A bonfire. Combustion consists of redox reactions involving free radicals.

In the combustion reaction, C-H and O=O bonds are being broken while stronger, polar bonds in carbon dioxide and water are being formed. Oxidation-reduction is an accounting system that systematizes the changes that occur when electronegative elements gain 'electron control' through a chemical reaction.

The topic of oxidation-reduction is another central domain of chemistry (and central MCAT topic) in which electronegativity is a crucial underlying concept. In fact, it is the manner in which electronegativity differences play out in chemical reactions that provides the underlying coherence of the oxidation-reduction system of chemistry. This is a somewhat complicated idea which we are going to be approaching from a number of directions as this MCAT course progresses. Let us summarize the basic gist here.

Oxidation-reduction is a systematic accounting procedure to reflect the changes in the bonding environment of electrons between products and reagents. Electronegativity reflects the strength of attraction an atom has for the electrons it shares in chemical bonds. When two atoms form a covalent bond, the more electronegative atom is assigned 'electron control' in the oxidation-reduction system. If an atom gains electron control through a chemical process, it is said to be 'reduced, ' while the atom that has lost electron control is said to be 'oxidized'.

The key to the system is that when a strongly electronegative atom is reduced, it draws the new electrons inwards towards its strongly attracting nucleus, and the bond becomes polarized. This closing of separation between like charges represents a potential energy decrease above and beyond the typical energy decrease that accompanies the formation of an ordinary bonding, molecular orbital.

In other words, the formation of a polar bond corresponds to a large potential energy decrease i.e. negative internal energy change and also likely negative enthalpy change and negative free energy change. As a general rule, polar bonds are stronger than nonpolar bonds (more energy is required to break them because the electrons have to be wrenched away from the oxidant).

Oxidation-reduction provides a systematic way to account for the tendency of polar bonds to be strong, low energy bonds. For this reason, the graph of reduction potentials of the various elements and their electronegativities are very similar.

Oxidation-reduction gives a systematic way to solve problems, but remember what the meaning is below the surface. In general, the chemical system decreases in energy when electronegative elements gain electron control through a chemical reaction. In the case of redox reactions where covalent bonds are being broken and formed, the plot is driven by electronegativity. Electronegative elements form stronger covalent bonds.