GCSE EDEXCEL Obtaining & Using Metals

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

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.

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.

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.

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

n the reactions of metals with water, steam and acids, the metals lose electrons and form cations. The metal is oxidised 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

Metals and displacement reactions

Displacement in solutions

A more reactive metal can displace a less reactive metal from its compounds. For example, magnesium is more reactive than copper. It displaces copper from copper sulfate solution:

magnesium + copper sulfate → magnesium sulfate + copper

Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)

In this displacement reaction:

• magnesium becomes coated with copper
• the blue colour of the solution fades as blue copper sulfate solution is replaced by colourless magnesium sulfate solution
  

  Determining a reactivity series

  A reactivity series can be deduced by carrying out several displacement reactions. A piece of metal is dipped into a salt solution. Different combinations of metal and salt solution are tested. The table shows the results of one of these investigations.

	Magnesium sulfate solution	Copper sulfate solution	Iron sulfate solution	Reactions
Magnesium	Not done	Brown coating	Black coating	2
Copper	No visible reaction	Not done	No visible reaction	0
Iron	No visible reaction	Brown coating	Not done	1

Question

Use the results in the table to deduce an order of reactivity, starting with the most reactive metal.

The order of reactivity is: magnesium > iron > lead. This is because magnesium could displace lead and iron, iron could only displace lead, but lead could not displace magnesium or iron.

Question

Explain why three combinations of metal and salt solution were not done in the investigation.

A metal cannot displace itself from a solution of one of its salts. There would be no reaction, so these combinations were not done.

Displacement reactions as redox reactions - Higher

A balanced equation for the reaction between magnesium and copper sulfate solution can be written in terms of the ions involved:

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

Sulfate ions, SO₄^2-, appear on both sides of the equation. They do not take part in the reaction and are called spectator ions. The equation can be rewritten without them:

Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)

This equation is an example of a balanced ionic equation. It can be split into two half equations:

Mg(s) → Mg²⁺(aq) + 2e^- (oxidation)

Cu²⁺(aq) + 2e^- → Cu(s) (reduction)

Notice that:

• magnesium atoms lose electrons - they are oxidised
• copper ions gain electrons - they are reduced

Reduction and oxidation happen at the same time, so the reactions are called redox reactions.

Displacement reactions are just one example of redox reactions. Electrolysis reactions are also redox reactions.

Note that the reaction of metals with acids can also be described as a displacement reaction or a redox reaction. Only metals above hydrogen in the reactivity series will react and displace hydrogen from acids.

For example:

Magnesium + sulfuric acid → zinc sulfate + hydrogen

Written with the ions involved:

Zn(s) + 2H⁺(aq) + SO₄^2-(aq) → Zn²⁺(aq) + SO₄^2-(aq) + H₂(g)

Removing the spectator ions this becomes an ionic equation:

Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)

The half equations are:

Zn(s) → Zn²⁺(aq) + 2e^-

2H⁺(aq) + 2e^- → H₂(g)