Problem 71 - Chemistry and Chemical Reactivity

Question

The products formed in the electrolysis of aqueous \(\mathrm{CuSO}_{4}\) are \(\mathrm{Cu}(\mathrm{s})\) and \(\mathrm{O}_{2}(\mathrm{g}) .\) Write equations for the anode and cathode reactions.

Step-by-Step Solution

Answer

Cathode: \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}(s) \); Anode: \( 2\text{H}_2 ext{O} \rightarrow \text{O}_2 + 4H^+ + 4e^- \).

Step 1: Determine the Electrolysis Environment

The aqueous solution of \( ext{CuSO}_4\) contains copper ions (\( ext{Cu}^{2+}\)) and sulfate ions (\( ext{SO}_4^{2-}\)), as well as water molecules (\( ext{H}_2 ext{O}\)) that can be involved in the electrolysis process. The setup involves two electrodes submerged in the solution.

Step 2: Identify the Cathode Reaction

During electrolysis, reduction occurs at the cathode. For aqueous \( ext{CuSO}_4\), \( ext{Cu}^{2+}\) ions will gain electrons to form copper metal. The cathodic reaction is thus: \[ \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}(s) \] This indicates that solid copper is deposited at the cathode.

Step 3: Determine the Anode Reaction

At the anode, oxidation occurs. Instead of \( ext{SO}_4^{2-}\) being oxidized, water is oxidized because it requires less energy. The anodic reaction involving water is: \[ 2\text{H}_2 ext{O}(l) \rightarrow \text{O}_2(g) + 4H^+ + 4e^- \] This shows oxygen gas is produced at the anode.

Step 4: Final Reaction Equations

The overall reactions at the electrodes during the electrolysis of \( ext{CuSO}_4\) are: At the cathode: \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}(s) \)At the anode: \( 2\text{H}_2 ext{O} \rightarrow \text{O}_2 + 4H^+ + 4e^- \)

Key Concepts

Cathode Reaction

In the process of electrolysis, the cathode is one of the two electrodes where reduction reactions occur. It is typically the negatively charged electrode, which attracts positive ions from the solution. For the electrolysis of copper sulfate (\(\mathrm{CuSO}_4\)) in an aqueous solution, the reaction at the cathode specifically involves copper ions.
These copper ions, \(\text{Cu}^{2+}\), are reduced as they gain electrons from the cathode to form solid copper, \(\text{Cu}(s)\). The equation representing this reduction is:

• \(\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}(s)\)

This transformation results in the deposition of solid copper onto the cathode surface. This is a crucial process in electroplating and refining, where pure copper is produced as a result. The reaction's efficiency comes from the strong affinity the \(\text{Cu}^{2+}\) ions have for gaining electrons and their relatively low reactivity in the presence of water molecules.

Anode Reaction

Opposite to the cathode, the anode is the electrode where oxidation reactions occur. During electrolysis of copper sulfate in water, the anode becomes the site where water molecules are oxidized rather than the sulfate ions.
Water oxidation is favored because it requires less energy than oxidizing sulfate ions, resulting in the production of oxygen gas, which is released at the anode. The equation describing this oxidation is:

• \(2\text{H}_2\text{O}(l) \rightarrow \text{O}_2(g) + 4\text{H}^+ + 4e^-\)

This reaction produces oxygen gas and protons, and uses up electrons. The oxygen can often be observed bubbling off the anode as gas during the process. This oxidation reaction is crucial in understanding the overall electrochemical behavior during the electrolysis of \(\mathrm{CuSO}_4\) and indicates how water's role is prominent, even when the solution's main components include metal ions like \(\text{Cu}^{2+}\).

Copper Sulfate

Copper sulfate, with the chemical formula \(\mathrm{CuSO}_4\), is a prominent compound in chemistry, especially when dissolved in water for electrolysis. Copper sulfate solutions appear bright blue due to the presence of \(\text{Cu}^{2+}\) ions, which provides important visual and chemical clues during experiments.
When dissolved in water, \(\mathrm{CuSO}_4\) dissociates into \(\text{Cu}^{2+}\) and \(\text{SO}_4^{2-}\) ions. In electrolysis experiments involving aqueous \(\mathrm{CuSO}_4\), it is the copper ions that are targeted for deposition as metal, while the sulfate ions remain in the solution.
Key points about copper sulfate include:

• Roles in electroplating and as an agricultural fungicide.
• High importance in educational experiments due to its vivid color and straightforward electrochemical properties.
• Solubility allows for easy preparation of electrolyte solutions.

Understanding copper sulfate is essential when exploring fundamental electrochemical techniques and reactions in chemistry laboratories.