2.8.2 Factors Influencing the Products of Electrolysis

Electrolysis involves the use of an electric current to drive a non-spontaneous redox reaction. In an aqueous solution, there are often multiple ions present, including those from the dissolved electrolyte and those from the autoionization of water (H^+ and OH^-). The question of which ions are discharged (i.e., oxidized or reduced) at the electrodes depends on several competing factors. This section explains the three main factors that determine the products of electrolysis.

Main Idea

• The state of the electrolyte (molten or aqueous).
• The position of the ions in the electrochemical (redox) series.
• The concentration of the electrolyte.

By considering these three factors together, you can predict which ions will be preferentially discharged at the anode and cathode.

Core Concept

The core concept is that electrolysis is a competitive process. At each electrode, multiple species may be present and capable of reacting. The species that requires the least energy (the one with the most favorable electrode potential) will be discharged first. However, concentration can also play a role: a high concentration of an ion can make it more likely to be discharged, even if its standard potential is less favorable.

Detailed Explanation

The textbook identifies three factors that influence the products of electrolysis. Each is explained below.

1. The State of the Electrolyte

Electrolytes are substances that conduct electricity when dissolved in water or in their molten state.

• Molten Electrolyte: When a molten electrolyte is used, only its own cations and anions are present. These are discharged at the electrodes. There is no water to compete with, so the products are simply the metal (from the reduction of the cation) and the non-metal (from the oxidation of the anion).
• Aqueous Solution: When an aqueous solution is used, it contains both ions from the dissolved electrolyte and ions from water (H^+ and OH^-). In this case, water molecules may be oxidized to produce oxygen gas at the anode or reduced to produce hydrogen gas at the cathode, instead of the electrolyte ions being discharged.

2. Position in the Electrochemical (Redox) Series

The position of substances in the electrochemical series gives an indication of their tendency to be reduced or oxidized during electrolysis.

• At the Cathode (Reduction): Cations that are less reactive than hydrogen (i.e., have a more positive reduction potential than hydrogen, such as Cu^{2+}, Ag^+) are preferentially discharged at the cathode. Cations that are more reactive than hydrogen (such as Na^+, Mg^{2+}, Ca^{2+}) are not discharged in aqueous solution. Instead, water molecules are reduced to liberate hydrogen gas.
• At the Anode (Oxidation): The ions with the lower (more negative) reduction potential are more easily oxidized. In aqueous solutions, the oxidation of water (or OH^-) to produce oxygen competes with the oxidation of anions like Cl^-, Br^-, I^-. The species that is more easily oxidized will be discharged.

3. Concentration of the Electrolyte

The concentration of ions in the electrolyte solution also affects the ease with which they are liberated during electrolysis.

• Higher concentrations can favor the liberation of certain ions. Even if an ion is lower in the electrochemical series, a sufficiently high concentration can cause it to be preferentially discharged.
• Ions with lower concentrations are often not discharged; instead, water molecules are discharged.

Examples from the Textbook

The following examples show how the identities of substances liberated in electrolysis depend on specific electrolytes, their constituent ions, as well as their position in the electrochemical series.

1. Electrolysis of Fused NaCl

• Since the electrolyte is molten, only Na^+ and Cl^- ions are present.
• At anode (oxidation): 2Cl^- → Cl_2 + 2e^-
• At cathode (reduction): 2Na^+ + 2e^- → 2Na
• Net reaction: 2Na^+ + 2Cl^- → 2Na + Cl_2

2. Electrolysis of Concentrated Aqueous NaCl (Brine)

• The solution contains a higher concentration of Cl^- ions. Due to this high concentration, they are discharged at the anode instead of OH^- ions.
• Na^+ ions being more reactive than hydrogen remain in solution and are not discharged. Instead, water molecules are discharged at the cathode.
• At anode: 2Cl^- → Cl_2 + 2e^-
• At cathode: 2H_2O + 2e^- → H_2 + 2OH^-
• Net reaction: 2H_2O + 2Cl^- → H_2 + Cl_2 + 2OH^-

3. Electrolysis of Dilute Aqueous NaCl

• In dilute aqueous NaCl solution, Cl^- ions have a lower concentration. So, chloride ions are not discharged at the anode. Instead, OH^- ions are discharged at the anode (producing oxygen gas).
• Here again, Na^+ ions being more reactive than hydrogen remain in the solution and water molecules are discharged at the cathode.
• At anode: 4OH^- → O_2 + 4e^- + 2H_2O
• At cathode: 4H_2O + 4e^- → 2H_2 + 4OH^-
• Net reaction: 4H_2O → 2H_2 + O_2

Intermediate Level

The products of electrolysis depend on a combination of thermodynamic and kinetic factors. The standard electrode potentials provide a thermodynamic ranking of what is easiest to oxidize or reduce. However, concentration can shift this ranking (as described by the Nernst equation). For example, in concentrated brine, the high concentration of Cl^- makes its oxidation more favorable than the oxidation of water, even though the standard potential for water oxidation is lower. Similarly, at the cathode, the reduction of water to hydrogen is favored over the reduction of Na^+ because Na^+ is too reactive (has a very negative reduction potential). The state of the electrolyte (molten vs. aqueous) determines whether water is present to compete.

