How to use Hess's Law to calculate enthalpy for the reaction?

Use Hess's law to calculate ΔH for the reaction below. N2(g) + 2 O2(g) 2 NO2(g)?

• Consider the following data. N2(g) + O2(g) 2 NO(g) ΔH = +180.7 kJ 2 NO(g) + O2(g) 2 NO2(g) ΔH = -113.1 kJ 2 N2O(g) 2 N2(g) + O2(g) ΔH = -163.2 kJ Use Hess's law to calculate ΔH for the reaction below. N2(g) + 2 O2(g) 2 NO2(g) ΔH = ?kJ

• Answer:

  You only need to use the first two equations. NO is a catalyst because it is a product in the first and a reactant in the second. Canceling it out, you get the reaction you want. All you have to do now is add the delta H for the first to reactions together. delta H = 180.7 - 113.1 = 67.6 kJ