4.2.3 Electrolysis of Dilute Aqueous Solution

Introduction

In the previous section, we studied the electrolysis of concentrated aqueous sodium chloride (brine). The products were chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in the solution.

But what happens if the solution is dilute instead of concentrated? The products can be different.

This section explains the electrolysis of four important dilute aqueous solutions:

• Dilute aqueous sodium chloride using platinum electrodes
• Dilute sulphuric acid using platinum electrodes
• Copper(II) sulphate using platinum electrodes
• Copper(II) sulphate using copper electrodes

The key factor that determines the products is the concentration of the electrolyte and the nature of the electrodes.

Definition

Electrolysis of Dilute Aqueous Solution: The process of passing an electric current through a dilute aqueous solution of an electrolyte, causing it to decompose into new substances.

In simple words: When you pass electricity through a dilute salt solution, it breaks down into new substances — but the products may be different from those obtained with a concentrated solution.

Main Idea

• In dilute aqueous solutions, water competes with the electrolyte ions for discharge at the electrodes.
• The products depend on:
• The concentration of the electrolyte
• The position of ions in the electrochemical series
• The type of electrodes used (inert or reactive)
• Dilute solutions tend to produce oxygen gas at the anode (from water) instead of chlorine.
• The cathode reaction depends on the reactivity of the metal ions present.

Core Concept

Key point: The products of electrolysis depend on the concentration of the electrolyte and the type of electrodes.

Detailed Explanation

a. Electrolysis of Dilute Aqueous Sodium Chloride Using Platinum Electrodes

What happens:

• Unlike electrolysis of concentrated aqueous sodium chloride, chloride ions are not oxidized at the anode.
• Instead, hydroxide ions are oxidized and oxygen gas is produced at the anode.
• Reaction at the cathode remains the same: hydrogen gas is produced.

Why this happens:

• In dilute solution, the concentration of Cl^- ions is relatively low compared to OH^- ions from water.
• The overpotential for the oxidation of Cl^- to chlorine gas is higher than that for the oxidation of OH^- to oxygen gas.
• This makes it energetically less favorable to discharge Cl^- ions.
• Therefore, OH^- ions are discharged instead.

Reactions:

At cathode (reduction): 4H⁺_(aq) + 4e^- → 2H_2(g)

At anode (oxidation): 4OH^-_(aq) → 2H₂O_(l) + 4e^- + O_2(g)

Overall reaction: 2H₂O_(l) → 2H_2(g) + O_2(g)

Products:

• Cathode: Hydrogen gas (H₂)
• Anode: Oxygen gas (O₂)

b. Electrolysis of Dilute Sulphuric Acid Using Platinum Electrodes

What happens:

• In an aqueous solution, sulphuric acid ionizes to form hydrogen ions and sulphate ions.
• Autoionization of water gives hydrogen ions and hydroxide ions.

Ionization reactions:

H₂SO_4(aq) → 2H⁺_(aq) + SO₄^2-_(aq)

H₂O_(l) ⇌ H⁺_(aq) + OH^-_(aq)

During electrolysis:

• Hydrogen ions move towards the cathode and are reduced to form hydrogen.
• Hydroxyl ions and sulphate ions move towards the anode.
• At the anode, hydroxyl ions are oxidized and sulphate ions remain unchanged.

Reactions:

At cathode (reduction): 4H⁺_(aq) + 4e^- → 2H_2(g)

At anode (oxidation): 4OH^-_(aq) → 2H₂O_(l) + 4e^- + O_2(g)

Overall reaction: 2H₂O_(l) → 2H_2(g) + O_2(g)

Products:

• Cathode: Hydrogen gas (H₂)
• Anode: Oxygen gas (O₂)

Key point: In the electrolysis of dilute aqueous H₂SO₄ using platinum electrodes, hydrogen gas is produced at the cathode and oxygen gas at the anode.

c. Electrolysis of Copper(II) Sulphate Using Platinum Electrodes

What happens:

• The aqueous solution of copper sulphate contains:
• Copper ions (Cu²⁺) and sulphate ions (SO₄^2-) from copper(II) sulphate
• Hydrogen ions (H⁺) and hydroxide ions (OH^-) from water
• During electrolysis, copper ions and hydrogen ions are attracted towards the cathode.
• At the cathode, only copper ions are reduced and hydrogen ions remain unchanged.
• The sulphate ions and hydroxide ions are attracted towards the anode.
• At the anode, only hydroxide ions are oxidized and sulphate ions remain unchanged.

