# What reactions that release energy?

Aug 9, 2017

$\text{Bond making reactions......., i.e. exothermic reactions}$

#### Explanation:

To a first approximation the.................... $\text{breaking of bonds REQUIRES ENERGY}$, and the $\text{formation of bonds RELEASES ENERGY}$. 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......

${H}_{3} C - C {H}_{2} C {H}_{3} \left(l\right) + {O}_{2} \left(g\right) \rightarrow 2 C {O}_{2} \left(g\right) + 4 {H}_{2} O \left(l\right) + \Delta$

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 $\Delta H$ 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.