CIE iGCSE Co-ordinated Sciences-C9.6 Extraction of metals- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C9.6 Extraction of metals – Study Notes
CIE iGCSE Co-ordinated Sciences-C9.6 Extraction of metals – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe the ease in obtaining metals from their ores, related to the position of the metal in the reactivity series
- State that iron from hematite is extracted by reduction of iron(III) oxide in the blast furnace
- State that main ore of aluminium is bauxite and that aluminium is extracted by electrolysis
Supplement
- Describe the extraction of iron from hematite in the blast furnace, limited to:
(a) the burning of carbon (coke) to provide heat and produce carbon dioxide C + O₂ → CO₂
(b) the reduction of carbon dioxide to carbon monoxide C + CO₂ → 2CO
(c) the reduction of iron(III) oxide by carbon monoxide Fe₂O₃ + 3CO → 2Fe + 3CO₂
(d) the thermal decomposition of calcium carbonate / limestone to produce calcium oxide CaCO₃ → CaO + CO₂
(e) the formation of slag CaO + SiO₂ → CaSiO₃
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Obtaining metals from their ores
The ease of extracting a metal from its ore depends on its position in the reactivity series because this indicates how reactive the metal is:
- Metals high in the reactivity series (e.g., potassium, sodium, calcium, aluminium, magnesium) are very reactive. They form strong compounds with oxygen or other non-metals, so simple heating or reduction with carbon is not sufficient to extract them. These metals are usually extracted by electrolysis of their molten compounds, such as molten aluminium oxide.
- Metals of intermediate reactivity (e.g., zinc, iron, lead) are less reactive. Their ores can often be reduced by carbon or carbon monoxide because these metals are more stable than carbon. For example, iron is extracted from hematite (\( \text{Fe}_2\text{O}_3 \)) in a blast furnace using carbon monoxide as the reducing agent:
\( \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \)
- Metals low in the reactivity series (e.g., copper, silver, gold, platinum) are unreactive. They are often found as native metals or in easily reduced ores and can be extracted by simple heating, chemical displacement, or even found in elemental form. For example, gold is often mined as pure metal.
Thus, the higher the metal is in the reactivity series, the more difficult and energy-intensive its extraction, while metals lower in the series are easier to obtain from their ores.
Extraction of Iron from Hematite
Iron is extracted from its ore, hematite (\(\mathrm{Fe_2O_3}\)), by the process of reduction in a blast furnace. In the blast furnace, iron(III) oxide is reduced to iron using carbon monoxide as the reducing agent.
Word Equation:
Iron(III) oxide + Carbon monoxide → Iron + Carbon dioxide
Chemical Equation:
\(\mathrm{Fe_2O_3 + 3CO → 2Fe + 3CO_2}\)
Explanation:
- Hematite (\(\mathrm{Fe_2O_3}\)) is the main iron ore.
- It is reduced (oxygen removed) in the blast furnace to form molten iron.
- Carbon monoxide (formed from coke reacting with oxygen) acts as the reducing agent.
- The reaction takes place at high temperatures (around 1500 °C).
Key Idea:
In the blast furnace, iron(III) oxide is reduced to iron by carbon monoxide — this is a reduction reaction because oxygen is removed from iron oxide.
Example Question :
State how iron is extracted from hematite in the blast furnace.
▶️ Answer/Explanation
Step 1: The ore used is hematite (\(\mathrm{Fe_2O_3}\)).
Step 2: Iron is extracted by reduction of iron(III) oxide with carbon monoxide in the blast furnace.
Step 3: The main chemical reaction is: \(\mathrm{Fe_2O_3 + 3CO → 2Fe + 3CO_2}\).
Final Answer: Iron is extracted from hematite by the reduction of iron(III) oxide with carbon monoxide in the blast furnace.
Extraction of aluminium from bauxite
- The main ore of aluminium is bauxite, which contains mainly aluminium oxide (\( \text{Al}_2\text{O}_3 \)) along with impurities such as iron oxides (\( \text{Fe}_2\text{O}_3 \)) and silica (\( \text{SiO}_2 \)).
- Aluminium is a very reactive metal and lies high in the reactivity series, so it cannot be extracted by reduction with carbon. Instead, it is extracted by the process of electrolysis.
Extraction process:
- Bauxite is first purified using the Bayer process to produce pure aluminium oxide (\( \text{Al}_2\text{O}_3 \)).
- Pure aluminium oxide is dissolved in molten cryolite (\( \text{Na}_3\text{AlF}_6 \)) to lower the melting point and increase electrical conductivity.
- Electrolysis is then carried out using a carbon-lined steel cell. During electrolysis:
At the cathode, aluminium ions are reduced to form aluminium metal:
\( \text{Al}^{3+} + 3e^- \rightarrow \text{Al} \)
At the anode, oxide ions are oxidized to form oxygen gas:
\( 2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^- \)
The molten aluminium collects at the bottom of the cell and is periodically removed.
