Question

A solution containing one mole per litre of each $Cu(NO_3)_2$, $AgNO_3,Hg_2(NO_3)_2$ and $Mg(NO_3)_2$ is being electrolysed by using inert electrodes. The values of standard electrode potentials in volts (reduction potential) are \hspace15mm Ag^+/ Ag =+0.80, Hg^{2+}_2/2Hg=+0.79 \hspace15mm Cu^+/ Cu =+0.34, Mg^{2+}/Mg=-2.37 With increasing voltage, the sequence of deposition of metals or the cathode will be

• A. $Ag, Hg Cu, Mg$
• B. $Mg, Cu, Hg, Ag$
• C. $Ag, Hg, Cu$
• D. $Cu, Hg, Ag$

Answer 1

Answer: (C)

$Ag, Hg, Cu$

Answer 2

In aqueous solution, only those ions who are less electropositive than hydrogen ($E^\circ$ > 0) 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 :
\hspace35mm Ag, Hg, Cu