Extraction of metals and corrosionIB MYP Chemistry: Revision notes

An ore is a rock that contains enough of a metal, usually as a compound such as an oxide, to make it worth extracting. Most metals are found combined with other elements because they are reactive.

How a metal is extracted depends on its position in the reactivity series:

• Metals below carbon (such as iron and zinc) are extracted by heating their oxides with carbon. Carbon is more reactive, so it takes the oxygen away. This is reduction.
• Metals above carbon (such as aluminium) hold on to oxygen too strongly, so they are extracted by electrolysis of the molten compound.
• Very unreactive metals such as gold are found as the uncombined metal.

Iron is made from haematite (iron(III) oxide, Fe₂O₃) in a blast furnace. Iron ore, coke (carbon) and limestone go in at the top and hot air is blown in near the bottom.

• Coke burns in the hot air: C + O₂ → CO₂
• Carbon dioxide reacts with more coke: CO₂ + C → 2CO
• Carbon monoxide reduces the iron oxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂
• Limestone breaks down: CaCO₃ → CaO + CO₂. The calcium oxide reacts with sand (silicon dioxide) impurities to form slag, CaO + SiO₂ → CaSiO₃, which floats on the molten iron.

The molten iron sinks to the bottom and is tapped off.

Aluminium is more reactive than carbon, so it cannot be extracted with carbon. Instead, aluminium oxide (Al₂O₃), purified from the ore bauxite, is electrolysed.

Aluminium oxide melts at over 2000 °C, which would waste huge amounts of energy. It is dissolved in molten cryolite, which lowers the temperature needed to about 950 °C.

• At the cathode (negative), aluminium ions gain electrons: Al³⁺ + 3e⁻ → Al. Molten aluminium collects at the bottom.
• At the anode (positive), oxygen forms: 2O²⁻ → O₂ + 4e⁻. It reacts with the graphite anodes to make carbon dioxide, so the anodes must be replaced regularly.

The process uses a great deal of electricity, which makes aluminium expensive to produce.

Mining gives us the metals we need, and brings jobs and income. But it also has costs: land is cleared and habitats are destroyed, waste and dust can pollute soil and water, and extraction uses energy that often produces carbon dioxide. Ores are also finite and will eventually run out.

Recycling metals reduces these problems. It saves energy (recycling aluminium needs much less energy than electrolysis), conserves ores, and reduces landfill. Its drawbacks are that scrap must be collected and sorted, which costs money.

Rusting is the corrosion of iron and steel. Both oxygen and water are needed.

Word equation: iron + oxygen + water → hydrated iron(III) oxide (rust)

To investigate rusting, put iron nails in different tubes: one with water and air (rusts), one with boiled water covered by oil so no air can dissolve (no rust), and one with a drying agent so the air is dry (no rust). The independent variable is the conditions, the dependent variable is whether rust forms, and the control variables include the type and size of the nail and the time.

Rust is flaky and falls away, exposing fresh iron underneath, so rusting continues.

Rust prevention either keeps oxygen and water away from the iron, or uses a more reactive metal.

• Painting, oiling/greasing and plastic coating form a barrier. If the layer is scratched, rusting starts.
• Galvanising coats the iron with zinc. It forms a barrier and, even when scratched, the zinc corrodes first because it is more reactive.
• Sacrificial protection: a block of a more reactive metal such as magnesium or zinc is attached to the iron (for example on ship hulls or buried pipes). The block corrodes instead of the iron and is replaced when it wears away.

Aluminium is more reactive than iron, yet aluminium objects last a long time. When fresh aluminium is exposed to air, it quickly reacts with oxygen to form a thin, tough layer of aluminium oxide. This layer sticks firmly to the metal and keeps oxygen and water away, so the metal underneath is protected.

Iron's rust is flaky and falls off, but aluminium oxide stays in place. This is why aluminium is used for drinks cans and aircraft.