To a first approximation the.................... #"breaking of bonds REQUIRES ENERGY"#, and the #"formation of bonds RELEASES ENERGY"#. As always, since energy is conserved, the energy released upon formation of a given bond is equal to the energy required to break that bond..........
When a hydrocarbon is combusted, for instance, propane, we can represent the combustion by the reaction......
#H_3C-CH_2CH_3(l) +O_2(g) rarr 2CO_2(g) + 4H_2O(l) +Delta#
Now the combustion reaction has to break the strong #C-H#, #C-C#, and #O=O# bonds present in the reactants, which certainly requires energy input..... But the reaction forms even stronger #C=O# and #O-H# bonds; and the collected difference in bond strengths is to a very good approx. the #DeltaH# of the reaction, which is experimentally measured.
And in fact we can even take standard tables of bond enthalpies, which lists the strengths of #C-C#, #C-O#, #C=O# etc. bonds, and with appropriate summation we can use these tables to give a very good estimate of the experimental value of the enthalpy of the reaction. And I think this is an excellent reason for studying the topic.
