Electricity and chemistry
Using inert electrodes (platinum or carbon)
| Electrolyte | Product at cathode | Product at anode |
| lead (II) bromide (l) | lead | bromine |
| concentrated HCl (aq) | Hydrogen | Chlorine |
| concentrated NaCl (aq) | Hydrogen | chlorine |
In general, metals or hydrogen are formed at the cathode (negative electrode, where reduction takes place), non-
metals except hydrogen are formed at the anode (positive electrode, where oxidation takes place).
How to predict products: (FIY they will only give compounds with 2 elements like water, but no more i.e. not CuSO4)
-if molten compound (e.g. iron oxide) then you just get the two components produced in elemental form (e.g. molten iron + oxygen gas)
-in a solution, at the cathode, the less reactive of the two positive ions will be reduced e.g. aqueous NaCl will produce hydrogen, not sodium.
-if a halide is in a concentrated solution, then a halogen will be produced at the anode. If the solution is dilute, oxygen, from the water, will be produced.
Electroplating (picture on the left) For electroplating, you need:
-an anode made of the metal that you want to electroplate the object with,
-ions of the same metal as the anode in solution
-object to be plated at cathode
It is used to make things look better (coating a watch with gold) or to prevent corrosion.
Aluminium is used for electricity cables (not wires) because it is light, does not corrode, is a good conductor, and cheaper and much lighter than copper. The cables have a steel core, for strength. Plastics and ceramics are used as insulators because they do not conduct electricity, and conduct heat poorly. Plastic is used for
the casing in electric plugs. Ceramics (inorganic, non-metallic solids) are used to support the cables in electricity pylons. Copper is used in electrical wires (not cables) as it is a very good conductor and ductile (not as good a silver, which is the best conductor used in circuitry in keyboards, phones etc., but it is cheaper)
Choice of electrodes can affect products. An example:
| Electrolyte | Cathode made of: | Anode made of: | Product at cathode: | at anode: |
| dilute copper (II) sulphate | inert material | copper | copper | Nothing but Copper is
lost |
| dilute copper (II) sulphate | inert material | inert material | copper | oxygen |
This concept is used to refine copper
Electrolysis is a way to decompose compounds, using electricity. Reduction of positive cations happens at the cathode (CATions at CAThode. Oxidation of negative anions happens at the anode (ANions at ANode) for example:
At the anode: 2Cl- → Cl2 + 2e- At the cathode: 2H+ + 2e- → H2
Extraction of aluminium and electrolysis of brine
Aluminium (III) oxide (alumina) is dissolved in molten cryolite (Na3AlF6). The mixture melts at a lower temperature. better conductor / reduces amount of energy needed / reduces cost / more economic / makes process viable / conserves energy
During electrolysis aluminium is produced at the carbon cathode and oxygen at the carbon anode. Electrolysis of Brine
Brine is concentrated NaCl solution
Chlorine (making PVC, medical drugs, pesticides, sterilising water, making chemicals) is produced at the titanium anode, hydrogen (making nylon, make hydrogen peroxide, hardening vegetable oils, fuel) is produced at the steel/nickel cathode (in the book it says steel in the text and nickel in the diagram) and sodium hydroxide (soaps, detergents, medical drugs, dyes, paper, ceramics) is left over.
Asbestos diaphragm lets ions pass through, but keeps gases apart.