Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Define the lithosphere and its importance as a resource.
• Explain the chemical processes for extracting gold and platinum.
• Discuss the environmental impacts of mining and energy use.
• Compare different energy resources like coal, solar, and nuclear.
• Write balanced chemical equations for key reactions.

Lesson summary

This week, we delve into the chemical systems within the lithosphere – the Earth's solid outer layer – focusing specifically on mining and energy resources. South Africa is exceptionally rich in mineral resources, and mining plays a significant role in our economy and our history. Understanding the chemistry behind these resources is crucial not only for sustainable resource management but also for understanding the environmental impact of mining and energy production.

Furthermore, the search for new and cleaner energy sources is a pressing global issue, particularly relevant in a country grappling with energy security.

Lesson notes

2.1 The Lithosphere: A Chemical System The lithosphere comprises the Earth's crust and the uppermost part of the mantle. It's a dynamic chemical system containing a vast array of minerals and elements. These elements combine to form compounds that we extract as resources. In South Africa, we are particularly concerned with minerals containing gold, platinum group metals (PGMs), coal, iron ore, manganese, and chrome, among others. Understanding the chemical composition of these resources and the surrounding rocks is crucial for efficient and environmentally responsible extraction. 2.2 Mining and Mineral Extraction: Chemical Processes Mining involves extracting valuable minerals from the Earth's crust. The process often involves several chemical reactions to separate the desired mineral from the surrounding rock (gangue).

Here are two examples: Gold Extraction (Cyanidation): Gold is often found in low concentrations, making direct extraction difficult. The cyanidation process is widely used. Finely ground ore is treated with a cyanide solution (typically sodium cyanide, NaCN) in the presence of oxygen. The gold dissolves to form a soluble complex: ``` 4Au(s) + 8NaCN(aq) + O₂(g) + 2H₂O(l) → 4Na[Au(CN)₂](aq) + 4NaOH(aq) ``` Explanation: Gold (Au) reacts with sodium cyanide (NaCN), oxygen (O₂), and water (H₂O) to form sodium cyanoaurate(I) (Na[Au(CN)₂]) and sodium hydroxide (NaOH). The cyanoaurate(I) complex is then recovered from the solution, often by adding zinc metal, which precipitates the gold: ``` 2Na[Au(CN)₂](aq) + Zn(s) → 2Au(s) + Na₂[Zn(CN)₄](aq) ``` Explanation: Zinc (Zn) reduces the gold cyanoaurate(I) complex, precipitating out pure gold (Au) and forming sodium tetracyanozincate(II) (Na₂[Zn(CN)₄]).

Platinum Group Metals (PGMs)

Extraction: PGMs, such as platinum, palladium, and rhodium, are often found together. A complex series of processes are used to separate and refine them. These processes include froth flotation, leaching with acids (like hydrochloric acid, HCl), and solvent extraction. For example, consider the leaching of PGMs with hydrochloric acid: ``` Pt(s) + 4HCl(aq) + 2NO₃⁻(aq) → [PtCl₄]²⁻(aq) + 2NO₂(g) + 2H₂O(l) ``` Explanation: Platinum (Pt) can be dissolved by a mixture of hydrochloric acid (HCl) and nitrate ions (NO₃⁻) to form a tetrachloroplatinate(II) complex ([PtCl₄]²⁻), nitrogen dioxide gas (NO₂), and water (H₂O). This leaching process often requires the addition of an oxidizing agent like nitric acid (HNO₃), which provides the nitrate ions. Subsequent steps involve selective precipitation or solvent extraction to isolate each PGM. 2.3 Energy Resources: Chemical Composition and Combustion South Africa relies heavily on coal for energy production. Coal is primarily composed of carbon, along with varying amounts of hydrogen, oxygen, nitrogen, and sulfur. When coal is burned (combustion), it reacts with oxygen to produce heat, carbon dioxide, and water.

Coal Combustion: The simplified equation for the combustion of carbon (representing coal) is: ``` C(s) + O₂(g) → CO₂(g) + Heat ``` Explanation: Solid carbon (C) reacts with gaseous oxygen (O₂) to produce gaseous carbon dioxide (CO₂) and releases heat. The heat is used to generate steam, which drives turbines to produce electricity.

However, the impurities in coal, especially sulfur, also react with oxygen to form sulfur dioxide (SO₂), a major air pollutant: ``` S(s) + O₂(g) → SO₂(g) ``` Sulfur dioxide contributes to acid rain and respiratory problems.

Other Energy Resources: Other energy resources include: Natural Gas: Primarily methane (CH₄), which combusts as follows: ``` CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat ``` Nuclear Fuels: Uranium-235 (²³⁵U) undergoes nuclear fission, releasing enormous amounts of energy. (Nuclear reactions are beyond the scope of balancing in this lesson, but the concept of controlled nuclear reactions providing energy is important). 2.4 Environmental Impacts Mining and energy production have significant environmental consequences: Air Pollution: Burning coal releases pollutants like SO₂, NOₓ (nitrogen oxides), and particulate matter, leading to respiratory problems and acid rain. Mining operations can also release dust containing harmful substances.

Water Pollution: Mine drainage, containing heavy metals and acids, can contaminate rivers and groundwater. Cyanide used in gold extraction can also pose a risk to water sources.

Land Degradation: Mining activities can destroy habitats and cause soil erosion.