Why copper is reduced instead of hydrogen:

• Copper is lower in the electrochemical series than hydrogen.
• This means copper ions (Cu²⁺) have a greater tendency to be reduced than hydrogen ions (H⁺).
• Therefore, copper metal is produced at the cathode.

Reactions:

At cathode (reduction): 2Cu²⁺_(aq) + 4e^- → 2Cu_(s)

At anode (oxidation): 4OH^-_(aq) → 2H₂O_(l) + O_2(g) + 4e^-

Overall reaction: 2CuSO_4(aq) + 2H₂O_(l) → 2Cu_(s) + 2H₂SO_4(aq) + O_2(g)

Products:

• Cathode: Copper metal (Cu)
• Anode: Oxygen gas (O₂)
• Observation: The blue colour of the copper sulphate solution gradually fades away as the reaction goes on.

Key point: In the electrolysis of aqueous copper(II) sulphate using a platinum electrode, copper metal is produced at the cathode and oxygen gas at the anode.

d. Electrolysis of Copper(II) Sulphate Using Copper Electrodes

What happens:

• If you use copper electrodes rather than platinum, the cathode reaction remains the same but the anode reaction differs.
• At the anode, copper atoms break away from the electrode forming Cu²⁺ ions, giving up electrons to the anode.
• These electrons are moved towards the cathode by the power source.

Why the anode reaction is different:

• Copper is a reactive metal.
• When copper electrodes are used, the copper anode can dissolve to form Cu²⁺ ions.
• This is more favorable than oxidizing water to produce oxygen gas.

Reactions:

At cathode (reduction): Cu²⁺_(aq) + 2e^- → Cu_(s)

At anode (oxidation): Cu_(s) → Cu²⁺_(aq) + 2e^-

Overall reaction: Cu_(s) (anode) → Cu_(s) (cathode)

Products:

• Cathode: Copper metal is deposited (cathode gains mass).
• Anode: Copper metal dissolves (anode loses mass).
• Observation: The concentration of CuSO₄ solution remains constant.
• Application: This process is used in electrorefining of copper.

Key point: In the electrolysis of aqueous copper(II) sulphate using copper electrodes, copper is transferred from the anode to the cathode. The concentration of the solution remains constant.

Summary Table

Intermediate Level

Why dilute solutions produce oxygen at the anode:

• In dilute solutions, the concentration of Cl^- is low.
• OH^- ions from water are discharged instead.
• This produces oxygen gas.

Why copper is reduced at the cathode:

• Copper is lower in the electrochemical series than hydrogen.
• Copper ions have a greater tendency to be reduced.

Why copper electrodes behave differently:

• Copper anode can dissolve to form Cu²⁺ ions.
• This is more favorable than oxidizing water.
• The concentration of the solution remains constant.

Key point:

- The products depend on the concentration of the electrolyte and the type of electrodes.

Solved Example

Example: Electrolysis of Dilute NaCl

Given: Dilute aqueous NaCl contains Na⁺, Cl^-, H⁺, and OH^- ions.

Required: Products at cathode and anode.

Solution:

At cathode (reduction): 4H⁺ + 4e^- → 2H₂

At anode (oxidation): 4OH^- → 2H₂O + 4e^- + O₂

Final Answer:

• Cathode: Hydrogen gas (H₂)
• Anode: Oxygen gas (O₂)

Example: Electrolysis of CuSO₄ with Copper Electrodes

Given: Aqueous CuSO₄ with copper electrodes.

Required: Products at cathode and anode.

Solution:

At cathode (reduction): Cu²⁺ + 2e^- → Cu

At anode (oxidation): Cu → Cu²⁺ + 2e^-

Final Answer:

• Cathode: Copper metal deposited (cathode gains mass)
• Anode: Copper metal dissolves (anode loses mass)
• Solution: Concentration of CuSO₄ remains constant

Concept Assessment Exercise 4.2

Sketch a voltaic cell labeling the cathode, the anode and the direction of flow of the electrons. Use the following chemicals:

Silver, Zinc, Silver Nitrate, and zinc sulphate (ZnSO₄).

