Redox reactions or oxidation reduction reactions have a number of similarities to acid base reactions Fundamentally redox reactions are a family of reactions that are concerned with the transfer of electrons between species Like acid base reactions redox reactions are a matched set you don t have an oxidation reaction without a reduction reaction happening at the same time Redox reactions primarily involve the transfer of electrons between two chemical species The compound that loses an electron is said to be oxidized the one that gains an electron is said to be reduced Each reaction by itself is called a half reaction simply because we need two 2 half reactions to form a whole reaction In notating redox reactions chemists typically write out the half reaction as Cu s Cu2 2 eThis half reaction says that we have solid copper with no charge being oxidized losing electrons to form a copper ion with a plus 2 charge Note that we have a balance between both sides of the reaction We have one 1 copper atom on both sides and the charges balance as well The symbol e represents a free electron with a negative charge that can now go out and reduce some other species such as in the other halfreaction 2 Ag aq 2 e 2 Ag s Here a silver ion silver with a positive charge is being reduced through the addition of two 2 electrons to form solid silver The abbreviations aq and s mean aqueous or solid respectively We can now combine the two 2 half reactions to form a redox equation Cu s Cu2 2 e2 Ag aq 2 e 2 Ag s Cu s 2 Ag aq 2 e 2 Ag s Cu2 2 eor Cu s 2 Ag aq 2 Ag s Cu2 A compound that is oxidized is refered to as a reducing agent while a compound that is reduced is referred to as the oxidizing agent In another way If a chemical causes another substance to be oxidized we call it the oxidizing agent In the equation above Ag is the oxidizing agent because it causes Cu s to lose electrons Oxidants get reduced in the process by a reducing agent Cu s is naturally the reducing agent in this case as it causes Ag to gain electrons Balancing Redox Reactions HALFREACTION METHOD Here are the steps to follow to balance a redox equation 1 Divide the equation into an oxidation halfreaction and a reduction half reaction 2 For each half reaction Balance all elements except H and O Balance the O by adding H2O Balance the H by adding H Balance the charge by adding electrons e 3 Multiply each half reaction by an integer such that the number of e lost in one equals the number gained in the other 4 Combine the half reactions and cancel identical species In considering redox reactions you must have some sense of the oxidation number ON of the compound An increase in oxidation number corresponds to oxidation A decrease in oxidation number corresponds to reduction Oxidation Number Assignments The determination of the oxidation number or oxidation state of chemical compounds can be made by following a few simple rules 1 The oxidation numbers of an atom or the atoms in a neutral molecule must add up to zero 2 If an atom or molecule is ionic it s oxidation number must add up to its overall charge 3 Alkali metal atoms Group I have an oxidation number equal to 1 within compounds Alkali earth atoms Group II have an oxidation number of 2 within compounds 4 All halogens besides fluorine have a 1 oxidation number in compounds 5 For hydrogen the oxidation number is 1 but in some cases it is 1 6 For oxygen the oxidation number is usually 2 In peroxide compounds the oxidation number of oxygen is 1 What is the oxidation number of sulfur in sulfur dioxide SO2 Given that each oxygen atom has a 2 charge and knowing that the molecule is neutral the oxidation number for sulfur must be 4 What about for a sulfate ion SO4 with a total charge of 2 Again the charge of all the oxygen atoms is 4 x 2 8 Sulfur must then have an oxidation number of 6 since 6 8 2 the total charge on the ion Since the sulfur in sulfate has a higher oxidation number than in sulfur dioxide it is said to be more highly oxidized Consider this example MnO4 aq 2 3 2 Fe aq Fe aq Mn aq What are the half reactions MnO4 Mn 2 3 2 Fe Fe What s the oxidation state of Mn in MnO4 Let s balance the reduction half reaction 2 MnO4 Mn 4H2O oxygens now in balance 2 MnO4 8H Mn 4H2O hydrogens now in balance Balance the charge MnO4 2 8H 5e Mn 4H2O Balanced Oxidation Reaction 2 3 Fe Fe 1e Multiply the oxidation rxn by 5 and add half reactions together Final answer 2 3 2 5Fe MnO4 8H 5Fe Mn 4H2O The chemical which gains electrons is reduced reduces its valency and is called the oxidizing agent The chemical which loses electrons is oxidized increases its valency and is called the reducing agent OIL Oxidation Is Loss of electrons RIG Reduction Is Gain of electrons Fe Cu2 Fe2 Oxidized Reduced Reducing Oxidizing Agent Agent Cu The chemical which is oxidized is the reducing agent The chemical which is reduced is the oxidizing agent In General When there is oxidation there is also reduction The substance which loses electrons is oxidised The substance which gains electrons is reduced
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