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Kirchhoff's Law (thermodynamics), change in heat capacity

  1. Nov 3, 2009 #1
    1. The problem statement, all variables and given/known data
    Calculate the standard enthalpy change ([tex]\Delta[/tex]H) at 25 degrees Celsius and 927 degrees Cesius for the reaction,
    WCl4(g) + CH4(g) = WC(s) + 4HCl(g)

    Data:

    WCl4(g): [tex]\Delta[/tex]H298 = -336 kJ/mole; Cp (heat capacity at constant pressure) = 105.6 J/mol*K
    HCl(g): [tex]\Delta[/tex]H298 = -92.3 kJ/mole; Cp = 30.5 J/mol*K
    CH4(g): [tex]\Delta[/tex]H298 = -74.8 kJ/mole; Cp = 59.1 J/mol*K
    WC(s): [tex]\Delta[/tex]H298 = -40.2 kJ/mole; Cp = 46.5 J/mole*K



    2. Relevant equations
    [tex]\Delta[/tex]H(final temp) = [tex]\Delta[/tex]H(initial temp) + [tex]\Delta[/tex]Cp(Tf - Ti)
    [tex]\Delta[/tex]Cp = [tex]\Sigma[/tex]Cp(products) - [tex]\Sigma[/tex]Cp(reactants)


    3. The attempt at a solution
    This is a take-home test, so it doesn't feel right getting help with the actual answer, but I have some specific questions I hope can be clarified for me:

    1.) When calculating [tex]\Delta[/tex]Cp, do you multiply each component by the number of moles involved? For instance, for the HCl factor in [tex]\Sigma[/tex]Cp, do you multiply Cp for HCl by 4? It seems like you should, but Cp stays in the same units, so the moles wouldn't cancel.

    2.) Can I apply the equation for [tex]\Delta[/tex]H (of reaction at 927 degrees Celsius) as I have written above (as in, is it applicable as is, or do I need to do further analysis of this specific situation as Kirchhoff's Law applies?

    Thanks!
     
  2. jcsd
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