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Homework Help: Simple Hess's Law Problem

  1. Apr 6, 2010 #1
    1. The problem statement, all variables and given/known data


    Here's the formula I was given
    CH4 (g) + 2O2 (g) >CO2(g) + 2H20 (l) : -803Kj
    CH4 (g) + 2O2 (g) . CO2 (g) + 2H2O (g) : -891 Kj

    Find enthalpy for vaporization of H2O







    2. Relevant equations



    3. The attempt at a solution
    My soln
    CO2(g) + 2H20 (l)>CH4 (g) + 2O2 (g) > : +803Kj
    CH4 (g) + 2O2 (g)>CO2 (g) + 2H2O (g) : -891 Kj

    Answer 88 Kj

    BUT Book says:

    1/2CO2(g) + 2H20 (l) > 1/2CH4 (g) + 2O2 (g) > : 1/2(+803Kj)
    1/2CH4 (g) + 2O2 (g) > 1/2CO2 (g) + 2H2O (g) : 1/2(-891 Kj)

    Answer 44Kj

    Why would the balanced eqn have to be be balanced by halving CO2's and CH4's? I'm stumped

    Any help would be great!

    Thanks
    Warren
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 6, 2010 #2
    Are you sure that everything wasn't suppose to be halved?
     
  4. Apr 6, 2010 #3
    Nope! That's what the book says!

    Makes no sense to me....but I'm new at this....

    W
     
  5. Apr 6, 2010 #4
     

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    Last edited: Apr 6, 2010
  6. Apr 6, 2010 #5
    Oh!
    So base it on an "empirical" formula and since we balanced the CH4 and all we deal with is H2O, 2H2O>2H2O becomes H2O>H2O

    Would it then follow if say (number totally made up) 10H2O(l)>10H20(g) 1000KJ
    The enthalpy of vaporization be: 100KJ? IE 1000/10? Since the "empirical" formula would be H2O>H2O? Regardless of the other reactants/products?

    I think that's it as I scan the problem! Thanks!
    Warren
     
  7. Apr 7, 2010 #6
    let me tell you something The enthalpy of vaporization, (symbol ΔHvap), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas it is constant for one substance for example water and it is 44 KJ/mol you cant have any number like 1000
     
  8. Apr 7, 2010 #7
    really...so despite any long winded problems 44kj would be the answer for the enthalpy of vaporization for H2O.....Thanks...W
     
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