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Chapter 9: Problem 168

In the electrolysis of aqueous \(\mathrm{NaCl}\) solution, side reactions taking place are: (1) \(2 \mathrm{OH}^{-}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{OCl}^{-}+\mathrm{H}_{2}\) (2) \(2 \mathrm{Na}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH}+\mathrm{H}_{2}\) (3) \(4 \mathrm{OH}^{-} \rightarrow \mathrm{O}_{2}+2 \mathrm{H}_{2}+4 \mathrm{e}^{-}\) Select the correct alternate: (a) 1 and 3 (b) 2 and 3 (c) 1 and 2 (d) 1,2 and 3

Short Answer

Expert verified
The correct alternate is (a) 1 and 3.

Step by step solution

01

Identify the Standard Reactions

For the electrolysis of aqueous \(\mathrm{NaCl}\), the primary reactions involve the decomposition of water and \(\mathrm{NaCl}\) into their respective ions: \(\mathrm{Na}^+,\,\mathrm{Cl}^-\), \(\mathrm{H}^+\), and \(\mathrm{OH}^-\). The reaction at the anode produces \(\mathrm{Cl}_2\), and at the cathode produces \(\mathrm{H}_2\).
02

Understand the Side Reactions

Side reactions often occur in the presence of additional ions or changes in pH which can induce competing reactions. All three given reactions (1, 2, and 3) involve such conditions where \(\mathrm{Cl}_2\) and \(\mathrm{OH}^-\) participate in forming other compounds, and reactions involving sodium can lead to the formation of \(\mathrm{NaOH}\) and \(\mathrm{H}_2\) through the hydroxyl ion.
03

Analyze Reaction 1

Reaction (1), \(2 \mathrm{OH}^- + \mathrm{Cl}_2 \rightarrow 2 \mathrm{OCl}^- + \mathrm{H}_2\), shows the formation of hypochlorite ions from hydroxyl ions and chlorine, which can occur under certain electrochemical conditions during electrolysis, making it plausible as a side reaction.
04

Analyze Reaction 2

Reaction (2), \(2 \mathrm{Na} + 2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NaOH} + \mathrm{H}_2\), involves sodium reacting with water to produce sodium hydroxide and hydrogen gas. This reaction is less probable in an aqueous solution of \(\mathrm{NaCl}\) since sodium ions are more stable in solution.
05

Analyze Reaction 3

Reaction (3), \(4 \mathrm{OH}^- \rightarrow \mathrm{O}_2 + 2 \mathrm{H}_2 + 4 \mathrm{e}^-\), represents the decomposition of hydroxide ions to produce oxygen and hydrogen gas along with electrons. This energy-favored reaction can take place during electrolysis particularly at the anode.
06

Select the Alternate

Since reactions 1 and 3 can both feasibly occur under the conditions of aqueous \(\mathrm{NaCl}\) electrolysis, they are plausible side reactions, while reaction 2 is unlikely due to the inert nature of \(\mathrm{Na}^+\) in aqueous solution. Therefore, alternate (a) 1 and 3 is correct.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Side Reactions
In the process of electrolysis, side reactions can often occur alongside the primary electrochemical reactions. Side reactions emerge when additional ions present in the solution interact under the conditions of the electrolysis.
For electrolysis of aqueous \(\text{NaCl}\), the standard reactions at the electrodes can result in different ions in the solution participating in alternative reactions. Here, we'll discuss the side reactions that occur:
  • **Reaction 1:** \(2 \text{OH}^- + \text{Cl}_2 \rightarrow 2 \text{OCl}^- + \text{H}_2\) is a side reaction where hydroxide ions and chlorine form hypochlorite ions and hydrogen gas.
  • **Reaction 3:** \(4 \text{OH}^- \rightarrow \text{O}_2 + 2 \text{H}_2 + 4 \text{e}^-\) involves the decomposition of hydroxide ions, creating oxygen, hydrogen gas, and releasing electrons.
These reactions highlight the complexity of electrolysis and the various pathways ions can follow.
Electrochemical Conditions
To understand the array of reactions during electrolysis, one must look at the electrochemical conditions. These conditions include the potential difference applied across the electrodes, the concentration of ions, and the solution's pH.
Aqueous solutions such as \(\text{NaCl}\) provide a mix of ions that can respond to these conditions:
  • **pH Influence:** The acidity or basicity of the solution can determine the availability of hydroxide ions, thus impacting reactions like the formation of \(\text{OCl}^-\).
  • **Concentration of Ions:** The abundance of \(\text{Na}^+\), \(\text{Cl}^-\), \(\text{H}^+\), and \(\text{OH}^-\) ions can shift reaction equilibria, permitting different side reactions.
  • **Electrode Potential:** Different potentials may favor certain reactions, with some ions requiring specific energies to undergo transformation.
By controlling or adjusting these electrochemical conditions, the primary and secondary reactions during electrolysis can be managed or predicted.
Anode and Cathode Reactions
Electrolysis involves reactions occurring at two distinct regions: the anode and the cathode. Each electrode plays a crucial role in determining the path and products of the reaction.
For aqueous \(\text{NaCl}\) electrolysis:
  • **Anode Reaction:** Typically generates chlorine gas, following \(2 \text{Cl}^- \rightarrow \text{Cl}_2 + 2 \text{e}^-\). However, it can also take paths that produce other compounds like hypochlorite ions (as seen in Reaction 1).
  • **Cathode Reaction:** This site facilitates the reduction process and typically generates hydrogen gas by \(2 \text{H}^+ + 2 \text{e}^- \rightarrow \text{H}_2\).
The division into separate anode and cathode reactions helps build an understanding of how different substances are formed and consumed during electrolysis. It delineates how electrons flow and implicates the electron-proton interactions that are paramount for such processes.
Formation of Hypochlorite Ions
During the electrolysis of aqueous \(\text{NaCl}\), hypochlorite ions \(\text{OCl}^-\) can form through certain side reactions. This is noted in Reaction 1, where chlorine interacts with hydroxide ions.
Hypochlorite ions are valuable for their use in various cleaning and bleaching processes. Understanding how they form involves recognizing these reactions:
  • **Primary Contributors:** Hydroxide ions \(\text{OH}^-\) play a significant role, reacting with \(\text{Cl}_2\) to form \(\text{OCl}^-\) during side reactions under specific electrochemical conditions.
  • **Environmental Factors:** The presence of chlorine in solution along with basic conditions (presence of OH-) accelerates the formation.
Recognizing the formation pathway of hypochlorite ions is not only fundamental for understanding electrolysis but also for its industrial applications in sanitation and cleaning.

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Most popular questions from this chapter

Carnallite on electrolysis gives: (a) \(\mathrm{Na}\) and \(\mathrm{CO}_{2}\) (b) \(\mathrm{Mg}\) and \(\mathrm{Cl}_{2}\) (c) \(\mathrm{Al}\) and \(\mathrm{Cl}_{2}\) (d) \(\mathrm{Ca}\) and \(\mathrm{Cl}_{2}\)

What are the products formed when an aqueous solution of magnesium bicarbonate is boiled? (a) \(\mathrm{Mg}, \mathrm{CO}_{2}, \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Mg}(\mathrm{OH})_{2}, \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{MgCO}_{3}, \mathrm{H}_{2} \mathrm{O}, \mathrm{CO}_{2}\) (d) \(\mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2}, \mathrm{H}_{2} \mathrm{O}\)

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