Which of the following reduction reaction CANNOT be carried out with coke?

Solution

Chemistry ma’am · 3 steps

Use the Ellingham diagram: carbon can only reduce an oxide whose line lies above the carbon line.

1. The rule. Carbon reduces a metal oxide only when forming CO from carbon releases more energy than forming that oxide. On the Ellingham diagram, the carbon line (2C + O₂ → 2CO) must lie below the metal oxide line at the working temperature.

   Oxide	Stability of oxide	Reduced by coke?
   Cu₂O	Low, line lies well above the carbon line	Yes, easily
   Fe₂O₃	Moderate, the carbon line crosses below it in the blast furnace	Yes, by coke and CO
   ZnO	Fairly high, the carbon line crosses below it at about 1270 K	Yes, at about 1673 K
   Al₂O₃	Very high, line lies far below the carbon line	No

2. Why aluminium fails. Δ_f G° of Al₂O₃ is so negative that Δ G° for Al₂O₃ + 3C → 2Al + 3CO stays positive at any temperature a furnace can reach. Aluminium is therefore extracted by electrolysis of molten alumina (Hall–Héroult process).
3. So. The reduction that cannot be done with coke is Al₂O₃ → Al, option (d).

The idea to keep. The lower an oxide's line on the Ellingham diagram, the harder it is to reduce. Very reactive metals like Al, Mg and Na sit far below carbon, so you extract them by electrolysis, not with coke.

Answer from the NTA final answer key. Solution by PrepWiser.