Electrochemistry 1 Question 22
22. A solution containing one mole per litre of each $\mathrm{Cu}\left(\mathrm{NO} _{3}\right) _{2}$, $\mathrm{AgNO} _{3}, \mathrm{Hg} _{2}\left(\mathrm{NO} _{3}\right) _{2}$ and $\mathrm{Mg}\left(\mathrm{NO} _{3}\right) _{2}$ is being electrolysed by using inert electrodes. The values of standard electrode potentials in volts (reduction potential) are
$ \begin{gathered} \mathrm{Ag}^{+} / \mathrm{Ag}=+0.80, \mathrm{Hg} _{2}^{2+} / 2 \mathrm{Hg}=+0.79 \\ \mathrm{Cu}^{2+} / \mathrm{Cu}=+0.34, \mathrm{Mg}^{2+} / \mathrm{Mg}=-2.37 \end{gathered} $
With increasing voltage, the sequence of deposition of metals on the cathode will be
(1984, 1M)
(a) $\mathrm{Ag}, \mathrm{Hg}, \mathrm{Cu}, \mathrm{Mg}$
(b) $\mathrm{Mg}, \mathrm{Cu}, \mathrm{Hg}, \mathrm{Ag}$
(c) $\mathrm{Ag}, \mathrm{Hg}, \mathrm{Cu}$
(d) $\mathrm{Cu}, \mathrm{Hg}, \mathrm{Ag}$
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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 $\mathrm{Ag}, \mathrm{Hg}$ and $\mathrm{Cu}$ would be deposited on passing electricity through aqueous solution of these ions, $\mathrm{Mg}$ 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 :
$ \mathrm{Ag}, \mathrm{Hg}, \mathrm{Cu} \text {. } $
