Question

A solution containing one mole per litre each of $Cu{(N{O_3})_2},AgN{O_3},H{g_2}{(N{O_3})_2}$ and $Mg{(N{O_3})_2}$ is being electrolysed by using inert electrodes. The values of standard electrode potentials in volt (reduction potentials) are:
$Ag/A{g^ + } = 0.80$
$2Hg/Hg_2^{2 + } = 0.79$
$Cu/C{u^{2 + }} = + 0.34$
$Mg/M{g^{2 + }} = - 2.37$
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. $Hg,Cu,Ag$

Answer 1

In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode (SHE). It is defined to have a potential of zero volts.

Complete answer:
During electrolysis, the concentration of the electrolyte remains unchanged and the number of electrons extracted at the cathode is equal to the number of electrons supplied at the cathode. Since metal atoms are deposited on the cathode, the mass of the cathode increases and the mass of the anode decreases by an equal amount. The standard potentials of electrodes are tabulated for reduction half reactions, indicating the tendencies of the electrodes to behave as cathodes towards SHE. Electrodes with positive $E^\circ$ values for reduction half reactions do in fact act as cathodes versus SHE, while those with negative $E^\circ$values of reduction half reactions behave instead as anodes versus SHE.
The deposition of metals on the cathode takes place in the order of decreasing oxidation potential or increasing order of reduction potential. Thus, with increasing voltage, the sequence of deposition of metals on the cathode will be in the order of: $Ag,Hg,Cu$. The external voltage should be greater than the reduction potential of the electrolytic cell.

Thus option C is the correct answer.

Note:
In electrochemistry, the Nernst equation is an equation that relates the reduction potential of an electrochemical reaction (half-cell or full cell reaction) to the standard electrode potential, temperature, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation.