Metal reactivity and displacement reactions

Potassium, sodium and calcium react with cold water to produce a metal hydroxide and hydrogen. Their different reactions reveal their relative reactivity: potassium reacts violently, sodium reacts quickly, and calcium reacts less strongly.

For calcium, the reaction is:

$$\mathrm{Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2}$$

The bubbles are hydrogen gas. The calcium atoms become calcium ions as calcium hydroxide forms.

Magnesium, zinc and iron react very slowly, if at all, with cold water under ordinary classroom conditions. Cold water therefore does not give an obvious comparison between every metal. Another reaction, such as a reaction with dilute acid or a salt solution, can provide the evidence needed.

Comparing reactions with dilute acids

Metals above hydrogen in the series react with dilute hydrochloric acid or dilute sulfuric acid to form a salt and hydrogen:

metal + acid → salt + hydrogen

For example:

$$\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}$$

Here magnesium chloride is the salt. With dilute sulfuric acid, magnesium sulfate forms instead:

$$\mathrm{Mg + H_2SO_4 \rightarrow MgSO_4 + H_2}$$

Under comparable conditions, magnesium reacts more vigorously than zinc, and zinc reacts more vigorously than iron. Faster production of hydrogen bubbles is evidence of greater reactivity. Copper is below hydrogen and does not react with these dilute acids to release hydrogen.

A useful comparison keeps the acid concentration and temperature the same and uses metal samples with comparable exposed surface areas. Otherwise, a difference in bubbling could result from the conditions rather than the metal's reactivity. Potassium and sodium react dangerously with acids and are not suitable for this comparison.

Using salt solutions to deduce an order

A more reactive metal displaces a less reactive metal from a solution of its salt. The added metal enters the solution as ions, while ions of the less reactive metal become solid metal.

For example, magnesium displaces copper from copper sulfate solution:

$$\mathrm{Mg + CuSO_4 \rightarrow MgSO_4 + Cu}$$

The blue colour of the copper sulfate solution fades as colourless magnesium sulfate solution forms. Copper coats the magnesium, and some copper may fall to the bottom of the container. These changes show that magnesium is more reactive than copper.

Several comparisons can be combined to build an order. Magnesium displaces iron from iron(II) sulfate solution, so magnesium is more reactive than iron. Iron displaces copper from copper(II) sulfate solution, so iron is more reactive than copper. Together, these observations give:

magnesium > iron > copper

A less reactive metal cannot displace a more reactive one. For example, copper does not displace calcium from calcium chloride solution. This tells us that copper is less reactive than calcium, although that comparison alone cannot tell us where every other metal belongs.

Explaining displacement through electrons — Higher Tier

Displacement is not simply one metal taking another metal's place. It involves a transfer of electrons. Oxidation is loss of electrons, and reduction is gain of electrons. Both happen together in a redox reaction.

In the magnesium–copper sulfate reaction, magnesium atoms lose two electrons each and become magnesium ions:

$$\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}$$

Magnesium is therefore oxidised. Copper ions gain those electrons and become copper atoms:

$$\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$$

The copper ions are therefore reduced. The same two electrons lost by magnesium are gained by copper ions.

Combining these half-equations gives the ionic equation:

$$\mathrm{Mg + Cu^{2+} \rightarrow Mg^{2+} + Cu}$$

Sulfate ions remain unchanged in solution, so they do not appear in this ionic equation. Magnesium's greater tendency to form cations explains why it can displace copper.

Reactivity and resistance to oxidation

A metal high in the reactivity series loses electrons readily, so it is readily oxidised. A metal low in the series loses electrons less readily, so it is more resistant to oxidation.

This is the same underlying pattern viewed in two ways: greater reactivity means a greater tendency to form cations and less resistance to oxidation. Gold, near the bottom of the series, is much more resistant to oxidation than a reactive metal such as magnesium.

• Cold water: potassium, sodium and calcium form a hydroxide and hydrogen. Their reactions become less vigorous down this sequence.
• Dilute hydrochloric or sulfuric acid: metals above hydrogen form a salt and hydrogen; metals below hydrogen do not release hydrogen.
• Salt solutions: a more reactive metal displaces a less reactive metal. Use each reaction to establish a pairwise comparison, then combine the comparisons.

Redox — Higher Tier

Oxidation = electron loss; reduction = electron gain.

In a metal displacement reaction, the added metal is oxidised and the displaced metal ions are reduced. Both changes occur together.