Important Points

• - Three factors: State of electrolyte, position in electrochemical series, and concentration.
• - Molten electrolyte: Only the electrolyte's ions are present; no water competition.
• - Aqueous solution: Water can be oxidized or reduced instead of the electrolyte ions.
• - Cathode: Cations less reactive than hydrogen (e.g., Cu^{2+}, Ag^+) are preferentially discharged. Cations more reactive than hydrogen (e.g., Na^+, Mg^{2+}) are not discharged; water is reduced instead.
• - Anode: The ion with the lower reduction potential is more easily oxidized.
• - Concentration: High concentration of an ion can favor its discharge.
• - Example: Concentrated brine → H_2 + Cl_2 + NaOH. Dilute brine → H_2 + O_2.

Remember

• - Molten = only electrolyte ions; no water.
• - Aqueous = water competes.
• - Cathode:
• - Na^+, Mg^{2+} → no discharge; water → H_2.
• - Cu^{2+}, Ag^+ → discharge; metal is deposited.
• - Anode:
• - High concentration of Cl^- → Cl_2 produced.
• - Low concentration of Cl^- → O_2 produced (from water).
• - Concentration can override the electrochemical series.

Common Mistakes

• - Assuming that the electrochemical series alone determines the products. Concentration can change the outcome. For example, concentrated Cl^- is oxidized even though OH^- has a lower standard potential.
• - Forgetting that water can be electrolyzed. In aqueous solutions, water can be oxidized at the anode or reduced at the cathode, depending on the potentials of the other ions present.
• - Confusing the anode and cathode reactions. Oxidation always occurs at the anode, and reduction at the cathode.
• - Thinking that reactive metals like Na can be produced by electrolysis of aqueous solutions. They cannot, because water is reduced instead of the metal ions.

Quick Revision

The products of electrolysis depend on three main factors: the state of the electrolyte (molten or aqueous), the position of the ions in the electrochemical series, and the concentration of the electrolyte. In a molten electrolyte, the ions of the electrolyte are the only ones present and are discharged. In an aqueous solution, water can be oxidized or reduced instead of the electrolyte ions. At the cathode, cations less reactive than hydrogen (like Cu^{2+}) are preferentially discharged, while more reactive cations (like Na^+) remain in solution, and water is reduced to produce hydrogen gas. At the anode, the ion with the lower reduction potential is more easily oxidized. However, concentration can override this; for example, concentrated Cl^- is oxidized to Cl_2 even though water would be more easily oxidized.

60-Second Revision

The products of electrolysis depend on three factors: the state of the electrolyte (molten/aqueous), the electrochemical series, and concentration. In a molten electrolyte, the electrolyte's ions are discharged. In aqueous solution, water competes. At the cathode, less reactive metals (like Cu^{2+}) are deposited, while more reactive metals (like Na^+) remain in solution and water is reduced to H_2. At the anode, higher concentrations can favor the discharge of ions like Cl^- to form Cl_2, even if water is more easily oxidized.

Frequently Asked Questions (FAQs)

• The state of the electrolyte (molten or aqueous).
• The position of the ions in the electrochemical series (their tendency to be oxidized or reduced).
• The concentration of the electrolyte.

Q2. How does the state of the electrolyte affect the products of electrolysis? Answer: In a molten electrolyte, only the ions of the electrolyte are present, and these are discharged at the electrodes. In an aqueous solution, water is also present and can be oxidized at the anode to produce oxygen or reduced at the cathode to produce hydrogen, competing with the electrolyte ions.

Q3. In an aqueous solution, why is water reduced at the cathode instead of Na^+ ions? Answer: Na^+ ions are more reactive than hydrogen (they have a more negative reduction potential). Therefore, it is harder to reduce Na^+ than to reduce water. In aqueous solutions, water is preferentially reduced to produce hydrogen gas at the cathode.

Q4. Why does concentrated brine produce Cl_2 at the anode, while dilute brine produces O_2? Answer: In concentrated brine, the high concentration of Cl^- ions favors their discharge at the anode, even though OH^- has a lower standard oxidation potential. In dilute brine, the concentration of Cl^- is low, so water (or OH^-) is preferentially oxidized, producing oxygen gas. This shows that concentration can override the thermodynamic tendency.

