From first principles, we know that #"bond formation"# releases energy, and #"bond-breaking"# requires the input of energy. And thus to a first approximation, bond-making is exothermic and bond-breaking is ENDOTHERMIC. The balance between bond-making, and a bond-breaking determines whether a given reaction is exothermic, energy neutral, or endothermic.
Now take a simple combustion reaction, that of methane, which combustion heats our homes, and cooks our dinners, we can write the balanced chemical equation.........
#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(g) + Delta#
And during this reaction, we manifestly have to break STRONG #C-H#, and #O=O# bonds, requiring an input of energy. But we make (stronger) #C=O#, and #H-O# bonds, and the energy balance is on the side of the bonds made. And thus the reaction as written is exothermic. Functional groups along the carbon chain could be oxidized to their simple oxides (or reduced as follows).......but a priori we don't know......
#2CH_3NO_2(l) + 3/2O_2(g) rarr 2CO_2(g) + 3H_2O(l) + N_2(g)+Delta#
