The Reactivity Series

Metals are arranged based on their reactions in the reactivity series.

They can be deduced by using displacement reactions, in which a less reactive metal is pushed out of its compound by a more reactive metal. For example, the reaction between magnesium and copper (II) oxide:

Mg + CuO -> MgO + Cu

Displacement reactions are examples of redox reactions, in which oxidation and reduction occurs in the same reaction.

The reducing agent is a substance that reduces something else. In this case, it’s magnesium. The oxidising agent is a substance that oxidises something else. In this case, it’s copper.

Remember OILRIG: Oxidation is gain (of electrons); Reduction is loss (of electrons)

Let’s look at this in terms of an ionic equation:

Mg + Cu²⁺ + O^2- -> Mg²⁺ + O^2- + Cu

This basically looks at what turns into ions and what does not. The magnesium turns into a positive ion because it loses two electrons to bond with oxygen. The copper ion, because it’s displaced, regains the two electrons it lost to the oxygen.

Notice in this equation, the oxygen ion does not change? It remains an ion. In this case, the oxygen ion is the spectator ion. Spectator ions aren’t included in the ionic equation, so the proper ionic equation of this displacement reaction should be:

Mg + Cu²⁺ -> Mg²⁺ + Cu

As mentioned earlier, the magnesium loses two electrons to bond with oxygen and copper gains two electrons. This can be written as half-equations:

Mg -> Mg²⁺ + 2e^–

Cu²⁺ + 2e^– -> Cu

Reactions with substances are different depending on its reactivity:

Metals	Reaction with Water	Reaction with Dilute Acids
Potassium, Sodium, Lithium	Very vigorous, produces hydroxides and hydrogen gas	Too reactive to add safely to acids
Calcium	Reacts gently and produces the same products as above	Can be added to very dilute acids – but it’s going to be violent!
Magnesium	Reacts with steam to produce magnesium oxide and hydrogen	Reacts vigorously – strongly exothermic – forms a salt and hydrogen
Zinc or Iron	Reacts slowly and forms an oxide and hydrogen	Reacts slowly
Anything Below	No reaction	No reaction

Zinc or Iron	Reacts slowly and forms an oxide and hydrogen	Reacts slowly
Anything Below	No reaction	No reaction