Reactivity of metals

Metal oxides

Metals can react with oxygen to form metal oxides. To form a metal oxide, an oxidation reaction occurs as oxygen is added to the metal. 

2Zn + O₂ ⟶ 2ZnO

Metal oxides can also lose their oxygen which is known as a reduction reaction. 

CuO + C ⟶ Cu + CO

What is the reactivity series of metals?

The reactivity series places metals in order of reactivity based on their reactions with acids and water. Metals lose electrons and form positive ions when they react with another element. This tendency to lose electrons and form positive metal ions determines the reactivity of the element. The greater the tendency to lose electrons, the greater the reactivity.  

Metals reacting with water 

Only metals above hydrogen in the reactivity series will react with water. The speed of the reaction depends on how high up the metal is in the reactivity series. Some metals, such as iron, have slow reactions with water, while others, like potassium, react vigorously with water. 

Some metals react with cold water to form a metal hydroxide and hydrogen gas:

sodium + water ⟶  sodium hydroxide + hydrogen

Other metals react with gaseous water to form a metal oxide and hydrogen gas:

magnesium + water  ⟶ magnesium oxide + hydrogen

Metals reacting with acids 

Only metals above hydrogen in the reactivity series will react with dilute acids. The hydrogen in the acid is replaced by the metal atom to produce a salt and hydrogen gas:

zinc + hydrochloric acid ⟶  zinc(II) chloride + hydrogen

Hydrogen is placed in the reactivity series because metals only react with acid if the metal is more reactive than hydrogen. Carbon is placed in the reactivity series because it allows us to see whether a metal oxide can be reduced by carbon or not for metal extraction.

Metal Extraction

Many metals exist as ores in the Earth’s crust. Ores are rocks containing enough metal that it is economically worthwhile extracting them. Usually, the ore is an oxide of the metal, for example, aluminium oxide. 

The method used to extract these metals from their ores depends on the reactivity of the metal:

• Metals above carbon in the reactivity series are extracted using electrolysis 
• Metals below carbon in the reactivity series are extracted using carbon as a reducing agent:

metal oxide + carbon   →   metal   +   carbon dioxide

Some metals, such as gold, are unreactive and found as an uncombined element so do not need to be chemically extracted. These are known as native metals. 

What are displacement reactions?

Displacement reactions occur when a more reactive metal takes the place of a less reactive metal in one of its compounds, for example:

zinc + copper oxide → zinc oxide + copper

Zinc, which is higher up the reactivity series than copper, displaces the copper from its oxide. Displacement also occurs between metals and aqueous solutions of metal salts, for example:

Mg + CuSO₄  →  MgSO₄ + Cu

Copper(II) sulfate solution is blue. As the magnesium displaces the copper in the compound, the blue colour will fade as the magnesium sulfate solution forms, which is colourless. 

Oxidation & Reduction in Terms of Electrons

During displacement, electrons are lost and gained. The loss of electrons is called oxidation, and the gain of electrons is called reduction. We can write ionic equations and half equations for displacement reactions. 

The word equation for the displacement of copper in copper sulfate by magnesium is:

Magnesium + copper sulfate  →  magnesium sulfate + copper 

The ions involved in the equation are:

Mg (s) + Cu²⁺ (aq) + SO₄^2- (aq) → Mg²⁺ (aq) + SO₄^2- (aq) + Cu (s)

To write the ionic equation, remove the ions that appear on both sides of the equation (known as spectator ions). In this case, it is the sulfate ions:

Mg (s) + Cu²⁺ (aq)  → Mg²⁺ (aq)  + Cu (s)

To write the half equations, identify the species that has been oxidised and reduced:

Mg → Mg²⁺ + 2e^–      

Cu²⁺ + 2e^– → Cu

• Magnesium atoms lose electrons so are oxidised 
• Copper ions gain electrons so are reduced

What keyword definitions do I need to know for the reactivity of metals? 

Some keyword definitions you need to know are:

• Oxidation - the gain of oxygen and loss of electrons
• Reduction - the loss of oxygen and gain of electrons
• Redox - oxidation and reduction occurring simultaneously
• Displacement - a more reactive element takes the place of a less reactive element 

This is a quick summary of some key concepts on the reactivity of metals - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!