Question

What is a redox reaction? Give two examples of redox reaction. Is it possible that a displacement reaction can be a redox reaction? Give examples of two redox reactions from your daily life.

Answer 1

1. Redox reaction
Redox reaction are reaction in which oxidation(loss of electron) and reduction(gain of electron) takes place simultaneously.

2. Example:
a. Reaction of magnesium with air

$2\mathrm{Mg}\left(\mathrm{s}\right)+\mathrm{O}{}_2(\mathrm{g})\to 2\mathrm{MgO}(\mathrm{S})$
(Magnesium) (Oxygen) (Magnesium oxide)
In above reaction oxidation of magnesium takes place which can be justified from two reasons:
i. Addition of oxygen is taking place.
ii. Oxidation number is changing from $0$(in $\mathrm{Mg}$) to $+2$(in $\mathrm{MgO}$) which shows loss of electron.
In above reaction reduction of oxygen takes place since oxidation number is changing from $0$ to $-1$ which shows gain of electrons.

b. Reaction of iron oxide with aluminium

$\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})+2\mathrm{Al}(\mathrm{s})\to 2\mathrm{Fe}(\mathrm{s})+\mathrm{Al}{}_2\mathrm{O}{}_3(\mathrm{s})$
Iron(III)oxide Aluminum Iron Aluminum oxide

• Calculation of oxidation state:

$\mathrm{Fe}{}_2\mathrm{O}{}_3$ $\mathrm{Al}{}_2\mathrm{O}{}_3$

Oxidation state of $\mathrm{Fe}$ will be: Oxidation state of $\mathrm{Al}$ will be:

Let $\mathrm{x}$ be the oxidation number of $\mathrm{Fe}$ Let $\mathrm{x}$ be the oxidation number of $\mathrm{Al}$

Oxidation state of $\mathrm{O}$ is $-2$ Oxidation state of $\mathrm{O}$ is $-2$

so for $\mathrm{Fe}{}_2\mathrm{O}{}_3$ $2\mathrm{x}+3(-2)=0$ so for $\mathrm{Al}{}_2\mathrm{O}{}_3$ $2\mathrm{x}+3(-2)=0$

$2\mathrm{x}=6$ $2\mathrm{x}=6$

$\mathrm{x}=+3$ $\mathrm{x}=+3$

Oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is $+3$. Oxidation state of $\mathrm{Al}$ in $\mathrm{Al}{}_2\mathrm{O}{}_3$ is $+3$.

⦁ Here oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is $+3$ and that of $\mathrm{Al}$ in $\mathrm{Al}$ is $0$ at reactant side but oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}$ is $0$ and that of $\mathrm{Al}$ in $\mathrm{Al}{}_2\mathrm{O}{}_3$ is $+3$ at product side.
⦁ So, we can say that $\mathrm{Al}$ is getting oxidised since its oxidation number is increasing from $0$ to $+3$ and $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is getting reduced since its oxidation number is decreasing from$+3$ to $0$ .
⦁ The one who gets oxidised act as a reducing agent and the one who get reduced act as an oxidising agent.
⦁ So here $\mathrm{Al}$ is a reducing agent and $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is an oxidising agent.

3. Yes, displacement reaction can also be a redox reaction.

Displacement reaction- Reaction in which more reactive element displaces less reactive element from its aqueous solution . $\mathrm{A}+\mathrm{BC}\to \mathrm{AC}+\mathrm{B}$
Example:

$\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})+2\mathrm{Al}(\mathrm{s})\to 2\mathrm{Fe}(\mathrm{s})+\mathrm{Al}{}_2\mathrm{O}{}_3(\mathrm{s})$
Iron(III)oxide Aluminum Iron Aluminum oxide

here $\mathrm{Al}$ is displacing $\mathrm{Fe}$ from $\mathrm{Fe}{}_2\mathrm{O}{}_3$and forming $\mathrm{Al}{}_2\mathrm{O}{}_3$
According to reactivity series: Aluminium is more reactive than iron.

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

Here oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is $+3$ and that of $\mathrm{Al}$ in $\mathrm{Al}$ is $0$ at reactant side but oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}$ is $0$ and that of $\mathrm{Al}$ in $\mathrm{Al}{}_2\mathrm{O}{}_3$ is $+3$ at product side.
⦁ So, we can say that $\mathrm{Al}$ is getting oxidised since its oxidation number is increasing from $0$ to $+3$ and $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is getting reduced since its oxidation number is decreasing from$+3$ to $0$ .
⦁ The one who gets oxidised act as a reducing agent and the one who get reduced act as an oxidising agent.
⦁ So here $\mathrm{Al}$ is a reducing agent and $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is an oxidising agent.

Therefore it is example of both displacement as well as redox reaction.

4. Example of redox reaction in daily life:
a. Combustion of fuels
In LPG cylinders majorly butane is present which on combustion produces carbon di oxide.
$\mathrm{C}{}_4\mathrm{H}{}_{12}(\mathrm{g})+\mathrm{O}{}_2(\mathrm{g})\to \mathrm{CO}{}_2(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})$
Butane Oxygen Carbon dioxide Water

Here oxidation state of $\mathrm{C}$ is $+3$ in $\mathrm{C}{}_4\mathrm{H}{}_{12}$ and that of $\mathrm{O}$ in $\mathrm{O}{}_2$ is $0$ at reactant side but oxidation state of $\mathrm{C}$ in $\mathrm{CO}{}_2$ is $+4$ and that of $\mathrm{O}$ in $\mathrm{H}{}_2\mathrm{O}$ is $-2$ at product side.
⦁ So, we can say that $\mathrm{C}$ is getting oxidised since its oxidation number is increasing from $+3$ to $+4$ and $\mathrm{O}{}_2$ is getting reduced since its oxidation number is decreasing from $0$ to $-2$ .
So butane get oxidised to carbon dioxide and oxygen is reduced to water.

b. Rusting of iron
Iron get rusted in presence of air and moisture.
$4\mathrm{Fe}\left(\mathrm{s}\right)+3\mathrm{O}{}_2(\mathrm{g})+\mathrm{xH}{}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{Fe}{}_2\mathrm{O}{}_3.\mathrm{xH}{}_2\mathrm{O}(\mathrm{s})$
Iron oxygen water Rust

Here oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}$ is $0$ and that of $\mathrm{O}$ in $\mathrm{O}{}_2$ is $0$ at reactant side but oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is $+3$ and that of $\mathrm{O}$ in $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is $-2$ at product side.
⦁ So, we can say that $\mathrm{Fe}$ is getting oxidised since its oxidation number is increasing from $0$ to $+3$ and $\mathrm{O}{}_2$ is getting reduced since its oxidation number is decreasing from $0$ to $-2$ .
So, iron get oxidised and oxygen get reduced.