Electrochemistry - Result Question 74
####22. A solution containing one mole per litre of each $Cu\left(NO _3\right) _2$, $AgNO _3, Hg _2\left(NO _3\right) _2$ and $Mg\left(NO _3\right) _2$ is being electrolysed by using inert electrodes. The values of standard electrode potentials in volts (reduction potential) are
$$ \begin{gathered} Ag^{+} / Ag=+0.80, Hg _2^{2+} / 2 Hg=+0.79 \ Cu^{2+} / Cu=+0.34, Mg^{2+} / Mg=-2.37 \end{gathered} $$
With increasing voltage, the sequence of deposition of metals on the cathode will be
(1984, 1M)
(a) $Ag, Hg, Cu, Mg$
(b) $Mg, Cu, Hg, Ag$
(c) $Ag, Hg, Cu$
(d) $Cu, Hg, Ag$
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Answer:
Correct Answer: 22. (c)
Solution:
- In aqueous solution, only those ions who are less electropositive than hydrogen $\left(E^{\circ}>0\right)$ would be deposited.
Therefore, in the present case, only $Ag, Hg$ and $Cu$ would be deposited on passing electricity through aqueous solution of these ions, $Mg$ will not be deposited.
Also, higher the value of $E^{\circ}$, easier will be their reduction, therefore, the sequence in which ions will be deposited on increasing voltage across the electrodes is :
$$ Ag, Hg, Cu \text {. } $$
