4.2.1 Electrolysis of Molten Electrolyte

Introduction

When an ionic compound is molten (melted), its rigid lattice structure breaks down, and the ions become free to move. This makes molten ionic compounds good electrolytes.

Electrolysis of molten electrolytes is used to produce reactive metals like sodium, potassium, and lead, and non-metals like chlorine.

This section describes the electrolysis of two important molten electrolytes:

• Molten sodium chloride (NaCl) — using a Down's Cell
• Molten lead(II) chloride (PbCl₂) — using platinum or graphite electrodes

Definition

Electrolysis of Molten Electrolyte: The process of passing an electric current through a molten ionic compound, causing it to decompose into its constituent elements.

In simple words: When you melt an ionic compound and pass electricity through it, it breaks down into its elements.

Main Idea

• In molten state, ionic compounds contain free-moving ions.
• When electricity is passed through the molten electrolyte:
• Cations move towards the cathode and are reduced to metal.
• Anions move towards the anode and are oxidized to non-metal.
• The products are the metal at the cathode and the non-metal at the anode.

Core Concept

Key point:

• Metal is always produced at the cathode.
• Non-metal (like chlorine) is always produced at the anode.

Detailed Explanation

(i) Electrolysis of Molten Sodium Chloride

On a large scale, sodium metal is produced by the electrolysis of fused sodium chloride. The electrolytic cell used in the process is called a Down's Cell.

Down's Cell uses:

• Iron cathode
• Carbon anode

During electrolysis:

• Sodium ions (Na⁺) are reduced at the cathode to liquid sodium metal.
• Chloride ions (Cl^-) are oxidized at the anode to chlorine gas.

Reactions:

At anode (oxidation): 2Cl^-_(l) → Cl_2(g) + 2e^-

At cathode (reduction): 2Na⁺_(l) + 2e^- → 2Na_(l)

Overall reaction: 2NaCl_(l) → 2Na_(l) + Cl_2(g)

Products:

• Sodium metal at the cathode (liquid)
• Chlorine gas at the anode

(ii) Electrolysis of Molten Lead(II) Chloride

On a large scale, lead metal is produced by the electrolysis of molten or fused lead(II) chloride. An electrolytic cell is used for this purpose.

Apparatus:

• Molten lead chloride is placed in the electrolytic tank.
• Platinum or graphite electrodes are dipped in it.
• The electrolysis is conducted above 327°C, which is the melting point of lead chloride.

What happens:

• Molten lead chloride contains free lead ions (Pb²⁺) and chloride ions (Cl^-).
• During electrolysis, chloride ions move towards the anode and lead ions towards the cathode.

Reactions:

At anode (oxidation): 2Cl^-_(l) → Cl_2(g) + 2e^-

At cathode (reduction): Pb²⁺_(l) + 2e^- → Pb_(l)

Overall reaction: PbCl_2(l) → Pb_(l) + Cl_2(g)

Products:

• Molten lead is collected at the bottom of the cathode.
• Chlorine gas at the anode.

Summary Table

Intermediate Level

Electrolysis of molten NaCl:

• Cell: Down's Cell
• Cathode: Iron
• Anode: Carbon
• Reactions:
• Anode: 2Cl^- → Cl₂ + 2e^-
• Cathode: 2Na⁺ + 2e^- → 2Na
• Products: Na metal (cathode), Cl₂ gas (anode)

Electrolysis of molten PbCl₂:

• Electrodes: Platinum or graphite
• Temperature: Above 327°C
• Reactions:
• Anode: 2Cl^- → Cl₂ + 2e^-
• Cathode: Pb²⁺ + 2e^- → Pb
• Products: Pb metal (cathode), Cl₂ gas (anode)

Key point:

• Metal is always produced at the cathode.
• Chlorine gas is always produced at the anode (for chloride electrolytes).

Solved Example

Example: Electrolysis of Molten NaCl

Given: Molten NaCl contains Na⁺ and Cl^- ions.

