Chapter 21.3, Problem 24LC

Interpretation Introduction

Interpretation: The half-reactions for the electrolysis in the Hall-Heroult process need to be written and the function of cryolite in the process needs to be explained.

Concept Introduction: In electrolytic cells, an electric current makes a non-spontaneous redox reaction take place. The process by which electrical energy is converted to chemical energy is known as electrolysis.

Explanation of Solution

In the Hall-Heroult reaction, the smelting of aluminum takes place. In this process, aluminum oxide is dissolved in molten cryolite.

The reactions at the cathode and anode are represented as follows:

Cathode-Reduction reaction:

  ${\text{Al}}^{3+}+3e\to \text{Al}$

Anode-Oxidation Reaction:

  ${\text{O}}^{2-}+C\to \text{CO}+\text{2e}$

The overall cell reaction will be:

  ${\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}+3\text{C}\to \text{2Al}+\text{3CO}$

Here, cryolite has a melting point of approximately $1009{}^{\text{o}}\text{C}$ . Cryolite is an electrolyte in this process because it dissolves in alumina and conducts electricity. It dissociates at higher voltage as compared to alumina and also has a lower density than aluminum.