Hess's Law.?

Question

Given the following data:


2O3 → 3O2 DH = −427 kJ


O2 → 2O DH = +495 kJ


NO + O3 → NO2 + O2 DH = −199 kJ


Calculate DH for the reaction


NO + O → NO2

I know that I have to manipulate the equation but I am getting some off answers. Can anyone point me in the right direction?

Someone answered me saying that Oxygen is not a monotomic element and yes i know that -- thats why the reaction is + it requires work. I need more direction than that.

Answer 1

2O3 → 3O2 DH = −427 kJ (i)


O2 → 2O DH = +495 kJ (ii)


NO + O3 → NO2 + O2 DH = −199 kJ (iii)

Last reaction is (iii) - 0.5(i + ii)

Which gives

del H = -199 -0.5(-427+495) = -173 kJ