Question

Write equations for the half-reactions that occur at the anode and cathode for the electrolysis of each aqueous solution: $$\text { a. }\operatorname{Ni}\left(\mathrm{NO}_{3}\right)_{2}(a q) \quad \text { b. } \mathrm{KCl}(a q) \quad \text { c. } \operatorname{CuBr}_{2}(a q)$$

Short answer

Anode reactions: Ni(NO3)2: 2NO3- -> 2NO2 + O2 + 4e-; KCl: 2Cl- -> Cl2 + 2e-; CuBr2: 2Br- -> Br2 + 2e-. Cathode reactions: Ni(NO3)2: Ni2+ + 2e- -> Ni; KCl: 2H2O + 2e- -> H2 + 2OH-; CuBr2: Cu2+ + 2e- -> Cu.

Step-by-step solution

Identify the Ions Present in the Solution

Determine the cations and anions present in the given solutions. For Ni(NO3)2, the cations are Ni2+ and the anions are NO3-. For KCl, the cations are K+ and the anions are Cl-. For CuBr2, the cations are Cu2+ and the anions are Br-.

Determine the Anode Reaction

The anode reaction involves the oxidation process where anions lose electrons. For Ni(NO3)2 and CuBr2, the NO3- and Br- will be oxidized to form NO2 gas and Br2 liquid, respectively. For KCl, Cl- will be oxidized to form Cl2 gas. Write the half-reactions: Ni(NO3)2: 2NO3- -> 2NO2 + O2 + 4e-; KCl: 2Cl- -> Cl2 + 2e-; CuBr2: 2Br- -> Br2 + 2e-.

Determine the Cathode Reaction

The cathode reaction involves the reduction process where cations gain electrons. For Ni(NO3)2, Ni2+ ions are reduced to Ni metal. For KCl, K+ ions are difficult to reduce, so water is reduced to form hydrogen gas and hydroxide ions. For CuBr2, Cu2+ ions are reduced to Cu metal. Write the half-reactions: Ni(NO3)2: Ni2+ + 2e- -> Ni; KCl: 2H2O + 2e- -> H2 + 2OH-; CuBr2: Cu2+ + 2e- -> Cu.

Key concepts

Anode and Cathode Reactions

Electrolysis is a chemical process that uses an electric current to drive a non-spontaneous reaction. It's essential to understand the reactions at the anode and cathode.
In the provided exercises, at the anode (which attracts anions), oxidation occurs. This is where anions lose electrons and become oxidized. For example, in the case of \textbf{Ni(NO\(_3\))\(_2\)}, the anion NO\(_3$$^-\) undergoes oxidation to form NO\(_2\) and O\(_2\). In \textbf{KCl}, Cl\(^-\) is oxidized to Cl\(_2\). Similarly, in \textbf{CuBr\(_2\)}, Br\(^-\) is oxidized to form Br\(_2\).
At the cathode (which attracts cations), reduction happens. Here, cations gain electrons. Ng(Ni\(_2$$^+\)) ions from \textbf{Ni(NO\(_3\))\(_2\)} get reduced to Ni metal, and Cu\(_2$$^+\) ions from \textbf{CuBr\(_2\)} are reduced to Cu metal. However, in \textbf{KCl}, the K\(^+\) ions are challenging to reduce, so water molecules are reduced to hydrogen gas (H\(_2\)) and hydroxide ions (OH\(^-\)) instead.

• At the anode: Anions undergo oxidation. This is an electron loss process.
• At the cathode: Cations undergo reduction, gaining electrons, or water is reduced if cations are difficult to reduce.

Remember that the specific reactions depend on the substances involved and the electrode materials.

Oxidation and Reduction

Oxidation and reduction are two halves of a redox reaction, central to understanding electrolysis. Oxidation is the loss of electrons, while reduction is the gain of electrons. A useful mnemonic is 'OIL RIG' - Oxidation Is Loss, Reduction Is Gain.
In the electrolysis examples provided, oxidation at the anode produces gas or liquid as a byproduct, such as NO\(_2\), Cl\(_2\), or Br\(_2\). Reduction at the cathode results in either a metal being deposited, like Ni or Cu, or the evolution of hydrogen gas and the formation of hydroxide ions if water is involved.
Here's how the processes look for each substance:

• Ni(NO\(_3\))\(_2\): 2NO\(_3$$^-\) (oxidation) -> 2NO\(_2\) + O\(_2\) + 4e\(^-\)
• KCl: 2Cl\(^-\) (oxidation) -> Cl\(_2\) + 2e\(^-\) 2H\(_2\)O + 2e\(^-\) (reduction) -> H\(_2\) + 2OH\(^-\)
• CuBr\(_2\): 2Br\(^-\) (oxidation) -> Br\(_2\) + 2e\(^-\)

The electrons lost by the anions during oxidation are the same electrons gained by the cations during reduction, keeping the reaction in balance.

Aqueous Solution Electrolysis

Electrolysis of aqueous solutions involves passing an electric current through the solution to induce a chemical change. The water molecules can also be participants in the reactions, especially when the ions from the solute are not easily oxidized or reduced on their own. For instance, in the electrolysis of \textbf{KCl}, rather than potassium ions being reduced at the cathode, it is the water molecules that get reduced to form hydrogen gas.
During aqueous solution electrolysis:

• The solvent (water) can undergo both oxidation and reduction.
• The products of electrolysis can include gases, metals, and hydroxides, depending on the ions present and the applied voltage.
• pH changes can occur near the electrodes due to the formation of H\(^+\) or OH\(^-\) ions.

It's important to predict the right reactions during the electrolysis of an aqueous solution, which will vary based on the electrolyte's ions and their respective reduction and oxidation potentials. This understanding enables precise control of the electrolysis process for desired outcomes in both laboratory and industrial settings.