Reactivity of Metals and Metal Extraction
The Reactivity Series
Metals can be arranged in order of their reactivity — this is called the reactivity series. Potassium, sodium, lithium and calcium react with cold water. Magnesium, zinc and iron react with dilute acids but only very slowly, or not at all, with cold water. Copper, silver and gold react with neither water nor dilute acid.
A more reactive metal will displace a less reactive metal from its compound. For example, if iron is placed in copper sulfate solution, the iron displaces the copper:
$$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$
The reactivity series (most to least reactive): K, Na, Li, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au. Carbon and hydrogen are included as reference points. A more reactive metal displaces a less reactive metal from its compound.
Extraction of Metals
Metals are found in the Earth’s crust, often combined with other elements in ores. The method of extraction depends on the metal’s position in the reactivity series:
- Metals less reactive than carbon (e.g. iron, copper, zinc) are extracted by reduction with carbon. Carbon is cheaper and more reactive, so it removes the oxygen from the metal oxide.
- Metals more reactive than carbon (e.g. aluminium, sodium, potassium) must be extracted by electrolysis, which is more expensive because it requires large amounts of electricity.
For example, iron is extracted from iron oxide using carbon in a blast furnace:
$$2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2$$
Oxidation is the gain of oxygen (or loss of electrons). Reduction is the loss of oxygen (or gain of electrons). Remember: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain. In a displacement reaction, the more reactive metal is oxidised (loses electrons) and the less reactive metal ion is reduced (gains electrons).
Metals below carbon in the reactivity series are extracted by reduction with carbon. Metals above carbon need electrolysis. Oxidation = gain of oxygen / loss of electrons. Reduction = loss of oxygen / gain of electrons.
Recycling metals saves energy (less needed than extraction from ore), reduces the use of limited ore resources, reduces waste going to landfill, and reduces environmental damage from mining and carbon emissions. However, collection and sorting can be difficult, and recycled metals may be impure.