Formation of water is as follows:
H2 (g)+ ½ O2(g)⟶H2O(g)
Bond energy or enthalpy is the amount of heat required to break a bond. Same amount of energy is released when that bond is formed.
We can calculate the total enthalpy changes by using bond enthalpies of reactants and products.
The bond energy of each bond is:
H-H = 433
O=O = 492
∆H= H(Bonds Broken) - H(Bonds Formed)
=(433+ ½ ×492)−(2×464)
This ∆H represents the heat changes in the above mentioned chemical reaction which shows the formation of water from its constituent atoms. Hence, the enthalpy of formation of water is -249 kJ.
The negative sign indicates that it is an exothermic reaction. An exothermic reaction is when energy is released from the system into the surroundings because the total energy level of reactants is higher than the products formed.