Question

A solution of nickel sulphate in which a nickel rod is dipped is diluted to 100 times its original volume. The reduction potential of Ni at STP (1 bar and 273 K):

• A. Decreases by 54 mV
• B. Increases by 54 mV
• C. Decreases by 30 mV
• D. Increases by 30 mV

Answer 1

The correct option is A Decreases by 54 mV

By applying Nernst equation at STP:
$E=E^{\circ }-\frac{RT}{nF}\ln \frac{1}{\left[N{i}^{2+}\right]}$
$E=E^{\circ }-\frac{8.314\times 273}{n\times 96500}\times 2.303\log \frac{1}{\left[N{i}^{2+}\right]}$
$E=E^{\circ }-\frac{0.0541}{2}\log \frac{1}{\left[N{i}^{2+}\right]}$
After dilution,
$E=E^{\circ }-\frac{0.0541}{2}\log \frac{100}{\left[N{i}^{2+}\right]}=E^{\circ }-\frac{0.0541}{2}\log \frac{1}{\left[N{i}^{2+}\right]}-\frac{0.0541}{2}\log 100=E^{\circ }-\frac{0.0541}{2}\log \frac{1}{\left[N{i}^{2+}\right]}-0.0541$
Hence, reduction potential decreases by 0.0541 $\approx$ 54 mV.