An iron nail in blue copper sulphate solution gets a brown copper coating and the blue fades. A more reactive metal displaces a less reactive one from its compound. The reverse does not occur.
Put a clean iron nail into blue copper sulphate solution and leave it for about 20 minutes. Two changes can be seen. The nail is covered with a reddish-brown coating. The blue colour of the solution becomes paler.
The blue colour of copper sulphate solution comes from dissolved copper ions, Cu²⁺. During the reaction, iron atoms from the nail go into solution as iron ions, Fe²⁺. Copper ions leave the solution and are deposited on the nail as copper metal. The brown coating is copper.
Fe + CuSO₄ → FeSO₄ + Cu
This is a displacement reaction: an element takes the place of another element in a compound, and the second element is released.
As the reaction continues, copper ions leave the solution and iron ions enter it. What colour change would you expect in the solution?
The blue becomes paler, because the number of copper ions in solution decreases. Iron(II) sulphate solution is pale green, so the solution turns from blue towards pale green.
The brown coating and the fading blue are two observations of one process: copper moving from the solution onto the nail, while iron moves from the nail into the solution.
Now put a clean copper strip into iron sulphate solution. No change occurs. Copper does not displace iron.
The difference is reactivity. A metal’s reactivity describes how readily its atoms give up electrons to form positive ions. Iron atoms give up electrons more readily than copper atoms. So when iron atoms meet copper ions, electrons pass from iron to copper: iron becomes ions, and copper ions become atoms. Copper atoms do not give electrons to iron ions, so the reverse reaction does not happen.
The general rule: a more reactive metal displaces a less reactive metal from a solution of its compound. Zinc and lead are both more reactive than copper, so both displace it:
Zn + CuSO₄ → ZnSO₄ + Cu
Pb + CuCl₂ → PbCl₂ + Cu
Part of the reactivity series. A metal higher in the list displaces any metal below it from a solution of its compound. A metal lower in the list does not displace one above it.
Silver is below copper in the reactivity series. Predict what happens when a silver strip is placed in copper sulphate solution.
No reaction. Silver is less reactive than copper, so silver atoms do not give electrons to copper ions. The strip and the solution stay unchanged.
A result of “no reaction” is also evidence. It shows that silver is below copper in the series.
In the equation, an element reacts with a compound. In the products, that element has become part of a new compound, and one element from the original compound is now on its own.
To predict whether a metal displaces another, compare their positions in the reactivity series. If the added metal is higher, the reaction occurs. If it is lower, no reaction occurs.
1In a displacement reaction…
2An iron nail in copper sulphate solution gets a brown coating. The coating is…
3A copper strip is placed in iron sulphate solution. What happens?
4Using the order Zn > Fe > Pb > Cu > Ag, does a reaction occur?
5The list of metals in order of decreasing reactivity is called the ____ series.
6What does it mean to say iron is more reactive than copper?
7Why is a “no reaction” result useful?
Next: reactions in which two compounds exchange ions and a solid forms.
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