Question

What reactions take place at the cathode and the anode when each of the following is electrolyzed? a. molten \(\mathrm{NiBr}_{2} \quad\) b. molten \(\mathrm{AlF}_{3} \quad\) c. molten \(\mathrm{MgI}_{2}\)

Short answer

For the electrolysis of molten compounds: a. \(NiBr_2\): Cathode (Reduction): \(Ni^{2+} + 2e^- \rightarrow Ni\) Anode (Oxidation): \(2Br^- \rightarrow Br_2 + 2e^-\) b. \(AlF_3\): Cathode (Reduction): \(Al^{3+} + 3e^- \rightarrow Al\) Anode (Oxidation): \(2F^- \rightarrow F_2 + 2e^-\) c. \(MgI_2\): Cathode (Reduction): \(Mg^{2+} + 2e^- \rightarrow Mg\) Anode (Oxidation): \(2I^- \rightarrow I_2 + 2e^-\)

Step-by-step solution

NiBr2

First, let's identify the ions in the compound: Cation: Ni2+ Anion: Br- Now, let's analyze the reactions that take place at the cathode and the anode during the electrolysis of molten NiBr2: Cathode (Reduction): The cation (Ni2+) will gain 2 electrons and be reduced to form metallic nickel. Ni2+ + 2e- → Ni Anode (Oxidation): The anion (Br-) will lose 1 electron and be oxidized to form bromine gas. Since there are two bromide ions for every nickel ion, this reaction occurs twice. 2Br- → Br2 + 2e-

AlF3

First, let's identify the ions in the compound: Cation: Al3+ Anion: F- Now, let's analyze the reactions that take place at the cathode and the anode during the electrolysis of molten AlF3: Cathode (Reduction): The cation (Al3+) will gain 3 electrons and be reduced to form metallic aluminum. Al3+ + 3e- → Al Anode (Oxidation): The anion (F-) will lose 1 electron and be oxidized to form fluorine gas. Since there are three fluoride ions for every aluminum ion, this reaction occurs 1.5 times (3/2). 2F- → F2 + 2e-

MgI2

First, let's identify the ions in the compound: Cation: Mg2+ Anion: I- Now, let's analyze the reactions that take place at the cathode and the anode during the electrolysis of molten MgI2: Cathode (Reduction): The cation (Mg2+) will gain 2 electrons and be reduced to form metallic magnesium. Mg2+ + 2e- → Mg Anode (Oxidation): The anion (I-) will lose 1 electron and be oxidized to form iodine gas. Since there are two iodide ions for every magnesium ion, this reaction occurs twice. 2I- → I2 + 2e-

Key concepts

Reduction

Reduction is a key process in electrolysis where a chemical species gains electrons. At the cathode, positively charged ions, known as cations, receive electrons. This gain of electrons is called reduction.
During the reduction process:

• Cations move towards the cathode.
• They gain electrons and undergo a change in their oxidation state, becoming neutral atoms.

For example, in the electrolysis of molten NiBr₂, Ni²⁺ ions are reduced by gaining two electrons to form nickel metal: \[ \text{Ni}^{2+} + 2\text{e}^- \rightarrow \text{Ni} \]
This process is essential for obtaining pure metals from ionic compounds.

Oxidation

Oxidation is the opposite of reduction. It involves the loss of electrons from a chemical species. In an electrolytic cell, oxidation occurs at the anode.
During oxidation:

• Anions, which are negatively charged ions, move towards the anode.
• They lose electrons, leading to the formation of neutral molecules or elements.

In the case of molten AlF₃ electrolysis, fluoride ions (F^-) are oxidized to form fluorine gas. This can be represented by the equation: \[ 2\text{F}^- \rightarrow \text{F}_2 + 2\text{e}^- \]
Oxidation is crucial for the liberation of gases or non-metal elements during electrolysis.

Electrode reactions

Electrode reactions are the specific processes occurring at the electrodes during electrolysis. These reactions are what drive the separation of elements or compounds.

• The cathode reaction involves reduction, where species gain electrons.
• The anode reaction involves oxidation, where species lose electrons.
• These reactions are influenced by the type of ions present in the molten or aqueous solution.

Understanding these reactions helps to predict the products obtained from different electrolysis processes. This includes the identification of elements at each electrode when compounds like MgI₂ are electrolyzed.

NiBr2 electrolysis

During the electrolysis of molten NiBr₂, we observe both reduction and oxidation at different electrodes. The nickel ions undergo reduction at the cathode, forming nickel metal: \[ \text{Ni}^{2+} + 2\text{e}^- \rightarrow \text{Ni} \]
At the anode, bromide ions undergo oxidation, releasing bromine gas: \[ 2\text{Br}^- \rightarrow \text{Br}_2 + 2\text{e}^- \]
This process demonstrates how electrolysis can separate the components of a compound into their elemental forms.

AlF3 electrolysis

In the electrolysis of molten AlF₃, aluminum and fluorine are separated through distinct electrode reactions.

• At the cathode, aluminum ions (Al³⁺) are reduced by gaining electrons:
• \[ \text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al} \]
• Simultaneously, at the anode, fluoride ions are oxidized, producing fluorine gas:
• \[ 2\text{F}^- \rightarrow \text{F}_2 + 2\text{e}^- \]

This method is effective for obtaining aluminum and fluorine, showcasing the utility of electrolysis in industry.

MgI2 electrolysis

The electrolysis of molten MgI₂ involves distinct reactions at the electrodes. Magnesium ions (Mg²⁺) are reduced at the cathode: \[ \text{Mg}^{2+} + 2\text{e}^- \rightarrow \text{Mg} \]
Conversely, iodide ions are oxidized at the anode, producing iodine gas: \[ 2\text{I}^- \rightarrow \text{I}_2 + 2\text{e}^- \]
These processes highlight electrolysis as a powerful technique to separate and obtain pure elements from ionic compounds.