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The smelter: which reducing agent works?

Pair ores with coke, carbon monoxide, hydrogen, aluminium powder and magnesium in a virtual smelter, set the furnace temperature, and let Ellingham thermodynamics decide whether metal comes out.

Difficulty ●●○ ≈ 20 min

Aim Find which reducing agents reduce each oxide, and above which temperature; explain why carbon can smelt iron and zinc but not aluminium.
Virtual experiment · straight-line Ellingham model

The smelter: which reducing agent works?

800 K
ΔG° > 0: the agent is not strong enough; the oxide stays

FeO+C→Fe+CO\ce{FeO + C -> Fe + CO}

ΔG° (per equation)
34.7 kJ
Equilibrium constant K
0.00542
ΔG° = n(O₂) × [G(C→CO) − G(FeO)]
0.5 × [−364 − (−433)] kJ
If metal forms, at this temperature it is
solid

ΔG° changes sign at 1019 K— hotter works

Furnace log

Nothing logged yet. Try: which agent reduces TiO₂? How hot must it be for carbon to reduce ZnO?

Computed: the reduction is balanced by the equation solver; ΔG°(T) is the gap between the two Ellingham lines times the moles of O₂ transferred (ΔH°, S° from the 298 K table, straight-line model, optional metal phase changes); K = exp(−ΔG°/RT) with pure solids and liquids at activity 1. The furnace glow is black-body colour from Planck’s law. Illustrative: powder colours are the usual appearance of these oxides (not computed), and the metal bead shows the thermodynamic verdict only — not a rate or a yield.

Procedure

  1. Start with FeO and coke at 800 K. Note the sign of ΔG°, then raise the temperature 100 K at a time until ΔG° changes sign.
  2. Repeat for ZnO: above 1180 K the zinc comes off as vapour. Switch “Include metal melting/boiling” on and off and compare the crossover temperatures.
  3. Switch to Fe₂O₃ with carbon monoxide (the blast furnace’s real reducing agent) and check ΔG° between 600 and 1800 K.
  4. Try every reducing agent on Cr₂O₃, TiO₂ and Al₂O₃, and log the results.
  5. Compare with the Ellingham diagram and explain your log by which line lies below which.

What to notice

  • Carbon (making CO) reduces FeO above about 1020 K and ZnO above about 1220–1260 K; for Al₂O₃ and MgO, ΔG° stays far above zero at ordinary furnace temperatures.
  • CO reduces Fe₂O₃ at every temperature, but for FeO + CO the ΔG° is slightly positive at high temperature — it needs a gas that is mostly CO.
  • Aluminium reduces Cr₂O₃ (thermite), and magnesium reduces TiO₂: their lines lie below those oxides.
  • The furnace glow runs from dull red through orange-yellow to near white — black-body colour from Planck’s law, which is how smiths judged temperature by eye.

In a real lab

  • Real smelting is also about rate: reactions between solids are extremely slow below about 700 K even when ΔG° < 0.
  • Carbon forms carbides with many metals (TiC, Al₄C₃), so the product is not the pure metal; titanium is made industrially by chlorination and magnesium reduction (the Kroll process).
  • Thermite exceeds 2000 °C and throws out molten metal, so it is demonstrated only under professional conditions; carbon monoxide is highly toxic.
  • The straight Ellingham lines ignore heat capacities and oxide melting, so crossover temperatures can be off by tens of kelvin.

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