posted by princess cunningham .
Use Hess's law and the following equations to calculate ÄH for the reaction N2(g) + 2O2(g) N2O4(g).
2NO2(g)->N2O4(g) ÄH = –57.0 kJ/mol
N2(g)+O2(g)->2NO(g) ÄH = 180.6 kJ/mol
2NO2(g)->2NO(g)+O2(gÄH = 114.4 kJ/mol
I didn't do this with a pencil and pad but try this
Add equn 1 to equan 2 to the reverse of equan 3 and see if that won't give you the equation you want. For the dH values, change the sign of those you reverse.