HS Chemistry - Redox Reactions

Displacement Reactions

Overview of The Page

This page will cover:

What are displacement reactions?
How are Group 17 displacement reactions different from metal displacement reactions?

Displacement reactions are a special type of redox reaction. Normally, they only occur with metals, but they also occur with Group 17 non-metals.

Displacement reactions are perhaps best explained through examples. The following redox reaction:

Ca + FeO → CaO + Fe

Is an example of a displacement reaction. If we break up the ionic compounds, we can see more clearly what is happening:

Ca + Fe²⁺ + O^2- → Ca²⁺ + O^2- + Fe

The redox reaction of that equation is:

Ca + Fe²⁺ → Ca²⁺ + Fe

At the beginning of the reaction, Calcium has a charge of 0 and Iron has a charge of +2, but at the end of the reaction, Calcium has a charge of +2 and Iron has a charge of 0. In the reaction, the Calcium takes the +2 charge away from the Iron $in reality, it gives two of its valence electrons to Iron$ . After this happens, Calcium now has a charge of +2 and Iron now has a charge of 0, which causes the Oxygen anion $O^2\-^$ to bond with the Calcium cation $Ca^2\+^$ . The Calcium has displaced the Iron from the Oxygen.

The reaction Ca + FeO → CaO + Fe is a metal displacement reaction in which Calcium displaces Iron from Oxygen.

Why does it happen this way? Why is Calcium able to displace Iron from Oxygen? Why not the other way around?

The metal reactivity rules state that metal elements become more reactive further down a Group, and to the left of the table. That is, if two metal elements are in the same Period (row) of the Periodic Table, the one that is further to the left on the Periodic Table $smaller Group number$ will be more reactive, and if they are not, the one that is further down the Periodic Table $has more electron shells$ will be more reactive. More reactive metals are able to displace less reactive elements in displacement reactions because of their greater reactivity.

On the Periodic Table, both Calcium and Iron are on the same Period, but Calcium $a Group 2 metal$ is further to the left than Iron $which is a Group 8 Transition Metal$ . Therefore, Calcium is more reactive than Iron, which allows Calcium to displace Iron in the displacement reaction.

This also means that since Iron is less reactive than Calcium, it can't displace Calcium in a displacement reaction. Thus, the reaction:

CaO + Fe → Ca + FeO

Will not work, as Iron can't displace Calcium from Oxygen.

Displacement reactions can also work with more complex ionic compounds. Try determining which element is displacing which element in the following reaction:

2K + Cu $NO~3~$ 2 → 2KNO₃ + Cu

To check your answer, hold and drag your mouse over the following colored bar: Potassium $K$ is displacing Copper $Cu$

Displacement with Halogens

Most non-metals are incapable of displacing one another. However, halogens $non-metals in Group 17, the Fluorine Group$ can displace one another in displacement reactions. Thus, the reaction:

2KBr + Cl₂ → 2KCl + Br₂

Is possible.

At first, this doesn't seem right. The metal reactivity rules say that metal elements become more reactive further down the Group, but Chlorine is higher up in Group 17 than Bromine. It doesn't seem like Chlorine should be able to displace Bromine.

But for halogens, the reactivity rules are different. As mentioned in Group 17 Physical & Chemical Properties, Group 17 elements become less reactive as you go down the Group. Thus, Bromine, which is further down the Group, is less reactive than Chlorine. This allows Chlorine to displace Bromine.

This also means that the reverse reaction:

2KCl + Br₂ → 2KBr + Cl₂

Is not possible, as Bromine is less reactive than Chlorine and therefore can't displace Chlorine.