1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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)


    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?

  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted