12 Chemistry 2.2 quantitative chemistry CR 07 Practical uses of Electrolysis Electrolysis is an expensive process because of the energy involved. Electrolysis is used to reduce metals from their ores however it is often necessary to have high temperature molten electrolyte because H + is easier to reduce than some metal ores

12 Chemistry 2.2 quantitative chemistry CR 07 Extraction of magnesium Sea water contains 0.13% Mg 2+ ions (by mass). Mg 2+ is precipitated from sea water: Mg 2+ (aq) + 2OH -  Mg(OH) 2(s) -this is filtered and dissolved in HCl

12 Chemistry 2.2 quantitative chemistry CR 07 Mg(OH) 2(s) + 2HCl (aq)  MgCl 2(aq) + 2 H 2 O (l) After evaporation hydrated MgCl 2.6H 2 O is heated to remove some of the water of crystallisation and then mixed with NaCl and CaCl 2 to prevent unwanted side reactions in electrolysis

12 Chemistry 2.2 quantitative chemistry CR 07 Reaction mix is then heated to C and electrolysed. Mg (s) forms at the cathode and Cl 2 forms at the anode. Redox: Red: Mg 2+ (aq) +2e -  Mg (s) Ox: 2Cl - (aq)  Cl 2(g) + 2e - Mg 2+ (aq) + 2Cl - (aq)  Mg (s) + Cl 2(g)

12 Chemistry 2.2 quantitative chemistry CR 07 Aluminium ‘Bauxite’ contains about 20% Al 2 O 3 (80% impurities) Al 2 O 3 is also called ‘alumina’ Ore purification (mined and purified in Australia. Bauxite is heated with NaOH – this dissolves Al 2 O 3 and silica only:

12 Chemistry 2.2 quantitative chemistry CR 07 Al 2 O 3 + NaOH + 3 H 2 O  2NaAl(OH) 4(aq) sodium aluminate The solution is cooled and seed crystals of Al(OH) 3 area added. Crystals grow leaving impurities in solution. They are 99.5% pure Al 2 O 3 NZ

12 Chemistry 2.2 quantitative chemistry CR 07 At Tiwai Point (Bluff) Electrolysis: Al 2 O 3 (alumina) is dissolved in cryolite (Na 3 AlF 6 ) to reduce MP from C to 970 o C The electrolyte is a mixture of Al 2 O 3 and cryolite

12 Chemistry 2.2 quantitative chemistry CR 07 Electrolysis occurs in large steel pots with carbon (graphite) electrodes. cathode reaction: Al e -  Al (l) -sinks to the bottom and is siphoned off

12 Chemistry 2.2 quantitative chemistry CR 07 Anode reaction: (anodes need replacing which is unusual) 2O 2- (l)  O 2(g) + 4e - Anodes: C (s) + O 2(g)  CO 2(g)

12 Chemistry 2.2 quantitative chemistry CR 07 Electroplating Electroplating puts a thin layer or ‘coat’ of one metal over another. Used in: -prevention of rust -cheaper versions of jewellery

12 Chemistry 2.2 quantitative chemistry CR 07 Chrome plating Chrome plating is when a thin layer of chromium metal is plated onto steel  shiny and prevents rust To chrome plate: 1.Clean object thoroughly

12 Chemistry 2.2 quantitative chemistry CR 07 2.Put the object on the cathode of an electrolytic cell containing Cr 3+ in the electrolyte. Similar processes are used for Cu, Ag and Zn plating