Ammonia (NH3 (g), Delta. Hf = –45.9 kJ/mol ) reacts with oxygen to produce nitrogen and water (H2 O(g), Delta. Hf = –241.8 kJ/mol) according to the equation below. 4 upper N upper H subscript 3 (g) plus 3 upper O subscript 2 (g) right arrow 2 upper N subscript 2 (g) plus 6 upper H subscript 2 upper O (g). What is the enthalpy of the reaction? Use Delta H r x n equals the sum of delta H f of all the products minus the sum of delta H f of all the reactants..