Chapter 20.3, Problem 25SSC

Interpretation Introduction

Interpretation:

In light of Hall-Heroult process, the significance of aluminum recycling is to be explained.

Concept introduction:

Electrochemistry is study of relation between electrical energy and chemical changes that takes place in redox reaction. There are three main aspects of electrochemistry branch, those are:

1. Electrolysis
2. Electrochemical cell
3. Electrolytic conduction.

Electrolysis is a process in which decomposition of electrolyte occurs on flow of electricity through its aqueous solution or molten state. The instrument employed to carry out electrolysis is electrolytic cell. Example of electrolytic cell is when a secondary battery is recharged.

Answer to Problem 25SSC

In Hall-Heroult process, $\text{Al}$ is obtained by electrolysis of ${\text{Al}}_2{\text{O}}_3$. This process employs large amount of electrical energy for electrolysis of ${\text{Al}}_2{\text{O}}_3$, therefore, recycling of aluminum becomes necessary. Recycled $\text{Al}$ has already undergone electrolysis therefore for making it again fit for usage requires only that much amount of heat which can melt it in the furnace.

Explanation of Solution

In Hall-Heroult process, $\text{Al}$ is obtained by electrolysis of ${\text{Al}}_2{\text{O}}_3$. In this process, cathode is graphite that surrounds the electrolytic cell and a graphite rod immersed in molten solution of ${\text{Al}}_2{\text{O}}_3$ dissolved at $1000{}^{°}\text{C}$ in synthetic cryolite act as anode.

The half cell reactions that occur at anode and cathode respectively are as follows:

$\begin{array}{l}{\text{Al}}^{\text{3+}}\left(\text{l}\right)+3{\text{e}}^{-}\to \text{Al}\left(\text{l}\right)\\ {\text{2O}}^{2-}\left(\text{aq}\right)\to {\text{O}}_2\left(\text{g}\right)+4{\text{e}}^{-}\end{array}$

${\text{Al}}^{\text{3+}}$ get reduced at anode and ${\text{O}}^{2-}$ gets oxidized at cathode into ${\text{O}}_2$ which react with carbon of anode liberate ${\text{CO}}_2$.

$\text{C}\left(\text{s}\right)+{\text{O}}_2\left(\text{g}\right)\to {\text{CO}}_2\left(\text{g}\right)$

Hall-Heroults’ process employs large amount of electrical energy for electrolysis of ${\text{Al}}_2{\text{O}}_3$, therefore, recycling of aluminum becomes necessary. Recycled $\text{Al}$ has already undergone electrolysis therefore for making it again fit for usage requires only that much amount of heat which can melt it in the furnace.

Conclusion

In Hall-Heroult process, $\text{Al}$ is obtained by electrolysis of ${\text{Al}}_2{\text{O}}_3$. This process employs large amount of electrical energy for electrolysis of ${\text{Al}}_2{\text{O}}_3$, therefore, recycling of aluminum becomes necessary. Recycled $\text{Al}$ has already undergone electrolysis therefore for making it again fit for usage requires only that much amount of heat which can melt it in the furnace.