Extraction of metals
The method used to extract a metal depends on its position in the reactivity series relative to carbon.
- Metals more reactive than carbon (e.g., aluminium, potassium, sodium): These metals hold onto their oxygen very strongly. Carbon cannot remove the oxygen from them. Therefore, they must be extracted using electrolysis (using electricity).
- Metals less reactive than carbon (e.g., iron, zinc, copper): These metals hold onto their oxygen less strongly than carbon does. Carbon can displace (remove) the oxygen from the metal oxide. Therefore, they are extracted by reduction with carbon (heating the ore with coke).
Building on the concept of reactivity: The higher a metal is in the series, the more energy is required to separate it from its ore.
- Main ore of aluminium: Bauxite (which is impure aluminium oxide, Al_2O_3).
- Extraction method: Electrolysis.
This is because aluminium is higher in the reactivity series than carbon.
Iron is extracted from hematite (Fe_2O_3) in a tall tower called a blast furnace. The raw materials loaded into the top are:
- Hematite (iron ore, Fe_2O_3)
- Coke (carbon, C)
- Limestone (calcium carbonate, CaCO_3)
- Hot air (oxygen, O_2)
The process involves several key reactions occurring at different temperatures within the furnace:
At the bottom of the furnace, hot air is blown in. The coke burns in the oxygen to produce heat (raising the temperature) and carbon dioxide.
Equation: C + O_2 \rightarrow CO_2
Why this matters: This reaction is exothermic, providing the high temperatures needed for the rest of the process.
As the carbon dioxide gas rises up through the furnace, it meets more hot coke. The carbon dioxide is reduced to carbon monoxide.
Equation: C + CO_2 \rightarrow 2CO
Why this matters: Carbon monoxide (CO) is the actual reducing agent that removes oxygen from the iron ore.
The carbon monoxide rises and reacts with the hematite falling down. The carbon monoxide takes the oxygen from the iron oxide, leaving pure molten iron.
Equation: Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2
Why this matters: This is a reduction reaction because the iron(III) oxide loses oxygen. The iron produced is molten (liquid) due to the high heat and sinks to the bottom.
Hematite contains impurities like silicon(IV) oxide (SiO_2, also called silica). This is an acidic oxide.
Limestone (CaCO_3) undergoes thermal decomposition due to the heat:
Equation: CaCO_3 \rightarrow CaO + CO_2
The calcium oxide (CaO) produced is a basic oxide. It reacts with the acidic silicon(IV) oxide impurity to form slag (calcium silicate).
Equation: CaO + SiO_2 \rightarrow CaSiO_3
Why this matters: Slag is molten and less dense than iron, so it floats on top of the iron. It can be easily removed. The slag is often used in road construction.
Learning Objective 5: Describe the extraction of aluminium from purified bauxite/aluminium oxide.
Since aluminium is more reactive than carbon, we use electrolysis.
- Purification: Bauxite is purified to obtain pure aluminium oxide (Al_2O_3).
- Melting Point Issue: Pure aluminium oxide has a very high melting point, which would make electrolysis too expensive (too much energy needed to melt it).
- Role of Cryolite: The aluminium oxide is dissolved in molten cryolite (Na_3AlF_6). This lowers the melting point of the mixture, saving energy and cost.
- Electrolysis Setup:
- Cathode (Negative electrode): Lined with carbon at the bottom of the tank.
- Anode (Positive electrode): Made of carbon (graphite) blocks suspended from the top.
Reactions at the Electrodes
The aluminium oxide dissociates into ions: Al_2O_3 \rightarrow 2Al^{3+} + 3O^{2-}
At the Cathode (Reduction):
The positive aluminium ions (Al^{3+}) are attracted to the negative cathode. They gain electrons to form aluminium metal.
Half-equation: Al^{3+} + 3e^- \rightarrow Al
The molten aluminium sinks to the bottom and is tapped off.At the Anode (Oxidation):
The negative oxide ions (O^{2-}) are attracted to the positive anode. They lose electrons to form oxygen gas.
Half-equation: 2O^{2-} \rightarrow O_2 + 4e^-
The oxygen gas produced at the anode reacts with the hot carbon anode itself to form carbon dioxide.
Equation: C + O_2 \rightarrow CO_2
Because the carbon anode burns away (oxidizes) during this reaction, it must be regularly replaced.
In the blast furnace, limestone undergoes thermal decomposition:
CaCO_3 \rightarrow CaO + CO_2
This is distinct from reduction (gain of electrons/loss of oxygen) or combustion (reaction with oxygen).
Chemical name: Calcium silicate (CaSiO_3).
It is molten, less dense than iron, and floats on top, allowing it to be separated.
Equation: Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2
- Check atoms:
- Fe: 2 on left, 2 on right. (Balanced)
- C: 3 on left, 3 on right. (Balanced)
- O: 3 + 3 = 6 on left; 3 \times 2 = 6 on right. (Balanced)
Example 2: Formation of Slag
Equation: CaO + SiO_2 \rightarrow CaSiO_3
- Check atoms:
- Ca: 1 on left, 1 on right.
- Si: 1 on left, 1 on right.
- O: 1 + 2 = 3 on left; 3 on right.
Note: Do not use Fe_2 as a formula for iron. Iron is a metal and exists as individual atoms (Fe) in equations.
Correct Understanding: The ease of extraction depends only on the position in the reactivity series relative to carbon. Aluminium is more reactive than carbon, so carbon cannot displace it. Iron is less reactive than carbon, so carbon can displace it.
Mistake: Confusing the role of coke and limestone.
Correct Understanding:
- Coke (Carbon): Provides heat (by burning) AND acts as a reducing agent (via carbon monoxide).
- Limestone (Calcium Carbonate): Removes impurities by forming slag. It does NOT reduce the iron ore.
Correct Understanding: The product is pure metal, so it is written as Fe. Ions (Fe^{3+}) only exist in compounds like hematite (Fe_2O_3).
Examiner Accepts: Use the OIL RIG mnemonic or oxygen transfer definitions.
- For reduction: State that the substance loses oxygen. Example: 'Iron(III) oxide is reduced because it loses oxygen to form iron.'
- For oxidation: State that the substance gains oxygen. Example: 'Carbon monoxide is oxidised because it gains oxygen to form carbon dioxide.'
Why: Examiners look for the specific transfer of oxygen atoms in these syllabus contexts. Mentioning electron transfer is correct in advanced chemistry, but 'loss/gain of oxygen' is the primary accepted answer for this level unless oxidation numbers are explicitly requested.
Examiner Accepts: State that 'the oxygen produced at the anode reacts with the carbon anode to form carbon dioxide.'
Why: Simply saying 'they burn' is vague. You must identify the reactant (oxygen from the half-equation) and the product (CO_2). This shows you understand the chemical process, not just the physical wear.
Examiner Accepts: State that it 'lowers the melting point of aluminium oxide' or 'lowers the temperature required for electrolysis.'
Why: Do not say it lowers the melting point of aluminium. It lowers the melting point of the mixture containing aluminium oxide. This distinction is crucial for full marks.