Question

Identify the type of chemical reaction taking place in each of the following:

Iron nails when left dipped in blue copper(II) sulphate become brownish in colour and the blue colour of copper(II) sulphate fades away.

Answer 1

1. Type of reaction

Combination reaction: Reaction in which two or more reactants combine to form a single product. $\mathrm{A}+\mathrm{B}\to \mathrm{C}$

Decomposition reaction: Reaction in which reactant is broken down into two or more products in the presence of heat, light or electricity. $\mathrm{A}\to \mathrm{B}+\mathrm{C}$

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)

Double displacement reaction: Reaction in which ions of both the reactants interchange their place and form new compounds. $\mathrm{AB}+\mathrm{CD}\to \mathrm{AD}+\mathrm{CB}$

2. Reason

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.

3. Reaction

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

Copper(II)sulphate Iron Iron sulphate Copper

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

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

4. 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.