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NCERT Class XI Chemistry Redox Reactions Solutions

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Question : 3
Total: 30
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) 4NH3(g)+5O2(g) → 4NO(g)+6H2O(g)
Solution:  
(a)
+2
Cu
−2
O(s)
+
0
H2(g)
 →
0
Cu(s)
+
+1
H2
−2
O(g)
O.N. of Cu decreases from + 2 in CuO to 0 in Cu but that of H increases from 0 in H2 to + 1 in H2O. Therefore, CuO is reduced to Cu but H2 is oxidised to H2O. Thus, this is a redox reaction.
(b) F
+3
e2
−2
O3(s)
+3
+2
C
O(g) → 2
0
Fe
(s)+3
+4
C
O2(g)
O.N. of Fe decreases from +3 in Fe2O3 to 0 in Fe while that of C increases from +2 in CO to +4 in CO2. Thus, Fe2O3 is reduced and CO is oxidised.
Thus, this is a redox reaction.
(c) 4
+3
B
C
−1
l3(g)
+3
+1
Li
+3
Al
−1
H4(s)
 → 2
−3
B
2
+1
H6(g)
+3
+1
Li
C
−1
l(s)
+3
+3
Al
C
−1
l3(s)
O.N. of B decreases from +3 in BCl3 to –3 in B2H6 while that of H increases from –1 in LiAlH4 to + 1 in B2H6. Therefore, BCl3 is reduced while LiAlH4 is oxidised. Thus, this is a redox reaction.
(d) 2K(s)+F2(g) → 2K+F(s)–
Each K atom has lost one electron to form K+ while F2 has gained two electrons to form two F– ions. Therefore, K is oxidised while F2 is reduced. Thus, it is a redox reaction.
(e) 4
−3
N
+1
H3(g)
+5
0
O2(g)
 → 4
+2
N
−2
O(g)
+6
+1
H
2
−2
O(g)
O.N. of N increases from –3 in NH3 to +2 in NO while that of O decreases from 0 in O2 to –2 in NO or H2O. Therefore, NH3 is oxidised while O2 is reduced. Thus, it is a redox reaction.
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