Introduction to displacement reactions
Key points
- In displacement reactions a more reactive metal will displace a less reactive metal from its compound.
- The reactivity series is a list of metals from the most reactive at the top to the least reactive at the bottom. It can be used to predict displacement reactions.
- Carefully planned displacement experiments can be used to put metals into a reactivity series.
Displacement reactions
A video about displacement reactions
We鈥檙e surrounded by metals like iron, silver, copper, gold and magnesium.
Most metals go through a chemical reaction when they are combined with other substances, but some metals react more easily than others 鈥 they are more reactive.
When you list metals in order of reactivity, you make what鈥檚 known as a reactivity series.
We can use the reactivity series to predict what will happen when we add metals to solutions of metal compounds.
Here we鈥檝e got an iron nail and some copper sulfate solution. Copper sulfate is a compound that contains the metal copper.
We start by filling the beaker with copper sulfate solution about halfway.
Then put the iron nail into the solution.
Quite quickly we can see a reaction starting to happen. The section of the iron nail that is in the solution becomes coated in a pinkish/brownish colour. It looks like the nail has rusted but this colour is not rust 鈥 it's actually copper. So what has happened?
Iron is more reactive than copper. The iron pushes copper out of its compound 鈥 iron displaces copper.
This is a displacement reaction.
Two new products have formed: copper and iron sulfate. In fact, we can see the colour of the solution changes as the copper sulfate gets replaced by iron sulfate. The word equation for the reaction is: iron + copper sulfate 鈫 copper + iron sulfate.
With this information we can predict what the result would be if we put copper into iron sulfate. As iron is more reactive than copper there will be no reaction. Copper is less reactive than iron, so cannot displace it from its compound.
Displacement reactions can help us arrange metals into a reactivity series. These word equations shows that zinc displaces iron but not magnesium. So where should we put zinc in our reactivity series?
Zinc must be more reactive than iron, but less reactive than magnesium so it goes in between them 鈥 here.
A chemical reaction in which a more reactive element displaces (pushes out) a less reactive element from a compound and takes its place. reactions are When chemical bonds are broken and made between atoms, so that new substances (compounds or elements) are made. which involve a metal and a A pure substance made from two or more elements which are chemically bonded in a fixed ratio. containing a different metal.
These compounds containing metals and non-metals are called salts. For example, iron (a metal) reacts with copper sulfate (a salt containing copper).
In a displacement reaction, a less reactive metal is displaced from its compound by a more reactive metal.
There is no reaction between a metal and a salt of the same metal. For example, iron cannot When an element is displaced, it is pushed out of a compound by a more reactive element. iron from iron chloride (a salt).
A more reactive metal displaces a less reactive metal. If there is no difference in reactivity between two metals, then no reaction can take place.
For example:
iron oxide + aluminium 鈫 iron + aluminium oxide
Aluminium is more reactive than iron. This means the aluminium takes the oxygen from the iron oxide to produce aluminium oxide. The iron has been displaced from its compound so it is not bonded to anything after the reaction.
What happens in a displacement reaction?
Displacement reactions can be seen when a metal salt containing a less reactive metal is dissolved into water to make a A mixture made when a solute (usually a solid) dissolves into a solvent (a liquid). Sea water is a solution of salt dissolved into water.. A more reactive metal is then dipped into it.
During this displacement reaction:
- the more reactive metal becomes less visible as it dissolves into the solution
- the less reactive metal from the salt coats the surface of the more reactive metal as it is displaced from its compound
Example
Magnesium is more reactive than copper. When magnesium is added to blue copper sulfate solution:
- the blue colour fades as a colourless magnesium sulfate solution forms
- brown copper coats the surface of the magnesium as the copper is displaced
The word equation for this reaction is:
magnesium + copper sulfate 鈫 magnesium sulfate + copper
The symbol equation is:
Mg + CuSO鈧 鈫 MgSO鈧 + Cu
The diagram shows the displacement reaction between magnesium and copper sulfate.
Iron is dipped into a copper sulfate solution. The products are iron sulfate and copper. Is iron or copper the more reactive metal?
Iron.
Iron has displaced copper from its copper sulfate compound. This shows that iron is the more reactive metal.
Order of reactivity
A series of displacement experiments can be used to put metals in order of reactivity. This involves placing metal pieces in different metal salt solutions.
In this diagram, an iron nail is placed in a solution of copper sulfate. The iron displaces the copper and copper builds up on the exterior of the nail.
The table shows the results of a series of experiments involving four metals and solutions of their salts. A tick shows where there was a visible reaction and a cross shows where there was no visible reaction.
| Magnesium | Zinc | Iron | Copper | |
|---|---|---|---|---|
| Magnesium salt | 鉁 | 鉁 | 鉁 | |
| Zinc salt | 鉁 | 鉁 | 鉁 | |
| Iron salt | 鉁 | 鉁 | 鉁 | |
| Copper salt | 鉁 | 鉁 | 鉁 | |
| Reactions seen | 3 | 2 | 1 | 0 |
Magnesium displaces three metals, zinc displaces two metals, iron displaces one metal and copper does not displace any of the other three metals.
From the results, it can be concluded that the order of reactivity, starting with the most reactive is:
- magnesium
- zinc
- iron
- copper
There is a gap in the results table because there is no reaction between a metal and its salt so these experiments aren鈥檛 carried out.
Gold, silver and platinum are used in jewellery. Where are they found in the reactivity series?
They are at the bottom of the reactivity series.
Gold, silver and platinum don鈥檛 react easily with air and water. This means they stay shiny when made into jewellery.
Temperature measurements
When displacement reactions occur, the temperature rises. The larger the temperature rise, the higher the metal is in the A list of elements in order of reactivity, with the most reactive one at the top. The reactivity series usually includes metals and occasionally some non-metals..
The table shows the reaction of five metals with copper sulfate solution. The metals are placed in order of reactivity, with the most reactive (Magnesium) at the top and the least reactive (Silver) at the bottom. Copper sulfate is chosen for the solution because copper is low in the reactivity series.
| Metal | Temperature rise (掳C) |
|---|---|
| Magnesium | 14 |
| Zinc | 16 |
| Iron | 9 |
| Tin | 5 |
| Silver | 0 |
The results show that:
- zinc had the highest temperature rise at 16 掳C
- silver had the lowest with no temperature rise
The results mostly show the same order as the reactivity series, except for magnesium and zinc.
Comparing the reactivity series with the results, it would be expected that magnesium gave a larger temperature rise than zinc. Magnesium is the most reactive metal of those tested. It reacts slowly with oxygen in the air meaning pieces of magnesium often have a coating of magnesium oxide. This coating slows down the reaction so the temperature rise is not as high as expected and lower than the rise for zinc.
There are often scientific explanations for unexpected results. We call unexpected results that do not fit the pattern or our scientific understanding, anomalies or outliers.
Find out more about concluding and evaluating in this guide.
Did you know?
Gold is so unreactive it doesn鈥檛 react with oxygen, water or most acids. One of the few things that it will react with is a strong acid mixture called aqua regia.
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