Chapter U5.11, Problem 2E

Interpretation Introduction

Interpretation: The exothermic nature of combustion reactions needs to be explained using the energies involved in bond breaking and bond making.

Concept introduction: In order to estimate the net energy of a chemical energy, we have to consider that a chemical reaction takes place in two parts which are energy needed in bond breaking and energy needed in bond making.

Answer to Problem 2E

Sum of bond energies are used to determine the net energy of a reaction. The negative value of the energy of a reaction determines that the reaction is an exothermic reaction.

Explanation of Solution

In a chemical reaction the energies used in bond making and bond breaking can be used to explain why the combustion reaction are exothermic.

In the combustion of methane,

  ${\text{CH}}_{\text{4}}\text{+}{\text{2O}}_{\text{2}}\to {\text{CO}}_{\text{2}}\text{+}{\text{2H}}_{\text{2}}\text{O}$

The bond breaking total energy is 2642 kJ and the total net bond making energy is -3466 kJ.

Therefore, the total or the net energy can be calculated as,

  $\begin{array}{c}\text{Net}\text{energy}\text{exchange}\text{=}\left(\text{2642}\text{kJ}\right)\text{+}\left(-\text{3466}\text{kJ}\right)\\ =-824\text{kJ}\end{array}$

In the above reaction, the magnitude of energy out is greater than the magnitude of energy in. Therefore, this reaction is an exothermic reaction. By exothermic we mean that the system loses energy to the surrounding.

Conclusion

An exothermic reaction is identified by the lose of energy to the surrounding in a chemical reaction. A negative sign is used in the net energy value in an exothermic reaction.