Chemistry in Society

Extraction of Metals – Revision Notes

Key Definitions and Terminology

• **Ore**: A naturally occurring rock that contains enough of a metal or metal compound to make it worthwhile extracting the metal.
• **Reduction**: The loss of oxygen from a compound, or the gain of electrons by an ion. This is the key process by which metals are obtained from their oxides.
• **Oxidation**: The gain of oxygen by a substance, or the loss of electrons. In extraction, carbon is oxidised when it removes oxygen from a metal oxide.
• **Reactivity series**: An arrangement of metals in order of their reactivity (most reactive at the top, least reactive at the bottom), which determines the method of extraction used.
• **Electrolysis**: The decomposition of an ionic compound (molten or in solution) by passing an electric current through it, used to extract the most reactive metals.
• **Smelting**: The process of heating a metal ore with carbon (or carbon monoxide) in a furnace to obtain the metal by reduction.

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Main Concepts

1. The Link Between Reactivity and Extraction Method

The position of a metal in the reactivity series determines how it is extracted from its ore:

| Reactivity | Metals | Extraction Method |

|---|---|---|

| Most reactive | Potassium, sodium, calcium, magnesium, aluminium | Electrolysis of molten compound |

| Medium reactivity | Zinc, iron, tin, lead | Reduction with carbon (or carbon monoxide) |

| Least reactive | Copper, silver, gold, platinum | Found native (uncombined) or by reduction with carbon; can also be purified by electrolysis |

• Metals more reactive than carbon **cannot** be reduced by carbon — they hold onto oxygen too strongly. Electrolysis is the only viable method.
• Metals less reactive than carbon **can** be reduced by carbon because carbon displaces them from their oxides.

2. Reduction of Metal Oxides with Carbon

• Carbon acts as the **reducing agent** — it removes oxygen from the metal oxide.
• The carbon is **oxidised** to carbon dioxide.
• This is a **redox** reaction: reduction and oxidation happen simultaneously.
• General equation:

> metal oxide + carbon → metal + carbon dioxide

3. Extraction of Aluminium by Electrolysis

• Aluminium is too reactive to be reduced by carbon, so **electrolysis of molten aluminium oxide (Al₂O₃)** is used.
• Aluminium oxide has a very high melting point (~2072 °C), so it is dissolved in **molten cryolite (Na₃AlF₆)** to lower the melting point and reduce energy costs.
• **Carbon (graphite) electrodes** are used.
• At the **cathode (negative electrode)**: aluminium ions gain electrons (reduction) and molten aluminium is produced.
• Al³⁺ + 3e⁻ → Al
• At the **anode (positive electrode)**: oxide ions lose electrons (oxidation) and oxygen gas is produced.
• 2O²⁻ → O₂ + 4e⁻
• The carbon anodes gradually **burn away** (react with the oxygen produced to form CO₂) and must be **regularly replaced**.

4. Extraction of Iron in the Blast Furnace

• Iron ore (haematite, Fe₂O₃) is reduced using **carbon monoxide** (produced from coke) in a blast furnace.
• Raw materials: **iron ore, coke (carbon), limestone, hot air**.
• Key reactions:
• Carbon burns: C + O₂ → CO₂
• Carbon dioxide is reduced: CO₂ + C → 2CO
• Iron oxide is reduced: Fe₂O₃ + 3CO → 2Fe + 3CO₂
• The iron produced is **molten** and collects at the bottom of the furnace.
• Limestone removes sandy impurities (SiO₂) as **slag** (calcium silicate).

5. Why Electrolysis Is More Expensive

• Electrolysis requires a **continuous supply of electricity**, making it significantly more expensive than reduction with carbon.
• This is why aluminium is more costly than iron, even though aluminium ore is more abundant in the Earth's crust.

6. Oxidation and Reduction in Terms of Electron Transfer (Higher Tier)

• **Reduction Is Gain** of electrons (remember: **OILRIG** — Oxidation Is Loss, Reduction Is Gain).
• At the cathode during electrolysis, metal ions are **reduced** (they gain electrons).
• At the anode, non-metal ions are **oxidised** (they lose electrons).

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Worked Examples and Real-World Applications

Worked Example 1: Extraction of Zinc

Zinc is extracted from zinc oxide by heating with carbon:

> 2ZnO + C → 2Zn + CO₂

• Zinc is **below** carbon in the reactivity series, so carbon can reduce it.
• The zinc oxide is **reduced** (it loses oxygen).
• The carbon is **oxidised** (it gains oxygen).

Identify the oxidation and reduction:

• ZnO → Zn — **reduction** (oxygen removed / Zn²⁺ gains electrons)
• C → CO₂ — **oxidation** (oxygen gained / carbon loses electrons)

Worked Example 2: Extraction of Lead from Lead(II) Oxide

> 2PbO + C → 2Pb + CO₂

• Lead is below carbon in the reactivity series.
• Carbon is the reducing agent.
• This is used industrially; lead was historically extracted this way for pipes and roofing.

Real-World Application: Recycling Aluminium

• Extracting aluminium by electrolysis uses enormous amounts of electrical energy.
• **Recycling aluminium** uses only about **5%** of the energy needed to extract it from its ore.
• This reduces costs, conserves bauxite ore reserves, and reduces carbon dioxide emissions from power generation and the burning of carbon anodes.

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Exam Technique Tips

Tip 1: "State and explain" questions on extraction method

When asked why a particular metal is extracted by electrolysis rather than by carbon reduction, always make two linked points:

1. The metal is **more reactive than carbon** (or "higher than carbon in the reactivity series").
2. Therefore carbon **cannot reduce** / **cannot displace** the metal from its oxide.

Edexcel mark schemes award one mark for stating the relative reactivity and a second mark for the consequence. Simply writing "it is too reactive" alone often scores only 1 out of 2.

Tip 2: Writing electrode half-equations for aluminium extraction

Edexcel frequently asks for half-equations at the cathode and anode. Make sure your equations are balanced for both atoms and charge:

• Cathode: **Al³⁺ + 3e⁻ → Al**
• Anode: **2O²⁻ → O₂ + 4e⁻**

Always state which electrode is positive/negative and label the process as reduction (cathode) or oxidation (anode). The mark scheme specifically requires correct use of these terms — do not mix them up.