Electrolysis Flashcards
Electrolysis
Used to separate a metal from its compound, Negative ions (anions) move to the anode (positive), Positive ions (cations) move to the cathode (negative), If in an aqueous/solution- must consider half equation of water
Why does electrolysis often use a high temperature
Electrolyte needs to be molten to allow for ion movement, Strong electrostatic attraction between cations and anions throughout lattice that needs a lot of energy to partially separate the ions (melt)
Downs cell
Purpose- electrolysis of molten sodium chloride, only possible way to make sodium metal and chloride gas, Uses molten NaCl/CaCl2 to reduce melting point from 801 degrees to around 600 degrees (saves money), An iron screen is used to allow for migration of ions but ensure that sodium ions and chloride gas don’t collide (would explode), Iron cathode is used as it is cheap and only acts as an electrical conductor, Forms Na (l) which is less dense than the electrolyte (NaCl/CaCl2) meaning it floats to the top for removal, Chlorine gas is collected by a fume hood (used for water purification and bleach)
Hall-Heroult process
Electrolysis of aluminium oxide, hydrated aluminium oxide (melting point of 2045 degrees) is dissolved in cryolite to lower the melting point to around 950-1000 degrees (mixture is 20% alumina and 80% cryolite), Must use molten aluminium oxide as if it was in a solution, water would preferentially reduce, Carbon anode is lowered to touch the carbon cathode lining creating a short circuit which sparks and releases energy/heat, Cathode: Al3+ (l) + 3e- —> Al (l), Aluminium produced is more dense than electrolyte and sinks to the bottom allowing for removal, Anode: C (s) + 2O2- —> CO2 (g) + 4e, Oxygen strips carbon forming carbon dioxide making this process a big generator of greenhouse gases, Carbon anodes are replaced once a month due to the stripping of carbon/breaking down whereas cathode is replaced every 5 years
Electroplating
Purpose: Add a thin layer of one metal on another to improve appearance or increase corrosion resistance, Examples- silver and gold plating jewellery, Sn in steel cans, galvanised iron, chrome plated steel, Cathode- object being plated, Anode- source of plating material (gold, silver, etc), Electrolyte- aqueous solution of a salt of the plating material, Best plating occurs when there is a low concentration of plating ions (when pure), low voltage and clean cathode
Electroplating silver
Anode- bar of silver, Electrolyte- solution contains silver ions (silver nitrate and sodium cyanide), Cyanide reacts with Ag+ ions to keep their concentration low, E value= 0 meaning more than 0 volts are needed
Electrorefining of copper
Purpose: electrolytically refine blister copper (not pure) to obtain the 99.99% purity required for electrical wiring which needs good conductivity (higher the purity, lower the resistance), Anode- impure copper cast into slabs, Cathode- thin sheet of pure copper, Electrolyte- Copper sulfate solution acidified with sulfuric acid meaning water doesn’t undergo reduction and produce oxygen gas, Impure copper (blister copper) is made of more reactive metals and less reactive metals, Reactive metals (High E value for oxidation) such as Ni, Fe and Cr enter the electrolyte and at low concentrations don’t do anything however as concentrations increase they start to plate on the cathode as they readily oxidise meaning the electrolyte has to be tested/monitored and changed which can then be purified, Less reactive metals (Low E value for oxidation) such as Au, Ag and Pt form the anode slime as they don’t preferentially react and is instead collected and refined, E value= 0 meaning a small voltage is used
