Decomposition Reactions and Metal Extraction

Decomposition Reactions

5.5.1 Definition and Characteristics

• Decomposition Reaction: A reaction in which a compound breaks down into two simpler substances.
    - General Form:
      $AB <br>ightarrow A + B$
    - Typically endothermic, requiring energy in the form of light, heat, or electricity to break the bonds in the reactants.
    - The reaction container may feel cool as a result of the energy absorption.
    - Electrolysis: When the process involves electricity.

Common Types of Decomposition Reactions

1. Heating Metal Hydroxides: Decompose to form metal oxides and water.
       - Example: Heating sodium hydroxide results in water vapor being released and a white powder remaining:
         $2NaOH(s) <br>ightarrow Na_2O(s) + H_2O(g)$

2. Heating Metal Carbonates: Decompose upon heating to produce metal oxides and carbon dioxide.
       - Example: Heating zinc carbonate produces zinc oxide and carbon dioxide gas:
         $ZnCO_3(s) <br>ightarrow ZnO(s) + CO_2(g)$

Learning Intention

• After completing this lesson, students should be able to describe and provide examples of decomposition reactions.

Testing for Carbon Dioxide

• Confirming Carbon Dioxide Presence: Direct the gas through a tube into limewater, which is calcium hydroxide, observing the solution's cloudiness as an indication of carbon dioxide presence.
    - Chemical Reaction:
      $CO_2(g) + Ca(OH)_2(aq) <br>ightarrow CaCO_3(s) + H_2O(l)$

• Properties of Carbon Dioxide: Does not support combustion; utilized in fire extinguishers as it can extinguish a lit match.

Extraction of Metals

Overview of Metal Extraction

• Process: Methods are used to extract metals from their ores, involving chemical reactions, often through smelting.

• Smelting: The process where mineral ores are melted to extract metals.
    - Used for Metals: Iron, copper, zinc.

Metal Extraction and Reactivity

• Reactivity Influence: The ease of metal extraction is influenced by metal reactivity.
    - Reactive Examples: Silver and gold are found naturally in their elemental state due to their low reactivity. In contrast, more reactive metals must be extracted from ores.

• Definition of Ores: Rocks containing mixtures of different compounds (e.g., oxygen, carbon) from which metals can be economically extracted.

Extraction Processes

Electrolysis of Very Reactive Metals

• Extraction of very reactive metals is accomplished through electrolysis of molten ores, requiring significant electrical energy.
    - Example: Aluminium extraction is performed through electrolysis, highlighting the importance of recycling due to high extraction costs.

Extraction of Iron

• Iron Extraction Method: Iron is predominantly mined and used for steel production.
    - Blasting Furnace: The smelting process is crucial for iron extraction from ores like haematite and magnetite.
      - Makes iron oxide react with carbon monoxide to remove oxygen.

Steps in Iron Extraction

1. Creation of Carbon Monoxide:
       1.1.
       $C + O_2 <br>ightarrow CO_2$
       1.2.
       $CO_2 + C <br>ightarrow 2CO$

2. Producing Iron Metal:
      $Fe_2O_3 + 3CO <br>ightarrow 2Fe + 3CO_2$

3. Removing Impurities:
      - Calcium carbonate decomposes to calcium oxide:
      $CaCO_3(s) <br>ightarrow CaO(s) + CO_2(g)$

4. Reactions with Silica: Calcium oxide reacts with silica to form calcium silicate (slag):
      $CaO(s) + SiO_2(s) <br>ightarrow CaSiO_3(l)$

5. Purity of Iron: Iron obtained from this process is impure, containing residual carbon, necessitating further treatment based on intended use.

Extension: Precious Metals

List of Precious Metals

• Rhodium (Rh): One of the rarest and most valuable precious metals, mainly used in catalytic converters in vehicles.

• Ranking of Precious Metals:
    1. Rhodium
    2. Platinum
    3. Gold
    4. Ruthenium
    5. Iridium
    6. Osmium
    7. Palladium
    8. Rhenium
    9. Silver
    10. Indium

Resources

• eWorkbook: Decomposition (ewbk-13135).

Quick Quiz

• Questions: Q1, Q2, Q3, Q4, Q5 (Details not provided).