• Q5. What are the products of the electrolysis of fused (molten) NaCl?
• Answer: In molten NaCl, the products are sodium metal at the cathode and chlorine gas at the anode:
• 2Na^+ + 2Cl^- → 2Na + Cl_2.

Q6. Explain why the electrolysis of concentrated and dilute aqueous NaCl gives different products. Answer: The difference in products is due to the concentration of the chloride ions. In concentrated brine, the high concentration of Cl^- leads to its discharge at the anode (Cl_2). In dilute brine, the low concentration of Cl^- means that water is preferentially oxidized at the anode (O_2). At the cathode, water is reduced to H_2 in both cases because Na^+ is too reactive to be reduced.

Q7. What is the role of the electrochemical series in determining the products of electrolysis? Answer: The electrochemical series provides a ranking of the ease of oxidation or reduction of different species. At the cathode, the cations with more positive reduction potentials (like Cu^{2+}) are more easily reduced and are preferentially discharged. At the anode, the ions with lower reduction potentials are more easily oxidized. The series helps predict which species is thermodynamically favored to react.

Q8. Why is the electrolysis of aqueous solutions more complex than the electrolysis of molten electrolytes? Answer: In aqueous solutions, water is present and can be oxidized or reduced, providing additional competing reactions. In molten electrolytes, only the ions of the electrolyte are present, making the prediction of products simpler.

• Q9. What happens at the anode during the electrolysis of dilute aqueous NaCl?
• Answer: At the anode, water (or OH^-) is oxidized to produce oxygen gas:
• 4OH^- → O_2 + 4e^- + 2H_2O.

Multiple Choice Questions (MCQs)

• Q1. Which of the following factors does NOT influence the products of electrolysis?
• A. The state of the electrolyte
• B. The position of the ions in the electrochemical series
• C. The concentration of the electrolyte
• D. The color of the electrolyte
• Correct Answer: D
• Explanation: The products depend on the state (molten/aqueous), the electrochemical series, and concentration. Color is not a relevant factor.

• Q2. In the electrolysis of molten NaCl, what is produced at the cathode?
• A. Chlorine gas
• B. Sodium metal
• C. Hydrogen gas
• D. Oxygen gas
• Correct Answer: B
• Explanation: In molten NaCl, Na^+ ions are reduced at the cathode to form sodium metal.

• Q3. In the electrolysis of concentrated aqueous NaCl, what is produced at the anode?
• A. Oxygen gas
• B. Hydrogen gas
• C. Chlorine gas
• D. Sodium metal
• Correct Answer: C
• Explanation: In concentrated brine, the high concentration of Cl^- leads to its discharge at the anode, producing chlorine gas.

Q4. In the electrolysis of dilute aqueous NaCl, what is produced at the anode? A. Chlorine gas B. Oxygen gas C. Hydrogen gas D. Sodium metal Correct Answer: B Explanation: In dilute brine, the low concentration of Cl^- means water (or OH^-) is preferentially oxidized at the anode, producing oxygen gas.

• Q5. Why is hydrogen gas produced at the cathode during the electrolysis of aqueous NaCl?
• A. Because Na^+ is less reactive than hydrogen.
• B. Because Na^+ is more reactive than hydrogen, so water is reduced instead.
• C. Because Cl^- is oxidized at the cathode.
• D. Because the cathode is positive.
• Correct Answer: B
• Explanation: Na^+ is more reactive than hydrogen, so it is not reduced in aqueous solution. Instead, water is reduced to produce hydrogen gas.

Q6. Which of the following ions is preferentially discharged at the cathode? A. Na^+ B. Mg^{2+} C. Cu^{2+} D. Ca^{2+} Correct Answer: C Explanation: Cu^{2+} is less reactive than hydrogen, so it is preferentially discharged (reduced) at the cathode. Na^+, Mg^{2+}, and Ca^{2+} are more reactive and are not discharged in aqueous solution.

• Q7. The high concentration of Cl^- in brine favors the formation of:
• A. O_2
• B. H_2
• C. Cl_2
• D. Na
• Correct Answer: C
• Explanation: A high concentration of Cl^- ions favors their discharge at the anode, producing chlorine gas (Cl_2).

• Q8. What is the effect of concentration on the products of electrolysis?
• A. Higher concentration of an ion can favor its discharge.
• B. Lower concentration of an ion always favors its discharge.
• C. Concentration has no effect.
• D. Concentration only affects the cathode.
• Correct Answer: A
• Explanation: Higher concentrations can favor the liberation of certain ions, even if they are not the most thermodynamically favored.

• Q9. In the electrolysis of aqueous CuSO_4, what is produced at the cathode?
• A. H_2 gas
• B. Cu metal
• C. O_2 gas
• D. SO_2 gas
• Correct Answer: B
• Explanation: Cu^{2+} ions are less reactive than hydrogen, so they are preferentially discharged at the cathode, depositing copper metal.

