Electrolysis

Ionic compounds do not conduct electricity when solid since the ion are not free to move around. However, ionic compounds do conduct electricity when molten, or through an aqueous solution of an ionic compound. Electrolysis of Lead Bromide Lead bromide must be heated until it is molten before it will conduct electricity. Electrolysis separates the molten ionic compound into its elements. The reactions at each electrode are called half equations. The half equations are written so that the same number of electrons occur in each equation. Pb2+ + 2e- ==> Pb (lead metal at the (-)cathode). 2Br- - 2e- ==> Br2 (bromine gas at the (+)anode). Lead ions gain electrons (reduction) to form lead atoms. Bromide ions lose electrons (oxidation) to form bromine atoms. The bromine atoms combine to form molecules of bromine gas. The overall reaction is: PbBr2(l) ==> Pb(s) + Br2(g) Electrolysis of aqueous solutions (Sodium Chloride) The electrolyte sodium chloride solution (brine), provides a high concentration of sodium ions Na+ and chloride ions Cl– to carry the current during the electrolysis process. Initially there are only traces of hydrogen ions H+ and hydroxide ions OH– from the self-ionisation of water. Brine is moderately concentrated sodium chloride solution (brine) with carbon (graphite) gives equal volumes of hydrogen gas (hydrogen ions H+ discharged at the –ve cathode) and green chlorine gas (chloride ions Cl– discharged at the +ve anode) with sodium hydroxide left in solution. The electrolysis will only take place when electricity is passed through the sodium chloride solution. The electrode equations and the theory of what happens in the electrolysis of aqueous sodium chloride The half-equations for the electrolysis of sodium chloride solution (the electrolyte brine). (a) The negative cathode electrode reaction for the electrolysis of brine (sodium chloride solution) The negative (–) cathode attracts the Na+ (from sodium chloride) and H+ ions (from water). Only the hydrogen ions are discharged at the cathode. The more reactive a metal, the less readily its ion is reduced on the electrode surface. The hydrogen ions are reduced by electron (e–) gain to form hydrogen molecules at the negative electrode which attracts positive ions. 2H+(aq) + 2e– ==> H2(g) positive ion reduction by electron gain other equations 2H2O(l) + 2e– ==> H2(g) + 2OH-(aq) or 2H3O+(aq) + 2e– ==> H2(g) + 2H2O(l) Nothing happens to the sodium ion, but it is still important (see after the anode reaction has been described). In fact, if sodium was released (which it isn't), it would immediately react with water to give hydrogen, the same product you get from the reduction of the hydrogen ion. Test for the cathode gas - colourless gas gives a squeaky pop with a lit splint – hydrogen (b) The positive anode electrode reaction for the electrolysis of brine (sodium chloride solution) The positive anode attracts the negative hydroxide OH– ions (from water) and chloride Cl– ions (from sodium chloride). Only the chloride ion is discharged in appreciable quantities i.e. it is preferentially oxidised to chlorine. The…