Problem 40 - A Complete Resource Book in Chemistry for JEE Main

Question

Aqueous copper sulphate solution is electrolyzed using platinum electrodes. The electrode reaction occurring at cathode is (a) \(\mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})\) (b) \(\mathrm{Cu}(\mathrm{s}) \longrightarrow \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}\) (c) \(2 \mathrm{H}_{2} \mathrm{O}\) (I) \(\longrightarrow \mathrm{O}_{2}(\mathrm{~g})+4 \mathrm{H}^{+}(\mathrm{aq})+4 \mathrm{e}^{-}\) (d) \(\mathrm{O}_{2}(\mathrm{~g})+4 \mathrm{H}^{+}(\mathrm{aq})+4 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}\) (1)

Step-by-Step Solution

Answer

(a) \(\mathrm{Cu}^{2+}(\mathrm{aq}) + 2 \mathrm{e}^{-} \rightarrow \mathrm{Cu}(\mathrm{s})\) is the cathode reaction.

Step 1: Identify the Nature of the Process

In the electrolysis of copper sulphate solution with platinum electrodes, we are primarily concerned with the reduction processes that occur at the cathode, where cations gain electrons.

Step 2: Analyze the Given Reactions

Out of the given reactions, (a) and (b) show processes involving copper ions, while (c) and (d) involve water and oxygen. We are interested in the reaction that involves the gain of electrons, which is a reduction process.

Step 3: Determine the Reduction Reaction at the Cathode

Reaction (a), \(\mathrm{Cu}^{2+}(\mathrm{aq}) + 2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})\), represents the reduction of copper ions to solid copper, which typically occurs at the cathode during electrolysis.

Key Concepts

Copper sulfate solution

Electrolysis involves passing an electric current through a solution or molten substance to cause a chemical change. In this case, the solution is aqueous copper sulfate or CuSO extsubscript{4}. This solution contains copper ions ( ext{Cu}^{2+}) and sulfate ions (SO extsubscript{4}^{2-}).
The Cu extsuperscript{2+} ions play a vital role in the electrolysis process, particularly at the cathode, where they undergo a significant transformation.
The solution also conducts electricity, allowing electrons to flow, facilitating the desired chemical reactions.

• This process is driven by the need for cations in the solution to achieve a more stable state by gaining electrons.
• The entire electrolysis setup relies on this solution to sustain the reactions occurring at the electrodes.

Understanding the composition and behavior of the copper sulfate solution is crucial to predicting and explaining the electrochemical reactions in this setup.

Cathode reaction

During electrolysis, two electrodes are typically immersed in the electrolytic solution: the anode and the cathode.
The cathode is the electrode where reduction reactions occur. In the context of copper sulfate electrolysis, the cathode sees a particularly interesting transformation.
Here, copper ions ( ext{Cu}^{2+}) in the solution receive electrons and are converted to solid copper metal. This reduction is crucial, as it transforms soluble copper ions into insoluble copper metal, which accumulates on the cathode surface.

• The cathode reaction is represented by the chemical equation: \( \text{Cu}^{2+}(\text{aq}) + 2 \text{e}^{-} \rightarrow \text{Cu}(\text{s}) \).
• This reaction emphasizes the gain of electrons by copper ions, a defining characteristic of reduction processes.
• This transition makes the cathode an essential component in depositing pure copper in processes like electroplating and purification.

Mastering the cathode reaction provides insight into how subatomic changes lead to tangible outcomes.

Platinum electrodes

Platinum electrodes serve as a versatile option in the electrolysis of copper sulfate owing to platinum's chemical inertness and conductivity.
These traits ensure that the electrodes do not participate in the reaction, leaving only the intended ionic changes within the solution.
Platinum electrodes are preferred when the material of the electrodes should not influence the solution or products, offering reliability and consistency.

• Platinum's resistance to corrosion makes it particularly useful in processes involving various electrolytes and chemical reactions.
• Although often more expensive, platinum's durability and minimal reactivity justify its choice in precise experimental setups.
• These electrodes act as mere conductors, ensuring that electrical energy is thoroughly applied to initiate and sustain the electrochemical changes.

Understanding the role of platinum electrodes shines a light on why particular materials are chosen for specific electrochemical tasks.

Reduction process

The reduction process is central to the workings of electrolysis, particularly evident at the cathode.
In our context involving copper sulfate, reduction specifically refers to the process where copper ions ( ext{Cu}^{2+}) gain electrons and convert into metallic copper. This transformation is foundational to various applications, like metal refining and coatings.

• Reduction involves migrating electrons from a donor to the target ion, facilitating the conversion to a more stable state.
• The copper reduction showcases the fundamental principle that underlies a myriad of electrochemical reactions.
• Recognizing reduction as an electron gain process helps to predict the outcome and behavior of different ions under electrolysis.

Mining deep into the reduction process unravels the complexities underlying efficient and targeted electrochemical transformations.