Topic C: Metals

Metals are central to daily life and industry, and their chemistry follows a clear pattern set by the reactivity series. This part shows how reactivity explains the way metals react with water, acid and oxygen, how more reactive metals displace less reactive ones, how metals are extracted, and how iron corrodes and can be protected.

The reactivity series

The reactivity series ranks metals by their tendency to react, that is to lose electrons and form positive ions. A common order from most to least reactive is potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, copper, silver and gold. The more reactive a metal, the more vigorously it reacts and the more stable its compounds. This single list predicts a wide range of behaviour: which metals react with water or acid, which displace others from solution, how the metal is extracted, and how readily it corrodes. Learning the order and the position of carbon and hydrogen relative to the metals is essential, as these reference points decide extraction and acid reactions.

Reactions of metals with water and steam

Reactivity with water shows the trend clearly. Very reactive metals such as potassium and sodium react vigorously with cold water to give a hydroxide and hydrogen, for example 2 Na + 2 H2O -> 2 NaOH + H2, with sodium melting into a ball that fizzes across the surface. Calcium reacts more steadily with cold water. Magnesium reacts very slowly with cold water but burns in steam to give the oxide and hydrogen, Mg + H2O -> MgO + H2. Less reactive metals such as iron react only slowly with steam, while copper, silver and gold do not react with water or steam at all. The vigour of the reaction decreases steadily down the series.

Reactions of metals with dilute acids

Metals above hydrogen in the reactivity series react with dilute acids to give a salt and hydrogen gas, for example Zn + H2SO4 -> ZnSO4 + H2 and Mg + 2 HCl -> MgCl2 + H2. The more reactive the metal, the faster the fizzing and the more rapidly hydrogen is produced. Magnesium reacts briskly, zinc more slowly, and iron slowly. Metals below hydrogen, such as copper, silver and gold, do not react with dilute acids because they cannot displace hydrogen. The gas can be tested with a lighted splint, which gives a squeaky pop, confirming hydrogen. This pattern is a practical way to place an unknown metal in the reactivity series.

Displacement reactions

A more reactive metal will displace a less reactive metal from a solution of its salt, because the more reactive metal loses electrons more readily. For example, adding iron to copper(II) sulfate solution gives Fe + CuSO4 -> FeSO4 + Cu, with reddish copper deposited and the blue colour fading as green iron(II) sulfate forms. The reaction is a redox process: the iron is oxidised while the copper ions are reduced. Displacement provides a direct experimental method to order metals by reactivity, since a metal can only displace those below it in the series. The same idea explains the thermite reaction, in which aluminium displaces iron from iron(III) oxide, releasing much heat.

Extraction linked to reactivity

The reactivity of a metal determines how its ore is processed. Metals above carbon in the series, such as aluminium and magnesium, are extracted by electrolysis of their molten compounds because carbon cannot reduce their oxides. Metals below carbon but above silver, such as zinc, iron and lead, are extracted by heating their oxides with carbon or carbon monoxide, which removes the oxygen by reduction. The least reactive metals, gold and silver, are found native and need no chemical extraction. This means very reactive metals require the most energy and cost to obtain, which is why aluminium was once more valuable than gold, while gold has always been found ready to use.

Corrosion of iron and its prevention

Corrosion is the gradual reaction of a metal with substances in its environment. The rusting of iron needs both oxygen and water; salt speeds it up. Rust is hydrated iron(III) oxide, and because it flakes away, fresh metal is exposed and corrosion continues. Several methods prevent rusting. Barrier methods, such as painting, oiling, greasing or coating with plastic, keep out air and water. Galvanising coats iron with zinc, which also gives sacrificial protection: zinc is more reactive, so it corrodes in preference to the iron even if the coating is scratched. Attaching blocks of a more reactive metal such as zinc or magnesium to ships and pipelines gives the same sacrificial protection.