Question

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 potentials) are $Ag ^{+} / Ag$ $=+0.80, Hg _{2}^{2+} / 2 Hg =+0.79, Cu ^{2+} / Cu =+0.34$, and $Mg ^{2+} /$ $Mg =-2.37$. With increasing voltage, the sequence of deposition of metals on 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: (B)

A cation having highest reduction potential will be reduced first and so on. However, $Mg ^{2+}$ in aqueous solution will not be reduced
$\left(E_{ Mg ^{2+} / Mg }^{\circ}< E_{ H _{2} O / \frac{1}{2}} H _{2}+ OH ^{-}\right)$
Instead water would be reduced in preference.