Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Define key mining terms like 'ore', 'smelting', and 'leaching'.
• Describe the chemical extraction of Gold, Iron, and Aluminium.
• Explain the formation and environmental impact of coal and petroleum.
• Describe methods of beneficiation.
• Discuss the importance of sustainable mining.

Lesson summary

The lithosphere, the Earth's rigid outer layer composed of the crust and the uppermost part of the mantle, is the source of nearly all the raw materials upon which modern society depends. In South Africa, mining and energy resources are particularly crucial due to our rich mineral deposits and reliance on coal for energy production. Understanding the chemical processes involved in the extraction, processing, and utilisation of these resources is vital. This knowledge is not just academic; it directly impacts our economy, environment, and the future of South Africa.

Lesson notes

The Lithosphere and Mineral Resources: The lithosphere is a treasure trove of mineral resources. These resources are not found in pure form but are typically embedded within rocks and other materials.

Ore: A naturally occurring solid material from which a valuable mineral or minerals can be extracted profitably. The profitability depends on the concentration of the desired mineral and the cost of extraction.

Gangue: The unwanted materials (e.g., sand, clay) associated with the ore. Separating the gangue from the desired mineral is a crucial step in mining.

Beneficiation: Processes used to separate the valuable minerals from the gangue and concentrate them. Examples include crushing, grinding, froth flotation, and magnetic separation.

Smelting: A process involving heating an ore to a high temperature in the presence of a reducing agent (like carbon) to extract the metal. This usually results in the formation of a molten metal and a slag (molten waste).

Leaching: A process where a chemical solvent is used to dissolve the desired mineral from the ore. The solvent containing the dissolved mineral is then separated, and the mineral is recovered.

Extraction of Key Minerals: Gold (Au): Gold often occurs in small concentrations in quartz veins.

Process:* Cyanide leaching is commonly used. Finely ground ore is treated with a cyanide solution (NaCN or KCN) in the presence of oxygen. Gold dissolves, forming a gold cyanide complex: 4Au(s) + 8CN⁻(aq) + O₂(g) + 2H₂O(l) → 4[Au(CN)₂]⁻(aq) + 4OH⁻(aq) The gold is then recovered by adding zinc dust, which reduces the gold complex back to metallic gold (Merrill-Crowe process): 2[Au(CN)₂]⁻(aq) + Zn(s) → 2Au(s) + [Zn(CN)₄]²⁻(aq)

Iron (Fe): Iron ore (e.g., hematite Fe₂O₃) is abundant in South Africa.

Process:* The blast furnace is used. Iron ore, coke (carbon), and limestone (CaCO₃) are fed into the furnace.

Coke burns to produce heat: C(s) + O₂(g) → CO₂(g) Carbon dioxide reacts with more coke to form carbon monoxide (reducing agent): CO₂(g) + C(s) → 2CO(g)

Iron oxide is reduced by carbon monoxide: Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g) Limestone decomposes to form calcium oxide (lime), which reacts with silica (SiO₂) in the ore to form slag: CaCO₃(s) → CaO(s) + CO₂(g); CaO(s) + SiO₂(s) → CaSiO₃(l) (slag) The molten iron and slag are tapped from the bottom of the furnace.

Aluminium (Al): Bauxite (Al₂O₃.xH₂O) is the primary ore.

Process:* The Bayer process is used to purify bauxite into alumina (Al₂O₃). Al₂O₃.xH₂O(s) + NaOH(aq) → 2[Al(OH)₄]⁻(aq) (dissolves aluminium oxide) Impurities remain undissolved and are filtered off. The solution is then cooled and seeded with aluminium hydroxide to precipitate Al(OH)₃: [Al(OH)₄]⁻(aq) → Al(OH)₃(s) + OH⁻(aq) The Al(OH)₃ is calcined (heated strongly) to produce alumina: 2Al(OH)₃(s) → Al₂O₃(s) + 3H₂O(g) The Hall-Héroult process is then used to electrolytically reduce alumina to aluminium. Alumina is dissolved in molten cryolite (Na₃AlF₆) to lower its melting point. An electric current is passed through the mixture, reducing aluminium ions to metallic aluminium at the cathode: Al³⁺(l) + 3e⁻ → Al(l) Oxygen is produced at the anode, which reacts with the carbon anode, forming carbon dioxide: 2O²⁻(l) + C(s) → CO₂(g) + 4e⁻ Energy Resources (Coal and Petroleum): Coal Formation: Coal is formed over millions of years from accumulated plant matter in swampy environments. Peat is formed first, followed by lignite, bituminous coal, and finally anthracite, as the carbon content increases and moisture content decreases under increasing pressure and temperature.

Petroleum Formation: Petroleum (crude oil) is formed from the remains of marine organisms that accumulated on the seabed millions of years ago. Under pressure and heat, the organic matter transforms into petroleum and natural gas.

Combustion of Fossil Fuels: The combustion of coal and petroleum releases energy but also produces harmful pollutants: Complete Combustion: Produces carbon dioxide and water: e.g., CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Incomplete Combustion: Produces carbon monoxide (toxic) and soot: e.g., 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g); CH₄(g) + O₂(g) → C(s) + 2H₂O(g)

Acid Rain: Sulfur dioxide (SO₂) and nitrogen oxides (NOx) are also released during the combustion of fossil fuels. These gases react with water in the atmosphere to form sulfuric acid and nitric acid, leading to acid rain: SO₂(g) + H₂O(l) → H₂SO₃(aq); 2SO₂(g) + O₂(g) → 2SO₃(g); SO₃(g) + H₂O(l) → H₂SO₄(aq)

Greenhouse Effect: Carbon dioxide is a major greenhouse gas that traps heat in the atmosphere, contributing to global warming. Guided Practice (With Solutions)

Question 1: Describe the role of carbon monoxide (CO) in the extraction of iron from iron ore in a blast furnace. Write a balanced chemical equation for the reaction.

Solution:* Carbon monoxide acts as a reducing agent in the blast furnace. It removes oxygen from the iron oxide, converting it to metallic iron.