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WAEC Chemistry 2021 Paper

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Question : 26

Total: 50

Consider the following diagram and use it to answer the question below

Which of the following half reaction equations represent the reaction at the cathode

$\mathrm{A}^{3+}(\mathrm{aq})^{+} 3 \mathrm{e}^{-} \rightarrow \mathrm{A}(s)$
$\mathrm{B}^{2+} + (\mathrm{aq}) + 2 \mathrm{e}^{-} \rightarrow \mathrm{B}(s).$
$\mathrm{A}_{(s)} \rightarrow \mathrm{A}^{3+}(\mathrm{aq})^{+} + 3 \mathrm{e}^{-}$
$\mathrm{B}(s) \rightarrow \mathrm{B}^{2+}(\mathrm{aq}) + 2 \mathrm{e}^{-}$

Solution:

Concept:

The cathode is the electrode where reduction (gain of electrons) takes place.

Explanation:

In the given electrochemical cell, the cathode is electrode

\mathrm{B}

.

At the cathode, ions in the solution accept electrons and get reduced to the metal.

Thus,

\mathrm{B}^{2+}(\mathrm{aq})

ions gain

2

electrons to form

\mathrm{B}(\mathrm{s})

.

The correct half-cell reaction is:

\mathrm{B}^{2+}(\mathrm{aq}) + 2\mathrm{e}^- \rightarrow \mathrm{B}(\mathrm{s})

This corresponds to option

\mathrm{B}

.

Answer:

Option B.

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