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Which of the following sets consists of oxides that are not reducible by carbon to the corresponding metals?

1. Cu2O, SnO2
2. Fe2O3, ZnO
3. CaO, K2O
4. PbO, Fe3O4

Correct answer: CaO, K2O

Solution

Carbon cannot reduce oxides of very electropositive metals like CaO and K2O because these metals have a far greater affinity for oxygen than carbon does (their oxides lie below the carbon line in the Ellingham diagram). Oxides such as Cu2O, SnO2, Fe2O3, ZnO, PbO and Fe3O4 are all reducible by carbon.

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