Question

A cell constructed by coupling a standard copper electrode and a standard magnesium electrode has emf of $2.7$ volts. If the standard reduction potential of the copper electrode is $+0.34$ volt, that of magnesium electrode is:

• A. $+3.04$ volts
• B. $-3.04$ volts
• C. $+2.36$ volts
• D. $-2.36$ volts

Answer 1

Answer: (D)

$-2.36$ volts

In electro-chemical series, $Hg$ lies above $Cu$.

Hence, $Cu$ electrode acts as the cathode and $Mg$ electrode acts as an anode.

Given, $E_{mf}$ of cell $=2.7V$

The standard reduction potential of $Cu=+0.34V$

$E_{cell}^0=E_{{Cu}^{++}/Cu}^0-E_{{Mg}^{++}/Mg}^0$

$2.7=0.34-E_{{Mg}^{++}/Mg}^0$

$E_{{Mg}^{++}/Mg}^0=-2.36V$

$\therefore$ Standard reduction potential of $Mg$ electrode is $-2.36V$.

Hence, option $D$ is correct.