Electrolytic Reduction of Reactive Metals | Class 10 Science

Quick answer: Metals high in the reactivity series, such as sodium, magnesium, calcium and aluminium, have a greater affinity for oxygen than carbon, so carbon cannot reduce their oxides. They are extracted by electrolytic reduction. For example, molten sodium chloride is electrolysed: sodium metal forms at the cathode ($\mathrm{Na^+ + e^- \rightarrow Na}$) and chlorine gas at the anode ($\mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}$). Aluminium is obtained by the electrolytic reduction of aluminium oxide.

Metals in the middle of the reactivity series, such as zinc, iron and lead, are usually extracted by heating their oxides with carbon. For example, carbon reduces zinc oxide to zinc:

$\mathrm{ZnO + C \longrightarrow Zn + CO}$

But heating carbon with the oxides of very reactive metals, such as potassium, sodium, calcium, magnesium and aluminium, does not work. These metals hold on to oxygen more strongly than carbon does, so another method is needed: electrolytic reduction.

In CBSE Class 10 Science, Chapter 3 (Metals and Non-metals), why reactive metals cannot be obtained by carbon reduction and how they are extracted by electrolysis are frequently asked board exam questions.

What You Will Learn

• Why carbon cannot reduce the oxides of highly reactive metals
• What electrolytic reduction is
• How sodium is extracted by the electrolysis of molten sodium chloride
• The reactions at the cathode and the anode
• Why solid or aqueous sodium chloride cannot be used
• How aluminium is extracted from aluminium oxide

1. Why Can't Carbon Reduce the Oxides of Highly Reactive Metals?

Common board question: Why can't sodium, magnesium and aluminium be obtained by heating their oxides with carbon?

Answer: These metals are above carbon in the reactivity series. They have a greater affinity for oxygen than carbon, so carbon cannot remove the oxygen from their oxides. These metals are therefore obtained by electrolytic reduction of their molten chlorides or oxides.

2. Extraction of Sodium from Molten Sodium Chloride

Sodium is obtained by passing an electric current through molten (fused) sodium chloride.

In the molten state, sodium chloride contains free ions:

$\mathrm{NaCl \longrightarrow Na^+ + Cl^-}$

At the cathode (reduction): sodium ions gain electrons and form sodium metal.

$\mathrm{Na^+ + e^- \longrightarrow Na}$

At the anode (oxidation): chloride ions lose electrons and form chlorine gas.

$\mathrm{2Cl^- \longrightarrow Cl_2 + 2e^-}$

Key point: In electrolytic reduction, the metal is always obtained at the cathode, because metal ions are positive and move towards the negative electrode.

3. Why Must the Sodium Chloride Be Molten?

Why Not Solid Sodium Chloride?

In solid sodium chloride, the ions are held in fixed positions and cannot move. Solid sodium chloride therefore does not conduct electricity. When it is melted, the ions become free to move and carry the current.

Why Not an Aqueous Solution of Sodium Chloride?

If a solution of sodium chloride in water is electrolysed, water is reduced more easily than sodium ions, so hydrogen gas forms at the cathode instead of sodium metal. Sodium is also so reactive that it would react with water anyway.

Remember: Electrolysis of aqueous sodium chloride (brine) is the chlor-alkali process. It produces sodium hydroxide, chlorine and hydrogen, not sodium metal.

4. Extraction of Aluminium

Aluminium is obtained by the electrolytic reduction of aluminium oxide ($\mathrm{Al_2O_3}$), which is obtained from bauxite ore.

• Aluminium is formed at the cathode.
• Oxygen is formed at the anode.

$\mathrm{2Al_2O_3 \xrightarrow{\text{electrolysis}} 4Al + 3O_2}$

Extra facts (beyond the Class 10 textbook):

• Pure aluminium oxide has a very high melting point (above $2000^\circ\text{C}$). In industry, it is dissolved in molten cryolite ($\mathrm{Na_3AlF_6}$), which lowers the operating temperature to about $950^\circ\text{C}$ and improves conductivity, saving energy.
• The anodes are made of carbon. The oxygen released at the anode reacts with the hot carbon to form carbon dioxide, so the anodes burn away and must be replaced from time to time.

5. Summary Table

6. Exam Tips and Common Mistakes

Exam Tip: When asked where the metal is deposited during electrolytic reduction, answer the cathode (negative electrode).

Exam Tip: Write the cathode and anode equations separately and label which is reduction and which is oxidation.

Common Mistake: Suggesting that aluminium can be extracted by heating its oxide with carbon. Aluminium has a greater affinity for oxygen than carbon, so this does not work.

Common Mistake: Saying that electrolysis of aqueous sodium chloride gives sodium metal. It gives hydrogen at the cathode.

Frequently Asked Questions

Why can't sodium be extracted by heating its oxide with carbon?

Sodium is more reactive than carbon and has a greater affinity for oxygen, so carbon cannot remove oxygen from sodium oxide.

What is electrolytic reduction?

It is the extraction of a metal by passing an electric current through its molten compound. The metal ions gain electrons at the cathode and form the metal.

At which electrode is sodium formed during electrolysis of molten NaCl?

Sodium is formed at the cathode (negative electrode), and chlorine gas is given off at the anode.

Why is molten sodium chloride used and not its aqueous solution?

In aqueous solution, water is reduced in preference to sodium ions, so hydrogen gas forms at the cathode instead of sodium.

How is aluminium extracted?

Aluminium is extracted by the electrolytic reduction of aluminium oxide, with aluminium forming at the cathode.