Required: Products at cathode and anode.

Solution:

At anode (oxidation): 2Cl^- → Cl₂ + 2e^-

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

Final Answer:

• Cathode: Sodium metal (Na)
• Anode: Chlorine gas (Cl₂)

Example: Electrolysis of Molten PbCl₂

Given: Molten PbCl₂ contains Pb²⁺ and Cl^- ions.

Required: Products at cathode and anode.

Solution:

At anode (oxidation): 2Cl^- → Cl₂ + 2e^-

At cathode (reduction): Pb²⁺ + 2e^- → Pb

Final Answer:

• Cathode: Lead metal (Pb)
• Anode: Chlorine gas (Cl₂)

Important Points

• - Molten electrolytes contain free-moving ions.
• - When electricity is passed:
• - Cations move towards the cathode and are reduced to metal.
• - Anions move towards the anode and are oxidized to non-metal.
• - Electrolysis of molten NaCl:
• - Cell: Down's Cell
• - Cathode: Iron
• - Anode: Carbon
• - Products: Na metal (cathode), Cl₂ gas (anode)
• - Electrolysis of molten PbCl₂:
• - Electrodes: Platinum or graphite
• - Temperature: Above 327°C
• - Products: Pb metal (cathode), Cl₂ gas (anode)
• - Metal is always produced at the cathode.
• - Non-metal (like chlorine) is always produced at the anode.

Remember

• - Molten electrolytes contain free ions.
• - Cations → cathode → reduction → metal.
• - Anions → anode → oxidation → non-metal.
• - Electrolysis of molten NaCl:
• - Down's Cell
• - Iron cathode, carbon anode
• - Products: Na metal, Cl₂ gas
• - Electrolysis of molten PbCl₂:
• - Platinum or graphite electrodes
• - Above 327°C
• - Products: Pb metal, Cl₂ gas
• - Metal at cathode; non-metal at anode.

Common Mistakes

Quick Revision

Electrolysis of Molten Electrolyte:

What happens:

• In molten state, ionic compounds contain free-moving ions.
• When electricity is passed through the molten electrolyte:
• Cations move towards the cathode and are reduced to metal.
• Anions move towards the anode and are oxidized to non-metal.

Electrolysis of Molten Sodium Chloride:

• On a large scale, sodium metal is produced by the electrolysis of fused sodium chloride.
• The electrolytic cell used in the process is called a Down's Cell.
• Down's Cell uses an iron cathode and carbon anode.
• Reactions:
• At anode: 2Cl^-_(l) → Cl_2(g) + 2e^- (oxidation)
• At cathode: 2Na⁺_(l) + 2e^- → 2Na_(l) (reduction)

Electrolysis of Molten Lead(II) Chloride:

• On a large scale, lead metal is produced by the electrolysis of molten or fused lead(II) chloride.
• Molten lead chloride is placed in the electrolytic tank and platinum or graphite electrodes are dipped in it.
• The electrolysis is conducted above 327°C, which is the melting point of lead chloride.
• Reactions:
• At anode: 2Cl^-_(l) → Cl_2(g) + 2e^- (oxidation)
• At cathode: Pb²⁺_(l) + 2e^- → Pb_(l) (reduction)
• Molten lead is collected at the bottom of the cathode and chlorine gas at the anode.

60-Second Revision

• Molten electrolytes contain free ions.
• Cations → cathode → reduction → metal.
• Anions → anode → oxidation → non-metal.
• Electrolysis of molten NaCl:
• Down's Cell
• Iron cathode, carbon anode
• Products: Na metal, Cl₂ gas
• Electrolysis of molten PbCl₂:
• Platinum or graphite electrodes
• Above 327°C
• Products: Pb metal, Cl₂ gas
• Metal at cathode; non-metal at anode.
• ⚠️ Warning: metal is always produced at the cathode, and non-metal at the anode — do not mix them up.