Introduction to displacement reactions

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Key points

  • The word displace means to push out.
  • A more reactive element can displace a less reactive element out of its compound during a chemical reaction.
  • Displacement reactions can be used to investigate the reactivity of metals and extract metals from metal oxides.

Video - displacement reactions

Watch this video which explains displacement reactions involving metals.

Displacement reactions in metals

In a solution containing copper compounds, what metal can be used to extract copper?

Iron can be dipped into a solution of copper compounds to produce valuable metallic copper.

The displacement reactions of metals

A chemical is described as being reactive if it takes part easily and quickly in .

Some metals are more reactive than others. Metals can be arranged in order of their reactivity. This is called a .

Unreactive metals take part more slowly in chemical reactions or don’t react at all.

reactions involve a metal and the of a different metal. A more reactive metal will displace or push out a less reactive metal from its compound in a displacement reaction. The less reactive metal is left uncombined after the reaction. It is no longer chemically bonded to any other . It is now a pure element.

Example

Magnesium is more than copper. When a piece of magnesium is dipped into blue copper sulfate solution, a displacement reaction occurs.

The magnesium displaces the copper, and the products are copper and a solution of magnesium sulfate.

This is the word equation:

magnesium + copper sulfate → copper + magnesium sulfate

This is the symbol equation:

Mg(s) + CuSO(aq) → Cu(s) + MgSO(aq)

This reaction cannot happen in reverse. Copper cannot displace magnesium from a compound because copper is less reactive than magnesium.

A displacement reaction
A displacement reaction

Can copper displace magnesium from magnesium chloride solution?

No, copper is less reactive than magnesium. Only a more reactive metal will displace a less reactive metal from its compound.

The reactivity series for common metals

Working scientifically

Observing displacement reactions

When observing displacement reactions look for a colour change or change in temperature. Record any observations in a results table, using words instead of numbers with units.

When recording a colour change, always include both the starting colour as well as the end colour. For example, from orange to brown.

If it looks like nothing happened, it is better to write down “no visible change” rather than “nothing”. Something might have happened which wasn’t visible.

Find out more about observation and measurement skills.

Using displacement reactions to extract metals from their oxides

Displacement reactions can be used to extract metals from their oxides.

Carbon is a non-metal element which is cheap and easy to find. It is more reactive than some metals, such as copper, iron and lead. This means that some metals can be extracted from their metal oxides using carbon.

The crushed rock is mixed with carbon and heated strongly. An example reaction is:

copper oxide + carbon → copper + carbon dioxide

This method will not work for metals like aluminium, which are more reactive than carbon. Carbon cannot displace these metals.

The reactivity series showing that carbon fits between aluminium and zinc
Carbon fits between aluminium and zinc in the reactivity series

Write an equation to show how carbon can be used to displace lead from lead oxide.

lead oxide + carbon → lead + carbon dioxide

Displacement reactions in non-metals

Chlorine, bromine and iodine all react in similar ways. Chlorine is the most reactive and iodine is the least reactive. In these non-metals:

  • Chlorine can displace bromine and iodine from their compounds.
  • Bromine can displace iodine but not chlorine.
  • Iodine cannot displace chlorine or bromine.
  • If chlorine is added to a solution of sodium bromide, a reaction occurs:

chlorine + sodium bromide → bromine + sodium chloride

  • If bromine is added to a solution of sodium iodide, a reaction occurs:

bromine + sodium iodide → iodine + sodium bromide

  • But if iodine is added to sodium chloride or sodium bromide, no reaction happens. The iodine is not reactive enough to displace those elements.
Chlorine water is added to three solutions - potassium chloride, potassium bromide and potassium iodide

Chlorine water is added to three solutions - potassium chloride, potassium bromide and potassium iodide.

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If bromine is added to a solution of sodium chloride. What will happen?

No displacement reaction will occur because the bromine is less reactive than the chlorine.

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