Displacement reactions are a type of chemical reaction in which a more reactive metal replaces a less reactive metal in a compound. These reactions are known as redox (reduction-oxidation) reactions.
During a displacement reaction:
An example of a displacement reaction is when iron is added to copper sulfate solution. Since iron is more reactive than copper, it replaces the copper in the compound, forming iron(II) sulfate and copper. The equation for this is:
Iron + Copper sulfate → Iron(II) sulfate + Copper
During the reaction, the blue colour of the copper sulfate solution changes as iron(II) sulfate forms. Also, copper coats the surface of the iron nail.
We can use displacement reactions to determine the relative positions of metals in the reactivity series. To do this, we add a piece of metal to a solution of a metal salt and observe the reaction. Then we can predict the order of reactivity of the metals based on the results.
For example, let’s consider what happens when we add metals to three different metal salt solutions: silver nitrate, magnesium chloride, and zinc sulfate. The results are recorded in the table below:
|Metal||Silver nitrate||Magnesium chloride||Zinc sulphate|
|Silver||No reaction||No reaction||No reaction|
|Magnesium||Magnesium nitrate and silver||No reaction||Magnesium sulfate and zinc|
|Zinc||Zinc nitrate and silver||No reaction||No reaction|
Based on these results, we can determine the relative order of reactivity of these metals.
Therefore, the order of reactivity, from most reactive to least, is magnesium, zinc and silver.
Mg > Zn > Ag
Displacement reactions are redox (reduction-oxidation) reactions, meaning they involve the transfer of electrons between the reactants. Half equations can be used to represent the redox reactions that occur during displacement reactions.
For example, let’s look at the reaction between zinc (Zn) and copper sulfate (CuSO4):
Zn (s) + CuSO4 (aq) → Cu(s) + ZnSO4 (aq)
During this reaction, zinc displaces copper from the copper sulfate solution.
Zn + Cu²⁺ → Cu + Zn²⁺
Zinc atoms lose electrons to form zinc ions.
The equations for the oxidation and reduction that occur during this reaction are:
Oxidation: Zn → Zn²⁺ + 2e⁻
Reduction: Cu²⁺ + 2e⁻ → Cu
The oxidation half equation shows that zinc atoms lose electrons to form zinc ions, while the reduction half equation shows that copper ions (Cu²⁺) gain electrons to form copper atoms (Cu). These half equations show the transfer of electrons between the reactants.
The sulfate ion (SO4²⁻) is unchanged on both the product and the reactant side of the reaction. Therefore, it is considered a spectator ion, which means it does not participate in the redox reaction and does not need to be included in the half equations.
Oxidation is the loss of electrons (or the gain of oxygen), while reduction is the gain of electrons (or the loss of oxygen.
When a substance undergoes oxidation, it is called the reducing agent, because it causes another substance to be reduced. In contrast, when a substance undergoes reduction, it is called the oxidising agent, because it causes another substance to be oxidised.
It’s important to remember that the oxidising and reducing agents are not always the same in different reactions. The identity of the oxidising and reducing agents depends on the specific reaction and the substances involved.
Let’s look at some examples to see how this works:
Calcium + Zinc sulphate → Calcium sulfate + Zinc
In this reaction, zinc sulfate is reduced to form zinc metal, while calcium metal is oxidised to form calcium sulfate.
Therefore, zinc sulfate is the oxidising agent, because it is causing the oxidation of calcium metal. Meanwhile, calcium metal is the reducing agent, because it is causing the reduction of zinc sulfate.
Aluminium + Copper oxide → Aluminium oxide + Copper
In this reaction, copper oxide is reduced to form copper metal, while aluminium metal is oxidised to form aluminium oxide.
Therefore, copper oxide is the oxidising agent, because it is causing the oxidation of the aluminium metal. Meanwhile, aluminium metal is the reducing agent, because it is causing the reduction of copper oxide.