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Hess's Law!!!!!!


Full Member
10+ Year Member
7+ Year Member
Jan 8, 2007
  1. Pre-Dental
    Using Hess's law what is the Hrxn for 2C(s) + O2(g) -> 2CO(g):

    C(s) + O2(g) -> CO2(g); H=-393.7 KJ/mol
    CO(g) + 1/2 O2(g) -> CO2(g); H= -283.3 KJ/mol

    answer is -220.8 KJ
    WHY?!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!:eek: :eek: :eek:
    Please Help!


    Full Member
    10+ Year Member
    Dec 1, 2006
    1. Pre-Dental
      Thank you, Ex_EE!
      I'm just wondering,
      isn't it Hrxn= Hpro-Hrea?
      Why do you add those two?

      Well if you take a look, both the products and the reactants are negative. So basically you are doing [2(-prod) - 2(-reactact)] and if you simplify the minus with the negative reactant, it becomes a plus, hence :
      it is (-product) + (reactact). Hope this helps!
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