Ionic Half Equations

Oxidation is the loss of electrons. Reduction is the gain of electrons. Redox equations can be separated into two half equations that show the gain and loss of electrons.The reaction:Cu(s) + ½O₂(g) → CuO(s)can be written as two half equations:Cu(s) → Cu ²⁺(s) +2e^-½O₂(g) +2e^- → O ^2-(s)
Metals react with acids to form salts and hydrogen gas.Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen gas.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
becomes:Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)The Chloride ions are not shown because they go from Cl^-(aq) to Cl^-(aq) and do not change in the reaction. As a result they are called spectator ions.The half equations for the reaction are:Mg(s) → Mg²⁺(aq) + 2e^- and 2H⁺(aq) + 2e^- → H₂(g)