calculate the ΔHrxn given three reactions

Question

calculate the ΔHrxn to the correct number of significant figures for N2O (g) + NO2 (g) -> 3NO (g) given the following data:

N2(g) + O2(g) -> 2NO(g) ΔH= +180.7 kJ

2NO(g) + O2(g) -> 2NO2(g) ΔH= -113.1 kJ

2N2O(g) -> 2N2(g) + O2(g) ΔH= -163.2 kJ

Answer 1

This is a problem involving Hess Law.

N2O(g) + NO2(g) ===> 3NO(g) ... Target equation

Eq #1: N2(g) + O2(g) ===> 2NO(g) ∆H = +180.7 kJ

Eq #2: 2NO(g) + O2(g) ===> 2NO2(g) ∆H = -113.1 kJ 

Eq #3: 2N2O(g) ===> 2N2(g) + O2(g) ∆H = -163.2 kJ 

Then copy eq #1 as is, reverse eq#2 and divide it by 2 and then divide eq#3 by 2 to obtain...

N2(g) + O2(g) ===> 2NO(g) ∆H = +180.7 kJ

NO2(g) ===> NO(g) + 1/2O2(g) ∆H = +56.55 kJ

N2O(g) ===> N2(g) + 1/2O2(g) ∆H = -81.6 kJ

Add them up and cancel like items on opposite sides of the equation....

NO2(g) + N2O(g) ===> 3NO(g) ∆H = 155.7 kJ (to 4 significant figures)