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) \(2 \mathrm{H}_{2} \mathrm{O}(\mathrm{I}) \longrightarrow \mathrm{O}_{2}(\mathrm{~g})+4 \mathrm{H}^{+}(\mathrm{aq})+4 \mathrm{e}^{-}\) (d) \(\mathrm{O}_{2}(\mathrm{~g})+4 \mathrm{H}^{+}(\mathrm{aq})+4 \mathrm{e} \stackrel{\longrightarrow}{2} \mathrm{H}_{2} \mathrm{O}(\mathrm{l})\)

Step-by-Step Solution

Answer

The correct cathodic reaction is (a) \(\mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})\).

Step 1: Identify the Electrolysis Setup

In this electrolysis process, we have an aqueous solution of copper sulphate. Platinum electrodes are used, one serving as the cathode and the other as the anode.

Step 2: Determine Reactions at the Cathode

At the cathode, a reduction reaction takes place, where cations gain electrons. The relevant possible reactions are given, and we are looking for the reduction of a species to a solid form.

Step 3: Examine Given Options for the Cathodic Reaction

Among the options, (a) \(\mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})\) represents a reduction occurring at the cathode, where copper ions gain electrons to form solid copper.

Step 4: Conclude the Reaction Occurring at the Cathode

The correct cathodic reaction for the electrolysis of an aqueous copper sulphate solution is option (a), as copper ions in the solution are reduced to form solid copper on the electrode.

Key Concepts

Copper Sulphate Solution

In an electrolysis setup, the role of the copper sulphate solution is fundamental. This solution acts as an electrolyte, which means it contains charged particles that allow the passage of electricity. Here, the copper sulphate solution, made by dissolving copper sulphate (\(CuSO_4\) ) in water, yields copper ions (\(Cu^{2+}\)) and sulfate ions (\(SO_4^{2-}\)).

• The copper ions are positively charged, acting as cations, which prefer to move towards the cathode during electrolysis.
• The sulfate ions, on the other hand, being anions, typically don't participate in the electrode reactions.

This separation of ions enables the transfer of electric current through the solution, facilitating the entire electrolysis process.

Platinum Electrodes

In the electrolysis of copper sulphate solution, platinum electrodes are employed due to their extraordinary chemical stability. Unlike other materials, platinum does not easily react or corrode, making it an ideal conductor for this process.

• Platinum serves as an inert electrode, meaning it doesn't participate directly in the chemical reaction.
• The primary function of these electrodes is to facilitate the transfer of electrons:
  • At the cathode: Electrons are provided for reduction reactions to occur.
  • At the anode: Electrons are withdrawn to allow oxidation reactions.

Since platinum electrodes do not influence the chemical reactions themselves, they offer a clear and controllable pathway for observing electrolysis reactions.

Cathodic Reaction

The cathodic reaction is pivotal to the electrolysis process. It is here that reduction, a fundamental type of chemical reaction, takes place. In the context of a copper sulphate electrolysis setup, this involves the transformation of copper ions:\[\text{\(Cu^{2+}(aq) + 2 e^- \rightarrow Cu(s)\)}\]

• Copper ions in the solution gain electrons (two in this case), a process called reduction.
• The outcome is solid copper being deposited on the cathode surface.

This reaction solidifies and confirms that, at the cathode, cations like copper ions undergo a gain of electrons, resulting in the crystallization of copper metal.

Reduction Reaction

The core principle of a reduction reaction is the gain of electrons by a substance. In electrolytic processes, this type of reaction primarily occurs at the cathode where electron input into the cation happens.For the aqueous copper sulphate solution, the focus is on copper ions:

• The reduction reaction is given by: \[\text{\(Cu^{2+}(aq) + 2 e^- \rightarrow Cu(s)\)}\]This equation reveals that two electrons are combined with copper ions to yield solid copper. Note that this transformation is crucial as it leads to the visible deposition of copper metal at the cathode, allowing for easy observation and measurement of electrolysis efficiency.
• Reduction reactions are essential for converting ions in solution into stable, often usable forms like solid metals.

Thus, electrolysis not only demonstrates the practical application of reduction but provides an avenue for chemical synthesis, recovery, and purification techniques.