• AMA with Certified Student Loan Professional

    Join SDN on December 7th at 6:00 PM Eastern as we host Andrew Paulson of StudentLoanAdvice.com for an AMA webinar. He'll be answering your questions about how to best manage your student loans. Register now!

Hess's Law question

jh311

Full Member
10+ Year Member
Jan 20, 2009
37
0
utah
  1. Pre-Dental
    Using Hess's Law, what is the ▲H rxn for:

    2C(s) + O2(g) >>>>> 2CO(g)

    C(s) + O2 >>>>> CO2(g) ▲H= -393.7KJ/mol
    CO(g) + ½O2 >>> CO2(g) ▲H= -283.3KJ/mol

    When I did this problem I got an answer that was way off apparently. By using ▲H = Σ▲H(products) - Σ▲H(rxtnts) Shouldn't you write(?):
    -((2(-283)) - 2(-393)? This would give me +566 -(-786) = +1352KJ. I was really confused to see that the answer was -220KJ and the way that it was set up seemed opposite to me. The answer said 2(-393KJ)-(-2(283KJ)) = -220KJ. To me it looks like the ▲H = reactants - products. Anyways, for this particular problem there's something i'm not seeing... please help!! Thanks
     

    doc1986

    Full Member
    10+ Year Member
    Mar 2, 2009
    134
    0
    1. Pre-Dental
      First of all, Hess's Law states that enthalpies of reactions are additive. Therefore, after you manipulate the reactions to look like the reaction that you want, you add them, not subtract.

      So, the first reaction should be multiplied by 2 and the second should be flipped (multiply by -1) and multiplied by 2. When you do this to both equations you will see that the 2CO2's will cancel out and 2O2 and O2 will cancel to become 1O2 on reactant side. You will also still have 2C on the reactant side to give you a net reaction of:
      2C + O2 --> 2CO which is what you want.

      So, now adding what you manipulated you get:
      (2(-393.7)) + (2(283.3)) = -220.8 kJ

      Hope this helps.
       
      This thread is more than 12 years old.

      Your message may be considered spam for the following reasons:

      1. Your new thread title is very short, and likely is unhelpful.
      2. Your reply is very short and likely does not add anything to the thread.
      3. Your reply is very long and likely does not add anything to the thread.
      4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
      5. Your message is mostly quotes or spoilers.
      6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
      7. This thread is locked.