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In the Ellingham diagram, the line for C + O2 -> CO2 is approximately horizontal, while the line for 2C + O2 -> 2CO has a steep negative slope. Consider a metal oxide MO whose Ellingham line intersects the carbon line at a certain temperature. From the diagram, what is the approximate minimum temperature at which MO can be reduced to metal M by carbon?

1. 1750°C
2. 277°C
3. 1200°C
4. 2200°C

Correct answer: 1200°C

Solution

On the Ellingham diagram, carbon reduces MO when the free energy of formation of CO becomes more negative than that of MO. The intersection temperature for many transition-metal oxides (e.g., Cr2O3-type MO) lies near 1200°C, making it the minimum temperature for feasible carbothermic reduction.

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