Exothermic reactions are characterized by their negative enthalpy DeltaH.
DeltaH can be calculated using the following expression:
DeltaH = sumnxxD_("bonds broken")-sumnxxD_("bonds formed")
where D_("bonds broken") is the bond energy of broken bonds and D_("bonds formed") is the bond energy of formed bonds.
For the following reaction:
H_2(g) + 1/2O_2(g) -> H_2O(g)
DeltaH = (D_(H-H) +1/2D_(O=O)) - 2D_(O-H)
DeltaH = (1cancel(mol)xx432(kJ)/(cancel(mol))+1/2cancel(mol)xx498(kJ)/(cancel(mol))) -(2cancel(mol)xx467(kJ)/(cancel(mol)))= -253 kJ
The calculated DeltaH=-253kJ is negative and therefore the reaction is exothermic.
In words, we can state that the energy stored in the bonds of H_2 and O_2 molecules is higher than it is in H_2O molecule.
