Eating, putting gas in a car and throwing a log on a campfire all involve adding energy to a system. In each case, the energy is added in the form of covalent bond*s that hold atoms together in molecules.
Covalent bonds are one of four types of chemical bonds. The other three are ionic bonds, metallic bonds and hydrogen bonds. Each bond type differs in the way atom share electrons. In covalent bonds, two atoms completely share one or more pairs of electrons. These bonds are quite strong.
Covalent bonds form between atoms when the total energy present in the newly formed molecule is lower than the energy present in each of the atoms alone. The lower energy when bonded results from the fact that atoms are more stable when their outer electron shells are full. Atoms can fill their outer shells by sharing electrons with other atoms though the formation of covalent bonds.
There is a symmetrical relationship between the amount of energy released during the formation of a covalent bond the amount of energy needed to break the bond. Note the flow of energy. Breaking covalent bonds requires energy, and covalent bond formation releases energy.
The term used to describe the energy in a system is Gibbs Free Energy. Gibbs Free Energy can be thought of as energy released during bond formation. When released, this energy is free to do other work. This energy is measured as heat using the units joules or calories or kilocalories.
The amount of energy released during molecule formation can be estimated by counting the number and types of bond in a molecule. For example, a methane molecule has one carbon atom bound to four hydrogen atoms via four single carbon-hydrogen covalent bonds. Carbon-hydrogen bonds release 100 kcal/mole of energy when formed, so the total energy needed to break all the bonds in a methane molecule is 100 kcal x 4 or 400 kcal.
n the following illustration, explores the amount of energy associated with covalent bonds in a selection of molecules containing between one and five carbon atoms. You can also test your understanding with covalent bond energy calculation practice problems.