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

1. Nov 3, 2009

### pillanoid

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

Data:

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

2. Relevant equations
$$\Delta$$H(final temp) = $$\Delta$$H(initial temp) + $$\Delta$$Cp(Tf - Ti)
$$\Delta$$Cp = $$\Sigma$$Cp(products) - $$\Sigma$$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 $$\Delta$$Cp, do you multiply each component by the number of moles involved? For instance, for the HCl factor in $$\Sigma$$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 $$\Delta$$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!