(Hint: Zn is more active than Ag)

Answer:

• Anode: Zinc (Zn) — oxidation occurs here
• Cathode: Silver (Ag) — reduction occurs here
• Direction of electron flow: From Zn (anode) to Ag (cathode) through the external circuit
• Reactions:
• At anode: Zn → Zn²⁺ + 2e^-
• At cathode: Ag⁺ + 1e^- → Ag

Important Points

• - In dilute aqueous solutions, water competes with the electrolyte ions for discharge.
• - Dilute NaCl: O₂ at anode, H₂ at cathode.
• - Dilute H₂SO₄: O₂ at anode, H₂ at cathode.
• - CuSO₄ with Pt electrodes: O₂ at anode, Cu at cathode. Blue colour fades.
• - CuSO₄ with Cu electrodes: Cu dissolves at anode, Cu deposits at cathode. Concentration remains constant.
• - The products depend on:
• - Concentration of the electrolyte
• - Position of ions in the electrochemical series
• - Type of electrodes used
• - Key difference: Dilute solutions tend to produce O₂ at the anode, while concentrated solutions tend to produce Cl₂.

Remember

• - Dilute NaCl → O₂ at anode, H₂ at cathode.
• - Dilute H₂SO₄ → O₂ at anode, H₂ at cathode.
• - CuSO₄ with Pt → O₂ at anode, Cu at cathode. Blue colour fades.
• - CuSO₄ with Cu → Cu dissolves at anode, Cu deposits at cathode. Concentration constant.
• - Dilute solutions → O₂ at anode.
• - Concentrated solutions → Cl₂ at anode.
• - Copper is lower than hydrogen in the electrochemical series → Cu is reduced at cathode.
• - Reactive electrodes can participate in the reaction.

Common Mistakes

Quick Revision

Electrolysis of Dilute Aqueous Solution:

a. Electrolysis of dilute aqueous Sodium chloride using platinum electrodes:

• Unlike electrolysis of concentrated aqueous sodium chloride, chloride ions are not oxidized at the anode.
• Instead, hydroxide ions are oxidized and oxygen gas is produced at the anode.
• Reaction at the cathode remains the same. And hydrogen is produced at the cathode.
• At cathode: 4H⁺_(aq) + 4e^- → 2H_2(g)
• At anode: 4OH^-_(aq) → 2H₂O_(l) + 4e^- + O_2(g)

b. Electrolysis of dilute sulphuric acid using platinum electrodes:

• In an aqueous solution, sulphuric acid ionizes to form hydrogen ions and sulphate ions. Autoionization of water gives hydrogen ions and hydroxide ions.
• At cathode: 4H⁺_(aq) + 4e^- → 2H_2(g)
• At anode: 4OH^-_(aq) → 2H₂O_(l) + 4e^- + O_2(g)

c. Electrolysis of copper(II) sulphate using platinum electrodes:

• At cathode: 2Cu²⁺_(aq) + 4e^- → 2Cu_(s)
• At anode: 4OH^-_(aq) → 2H₂O_(l) + O_2(g) + 4e^-
• The blue colour of the copper sulphate solution gradually fades away.

d. Electrolysis of copper(II) sulphate using copper electrodes:

• At cathode: Cu²⁺_(aq) + 2e^- → Cu_(s)
• At anode: Cu_(s) → Cu²⁺_(aq) + 2e^-

Key Rule:

• Dilute Solution: Oxygen at the anode.
• Concentrated Solution: Halogen at the anode.
• The cathode always produces hydrogen gas in both cases.

60-Second Revision

• Dilute NaCl: O₂ at anode, H₂ at cathode.
• Dilute H₂SO₄: O₂ at anode, H₂ at cathode.
• CuSO₄ with Pt: O₂ at anode, Cu at cathode. Blue colour fades.
• CuSO₄ with Cu: Cu dissolves at anode, Cu deposits at cathode. Concentration constant.
• Dilute solutions → O₂ at anode.
• Concentrated solutions → Cl₂ at anode.
• Copper is lower than hydrogen → Cu is reduced at cathode.
• Reactive electrodes can participate in the reaction.
• ⚠️ Warning: dilute and concentrated solutions can give different products at the anode.