Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following aqueous solutions during electrolysis, \(\mathrm{H}_{2}\) and \(\mathrm{Cl}_{2}\) are liberated? (a) \(\mathrm{CuCl}_{2}(\mathrm{aq})\) (b) \(\mathrm{KCl}(\mathrm{aq})\) (c) \(\mathrm{MgCl}_{2}(\mathrm{aq})\) (d) \(\mathrm{NaCl}(\mathrm{aq})\)

Step-by-Step Solution

Answer

Solutions (b) KCl(aq), (c) MgCl_2(aq), and (d) NaCl(aq) produce H_2 and Cl_2 during electrolysis.

Step 1: Understanding Electrolysis

Electrolysis is a chemical process that uses electric current to drive a non-spontaneous chemical reaction. When an aqueous solution is electrolyzed, the cation (positive ion) is reduced at the cathode (negative electrode), and the anion (negative ion) is oxidized at the anode (positive electrode).

Step 2: Identifying Possible Reactions at the Electrodes

For each option, determine what ions are present in the solution and what will occur at each electrode. In general, water reduction and oxidation need to be considered as well, which can lead to the production of hydrogen gas at the cathode and oxygen gas at the anode.

Step 3: Considering CuCl_2(aq)

In the electrolysis of CuCl_2(aq), copper ions and chloride ions are present in the solution. At the cathode, copper ions (Cu^{2+}) will be reduced to solid copper, and at the anode, chloride ions (Cl^- ) will be oxidized to Cl_{2} gas.

Step 4: Analyzing KCl(aq)

With KCl(aq), potassium ions and chloride ions are in solution. At the cathode, water is reduced more easily than potassium ions, producing H_{2} gas, and at the anode, chloride ions will be oxidized to Cl_{2} gas.

Step 5: Examining MgCl_2(aq)

In the electrolysis of MgCl_2(aq), magnesium ions and chloride ions are present. Similar to KCl, H_{2} gas is produced at the cathode by water reduction, while Cl_{2} gas is produced at the anode from chloride ion oxidation.

Step 6: Evaluating NaCl(aq)

For NaCl(aq), sodium ions and chloride ions are in the solution. Water is again reduced at the cathode to produce H_{2} gas, and chloride ions are oxidized at the anode to produce Cl_{2} gas.

Step 7: Identifying Solutions Producing H_2 and Cl_2

Among the options, KCl(aq), MgCl_2(aq), and NaCl(aq) produce both H_2 and Cl_2 during electrolysis. Only CuCl_2(aq) does not produce H_2, as copper is deposited instead.

Key Concepts

Aqueous Solutions

In chemistry, an aqueous solution is a solution in which the solvent is water. When salts like \(\mathrm{KCl}\) and \(\mathrm{NaCl}\) are dissolved in water, they break down into their respective ions due to the polar nature of water molecules.
This process is called dissociation.These ions become free to move in the solution, leading to interesting chemical behaviors when influenced by electricity.
The ability of these ions to conduct electricity makes aqueous solutions important for the process of electrolysis.For electrolysis to occur, the solution must contain ions that can move freely towards electrodes to undergo chemical changes.
Water itself can also participate in electrolysis, producing hydrogen and oxygen gases under suitable conditions.

Electrode Reactions

Electrode reactions are central to the process of electrolysis, whereby an electric current causes chemical changes at each electrode.
In a typical electrolysis setup:

• The cathode is the negative electrode. Here, reduction occurs, meaning ions gain electrons.
• The anode is the positive electrode. Here, oxidation happens, and ions lose electrons.

When an aqueous salt solution undergoes electrolysis, various ions in the solution will migrate towards the electrodes.
At the cathode, reductions of cations occur, while the oxidation of anions happens at the anode. Water itself can also be reduced or oxidized, depending on the specific ions present and their electrode potentials.

Reduction and Oxidation

Reduction and oxidation are two complementary processes that are essential in electrochemical reactions like electrolysis.
These processes are often described using the term "redox".

• Reduction: This is the gain of electrons. It happens at the cathode. For instance, when electrolysis occurs in \(\mathrm{KCl}(\text{aq})\), water reduces more easily than potassium ions producing \(\mathrm{H}_{2}\) gas.
• Oxidation: This is the loss of electrons. It takes place at the anode. Chloride ions in solutions like \(\mathrm{NaCl}(\text{aq})\) lose electrons to form chlorine gas (\(\mathrm{Cl}_{2}\)).

These redox processes are essential in determining which gases or elements will be produced at each electrode, and they provide the foundation for understanding the changes during electrolysis.

Gas Production During Electrolysis

One of the notable outcomes of electrolysis is the production of gas at the electrodes.
In many aqueous solutions, particularly those containing halide ions like \(\mathrm{Cl}^-\), gases such as hydrogen and chlorine are liberated:

• At the cathode: The reduction of water molecules often results in the production of hydrogen gas, especially if the metal cations present are less favorable to reduction than water.
• At the anode: Chloride ions are oxidized to form chlorine gas, which bubbles out of the solution.

For example, in solutions like \(\mathrm{KCl}(\text{aq})\), \(\mathrm{MgCl}_2(\text{aq})\), and \(\mathrm{NaCl}(\text{aq})\), this gas production is observed as both \(\mathrm{H}_2\) and \(\mathrm{Cl}_2\) gases are produced during electrolysis.
Each gas production depends on multiple factors including ion concentration, type of ions, and the standard electrode potential. These processes are essential not only for understanding the nature of the solutions but also for practical applications like hydrogen production and chlorine generation in industrial settings.