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Homework Help: Need help determining the molar enthalpy of Magnesium with HCl

  1. Feb 12, 2013 #1
    Picture to show more clearly: http://i.imgur.com/JmARtQM.png

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

    What is the molar enthalpy of combustion ΔH(c) of magnesium?

    Mg(s) + 2HC(aq) → H2(g) + MgCl2(aq)

    MgO(s) + 2HCl(aq) → H2O(l) + MgCl2(aq)

    m(Mg) = 0.5 g
    m(MgO) = 1.0 g
    V(HCl) = 100.0 mL
    C(HCl) = 1.0 mol/L
    T(HCl) = 19° C
    T(rxn1) = 38° C
    T(rxn2) = 25° C

    rxn1 relates to Mg(s) + 2HCl(aq)

    rxn2 relates to MgO(s) + 2HCl(aq)

    2. Relevant equations

    n(Mg) = m/M
    q = mcΔT
    ΔH(C) = -q/n

    3. The attempt at a solution

    n(Mg) = m/M
    n(Mg) = (0.5 g)/(24.31 g/mol)
    n(Mg) = 0.02056766762649115590292060880296 mol
    n(Mg) = 0.021 mol

    q = mcΔT
    q = (3.6 g)(4.18 J/g °C)(38° C – 19° C)
    q = 285.912 J
    q = 285.9 J

    ΔHC = -q/n
    ΔHC = -285.9 J/0.021 mol
    ΔHC = 13614.285714285714285714285714286
    ΔHC = 13.6 kJ/mol


    If anyone can help me with this it would be greatly appreciated!
  2. jcsd
  3. Feb 13, 2013 #2


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    Staff: Mentor

    Wouldn't hurt if you will explain what you are doing and what the data is about.

    Do you know the Hess law?
  4. Feb 13, 2013 #3
    My class touched on Hess's Law for a bit but not enough to make much of an impact on me. I would edited the post above, but it shows no edit button to use. Basically, I am finding the moles of magnesium for molar enthalpy and the heat quality of the HCl for the other part of molar enthalpy and plug values into the molar enthalpy equation.

    Forgot to mention, the data above is experimental
    Last edited: Feb 13, 2013
  5. Feb 13, 2013 #4


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    Staff: Mentor

    This is a Hess law problem, so you can't solve it without understanding the Hess law.
  6. Feb 14, 2013 #5
    all right, I think I've got it after checking with Hess's Law and doing a full solution.

    this is what I have come up with: http://imgur.com/a/Cs95f (sorry, I am lazy)

    overall, I got a value of -563.7 kJ/mol.
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