The reactivity series of metals

The reactivity series of metals is a chart showing metals in order of decreasing reactivity. In general, the more reactive a metal is:

  • the more vigorous its reactions are
  • the more easily it loses electrons in reactions to form cations

The table summarises some reactions of metals in the reactivity series. Hydrogen is shown for comparison.

Table showing the reactivity series of metals, from least to most reactiveReactivity series of metals

Reaction of metals with water

When a metal reacts with water, a metal hydroxide and hydrogen are formed. For example, sodium reacts rapidly with cold water:

sodium + water → sodium hydroxide + hydrogen

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

In general, the more reactive the metal, the more rapid the reaction is. Aluminium is unusual because it is a reactive metal that does not react with water. Its surface naturally forms a very thin layer of aluminium oxide that keeps water away from the metal below.

Reactions with steam

Magnesium reacts slowly when it is first added to water, but a layer of insoluble magnesium hydroxide forms. This protects the metal and stops it reacting. However, if steam is passed over hot magnesium, a vigorous reaction happens:

magnesium + steam → magnesium oxide + hydrogen

Mg(s) + H2O(g) → MgO(s) + H2(g)

In this reaction:

  • magnesium is oxidised because it gains oxygen to form magnesium oxide
  • water is reduced because it loses oxygen to form hydrogen
Question

State the difference between the products formed when calcium reacts with cold water and when it reacts with steam.

Calcium hydroxide forms when it reacts with water, but calcium oxide forms when it reacts with steam.

Reaction of metals with dilute acids

When a metal reacts with a dilute acid, a salt and hydrogen are formed. For example, magnesium reacts rapidly with dilute hydrochloric acid:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

The more reactive the metal, the more rapid the reaction is. A metal below hydrogen in the reactivity series will not react with dilute acids.

Question

Platinum is placed below gold in the reactivity series. Predict its reaction with dilute acids and explain your answer.

Platinum will not react with dilute acids. Metals below hydrogen in the reactivity series do not react with dilute acids. Both gold and platinum are placed below hydrogen.