Do Chemical Reactions Involve Energy?

Molecules house energy in the bonds between atoms. So, when a chemical reaction takes place and the molecules are broken at their bonds and bonds are formed for the new (product) molecules, energy has to be involved. Generally speaking there are two types of chemical reactions— those that release energy (energy releasing) and those that require the input of energy (energy demanding). If a chemical reaction is said to be energy releasing, it means that more energy will be released in the disruption of the bonds of the reacting molecule than is needed to form the new bonds in the product molecule(s), as shown in Figure 1.5.
 

Said differently, if the energy within the bonds of the products is less than the energy associated with the initial energy in the bonds of the reactants, then the reaction can proceed without a need for an input of outside energy. In this situation, there is leftover energy. On the other hand, if the energy that is required to form the bonds of a new molecule(s) is greater than the energy that will be released by disrupting the reacting molecule(s), then an outside energy source will be needed. This is often the case when complex molecules are being built in our body. To do so, the energy released from energy-releasing reactions is used to “drive” the energy-demanding reactions.
 

Beyond those chemical reactions that either release or require appreciable amounts of energy, there are many chemical reactions that take place without a release or demand for energy. Here the energy associated with the bonds of the reactants and products of chemical reactions is the same. These would be the reversible reactions we discussed earlier, where one enzyme catalyzes the reaction in both directions.