Oxidation-Reduction Reactions Many of the chemical reactions that occur around us can be classified as oxidation-reduction reactions (redox). The terms oxidation and reduction go together because you cannot have one occurring without the other taking place at the same time. The terms oxidation and reduction have been around for a long time and have had different meanings throughout this time. The original definitions are listed below along with some examples of each. oxidation - the combination of an element or a compound with oxygen to form oxides. C + O2 ----------------------> CO2 CH4 + 2O2 ------------------------> CO2 + 2H2O reduction - the loss of oxygen from a compound. 2Fe2O3 + 3C -------------------------> 4Fe + 3CO2 The definitions were later extended to include the loss or gain of electrons. The extended definitions are listed below. oxidation - oxidation occurs when a substance loses electrons or gains oxygen reduction - reduction occurs when a substance gains electrons or loses oxygen With the extended definition oxidation cannot occur without reduction. If a substance is gaining electrons, the electrons must be gained from another substance, which is losing electrons. The following reaction illustrates the loss and gain of electrons between two substances in a chemical reaction. Mg + S ------------------------> Mg2+S2- In this reaction magnesium loses its two valence electrons and forms the magnesium ion. Sulfur gains the two electrons lost by the magnesium and completes its valence shell to form the sulfide ion. The opposite charged ions are attracted and form magnesium sulfide. Two electrons are transferred between the two atoms so oxidation and reduction have both occurred. The substance that loses electrons is oxidized and it is also called the reducing agent. The substance that gains electrons is reduced and it is also called the oxidizing agent. Here is another example of a redox reaction. See if you can identify what is being oxidized and reduced and identify the oxidizing agent and the reducing agent. 2K + Cl2 ----------------------> 2KCl Solution: Two potassium (K) atoms are being oxidized because they go from a neutral charge to a +1 charge, therefore each atom is losing 1 electron. The two chlorine atoms in Cl2 are being reduced because they go from a neutral charge to a -1 charge on each atom, therefore each atom is gaining 1 electron. A total of two electrons are lost by the potassium atoms and a total of two electrons are gained by the chlorine atoms. The total number of electrons lost by a substance must equal the total number of electrons gained by a substance or else the reaction is not balanced. The potassium atoms are the reducing agents because they provide the electrons for the chlorine to be reduced and the chlorine atoms are the oxidizing agents because they accept the electrons being lost by the potassium atoms.