Q10. Why does molten NaCl produce different products from aqueous NaCl? A. Because molten NaCl has no water to compete with the ions. B. Because molten NaCl has a higher concentration. C. Because molten NaCl is at a higher temperature. D. Because molten NaCl has a different electrochemical series. Correct Answer: A Explanation: In molten NaCl, only Na^+ and Cl^- ions are present, so they are discharged. In aqueous solution, water can be oxidized or reduced instead.

Multiple Choice Questions (MCQs)

Q1. What are the main factors that determine the products of electrolysis? A. Temperature, pressure, and volume B. State of electrolyte, electrochemical series, and concentration C. Color, density, and melting point D. Atomic mass, atomic number, and valency Correct Answer: B Explanation: The products of electrolysis depend on the state of the electrolyte (molten or aqueous), the position of ions in the electrochemical series, and the concentration of the electrolyte.

• Q2. In the electrolysis of molten NaCl, what is produced at the anode?
• A. Sodium metal
• B. Chlorine gas
• C. Hydrogen gas
• D. Oxygen gas
• Correct Answer: B
• Explanation: In molten NaCl, chloride ions are oxidized at the anode to produce chlorine gas.

• Q3. In the electrolysis of concentrated aqueous NaCl, why is Cl_2 produced at the anode instead of O_2?
• A. Because Cl^- has a lower concentration.
• B. Because Cl^- has a higher concentration, which favors its discharge.
• C. Because Na^+ is more reactive than hydrogen.
• D. Because the anode is negative.
• Correct Answer: B
• Explanation: In concentrated brine, the high concentration of Cl^- ions favors their discharge at the anode, producing chlorine gas.

• Q4. Why is hydrogen gas produced at the cathode during the electrolysis of aqueous NaCl?
• A. Because Na^+ is less reactive than hydrogen.
• B. Because Na^+ is more reactive than hydrogen, so water is reduced instead.
• C. Because Cl^- is oxidized at the cathode.
• D. Because the cathode is positive.
• Correct Answer: B
• Explanation: Na^+ is more reactive than hydrogen, so it is not reduced in aqueous solution. Instead, water is reduced to produce hydrogen gas.

• Q5. Which ion is preferentially discharged at the cathode in an aqueous solution?
• A. Na^+
• B. Mg^{2+}
• C. Cu^{2+}
• D. Ca^{2+}
• Correct Answer: C
• Explanation: Cu^{2+} is less reactive than hydrogen, so it is preferentially reduced at the cathode.

Q6. What is the product at the anode during the electrolysis of dilute aqueous NaCl? A. Cl_2 B. O_2 C. H_2 D. Na Correct Answer: B Explanation: In dilute brine, the low concentration of Cl^- means water (or OH^-) is preferentially oxidized at the anode, producing oxygen gas.

Q7. What is the effect of the state of the electrolyte on the products of electrolysis? A. Molten electrolytes have no water to compete, so only the electrolyte ions are discharged. B. Aqueous electrolytes always produce hydrogen gas. C. Molten electrolytes always produce oxygen gas. D. The state of the electrolyte has no effect. Correct Answer: A Explanation: In a molten electrolyte, only the ions of the electrolyte are present, so they are the only species available for discharge. In an aqueous solution, water can also be oxidized or reduced.

Q8. What is the role of the electrochemical series in electrolysis? A. It determines the color of the products. B. It ranks the ease of oxidation and reduction of species. C. It measures the pH of the solution. D. It determines the temperature of the cell. Correct Answer: B Explanation: The electrochemical series ranks species by their tendency to be oxidized or reduced. The species that is more easily oxidized or reduced is preferentially discharged.

Q9. In the electrolysis of aqueous CuSO_4, what is produced at the anode? A. Cu metal B. O_2 gas C. H_2 gas D. SO_2 gas Correct Answer: B Explanation: In the electrolysis of aqueous CuSO_4, water (or OH^-) is preferentially oxidized at the anode, producing oxygen gas. Copper is deposited at the cathode.

Q10. Why does the electrolysis of dilute aqueous NaCl produce different products from the electrolysis of concentrated aqueous NaCl? A. Because the concentration of Cl^- affects which ion is preferentially discharged at the anode. B. Because the concentration of Na^+ affects which ion is preferentially discharged at the cathode. C. Because the temperature of the solution is different. D. Because the electrochemical series changes with concentration. Correct Answer: A Explanation: The difference in products is due to the concentration of Cl^- ions. In concentrated brine, the high concentration of Cl^- favors its discharge at the anode (Cl_2). In dilute brine, the low concentration of Cl^- means water is preferentially oxidized (O_2).