Question

Name the reducing agent, oxidising agent, substance oxidised and substance reduced in the following redox reaction:

$\mathrm{Cr}{}_2\mathrm{O}{}_3+2\mathrm{Al}\to 2\mathrm{Cr}+\mathrm{Al}{}_2\mathrm{O}{}_3$

Answer 1

• Oxidation means loss of electron in this oxidation state increases.
• Reduction means gain of electron in this oxidation state decreases.
• The one who gets oxidised act as a reducing agent and the one who get reduced act as an oxidising agent.
• Reaction:

$\mathrm{Cr}{}_2\mathrm{O}{}_3+2\mathrm{Al}\to 2\mathrm{Cr}+\mathrm{Al}{}_2\mathrm{O}{}_3$
(Chromium oxide) (Aluminium) (Chromium) (Aluminium oxide)

• Calculation of oxidation state:

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

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

Let $\mathrm{x}$ be the oxidation number of $\mathrm{Cr}$ 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 $\mathrm{Cr}{}_2\mathrm{O}{}_3$ $2\mathrm{x}+3(-2)=0$ 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 $\mathrm{Cr}$ in $\mathrm{Cr}{}_2\mathrm{O}{}_3$ is $+3$. Oxidation state of $\mathrm{Al}$ in $\mathrm{Al}{}_2\mathrm{O}{}_3$ is $+3$.

⦁ Here oxidation state of $\mathrm{Al}$ in $\mathrm{Al}$ is $0$and that of $\mathrm{Cr}$ in $\mathrm{Cr}{}_2\mathrm{O}{}_3$ is$+3$ at reactant side but oxidation state of $\mathrm{Al}$ in $\mathrm{Al}{}_2\mathrm{O}{}_3$is $+3$ and that of $\mathrm{Cr}$ in $\mathrm{Cr}$ is $0$ at product side.
⦁ So, we can say that $\mathrm{Al}$ is getting oxidised since its oxidation number is increasing from $0$ to $+3$ and $\mathrm{Cr}{}_2\mathrm{O}{}_3$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 $\mathrm{Cr}{}_2\mathrm{O}{}_3$is an oxidising agent.