H₂ + ½ O₂ → H₂O + 285.8 kJ

The 285.8 kJ is the enthalpy of formation for liquid water: the energy released when one mole of liquid water forms from its elements.

The fact that the value is on the products side of the reaction shows this is an exothermic process. The value can also be written separately (as in a table). By convention, a negative sign is then applied to show that there is a net loss of enthalpy in the system as the reactants become products. That means heat is released to the surroundings. So we can say ΔH_f^o = -285.8 kJ/mol of water. All kinds of enthalpies of formation have been tabulated.