The Reactivity Series

What makes one metal more reactive than another?

When a metal reacts, its atoms form positive ions by losing electrons. This can be represented generally as:

$\text{M} \rightarrow \text{M}^{n+} + n\text{e}^-$

Different metals lose electrons with different degrees of ease. A metal that forms positive ions readily is more reactive than one that does so less readily. Under the same conditions, the more reactive metal will usually react more quickly and vigorously.

The reactivity series places elements in order from most reactive to least reactive. For the metals required here, including the usual non-metal reference points, the order is:

$\text{K} > \text{Na} > \text{Li} > \text{Ca} > \text{Mg} > \text{C} > \text{Zn} > \text{Fe} > \text{H} > \text{Cu}$

Carbon and hydrogen are not metals. They are included because their positions help us make predictions. Hydrogen separates metals that react with dilute acids from those that do not; carbon is useful when considering metal extraction.

Reactions with water at room temperature

Potassium, sodium, lithium and calcium react with cold water. The general reaction is:

$\text{metal} + \text{water} \rightarrow \text{metal hydroxide} + \text{hydrogen}$

Bubbles show that hydrogen gas is being produced. The metal becomes smaller because its atoms form positive ions in the metal hydroxide; the metal has not simply disappeared.

For example, sodium forms sodium hydroxide and hydrogen:

$2\text{Na}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{NaOH}(aq) + \text{H}_2(g)$

The sodium hydroxide remains in solution, so the resulting solution is alkaline. In this AQA topic, these comparisons are limited to room temperature: reactions with steam are not included.

Reactions with dilute acids

A metal above hydrogen in the reactivity series reacts with a dilute acid. The metal forms positive ions while hydrogen ions from the acid form hydrogen gas:

$\text{metal} + \text{dilute acid} \rightarrow \text{salt} + \text{hydrogen}$

The bubbles are hydrogen. With hydrochloric acid, the salt is a chloride; with sulfuric acid, it is a sulfate. For example:

$\text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g)$

Reaction vigour decreases down the series. Potassium and sodium would react dangerously vigorously with dilute acid and are not suitable for routine classroom comparisons. Lithium and calcium are also very reactive. Among the metals commonly compared safely using dilute acid, magnesium fizzes rapidly, zinc less rapidly and iron slowly. Copper does not react because it lies below hydrogen.

Using experimental evidence to build an order

A reactivity order can be deduced by comparing observations made under the same conditions. Useful evidence includes the rate of fizzing, the rate at which the metal becomes smaller and the rate of gas production. Faster change usually indicates a more reactive metal.

Suppose metal A fizzes rapidly in dilute acid, metal B fizzes slowly and metal C does not react. The results support the partial order $A>B>C$. Because A and B react with acid, both are above hydrogen; C may be below hydrogen.

A fair comparison requires the same temperature, acid concentration and volume, as well as metal pieces with similar surface areas. Surface coatings matter: an oxide coating on magnesium can delay its reaction and make it appear less reactive than it really is.

Water and acid results can be combined. Cold water distinguishes the highly reactive metals particularly well, while dilute acid distinguishes metals such as magnesium, zinc, iron and copper. Several comparisons may therefore be needed to construct the whole series.

Displacement reactions

The same order predicts displacement reactions. A more reactive metal can displace a less reactive metal from its compound because the more reactive metal has the greater tendency to form positive ions.

Zinc is above copper, so zinc displaces copper from copper sulfate solution:

$\text{zinc} + \text{copper sulfate} \rightarrow \text{zinc sulfate} + \text{copper}$

$\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Cu}(s)$

A red-brown deposit of copper forms, and the blue colour of the copper sulfate solution fades as copper ions are removed. Zinc atoms enter the solution as zinc ions.

The reverse combination gives no reaction:

$\text{copper} + \text{zinc sulfate} \rightarrow \text{no reaction}$

Copper is below zinc, so it cannot displace zinc from a zinc compound. This gives a direct rule for interpreting results: if A displaces B from a compound, A is more reactive than B; if it does not, A is less reactive than B, provided the experiment was carried out under suitable conditions.