http://www.ck12.org Chapter 22. Oxidation-Reduction Reactions
Oxygen’s Role in Reactions
Many elements simply combine with oxygen to form the oxide of that element. Heating magnesium in air allows it
to combine with oxygen to form magnesium oxide.
2 Mg(s) + O 2 (g)→2MgO(s)Many compounds react with oxygen as well, often in very exothermic processes that are generally referred to as
combustion reactions. For example, when methane burns, carbon dioxide and water are produced.
CH 4 (g) + 2O 2 (g)→CO 2 (g) + 2H 2 O(g)Carbon dioxide is an oxide of carbon, while water is an oxide of hydrogen. Early scientists viewed oxidation as a
process in which a substance was reacted with oxygen to produce one or more oxides. In the previous examples,
magnesium and methane are being oxidized.
Oxidation does not necessarily require heating. Iron that is exposed to air and water slowly oxidizes in a process
commonly known as rusting. Bleaches contain various compounds such as sodium hypochlorite (NaClO) that can
oxidize stains by the transfer of oxygen atoms, making the molecules in the stains more water-soluble and therefore
easier to rinse off. Hydrogen peroxide (H 2 O 2 ) releases oxygen as it spontaneously decomposes. It also acts as a
bleach and is used as an antiseptic that kills bacteria by oxidizing them.
The opposite of oxidation is called reduction. Since oxidation was originally defined as the addition of oxygen,
reduction was therefore the removal of oxygen from a substance. Many naturally occurring metal ores are present
as oxides. The pure metals can be extracted by reduction. For example, pure iron is obtained from iron(III) oxide by
reacting it with carbon at high temperatures.
2Fe 2 O 3 (s) + 3C(s)→4Fe(s) + 3CO 2 (g)The removal of oxygen from the Fe 2 O 3 means that it is being reduced to Fe. Note that an oxidation process is also
occurring simultaneously in this reaction; the carbon reactant is being oxidized to CO 2. This is an important concept.
Oxidation and reduction must happen together. Neither can happen alone in a reaction.
The Electron in Redox Reactions
The definitions of oxidation and reduction were eventually broadened to include similar types of reactions that do
not necessarily involve oxygen. Oxygen is more electronegative than any element except for fluorine. Therefore,
when oxygen is bonded to any element other than fluorine, electrons from the other atom are shifted away from that
atom and toward the oxygen atom. Anoxidation-reduction reaction(sometimes abbreviated as aredox reaction)
is a reaction that involves the full or partial transfer of electrons from one reactant to another.Oxidationinvolves
a full or partial loss of electrons, whilereductioninvolves a full or partial gain of electrons.
An easy saying to remember the definitions of oxidation and reduction is “LEO the lion says GER” (Figure22.1)!
LEO stands for Losing Electrons is Oxidation, while GER stands for Gaining Electrons is Reduction.
Redox Reactions and Ionic Compounds
In the course of a chemical reaction between a metal and a nonmetal, electrons are transferred from the metal atoms
to the nonmetal atoms. For example, when zinc metal is heated in the presence of sulfur, the compound zinc sulfide
is produced (Figurebelow). Two valence electrons from each zinc atom are transferred to each sulfur atom.