Calculating Enthalpy Changes in Chemical Reactions: Chemistry Homework Help

[lutetia]

Chemistry HW PLEASE HELP ME!

Hello, I have chemistry homework and I have to finish but I'm having a really hard time solving one problem. Anyone, could you please help me?
-----------------------------------------------------------------
Question. # 24

H2(g) + 1/2 O2(g) ----- H2O(l) H = -285.8 kJ -----(1)

N2O5(g) + H2O(l) ----- 2 HNO3(l) H = -76.6 kJ -----(2)

1/2 N2(g) + 3/2 O2(g) + 1/2 H2(g) ----- HNO3(l) H= -174.1 kJ ----(3)

calculate the enthalpy change for the fallowing reaction

2 N2(g) + 5 O2(g) ------ 2 N2O5(g)
-------------------------------------------------------------------------

 

[semc]

hmm...for a start, try drawing the born-haber cycle

 

[Stevedye56]

Well you have to manipulate those equations so that they cancel out and give you the bottom equation. Then just add up the Delta H.