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2020 O Level Chem P1 Q27 — Redox / Redox Reactions Solution

Redox / Redox Reactions 2020 O Level Pure Chemistry · Paper 1 · Q27

Question

The equations show reactions that take place in the blast furnace. 1 Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g) 2 C(s) + O₂(g) → CO₂(g) 3 CaCO₃(s) → CaO(s) + CO₂(g) Which of the reactions are redox reactions?

A1 only
B1 and 2 only
C2 and 3 only
D1, 2 and 3

Correct answer: B

Video Solution

Explanation

Answer: B (reactions 1 and 2 only). A redox reaction is one in which elements change oxidation state, with something oxidised and something reduced. In reaction 1, \(\text{Fe}_2\text{O}_3 + 3\text{CO} \to 2\text{Fe} + 3\text{CO}_2\), iron is reduced from +3 to 0 while carbon is oxidised from +2 to +4, so it is redox. In reaction 2, \(\text{C} + \text{O}_2 \to \text{CO}_2\), carbon is oxidised from 0 to +4 and oxygen is reduced from 0 to −2, so it is also redox.

Reaction 3, \(\text{CaCO}_3 \to \text{CaO} + \text{CO}_2\), is just thermal decomposition: calcium stays +2, carbon stays +4 and oxygen stays −2, so no oxidation states change and it is not redox.

The redox reactions are 1 and 2, giving answer B.

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