Question

Which of the following occurs due to redox reaction?

• A. Liberation of ${\mathrm{H}}_2$ gas on reaction of metal with dilute acid.
• B. Burning of $\mathrm{Mg}$ ribbon in air.
• C. Formation of Ferrous sulphide when iron is heated with sulphur.
• D. Precipitation reaction.

Answer 1

The correct option is A

Liberation of ${\mathrm{H}}_2$ gas on reaction of metal with dilute acid.

Explanation for correct option:

Redox reactions occur when simultaneous oxidation and reduction take place. It also involves a change of oxidation number.

(A) Liberation of Hydrogen (${\mathrm{H}}_2$) gas on reaction of metal with dilute acid is a redox reaction.

For example:

$\underset{\mathrm{Zinc}}{\stackrel{0}{\mathrm{Zn}\left(\mathrm{s}\right)}}+\underset{\mathrm{Hydrochloric}\mathrm{acid}}{\stackrel{+1}{2\mathrm{HCl}\left(\mathrm{aq}\right)}}\to \underset{\mathrm{Zinc}\mathrm{Chloride}}{\stackrel{+2}{{\mathrm{ZnCl}}_2\left(\mathrm{aq}\right)}}+\underset{\mathrm{Hydrogen}}{\stackrel{0}{{\mathrm{H}}_2\left(\mathrm{g}\right)}}$

In the above reaction oxidation occurs as oxidation number of Zinc ($\mathrm{Zn}$) increases from $0$ to $+2$ and reduction occurs due to the decrease in oxidation number of Hydrogen ($\mathrm{H}$) from $+1$ to $0$. So, this reaction is a redox reaction.

Explanation for incorrect options:

(B) Burning of Magnesium ribbon in the air gives the following reaction:

$\underset{\mathrm{Magnesium}}{2\mathrm{Mg}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Magnesium}\mathrm{oxide}}{2\mathrm{MgO}\left(\mathrm{s}\right)}+\mathrm{heat}$

This is an example of an exothermic reaction.

(C) When iron is heated with Sulphur following reaction occurs.

$\underset{\mathrm{Iron}}{\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Sulphur}}{\mathrm{S}\left(\mathrm{g}\right)}\to \underset{\mathrm{Iron}\mathrm{Sulphide}}{\mathrm{FeS}\left(\mathrm{s}\right)}$

This is an example of a combination reaction.

(D) Precipitation reaction involves the formation of one or more insoluble products.

$\underset{\mathrm{Silver}\mathrm{nitrate}}{{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Potassium}\mathrm{chloride}}{\mathrm{KCl}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Silver}\mathrm{Chloride}}{\mathrm{AgCl}\left(\mathrm{s}\right)}+\underset{\mathrm{Potassium}\mathrm{nitrate}}{{\mathrm{KNO}}_3\left(\mathrm{aq}\right)}$

In the above reaction, Silver chloride precipitates out.

Hence, the correct option is (A).