Question

Which of the following reactions occurs at the anode in a voltaic cell? Cu2+ (aq) + 2e– → Cu (s)
Cd2+ (aq) + Zn (s) → Zn2+ (aq) + Cd (s)
K+ (aq) + e– → K (s)
Mg (s) → Mg2+ + 2e–

Answer 1

Answer : The correct option is, (3) $Mg(s)\rightarrow Mg^{2+}(aq)+2e^-$

Explanation :

Voltaic cell : It is a device which is used for conversion of the chemical energy produces in a redox reaction into the electrical energy.

Voltaic cell is also known as the galvanic cell or electrochemical cell.

In the voltaic cell, the oxidation occurs at an anode which is a negative electrode and the reduction occurs at the cathode which is a positive electrode.

From the given reactions,

$Mg(s)\rightarrow Mg^{2+}(aq)+2e^-$ in this reaction, oxidation reaction takes place in which the oxidation of magnesium changes from (0) to (2+). This reaction reaction occurs at anode in voltaic cell.

While in the other reactions,

$Cu^{2+}(aq)+2e^-\rightarrow Cu(s)$ and $K^{+}(aq)+1e^-\rightarrow K(s)$ in both the reactions, the reduction reaction takes place.

And,

$Cd^{2+}+Zn\rightarrow Zn^{2+}+Cd$ this reaction shows both oxidation and reduction reaction.

Hence, the correct option is, (3) $Mg(s)\rightarrow Mg^{2+}(aq)+2e^-$

Answer 2

By definition, the correct answer is the last option: the reaction Mg (s) → Mg²⁺ + 2e⁻ occurs at the anode in a voltaic cell.

Definition of voltaic cell

The voltaic cell is an electrochemical cell that obtains an electrical current from chemical energy.

This occurs through a device in which the transfer of electrons occurs through an external circuit rather than directly between reactants; in this way the flow of electrons (electric current) can be used.

The important parts of a voltaic cell are the anode, where oxidation occurs, and the cathode, where reduction occurs. Electron transfer occurs from the oxidation half-reaction to the reduction half-reaction.

Oxidation and reduction

Oxidation is a reaction where an atom, ion or molecule loses electrons, increasing the oxidation state; while the reduction corresponds to the gain of electrons of an atom, ion or molecule, lowering the oxidation state.

Reactions in this case

1. Cu²⁺ (aq) + 2e⁻ → Cu (s) Reduction reaction takes place in which the oxidation of copper changes from 2+ to 0, and the copper gains 2 electrons.
2. Cd²⁺ (aq) + Zn (s) → Zn²⁺ (aq) + Cd (s) This reaction shows both oxidation and reduction reaction.
3. K⁺ (aq) + e⁻ → K (s) Reduction reaction takes place in which the oxidation of potassium changes from 1+ to 0, and the potassium gains 1 electrons.
4. Mg (s) → Mg²⁺ + 2e⁻ Oxidation reaction takes place in which the oxidation of magnesium changes from 0 to 2+, and the magnesium loses 2 electrons.

Remembering that oxidation occurs at the anode, the correct answer is the last option: the reaction Mg (s) → Mg²⁺ + 2e⁻ occurs at the anode in a voltaic cell.

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