[SolidChymist]

Hello,

I'm having a few problems with the following questions:

1. At 1077 K, ΔH°Rxn = -120.8 kJ/mol for the reaction 2H2(g) + O2(g) → 2H2O(g) with CP,m = 3.469R, 3.534R and 4.040R for H2(g), O2(g) and H2O(g), respectively. Assume the heat capacities are independent of temperature. Calculate ΔHf (in kJ/mol) for H2O(g) at 24.1 °C.
ΔH°f (in kJ/mol) for H2O =

2.
2.



A sample of Mg(s) of mass 2.438 g undergoes combustion in a constant volume calorimeter. The calorimeter constant is 1857 J K–1 and the measured temperature rise in the inner water bath containing 1301 g of water is 3.01 K. Calculate ΔU°f (in kJ/mol) and ΔH°f (in kJ/mol) for MgO(s). Round your answers to 2 places past the decimal.
ΔU°f (in kJ/mol) =

ΔH°f (in kJ/mol) =

3.   3.
Calculate ΔH°Rxn (in kJ/mol) at 796 K for the reaction CH3OH(g) + (3/2)O2(g) → 2H2O(g) + CO2(g) using the heats of formation and the temperature dependence of the heat capacities from the data tables in your book.
ΔH°Rxn (in kJ/mol) =

I know what the answers should be, but I don't know how to get there. For the second problem I tried:

-Ms/ms((mH2O/MH2O)*CH2O,m*change in temperature+Ccalorimeter*change in temperature, but I got nowhere near the value of -218.90 kj.

For the 3rd I tried to add the equations for the molar heat capacities of products and reactants and then integrating from 298.25 to 796 kelvins but I still did not get the value -667 kj

Any help in the right direction is appreciated.