Oxidation-Reduction Reactions

Certain processes that are vital to life, including photosynthesis, combustion, and corrosion, fall into the class of reactions called oxidation–reduction, or redox, reactions.

Redox reactions consist of two simultaneous processes: oxidation and reduction.

The term oxidation means an increase in oxidation number, which corresponds to the loss of electrons, while reduction means a decrease in oxidation number, which corresponds to the gain of electrons. To remember the role of electrons, use the acronym OIL RIG, which stands for: “oxidation is losing, reduction is gaining.”

Oxidation and reduction are complementary processes. In a redox reaction between two reactants, one reactant loses electrons and is oxidized, while the other reactant gains electrons and is reduced.

Consider the oxidation–reduction reaction between potassium — an alkali metal — and chlorine, a nonmetal.

The neutral potassium atom loses an electron to become a potassium ion. Potassium is oxidized, and its charge increases from zero in the neutral atom to one-plus in the cation.

The neutral chlorine atom gains an electron and becomes a chloride ion. Chlorine is reduced, and its charge decreases from zero in the neutral atom to one-minus in the anion.

Since potassium donates an electron, it is the reducing agent, or reductant. Chlorine accepts the electron, so it is the oxidizing agent, or an oxidant. The redox process leads to the formation of potassium chloride.

In general, in redox reactions between alkali or alkaline earth metals and nonmetals, the metal is oxidized and the nonmetal is reduced to form an ionic compound through complete electron transfer. This is often true for reactions between other metals or metalloids and nonmetals as well, but not always.

Another example of a redox process is the formation of gaseous hydrogen chloride. Here, both reactants — hydrogen and chlorine — are nonmetals, so there is no complete transfer of electrons. Instead, hydrogen shares an electron with chlorine in a partial, or formal, electron transfer.

Thus, in the formation of hydrogen chloride, hydrogen is oxidized and acquires a partial positive charge, while chlorine is reduced and acquires a partial negative charge. Since both oxidation and reduction processes occur, this is a redox reaction.

In general, redox reactions between nonmetals involve partial electron transfer between elements to form a covalent compound.