Question

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

Short answer

Solutions (b), (c), and (d) liberate \\( ext{H}_2 \\) and \\( ext{Cl}_2 \\); (a) does not.

Step-by-step solution

Identify Ions in Solution

For electrolysis, the dissolved ions in the aqueous solutions are identified. - In option (a) CuCl2 (aq), the ions present are Cu²⁺ and Cl⁻. - In option (b) KCl (aq), the ions present are K⁺ and Cl⁻. - In option (c) MgCl2 (aq), the ions present are Mg²⁺ and Cl⁻. - In option (d) NaCl (aq), the ions present are Na⁺ and Cl⁻.

Identify Possible Electrolysis Products

Consider what occurs at each electrode: - At the cathode, reduction occurs. Hydrolysis of water can liberate H₂ ( ext{H⁺} or ext{H₂O} ⇌ ext{H}_2). - At the anode, oxidation occurs. Halide ions can give Cl₂ ( ext{Cl⁻} ⇌ ext{Cl₂}). For H₂ to be liberated, absence of metal reduction (Cu, K, Mg, Na) at the cathode is required.

Cathode Reaction Analysis

Determine which metal ions, if any, are more readily discharged at the cathode compared to H⁺. - Cu²⁺ is lower than H⁺ in the electrochemical series, so will be reduced first. Thus, ext{CuCl₂} will not liberate H₂. - K⁺, Mg²⁺, and Na⁺ are higher than H⁺, so H₂ is liberated in their solutions.

Anode Reaction Analysis

At the anode, the Cl⁻ ion will be oxidized to Cl₂ over the oxidation of H₂O as Cl⁻ has a lower discharge tendency than OH⁻. This applies to each solution with Cl⁻ present: all can produce Cl₂.

Determine the Correct Solutions

Based on cathode and anode reactions, - KCl, MgCl₂, and NaCl solutions liberate H₂ and Cl₂ as their ions lead to these reactions. - CuCl₂ does not meet the requirement as it liberates Cu at the cathode instead.

Key concepts

Electrochemical series

The electrochemical series is a list of metal ions arranged according to their ability to gain electrons, which is known as their reduction potential. This series helps to predict which ions will be reduced at the cathode during the process of electrolysis. When deciding which reaction occurs at the electrodes in an electrolyte solution, the electrochemical series serves as a helpful guide. For instance:

• Ions with higher reduction potentials (lower in the series) are more readily reduced than those with lower reduction potentials (higher in the series).
• In the context of the given exercise, electrolytes like Cu²⁺ are lower in the series than H⁺, meaning Cu²⁺ will be reduced at the cathode instead of H⁺.

Understanding the position of each ion helps determine the course of reactions in electrolysis.

Cathode reaction

The cathode in an electrolysis setup is where reduction reactions occur. During electrolysis, different ions compete to receive electrons and get reduced. The choice of ions reduced at the cathode is largely influenced by the electrochemical series.In aqueous solutions involving chloride salts such as \(\mathrm{KCl}, \mathrm{MgCl}_{2}, \mathrm{NaCl}\), the following can occur:

• H₂ is liberated at the cathode since ions such as K⁺, Mg²⁺, and Na⁺ are positioned higher than H⁺ in the electrochemical series.
• Thus, instead of the metal ions, it is the hydrogen cations (H⁺) or water molecules that are reduced to hydrogen gas (H₂).

This results in the liberation of hydrogen gas in these electrolyte solutions.

Anode reaction

At the anode, oxidation reactions take place. For chloride-containing solutions, Cl⁻ ions are oxidized. The ease of this oxidation can be attributed to the position of chloride ions concerning hydroxide ions (OH⁻). In electrolysis:

• Cl⁻ ions are oxidized to produce chlorine gas (Cl₂) at the anode.
• This reaction is more favorable than the oxidation of hydroxide ions due to the discharge potential being lower for chlorine ions.

For any solution containing Cl⁻ ions, the expected outcome is the production of chlorine gas at the anode.

Reduction and oxidation processes

A key principle in electrolysis is understanding the reduction and oxidation processes at the electrodes. Reduction refers to the gain of electrons and occurs at the cathode, while oxidation refers to the loss of electrons and takes place at the anode.The processes can be summarized as:

• Reduction: An example is \(\mathrm{H^+ + e^- \rightarrow H_2}\), where hydrogen ions gain electrons to form hydrogen gas (H₂).
• Oxidation: For example, chloride ions undergo \(\mathrm{2Cl^- \rightarrow Cl_{2} + 2e^-}\), losing electrons to form chlorine gas (Cl₂).

Understanding these processes helps to predict the course of electrolysis reactions, making it possible to determine the gaseous products that will be liberated at the electrodes.