The combustion of hydrogen is:
H2(g) + ½ O2(g) → H2O(l)
The standard enthalpy of formation(∆H°f) of H2O(l) is -285.8 kJ/mol. This value can be found in books or reliable internet source. The negative value indicates that the reaction is exothermic.
Converting ∆H°f into kJ per kilogram
Hydrogen and methanol have both been proposed as alternatives to hydrocarbon fuels. To compare the energy of these fuels to that of octane (C8H18), calculate the energy released per kilogram of octane.