Reactivity, Metal Extraction and Alloys
The Reactivity Series
Metals can be arranged in order of their chemical reactivity — this is called the reactivity series. From most to least reactive: potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), aluminium (Al), carbon (C), zinc (Zn), iron (Fe), hydrogen (H), copper (Cu), silver (Ag), gold (Au). Carbon and hydrogen are included as non-metal reference points.
A more reactive metal will displace a less reactive metal from a solution of its compound. For example, iron displaces copper from copper(II) sulfate solution:
$$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$
The solution changes from blue to pale green, and a copper-coloured solid forms on the iron. This is evidence that a displacement reaction has occurred.
Reactivity series (most → least reactive): K, Na, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au. A more reactive metal displaces a less reactive metal from its compound. Metals above carbon must be extracted by electrolysis; those below carbon can be extracted by reduction with carbon.
Oxidation and Reduction (Redox)
Displacement reactions are redox reactions — one species is oxidised while another is reduced. The memory aid OIL RIG helps keep the definitions straight: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
In the iron/copper sulfate displacement reaction: iron atoms lose electrons to become \(Fe^{2+}\) ions — iron is oxidised. Copper(II) ions gain electrons to become copper atoms — copper is reduced.
OIL RIG — Oxidation Is Loss of electrons, Reduction Is Gain. In displacement reactions, the more reactive metal (higher in the series) is oxidised; the less reactive metal ion is reduced. Both changes happen simultaneously.
Extracting Metals from Their Ores
Most metals are found in the Earth's crust 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, zinc, copper) — extracted by reduction with carbon. Carbon (or carbon monoxide) is more reactive than these metals under smelting conditions, so it removes the oxygen from the metal oxide.
- Metals more reactive than carbon (e.g. aluminium, sodium, potassium) — must be extracted by electrolysis. This requires large amounts of electricity, making it much more expensive.
Iron is extracted from iron(III) oxide (\(Fe_2O_3\)) in a blast furnace. Coke (carbon) burns in hot air to form carbon dioxide, which then reacts with more coke to form carbon monoxide (CO). The carbon monoxide reduces the iron oxide:
$$Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$$
Limestone (\(CaCO_3\)) is added to remove acidic silicon dioxide impurities as liquid slag (calcium silicate), which is tapped off separately from the molten iron.
Rusting and Corrosion
Rusting occurs when iron or steel is exposed to both water and oxygen. The overall reaction produces hydrated iron(III) oxide (rust). Rusting is an oxidation process — iron is oxidised. Preventing rusting is commercially vital: methods include galvanising (zinc coating), painting, oiling, or using a sacrificial metal (attaching a more reactive metal that corrodes preferentially).
Alloys
A pure metal has a regular lattice of identical atoms that can slide over each other easily, making the metal soft. An alloy contains atoms of different sizes, which distort the lattice and make it harder for layers to slide. This makes alloys harder and stronger than the pure metals they contain.
Steel: iron + carbon (and sometimes other metals). Low-carbon steel is malleable; high-carbon steel is harder and more brittle; stainless steel (iron + chromium + nickel) is corrosion-resistant.
Bronze: copper + tin. Harder than copper alone; used for statues, bells and bearings.
Brass: copper + zinc. Harder than copper; used for musical instruments and decorative fittings.
Gold alloys (carat gold): pure gold is too soft for jewellery; it is alloyed with silver or copper. 18-carat gold contains 18 parts gold in 24 parts alloy (75% gold).
Alloys are harder than pure metals because the different-sized atoms disrupt the regular metallic lattice, preventing layers from sliding easily over each other.
Electroplating uses electrolysis to deposit a thin layer of metal onto an object. The object to be plated acts as the cathode (negative electrode), the plating metal acts as the anode (positive electrode), and both are placed in a solution containing ions of the plating metal. Metal ions are reduced at the cathode and deposited onto the object; the anode gradually dissolves to replenish the ions. Applications include chrome plating of car parts, silver plating of cutlery, and tin plating of steel cans.