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WAEC Chemistry 2018 Paper

Question : 3

Total: 50

Which of the following equations represents a redox reaction?

$\mathrm{HNO}_{3\text{(aq)}} + \mathrm{NaOH}_{\text{(aq)}} \to \mathrm{NaNO}_{3\text{(aq)}} + \mathrm{H}_{2}\mathrm{O}_{\text{(l)}}$
$\mathrm{AgNO}_{3\text{(aq)}} + \mathrm{KCl}_{\text{(aq)}} \to \mathrm{AgCl}_{\text{(s)}} + \mathrm{KNO}_{3\text{(aq)}}$
$\mathrm{CaCO}_{3\text{(s)}} \to \mathrm{CaO}_{\text{(s)}} + \mathrm{CO}_{2\text{(g)}}$
$2\mathrm{H}_{2}\mathrm{S}_{\text{(s)}} + \mathrm{SO}_{2\text{(g)}} \to 2\mathrm{HO}_{2} + 3\mathrm{S}_{\text{(g)}}$

Solution:

Concept:

A redox reaction involves a change in oxidation states, so one substance is oxidised while another is reduced.

Explanation:

Check whether oxidation numbers change in each option.

Options A, B, and C have no change in oxidation states, so they are neutralisation, precipitation, and thermal decomposition reactions respectively.

For option D, the balanced equation is:

2\mathrm{H_2S} + \mathrm{SO_2} \to 2\mathrm{H_2O} + 3\mathrm{S}

Sulfur in

\mathrm{H_2S}

has an oxidation state of

-2

It is oxidised to

0

in elemental sulfur,

\mathrm{S}

Sulfur in

\mathrm{SO_2}

has an oxidation state of

+4

It is reduced to

0

in elemental sulfur,

\mathrm{S}

Since both oxidation and reduction take place, the reaction is a redox reaction.

Answer:

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