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

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

Step 1: Identify the Cathode Reaction

In an electrolytic cell, reduction occurs at the cathode. During the electrolysis of copper sulphate solution with platinum electrodes, we look for the reaction that involves a gain of electrons at the cathode.

Step 2: Review Given Reactions

Consider the reactions:(a) \(\mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})\) involves the gain of electrons.(b) \(\mathrm{Cu}(\mathrm{s}) \longrightarrow \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-}\) involves the loss of electrons, so it cannot be the cathode reaction.(c) and (d) involve transformations of water and are relevant to oxygen evolution or consumption, not directly related to Cu at the cathode.

Step 3: Select the Correct Option

The reaction \(\mathrm{Cu}^{2+}(aq) + 2e^- \rightarrow \mathrm{Cu}(s)\) from option (a) involves the reduction of copper ions to solid copper. This reduction reflects the deposition of copper at the cathode.

Key Concepts

Cathode Reaction

The concept of a cathode reaction is crucial in understanding the process of electrolysis. In an electrolytic cell, the cathode is the electrode where reduction occurs.
The key characteristics of a cathode reaction include:

• It involves the gain of electrons.
• It leads to a decrease in oxidation state of the species involved.

For the electrolysis of aqueous copper sulphate, the reaction at the cathode is: \[\mathrm{Cu}^{2+}( ext{aq}) + 2\, ext{e}^- \rightarrow \mathrm{Cu}( ext{s})\]This equation illustrates the transformation of copper ions into solid copper. The copper ions ( ext{Cu}^{2+}) in the solution accept electrons and get deposited as solid copper ( ext{Cu}) on the cathode surface. This is a clear example of a cathode reaction occurring within an electrolytic process.

Reduction in Electrolysis

Reduction is a term that describes a chemical process where a substance gains electrons, leading to a decrease in its oxidation state. In the context of electrolysis, the reduction process plays a pivotal role at the cathode.For the electrolysis of copper sulphate, the reaction that signifies reduction is:\[\mathrm{Cu}^{2+}( ext{aq}) + 2\, ext{e}^- \rightarrow \mathrm{Cu}( ext{s})\]Key aspects of reduction in electrolysis include:

• The reaction results in the deposition of substances, such as metals, from ions in the solution.
• The energy supplied by the electrolytic cell drives the reaction forward by allowing electrons to be gained by the ions.

Therefore, reduction in electrolysis is at the heart of processes like metal plating, purification, and extraction, making it an essential concept in electrochemistry.

Electrode Reactions

Electrode reactions are fundamental to understanding how electrolysis works. These reactions occur at the electrodes—cathode and anode—of an electrolytic cell and represent the transformations of ions.In the electrolysis of copper sulphate:

• At the cathode, copper ions are reduced to form solid copper:

\[\mathrm{Cu}^{2+}( ext{aq}) + 2\, ext{e}^- \rightarrow \mathrm{Cu}( ext{s})\]

• At the anode, generally, oxygen is evolved from water oxidation if inert electrodes like platinum are used:

\[2\, ext{H}_2 ext{O(l)} \rightarrow \text{O}_2( ext{g}) + 4\, ext{H}^+( ext{aq}) + 4\, ext{e}^-\]Understanding electrode reactions helps elucidate the broader picture of electrolytic processes, where ions undergo transformations at each electrode. The overall efficiency and mechanism of the electrolysis depend heavily on these localized electrode reactions.