Thermodynamics-Heat of Reaction?

Question

Find the heat of formation for a mole of hydrazine (N2H4) given:


2N2H4(l) + 2O2(g) → 2N2(g) + 4H2O(g) D Ho = -1068.4 kJ
H2(g) + 1/2O2(g) → H2O(g) D Ho = -241.8 kJ

Answer 1

The first equation can't be right, because it is divisible by 2. The correct equation would be:

N2H4(l) + O2(g) ===> N2(g) + 2H2O(g) DHo = -? kJ

Then you write:

N2(g) + 2H2O(g) ===> N2H4(l) + O2(g) DHo = +? kJ

2H2(g) + O2(g) ===> 2H2O(g) DHo = -483.6 kJ

You add them together, the O2's and the H2O's cancel out, and you have:

N2(g) + 2H2(g) ===> N2H4(l) DHo = ?kJ - 483kJ =