The reactivity series of metals, displacement reactions of metals and their salts, the reactions of metals with water and acids, the extraction of metals by reduction with carbon, and writing ionic and half equations.

What this topic is asking

OCR wants you to recall the reactivity series, predict displacement reactions, describe the reactions of metals with water and acids, explain how metals are extracted by reduction with carbon, and write ionic and half equations.

The reactivity series

A more reactive metal loses its electrons more readily, so it forms positive ions more easily. The series predicts behaviour: the more reactive a metal, the more vigorously it reacts with water and acids, and the harder it is to extract from its ore. Carbon and hydrogen are included as reference points because they are used in extraction (carbon) and in acid reactions (hydrogen). You can confirm the order experimentally by comparing how metals react with water or dilute acid, or by displacement reactions.

Displacement and reactions of metals

A displacement reaction happens when a more reactive metal takes the place of a less reactive metal in a compound. For example, iron is more reactive than copper, so an iron nail placed in copper sulfate solution displaces copper: the blue solution fades and a brown copper coating forms on the nail (iron + copper sulfate produces iron sulfate + copper). Metals with water: very reactive metals (potassium, sodium, calcium) react with cold water to form a metal hydroxide and hydrogen, getting more vigorous up the series. Metals with acids: metals above hydrogen react with dilute acids to form a salt and hydrogen, with the reaction faster for more reactive metals; metals below hydrogen (copper, silver, gold) do not react with dilute acids.

Ionic and half equations

An ionic equation shows only the particles that take part in the change, leaving out spectator ions that do not change. For the displacement above, the ionic equation is:

A half equation shows the electrons lost or gained by one species. In that displacement, iron is oxidised: $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$ (loses electrons), and copper is reduced: $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$ (gains electrons). The same idea gives the electrode reactions in electrolysis, for example at the cathode $\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$. Half equations must balance both atoms and charge, which the electrons take care of.