Question

What reaction will take place at the cathode and the anode when each of the following is electrolyzed? a. molten \(\mathrm{KF} \quad\) b. molten \(\mathrm{CuCl}_{2} \quad\) c. molten \(\mathrm{MgI}_{2}\)

Short answer

During the electrolysis of the following molten salts: a. KF: Potassium metal forms at the cathode (\(K^+ + e^- \rightarrow K (s)\)) and fluoride gas forms at the anode (\(2F^- \rightarrow F_2(g) + 2e^-\)). b. CuCl2: Copper metal forms at the cathode (\(Cu^{2+} + 2e^- \rightarrow Cu (s)\)) and chloride gas forms at the anode (\(2Cl^- \rightarrow Cl_2(g) + 2e^-\)). c. MgI2: Magnesium metal forms at the cathode (\(Mg^{2+} + 2e^- \rightarrow Mg (s)\)) and iodine gas forms at the anode (\(2I^- \rightarrow I_2(g) + 2e^-\)).

Step-by-step solution

Electrolysis of molten KF

Cathode reaction (reduction): The cation in KF is K+ (potassium ion), which will migrate to the cathode and gain an electron to form potassium metal. \[K^+ + e^- \rightarrow K (s)\] Anode reaction (oxidation): The anion in KF is F- (fluoride ion), which will migrate to the anode, lose an electron, and form fluoride gas. \[2F^- \rightarrow F_2(g) + 2e^-\] So, during the electrolysis of molten KF, potassium metal will form at the cathode, and fluoride gas will form at the anode.

Electrolysis of molten CuCl2

Cathode reaction (reduction): The cation in CuCl2 is Cu2+ (copper (II) ion), which will migrate to the cathode and gain two electrons to form copper metal. \[Cu^{2+} + 2e^- \rightarrow Cu (s)\] Anode reaction (oxidation): The anion in CuCl2 is Cl- (chloride ion), which will migrate to the anode, lose an electron, and form chloride gas. \[2Cl^- \rightarrow Cl_2(g) + 2e^-\] So, during the electrolysis of molten CuCl2, copper metal will form at the cathode, and chloride gas will form at the anode.

Electrolysis of molten MgI2

Cathode reaction (reduction): The cation in MgI2 is Mg2+ (magnesium ion), which will migrate to the cathode and gain two electrons to form magnesium metal. \[Mg^{2+} + 2e^- \rightarrow Mg (s)\] Anode reaction (oxidation): The anion in MgI2 is I- (iodide ion), which will migrate to the anode, lose an electron, and form iodine gas. \[2I^- \rightarrow I_2(g) + 2e^-\] So, during the electrolysis of molten MgI2, magnesium metal will form at the cathode, and iodine gas will form at the anode.

Key concepts

Cathode Reaction

In the electrolysis process, the cathode reaction is where reduction occurs. Reduction means gaining electrons. When you electrolyze a molten compound, such as KF, CuCl2, or MgI2, positive ions (cations) move toward the cathode. Here's what happens at the cathode for each of these compounds:

• For molten KF: The potassium ion (\( K^+ \)) gains an electron and forms solid potassium (\( K(s) \)).
• For molten CuCl2: The copper ion (\( Cu^{2+} \)) gains two electrons to form solid copper (\( Cu(s) \)).
• For molten MgI2: The magnesium ion (\( Mg^{2+} \)) gains two electrons, resulting in solid magnesium (\( Mg(s) \)).

In all these cases, metals are formed because the cations reduce by gaining electrons at the cathode. It's essential to remember that the reduction at the cathode involves a decrease in the oxidation state of the cation.

Anode Reaction

At the anode, you witness oxidation reactions during the electrolysis process. Oxidation involves losing electrons. In molten compounds like KF, CuCl2, or MgI2, negative ions (anions) will drift toward the anode. The following reactions take place at the anode for the respective compounds:

• For molten KF: The fluoride ion (\( F^- \)) loses an electron, forming fluoride gas (\( F_2(g) \)).
• For molten CuCl2: The chloride ion (\( Cl^- \)) loses an electron to create chlorine gas (\( Cl_2(g) \)).
• For molten MgI2: The iodide ion (\( I^- \)) undergoes electron loss to produce iodine gas (\( I_2(g) \)).

The oxidation at the anode results in the release of gas from the anions. As you observe, oxidation is the process where electrons are removed from ions, which results in a change in their oxidation states.

Oxidation and Reduction Reactions

Understanding oxidation and reduction is vital in electrolysis.

• Oxidation: This is when an atom or ion loses electrons. It takes place at the anode where elements tend to lose electrons and become oxidized. The general reaction form is \( X^- \to X + e^- \)
• Reduction: This involves the gain of electrons. It occurs at the cathode, where ions gain electrons and reduce. The typical reaction is \( Y^{n+} + n e^- \to Y \)

In summary, electrolysis involves simultaneous oxidation at the anode and reduction at the cathode. It's a bit like two sides of the same coin. These reactions are fundamentally about the transfer of electrons. They are the driving force enabling the transformation of ions back into elemental forms, often seen as metals solidifying or gases being liberated in these processes. Therefore, grasping the basics of these reactions provides clarity on how materials are manipulated using electrolysis.