Question

Define displacement reaction and double displacement reaction and give one example of each reaction.

Answer 1

1. Displacement reaction:

Reaction in which one of the higher reactive element displaces a less reactive element from its solution.

$\mathrm{A}+\mathrm{BC}\to \mathrm{AC}+\mathrm{B}$ ( Here A is more reactive than B)

Example

When iron nails are dipped in ${\mathrm{CuSO}}_4$ solution, displacement reaction takes place and formation of ferrous sulphate takes place which is a green colour solution and on iron nail deposition of copper takes place which turns grey iron nail to brown in colour.

$\mathrm{CuSO}{}_4(\mathrm{aq})+\mathrm{Fe}(\mathrm{s})\to \mathrm{FeSO}{}_4(\mathrm{aq})+\mathrm{Cu}\left(\mathrm{s}\right)$

Copper(II)sulphate Iron Iron sulphate Copper

(Blue ) (Grey) (Green) (Brown)

Since $\mathrm{Fe}$ is more reactive than $\mathrm{Cu}$ displacement takes place.

Reactivity series

$\mathrm{K}>\mathrm{Na}>\mathrm{Ca}>\mathrm{Mg}>\mathrm{Al}>\mathrm{Zn}>\mathrm{Fe}>\mathrm{Ni}>\mathrm{Sn}>\mathrm{Pb}>\mathrm{H}>\mathrm{Cu}>\mathrm{Hg}>\mathrm{Ag}>\mathrm{Au}>\mathrm{Pt}$

Here potassium is the most reactive metal and platinum is the least reactive metal in this series.

So, from the reactivity series, we can see $\mathrm{Fe}$ is more reactive than $\mathrm{Cu}$ so it can displace copper from its solution and therefore the blue colour is destroyed.
This is a displacement reaction.

2. Double displacement reaction: Reaction in which ions of both the reactants interchange their place and form new compounds.

$\mathrm{AB}+\mathrm{CD}\to \mathrm{AD}+\mathrm{CB}$

Example:

$\mathrm{NaBr}\left(\mathrm{aq}\right)+{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)\to \mathrm{AgBr}\left(\mathrm{s}\right)+{\mathrm{NaNO}}_3\left(\mathrm{aq}\right)$

Sodium bromide Silver nitrate Silver bromide Sodium nitrate

Here $\mathrm{Na}$ and $\mathrm{Ag}$ are interchanging their position and forming new compounds.

Here yellow precipitate of $\mathrm{AgBr}$ is formed which is used in black and white photography.

Since new compounds are formed by interchanging positions between two ions it is called a double displacement reaction.