Electrochemical cells

Oxidation and reduction

Many reactions involve electrons moving from one substance to another. Chemists call these redox reactions (reduction and oxidation):

• Oxidation is the loss of electrons. When zinc reacts, Zn → Zn²⁺ + 2e⁻.
• Reduction is the gain of electrons. When copper ions become copper metal, Cu²⁺ + 2e⁻ → Cu.

A useful memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain. The two always happen together, because electrons lost by one substance must be gained by another. The words first meant gaining and losing oxygen: when iron rusts it is oxidised, and when oxygen is removed from iron ore it is reduced.

You can now see the blast furnace (3G) in a new way. In Fe₂O₃ + 3CO → 2Fe + 3CO₂, the iron(III) ions each gain three electrons and become iron atoms, so the ore is reduced, while carbon monoxide is oxidised to carbon dioxide. In green steel the hydrogen is oxidised instead (Fe₂O₃ + 3H₂ → 2Fe + 3H₂O). Every way of extracting a metal from its ore is a reduction; what changes is what supplies the electrons: carbon, hydrogen or, for the most reactive metals, an electric current.

Galvanic cells: chemical energy to electrical energy

If you put zinc into copper(II) sulfate solution, zinc atoms give electrons straight to copper ions. Copper coats the zinc, the blue colour fades and the solution warms up: the energy is lost as heat. A galvanic cell (also called a voltaic cell) separates the two halves of the reaction so the electrons have to travel through a wire, where they can do useful work.

• The anode is where oxidation happens. In a galvanic cell it is the negative electrode, made of the more reactive metal. It slowly dissolves as atoms become ions.
• The cathode is where reduction happens. In a galvanic cell it is the positive electrode. Metal ions from the solution gain electrons and coat it.
• Electrons flow through the external circuit (the wire) from anode to cathode.
• The salt bridge (paper or a tube soaked in a salt solution such as potassium nitrate) completes the circuit. Negative ions drift towards the anode and positive ions towards the cathode, so charge doesn't build up. Take it away and the current stops instantly.

The further apart two metals are in reactivity, the bigger the voltage. Chemists measure each metal's tendency to lose electrons as a standard electrode potential. The cell voltage is the difference between the two values.

For zinc and copper: 0.34 − (−0.76) = 1.10 V. The metal with the more negative value is the anode. These values apply when the solutions have a standard concentration at 25 °C; real cells give a little less.

Electrolytic cells: electrical energy to chemical energy

An electrolytic cell runs the other way. A power supply pushes electrons to force a reaction that would not happen by itself, storing energy in the products. This is electrolysis.

In an electrolytic cell, the anode is connected to the positive terminal and the cathode to the negative terminal, the opposite of a galvanic cell. But the rule doesn't change: oxidation always happens at the anode and reduction at the cathode.

Electroplating uses electrolysis to coat an object with a thin layer of metal, such as chromium on taps or gold on electrical connectors. The object is made the cathode in a solution of the plating metal's ions.

Aluminium is too reactive to be extracted with carbon, so it is made by electrolysis. Australia is one of the world's two biggest bauxite producers, with mines at Weipa in Queensland, Gove in the Northern Territory and in the Darling Range of Western Australia. Bauxite is refined into pure aluminium oxide (alumina), which is dissolved in molten cryolite and split by a huge current.

Smelting uses enormous amounts of electricity: about 14 kWh for every kilogram of aluminium. The Tomago smelter near Newcastle uses about a tenth of all the electricity in NSW. Recycling an aluminium can uses only about 5% of the energy needed to make new aluminium, which is why cans are worth collecting.

Batteries and recycling

A rechargeable battery is a galvanic cell when it is being used and an electrolytic cell when it is being charged. In a lithium-ion cell, lithium is stored between the layers of graphite at the anode. When the phone is on, lithium atoms lose electrons, the electrons flow through the phone and lithium ions travel through the electrolyte into the metal oxide cathode. A charger pushes everything back. Each cell gives about 3.7 V.

Australia produces more lithium than any other country, mostly as the mineral spodumene from Western Australia, and is starting to refine it into battery chemicals onshore. Other battery metals mined here include nickel, cobalt and manganese. As demand grows, chemists are developing batteries that use more common elements, such as sodium-ion and lithium iron phosphate cells, which avoid cobalt.

Used batteries should never go in household bins. Crushed lithium batteries can short-circuit and catch fire, and they cause thousands of fires in garbage trucks and recycling plants each year. The national B-cycle scheme collects batteries at supermarkets and hardware stores so their metals can be recovered.