Classical Ideas of Redox Reactions

Classically, oxidation was defined narrowly as combination with oxygen. Carbon burning, C(s) + O2(g) → CO2(g), and magnesium burning, 2Mg + O2 → 2MgO(s), are both oxidations by this definition, since both carbon and magnesium react directly with oxygen. The reverse process, removal of oxygen, is a reduction: in 2Fe2O3 + 3C(s) → 4Fe(s) + 3CO2(g), the iron(III) oxide loses its oxygen to become metallic iron, so this is a reduction (with carbon acting as the reducing agent, itself being oxidised to CO2). Oxidation can equally be seen as removal of hydrogen: in 2H2S(g) + O2(g) → 2S(s) + 2H2O(l), the sulfur in H2S loses its hydrogen (to oxygen, which is simultaneously reduced by gaining that hydrogen), so this too counts as an oxidation of sulfur, even though no new oxygen atom attaches to the sulfur itself. Beyond oxygen and hydrogen, the classical definition widens further to any electronegative or electropositive element. Combination with an electronegative element is oxidation: magnesium reacting with fluorine, Mg(s) + F2(g) → MgF2(s), or with sulfur, Mg(s) + S(s) → MgS(s), are oxidations of magnesium even though no oxygen is present at all. Oxidation can likewise be seen as removal of an electropositive element: in Hg2Cl2(s) → HgCl2(s) + Hg(s), the compound loses one of its two mercury atoms, so this is an oxidation. Symmetrically, reduction can also mean removal of an electronegative element -- 2FeCl3 + H2(g) → 2FeCl2(aq) + 2HCl, where FeCl3 loses a chlorine to become FeCl2 -- or addition of hydrogen -- CH2=CH2(g) + H2(g) → CH3-CH3(g), where ethene is hydrogenated to ethane -- or addition of an electropositive element -- 2HgCl2(aq) + SnCl2(aq) → Hg2Cl2(s) + SnCl4(aq), where HgCl2 gains mercury from SnCl2 to become Hg2Cl2. This last reaction is important because it shows oxidation and reduction happening together in a single equation: HgCl2 is reduced to Hg2Cl2 while SnCl2 i …