Question

Which one of the following is not a redox reaction?

• A. ${\mathrm{CaCO}}_3\to \mathrm{CaO}+{\mathrm{CO}}_2$
• B. $\mathrm{Na}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{NaOH}+\frac{1}{2}{\mathrm{H}}_2$
• C. $2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$
• D. ${\mathrm{MnCl}}_3\to {\mathrm{MnCl}}_2+\frac{1}{2}{\mathrm{Cl}}_2$

Answer 1

The correct option is A

${\mathrm{CaCO}}_3\to \mathrm{CaO}+{\mathrm{CO}}_2$

Explanation for correct option

(A) ${\mathbf{CaCO}}_{\mathbf{3}}\mathbf{\to }\mathbf{CaO}\mathbf{+}{\mathbf{CO}}_{\mathbf{2}}$

Redox reaction

• A chemical process in which electrons are transferred between two reactants is referred to as a redox reaction.
• The change in the oxidation states of the reacting species can be used to identify this electron transfer.

${\mathbf{CaCO}}_{\mathbf{3}}\mathbf{\to }\mathbf{CaO}\mathbf{+}{\mathbf{CO}}_{\mathbf{2}}$

• The reaction is as follows:

$\stackrel{+2}{\mathrm{Ca}}\stackrel{+4}{\mathrm{C}}{\stackrel{-2}{\mathrm{O}}}_3\to \stackrel{+2}{\mathrm{Ca}}\stackrel{-2}{\mathrm{O}}+\stackrel{+4}{\mathrm{C}}{\stackrel{-2}{\mathrm{O}}}_2$

• In this reaction, any species has no change in the oxidation number.
• It is not a redox reaction.
• Breaking down calcium carbonate into calcium oxide and carbon dioxide is a decomposition reaction.

Explanation for incorrect options

(B) $\mathbf{Na}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\to }\mathbf{NaOH}\mathbf{+}\frac{\mathbf{1}}{\mathbf{2}}{\mathbf{H}}_{\mathbf{2}}$

• The reaction is as follows:

$\stackrel{0}{\mathrm{Na}}+{\stackrel{+1}{\mathrm{H}}}_2\stackrel{-2}{\mathrm{O}}\to \stackrel{+1}{\mathrm{Na}}\stackrel{-2}{\mathrm{O}}\stackrel{+1}{\mathrm{H}}+\frac{1}{2}{\stackrel{0}{\mathrm{H}}}_2$

• In this reaction, there is a change in the oxidation number of species.
• It is a redox reaction.

(C) $\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{\to }\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$

• The reaction is as follows:

$2{\stackrel{0}{\mathrm{H}}}_2+{\stackrel{0}{\mathrm{O}}}_2\to 2{\stackrel{+1}{\mathrm{H}}}_2\stackrel{-2}{\mathrm{O}}$

• In this reaction, there is a change in the oxidation number of species.
• It is a redox reaction.

(D) ${\mathbf{MnCl}}_{\mathbf{3}}\mathbf{\to }{\mathbf{MnCl}}_{\mathbf{2}}\mathbf{+}\frac{\mathbf{1}}{\mathbf{2}}{\mathbf{Cl}}_{\mathbf{2}}$

• The reaction is as follows:

$\stackrel{+3}{\mathrm{Mn}}{\stackrel{-1}{\mathrm{Cl}}}_3\to \stackrel{+2}{\mathrm{Mn}}{\stackrel{-1}{\mathrm{Cl}}}_2+\frac{1}{2}{\stackrel{0}{\mathrm{Cl}}}_2$

• In this reaction, there is a change in the oxidation number of species.
• It is a redox reaction.

Therefore, the correct option is (A) ${\mathbf{CaCO}}_{\mathbf{3}}\mathbf{\to }\mathbf{CaO}\mathbf{+}{\mathbf{CO}}_{\mathbf{2}}$.