Nitrogen gas when combined with hydrogen gas forms ammonia gas according to the following equation N2(g)+3H2(g)->2NH3(g) deltaHrxn=-56 kJ.
The deltaHrxn was determined using Hess's Law and the following three related reactions.
2NH3(g)+4H2O(l)->NO2(g)+7H2(g) deltaH=-68 kJ
2NO2->N2(g)+2O2(g) deltaH=??
H2O(l)->H2(g)+(1/2)O2(g) delta H=-21 kJ