Reasons for using electrolysis:
Aluminium is highly reactive and cannot be extracted by carbon reduction. Electrolysis is the only practical method for obtaining pure aluminium.
Example
Explain why aluminium cannot be extracted by carbon reduction.
▶️ Answer/Explanation
Aluminium is very reactive and forms a very stable oxide (\( \text{Al}_2\text{O}_3 \)). Carbon is less reactive than aluminium, so it cannot reduce aluminium oxide to aluminium. Therefore, electrolysis is required to extract aluminium.
Example
Describe the role of cryolite in aluminium extraction.
▶️ Answer/Explanation
Cryolite (\( \text{Na}_3\text{AlF}_6 \)) is used to dissolve aluminium oxide and lower its melting point from about 2050°C to around 900°C. This reduces energy consumption and improves the conductivity of the molten mixture for electrolysis.
Example
Write the half-equations for the electrolysis of aluminium oxide.
▶️ Answer/Explanation
Cathode (reduction):
\( \text{Al}^{3+} + 3e^- \rightarrow \text{Al} \)
Anode (oxidation):
\( 2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^- \)
The aluminium metal produced is molten and collected at the bottom of the electrolytic cell, while oxygen gas is released at the carbon anode.
Iron extraction from hematite
Iron is extracted from its main ore, hematite (\( \text{Fe}_2\text{O}_3 \)), using a blast furnace. The process involves the reduction of iron(III) oxide to iron metal. Carbon monoxide, produced from the burning of coke, acts as the main reducing agent. The overall reaction is:
\( \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \)
This method is used because iron is moderately reactive and can be reduced by carbon monoxide at high temperatures. The blast furnace allows continuous production of molten iron, which can then be further processed to make steel.
Process of extraction:
(a) Burning of carbon (coke) to provide heat and produce carbon dioxide
Coke is mainly carbon. It is burned in the hot air blast to generate the high temperature needed for the reactions in the blast furnace and to produce carbon dioxide:
\( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \)
(b) Reduction of carbon dioxide to carbon monoxide
The carbon dioxide produced reacts with more coke to form carbon monoxide, which is a stronger reducing agent that can reduce iron(III) oxide to iron:
\( \text{C} + \text{CO}_2 \rightarrow 2\text{CO} \)
(c) Reduction of iron(III) oxide by carbon monoxide
Carbon monoxide reduces iron(III) oxide to molten iron, which collects at the bottom of the blast furnace:
\( \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \)
(d) Thermal decomposition of calcium carbonate (limestone) to produce calcium oxide
Limestone is added to the furnace to remove acidic impurities (silica) from the ore. On heating, calcium carbonate decomposes to form calcium oxide and carbon dioxide:
\( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)
(e) Formation of slag
The calcium oxide reacts with silica impurities in the ore to form slag, which is less dense than iron and floats on top, allowing easy removal:
\( \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \)
Overall, the blast furnace allows continuous production of iron by combining the reduction reactions with removal of impurities as slag, making the process efficient for large-scale iron extraction.
Example
Explain how iron is extracted from hematite in a blast furnace and identify the role of each reactant.
▶️ Answer/Explanation
Hematite (\( \text{Fe}_2\text{O}_3 \)) is reduced to iron using carbon monoxide in a blast furnace. The main steps are:
Coke burns to produce carbon dioxide:
\( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \)
Carbon dioxide reacts with more coke to produce carbon monoxide:
\( \text{C} + \text{CO}_2 \rightarrow 2\text{CO} \)
Carbon monoxide reduces iron(III) oxide to iron:
\( \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \)
Limestone decomposes to calcium oxide:
\( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)
Calcium oxide reacts with silica impurities to form slag:
\( \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \)
This process produces molten iron at the bottom and slag on top, which is removed.
Example
Decide whether a metal should be extracted by electrolysis or by reduction with carbon.
▶️ Answer/Explanation
Step 1: Check the position in the reactivity series.
Step 2: Metals high in the reactivity series (e.g., aluminium, sodium, magnesium) are extracted by electrolysis of molten compounds because carbon reduction is ineffective.
Step 3: Metals of intermediate reactivity (e.g., iron, zinc) can be extracted by reduction with carbon or carbon monoxide.
Step 4: Metals low in the reactivity series (e.g., copper, silver, gold) are often found as native metals or can be extracted by simple heating or chemical displacement.
Example
Explain why limestone is added to the blast furnace during iron extraction.
▶️ Answer/Explanation
Limestone (\( \text{CaCO}_3 \)) decomposes on heating to form calcium oxide:
\( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)
The calcium oxide reacts with silica impurities (\( \text{SiO}_2 \)) to form slag:
\( \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \)
Slag is less dense than iron and floats on top, allowing easy removal. This prevents impurities from mixing with the iron and protects the furnace lining.