Question 11 of 31intermediate🔧 ApplyShort Answer5 marks

Justify that the following reactions are redox reactions: (a) CuO(s) + H₂(g) → Cu(s) + H₂O(g) (b) Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g) (c) 4BCl₃(g) + 3LiAlH₄(s) → 2B₂H₆(g) + 3LiCl(s) + 3AlCl₃(s) (d) 2K(s) + F₂(g) → 2K⁺F⁻(s) (e) 4 NH₃(g) + 5 O₂(g) → 4NO(g) + 6H₂O(g)

(a)
Answer

Assigning oxidation numbers: +2−200+1−2+2 -2 \quad 0 \quad 0 \quad +1 -2

CuO(s)+H2(g)→Cu(s)+H2O(g)\text{CuO(s)} + \text{H}_2\text{(g)} \rightarrow \text{Cu(s)} + \text{H}_2\text{O(g)}

Here, Cu decreases from +2+2 to 00 (reduced) and H increases from 00 to +1+1 (oxidised). Hence, it is a redox reaction.

(b)
Answer

Assigning oxidation numbers: +3−2+2−20+4−2+3 -2 \quad +2 -2 \quad 0 \quad +4 -2

Fe2O3(s)+3CO(g)→2Fe(s)+3CO2(g)\text{Fe}_2\text{O}_3\text{(s)} + 3\text{CO(g)} \rightarrow 2\text{Fe(s)} + 3\text{CO}_2\text{(g)}

Fe decreases from +3+3 to 00 (reduced) and C increases from +2+2 to +4+4 (oxidised). Hence, it is a redox reaction.

(c)
Answer

Assigning oxidation numbers: +3−1+1+3−1−3+1+1−1+3−1+3 -1 \quad +1 +3 -1 \quad -3 +1 \quad +1 -1 \quad +3 -1

4BCl3(g)+3LiAlH4(s)→2B2H6(g)+3LiCl(s)+3AlCl3(s)4\text{BCl}_3\text{(g)} + 3\text{LiAlH}_4\text{(s)} \rightarrow 2\text{B}_2\text{H}_6\text{(g)} + 3\text{LiCl(s)} + 3\text{AlCl}_3\text{(s)}

B decreases from +3+3 to −3-3 (reduced) and H increases from −1-1 to +1+1 (oxidised). Hence, it is a redox reaction.

(d)
Answer

Assigning oxidation numbers: 00+1−10 \quad 0 \quad +1 -1

2K(s)+F2(g)→2K+F−(s)2\text{K(s)} + \text{F}_2\text{(g)} \rightarrow 2\text{K}^+\text{F}^-\text{(s)}

K increases from 00 to +1+1 (oxidised) and F decreases from 00 to −1-1 (reduced). Hence, it is a redox reaction.

(e)
Answer

Assigning oxidation numbers: −3+10+2−2+1−2-3 +1 \quad 0 \quad +2 -2 \quad +1 -2

4NH3(g)+5O2(g)→4NO(g)+6H2O(g)4\text{NH}_3\text{(g)} + 5\text{O}_2\text{(g)} \rightarrow 4\text{NO(g)} + 6\text{H}_2\text{O(g)}

N increases from −3-3 to +2+2 (oxidised) and O decreases from 00 to −2-2 (reduced). Hence, it is a redox reaction.

Explanation

A redox reaction is defined as a reaction which involves a change in the oxidation number of the interacting species. By assigning oxidation numbers to each element in the reactants and products, we can identify which species is oxidised (increase in oxidation number) and which is reduced (decrease in oxidation number), thereby justifying the reaction as a redox process.

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