S → FeS → SO2 → SO3 → Na2SO4
1) S + Fe → FeS (compound reaction; sulfur, iron, iron sulfide);
2) 4FeS + O2 → 2Fe2O3 + SO2 (substitution reaction; iron sulfide, oxygen, ferum (III) oxide, sulfur (IV) oxide);
3) 2SO2 + O2 → 2SO3 (compound reaction; sulfur (IV) oxide, oxygen, sulfur (VI) oxide);
4) SO3 + Na2O → Na2SO4 (compound reaction; sulfur (VI) oxide, sodium oxide, sodium sulfate).
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