Do Chemical Reactions Involve Energy?

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 disrup-
tion 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 appre-
ciable 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.