Extraction method vs reactivity
How a metal is extracted follows its position in the reactivity series. Metals ABOVE carbon (K, Na, Ca, Mg, Al) need electrolysis of a molten compound, because carbon cannot reduce their stable oxides. Metals below carbon but above hydrogen (Zn, Fe, Sn, Pb) have their oxides reduced cheaply by carbon or CO. Metals below hydrogen (Cu, Ag, Au) occur native. Iron comes from haematite by CO reduction; aluminium from alumina by electrolysis.
Blast furnace: materials and products
Four raw materials enter the blast furnace: haematite (Fe₂O₃, the iron source), coke (the fuel and source of the reducing agent), limestone (CaCO₃, to remove impurities) and hot air. The reducing agent is carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Coke burns (C + O₂ → CO₂), then CO₂ + C → 2CO. Limestone decomposes (CaCO₃ → CaO + CO₂) and CaO removes acidic silica as slag (CaO + SiO₂ → CaSiO₃). Products: molten iron, slag and waste gases.
(Extended) Aluminium by electrolysis
Aluminium is above carbon, so its oxide cannot be reduced by carbon — it is extracted by electrolysis of aluminium oxide (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆). Cryolite lowers the melting point of the electrolyte mixture from about 2050 °C to about 950 °C, saving energy. Cathode: Al³⁺ + 3e⁻ → Al (reduction); anode: 2O²⁻ → O₂ + 4e⁻ (oxidation). The oxygen burns the carbon anodes (C + O₂ → CO₂), so they are replaced regularly.
Drawn from real examiner reports.
Limestone forms slag, not a catalyst
The role of limestone is the most-penalised blast-furnace point. It is NOT a catalyst and does NOT lower the melting point of iron. Limestone decomposes to calcium oxide (CaCO₃ → CaO + CO₂); the basic CaO then reacts with acidic silica to form molten slag (CaO + SiO₂ → CaSiO₃), tapped off separately. Chain: limestone → CaO → removes silica → slag.
W22 P11 Q26; W22 P21 Q26; W22 P22 Q26; W22 P31 Q2(c)(iv) — role of calcium oxide poorly recalled; confused with a catalyst or "lowering the melting point of iron".
Cryolite lowers the mixture's melting point
State WHAT cryolite affects: the melting point of the ELECTROLYTE MIXTURE (alumina dissolved in cryolite), lowered from about 2050 °C to about 950 °C, so electrolysis runs cooler and saves energy. It does NOT lower the melting point of aluminium metal, and it is not a catalyst. "Lowers the melting point" alone scores partial credit.
Name the ore: haematite
Asked to NAME the iron ore, write haematite (hematite) — not "iron oxide". Iron(III) oxide, Fe₂O₃, is the main compound WITHIN the ore, but the ore itself is haematite. Students who give the compound name, or an unrelated metal, lose the mark. Keep the two separate: the ore is haematite; its useful compound is iron(III) oxide.
W22 P31 Q2(c)(i); S23 P32 Q5(a)(i) — ore name haematite poorly recalled; students give "iron oxide" (the compound).
Thermal decomposition means both words
Defining thermal decomposition, you must explain BOTH words: "thermal" = caused by heat, and "decomposition" = a compound broken into simpler substances. Answers like "separation" or "burning" earn nothing, and restating the term without explanation gains no credit. Example: CaCO₃ → CaO + CO₂ splits limestone into two simpler substances using heat.
W22 P31 Q2(c)(iii) — definitions must explain both "thermal" (heat) and "decomposition" (broken into simpler substances).
(Extended) CO is the reducing agent
The main reducing agent is carbon MONOXIDE, not solid carbon. In Fe₂O₃ + 3CO → 2Fe + 3CO₂, CO gains oxygen (CO → CO₂), so CO is oxidised and is the reducing agent; Fe₂O₃ loses oxygen, so it is reduced and is the oxidising agent. The species that is itself oxidised is the reducing agent, not Fe₂O₃.
Electrolysis only above carbon
Only metals ABOVE carbon in the reactivity series need electrolysis. Iron, zinc, tin and lead are below carbon, so they are reduced cheaply by carbon or CO — do not say they are electrolysed. A common slip is claiming copper needs electrolysis; copper is below hydrogen and is obtained by reduction. Reserve electrolysis for K, Na, Ca, Mg and Al.
(Extended) Blast furnace equations by zone
Build the furnace equations by zone. Bottom (excess air): C + O₂ → CO₂. Limited-air zone: CO₂ + C → 2CO, making the reducing agent. Middle: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Limestone decomposes (CaCO₃ → CaO + CO₂) and CaO + SiO₂ → CaSiO₃ forms slag.
(Extended) Cathode and anode half-equations
For electrolysis half-equations, cations go to the cathode and anions to the anode. Cathode (reduction): metal ion + electrons → metal, e.g. Al³⁺ + 3e⁻ → Al. Anode (oxidation): the non-metal ion loses electrons, e.g. 2O²⁻ → O₂ + 4e⁻. Add electrons to balance the charge.
Name vs formula; correct oxidation state
Answer in the form asked. If a NAME is wanted, give the word (haematite, calcium silicate), not a formula, and vice versa. Include the correct oxidation state: iron(III) oxide, not "iron oxide" or iron(II). Reading "name" versus "formula" carefully protects easy marks.
Haematite — the naturally occurring iron ore; it contains iron(III) oxide () mixed with silica () and other impurities.
Reduction — the removal of oxygen from a metal oxide (Core definition); more precisely, the gain of electrons (Extended). Carbon acts as a reducing agent in the blast furnace by removing oxygen from the iron ore.
Electrolysis — the decomposition of a substance using an electric current. Used to extract metals that are too reactive to be reduced by carbon.
Cryolite — the mineral sodium hexafluoroaluminate (), used as a solvent for alumina in the aluminium extraction cell. It lowers the melting point of the electrolyte mixture, not of aluminium itself.
Slag — calcium silicate (), the by-product formed when calcium oxide reacts with silica impurities in the blast furnace.
State the extraction method used for (a) aluminium and (b) iron. Explain in one sentence why different methods are used.
Iron is extracted from haematite in the blast furnace using carbon monoxide as a reducing agent.
(a) State the name and formula of the iron ore used in the blast furnace.
(b) Write a balanced symbol equation for the reaction between iron(III) oxide and carbon monoxide in the blast furnace.
(c) Identify the reducing agent in the reaction in (b) and explain why it is the reducing agent.
