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Homework Help: Hess's Law and Thermochemical Equations

  1. Dec 5, 2007 #1
    [SOLVED] Hess's Law and Thermochemical Equations

    1. The problem statement, all variables and given/known data
    Question: 5.63:
    The standard heat of formation of gaseous hydrogen bromine was first evaluated by means of the standard enthalpy values measured for the following reactions. The last three are standard heats of solution.

    Cl2(g) + 2KBr(aq) --> Br2(aq) + KCl(aq) Delta H = -96.2kJ
    H2(g) + Cl2(g) --> 2KCl (g) Delta H = -184kJ
    HCl(aq) + KOH(aq) --> KCl(aq) + H2O Delta H = -57.3kJ
    HBr(aq) + KOH(aq) --> KBr(aq) + H2O Delta H = -57.3kJ
    HCl(g) --> HCl(aq) Delta H = -77.0kJ
    Br2(g) --> Br2(aq) Delta H = -4.2kJ
    HBr(g) --> HBr(aq) Delta H = -79.9kJ

    Write the thermochemical equation for the formation of 1 mol of HBr(g) from its elements, including its value for Delta H final.

    2. Relevant equations
    (Included in question)

    3. The attempt at a solution
    I assumed you would have to isolate 1/2H2 and 1/2Br2 to make the final equation of 1/2H2 + 1/2Br2 --> HBr(g). No matter which way I work the equations I end up without H2 or Br2 depending on my method. However, I am wondering if I was on the right track. Is that what I am looking for?

    EDIT: Realized my mistake. Disregarded the states of compounds. SOLVED
    Last edited: Dec 5, 2007
  2. jcsd
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