In the thermite reaction $3\mathrm{Fe}_3\mathrm{O}_4(s) + 8\mathrm{Al}(s) \rightarrow 9\mathrm{Fe}(s) + 4\mathrm{Al}_2\mathrm{O}_3(s)$, which species undergoes reduction?
AAl, because it loses electrons to oxygen
B$\mathrm{Fe}_3\mathrm{O}_4$, because oxygen has been removed from it
CBoth Al and $\mathrm{Fe}_3\mathrm{O}_4$ are reduced
D$\mathrm{Al}_2\mathrm{O}_3$, because it gains oxygen during the reaction
Answer & Solution
Correct answer: B. $\mathrm{Fe}_3\mathrm{O}_4$, because oxygen has been removed from it
1. Apply the classical definition: reduction = removal of oxygen (or addition of hydrogen).
2. In $\mathrm{Fe}_3\mathrm{O}_4$, iron starts in mixed +2/+3 oxidation states; in product Fe(s) iron is 0. So Fe goes DOWN in oxidation number — reduction.
3. Aluminium goes from 0 in Al(s) to +3 in $\mathrm{Al}_2\mathrm{O}_3$, an INCREASE — oxidation, not reduction. So options A and C are wrong.
4. $\mathrm{Al}_2\mathrm{O}_3$ is a PRODUCT and is the oxidised form of Al; it is not undergoing further reduction in this reaction (option D wrong).
_Source: NCERT Class 11 Chemistry Part 2, Ch 7, Problem 7.1 (ii), p. 1–2._
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