third one by 4. Am I on the right track? Anything else need to be swapped? I feel like the next step should be obvious; I'm just stuck.
  2. jcsd
  3. Oct 10, 2013 #2
    You almost have it. Just don't multiply the middle one by 1/2.
  4. Oct 11, 2013 #3
    Ahh, okay thanks! I solved it by swapping the middle and just multiplying the bottom by 4. I figured out what stumped me; it was not knowing how to get the unwanted compounds in the reactions to cancel out. I get it now though.
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