Animated Solution for Chemistry - Metallurgy: Which of the following reduction reaction cannot be carried out with coke?

In the fascinating world of metallurgy, extracting a pure metal from its oxide ore is essentially a battle for oxygen. We need a reducing agent that loves oxygen more than the metal itself does. Carbon, in the form of coke, is the undisputed king of industrial reducing agents. It is cheap, abundant, and highly effective for metals like iron, zinc, and copper.

When we heat metal oxides with carbon, the carbon steals the oxygen to form carbon monoxide ($extCO$) or carbon dioxide ($ext{CO}_2$), leaving the pure metal behind. For example, zinc oxide is easily reduced by carbon at around $1673\text{ K}$:

To understand why carbon works for some metals and fails for others, we turn to the Ellingham Diagram. This diagram plots the standard Gibbs free energy of formation ($\Delta G^{\circ }$) of oxides against temperature ($T$).

The golden rule of the Ellingham diagram is simple: A metal (or carbon) can reduce the oxide of another metal if its $\Delta G^{\circ }$ curve lies below the curve of that oxide.

Interestingly, the curve for the formation of carbon monoxide ($2\text{C}+{\text{O}}_2⟶2\text{CO}$) slopes downwards. This happens because a solid (carbon) and a gas (oxygen) react to form two moles of a gas (carbon monoxide), leading to a significant increase in entropy ($\Delta S>0$). As temperature increases, the $-T\Delta S$ term becomes more negative, driving the $\Delta G^{\circ }$ down. Eventually, the carbon curve crosses below the curves of zinc, iron, and copper, making reduction thermodynamically feasible.

Now, let's look at aluminium. Aluminium has an incredibly high affinity for oxygen. Its $\Delta G^{\circ }$ curve for the formation of alumina (${\text{Al}}_2{\text{O}}_3$) lies very low on the Ellingham diagram.

For carbon to reduce alumina, we would need to heat the mixture to a temperature where the carbon curve finally crosses below the aluminium curve. This intersection occurs at an extreme temperature, well over $2000^{\circ }\text{C}$.

While thermodynamically possible on paper, it is practically impossible in a standard furnace. At such extreme temperatures, the aluminium metal would vaporize. Furthermore, aluminium reacts with carbon at these temperatures to form aluminium carbide (${\text{Al}}_4{\text{C}}_3$), completely defeating the purpose of extracting pure metal.

Because carbon reduction is off the table, we must use a different weapon: electricity. Aluminium is extracted commercially using the Hall-Heroult process.

In this process, alumina is dissolved in molten cryolite (${\text{Na}}_3{\text{AlF}}_6$) to lower its melting point and increase electrical conductivity. A massive electric current is passed through the molten mixture, forcing the reduction of aluminium ions at the cathode:

Thus, while coke is a fantastic reducing agent for moderately reactive metals, the sheer bond strength of alumina demands the brute force of electrometallurgy.