Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

Molten \(\mathrm{CuCl}_{2}\) is electrolyzed using platinum electrodes. The reaction occurring at anode is (a) \(2 \mathrm{Cl} \longrightarrow \mathrm{Cl}_{2}(\mathrm{~g})+2 \mathrm{e}^{-}\) (b) \(\mathrm{Cl}_{2}(\mathrm{~g})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Cl}\) (c) \(\mathrm{Cu}^{2+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})\) (d) \(\mathrm{Cu}(\mathrm{s}) \longrightarrow \mathrm{Cu}^{2+}+2 \mathrm{e}^{-}\)

Step-by-Step Solution

Answer

The correct anode reaction is (a) \(2 \mathrm{Cl}^- \rightarrow \mathrm{Cl}_{2}(\mathrm{g})+2 \mathrm{e}^{-}\).

Step 1: Understanding Anode Reactions

To determine the reaction occurring at the anode during electrolysis, we first need to recall that oxidation happens at the anode. During oxidation, a substance loses electrons.

Step 2: Identify Possible Anode Reactions

The options given are: a) Oxidation of chloride ions to chlorine gas: \(2 \mathrm{Cl}^- \longrightarrow \mathrm{Cl}_{2}(\mathrm{g})+2 \mathrm{e}^{-}\)b) Reduction of chlorine gas to chloride ions: \(\mathrm{Cl}_{2}(\mathrm{~g})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Cl}^-\)c) Reduction of copper ions to copper metal: \(\mathrm{Cu}^{2+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})\)d) Oxidation of copper metal to copper ions: \(\mathrm{Cu}(\mathrm{s}) \longrightarrow \mathrm{Cu}^{2+}+2 \mathrm{e}^{-}\)

Step 3: Select the Oxidation Reaction

The oxidation reaction is one where a species loses electrons. Among the options, option (a) is an oxidation process: \(2 \mathrm{Cl}^- \longrightarrow \mathrm{Cl}_{2}(\mathrm{g})+2 \mathrm{e}^{-}\). This involves chloride ions losing electrons to form chlorine gas.

Step 4: Eliminate Reduction Reactions

The reactions (b) and (c) involve reduction, where electrons are gained. Option (d) describes an oxidation of copper, but copper ions are not expected at the anode in a chlorine-salt electrolysis. Thus, option (a) is the correct anode reaction.

Key Concepts

Anode reaction

In electrolysis, the anode is the site where oxidation reactions occur. This process involves a species losing electrons. It's important because the anode reaction helps drive the entire electrolysis process by supplying the necessary electrons. During the electrolysis of molten compounds such as \(\text{CuCl}_2\), the anode plays a crucial role.

To determine the anode reaction, you need to identify which species present in the electrolyte will undergo oxidation. In this exercise, the correct anode reaction involves the oxidation of chloride ions to form chlorine gas. This occurs as per the equation:

• \(2 \text{Cl}^- \rightarrow \text{Cl}_2(\text{g}) + 2 \text{e}^-\)

This reaction illustrates the loss of electrons from chloride ions, confirming its oxidation process.

Oxidation

Oxidation is a fundamental chemical process occurring at the anode during electrolysis. In a broad sense, oxidation refers to the loss of electrons from a molecule, atom, or ion. It's often remembered through the acronym "OIL RIG," which stands for "Oxidation Is Loss, Reduction Is Gain."

During the electrolysis of a molten electrolyte, the oxidation reaction can be observed directly at the anode where certain ions lose electrons. In our example with \(\text{CuCl}_2\) electrolysis, chloride ions undergo oxidation:

• \(2 \text{Cl}^- \rightarrow \text{Cl}_2(\text{g}) + 2 \text{e}^-\)

The chloride ions give up their electrons to form chlorine gas. So essentially, via oxidation, the chemical identity and properties of the ions are changed.

Chloride ions

Chloride ions \((\text{Cl}^-)\) are crucial participants in the electrolysis process of molten \(\text{CuCl}_2\). These ions are found in many salts but act as essential reactants during electrolysis, especially at the anode. As a negatively charged ion, chloride is attracted to the positive anode.

Once at the anode, chloride ions undergo the oxidation process, losing electrons and forming chlorine gas. This transformation is not only key to understanding how electrolysis breaks down electrolytes but also highlights the role that these specific ions play in the creation of new products. In the given reaction:

• \(2 \text{Cl}^- \rightarrow \text{Cl}_2(\text{g}) + 2 \text{e}^-\)

The competence of chloride ions to easily shed electrons makes them vital for the process.

Chlorine gas

Chlorine gas, resulting from the oxidation of chloride ions, is a notable product of electrolysis. In industrial and laboratory settings, the efficient generation of chlorine gas is a primary aim of electrolyzing solutions or molten salts like \(\text{CuCl}_2\).

This gas forms when chloride ions lose their electrons at the anode, combining to create molecules of chlorine gas:

• \(2 \text{Cl}^- \rightarrow \text{Cl}_2(\text{g}) + 2 \text{e}^-\)

Chlorine is a highly reactive gas and finds substantial use in various applications, from water treatment to the production of household cleaning products. Electrolysis is an incredibly efficient method to produce chlorine gas because it allows for the direct conversion of available chloride ions into this valuable substance.