What reactions that release energy?

1 Answer
Aug 9, 2017

"Bond making reactions......., i.e. exothermic reactions"Bond making reactions......., i.e. exothermic reactions

Explanation:

To a first approximation the.................... "breaking of bonds REQUIRES ENERGY"breaking of bonds REQUIRES ENERGY, and the "formation of bonds RELEASES ENERGY"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.