Chapter 21, Problem 4STP

Interpretation Introduction

Interpretation: The true statement for the given half-reaction of electrolysis of molten ${\text{MgCl}}_{\text{2}}$ needs to be selected.

Concept Introduction: In an electrolytic cell, oxidation takes place at the anode and reduction takes place at the cathode. In oxidation, electrons are released and in reduction, electrons are added to the reactant. The oxidation state of the reactant increases during oxidation and decreases during reduction.

Answer to Problem 4STP

The correct answer is option (A)

Explanation of Solution

Reason for the correct option:

The given half-reaction is as follows:

  ${\text{Mg}}^{2+}\left(l\right)+2e^{-}\to \text{Mg}\left(l\right)$

In the above half-reaction, ${\text{Mg}}^{2+}$ gains 2 electrons to form Mg. The oxidation state of Mg decreases from +2 to 0 thus, reduction takes place. Since reduction occurs at the cathode, thus, this reaction occurs at the cathode.

Thus, option (A) is correct.

Conclusion

Reason for incorrect answer:

In a voltaic cell, the reduction of metal takes place at the cathode. Here, the oxidation state of the metal decreases. Since the oxidation state of metal Mg decreases in this case, thus, it is getting reduced not oxidized.

Thus, option (B) is incorrect.

Reduction takes place at the cathode, not at the anode. Also, oxidation of chloride ions takes place (oxidation state decreases). Thus, option (C) is incorrect.

Here, chloride ions lose electrons, in this case, to get oxidized. Thus, option (D) is also incorrect.