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 positive ions (cations)

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

Reactions of metals with water

When a metal reacts with water, a metal hydroxide and hydrogen are formed.

Key fact

Metal + water → metal hydroxide + hydrogen

For example, sodium reacts rapidly with cold water, melting into a ball, and ‘fizzing’ about the surface:

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.

Reactions with steam

Metals that react slowly with cold water can react quickly with steam. In these reactions a metal oxide and hydrogen are produced.

Metal + steam → metal oxide + hydrogen

For example, magnesium reacts slowly with cold water. However, if steam is passed over hot magnesium, a vigorous reaction occurs:

Magnesium + steam → magnesium oxide + hydrogen

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Question

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

answer

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

Reactions of metals with dilute acids

When a metal reacts with a dilute acid, a salt and hydrogen are formed.

Metal + acid → salt + hydrogen

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.

answer

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

Key fact

Hydrogen is always given off when a metal reacts with water, steam or a dilute acid.

In the reactions of metals with water, steam and acids, the metals lose electrons and form cations. The metal is oxidized and the water is reduced.

A metal’s relative tendency to form cations and its resistance to oxidation are both related to its position in the reactivity series. In general:

• the higher up a metal, the greater the tendency to form cations
• the lower down a metal, the greater its resistance to oxidation