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.
