1.
[15] Balance the following redox reactions: {3 points each}
a) Cr2O72-(aq) + Cl-(aq) Cr3+(aq) + Cl2(g) (acidic medium)
b) Al(s) + MnO4-(aq) MnO2(s) + Al(OH)4-(aq) (basic medium)
Solution
a) Cr2O72-(aq) + Cl-(aq) ? Cr3+(aq) + Cl2(g) (acidic medium)
O.H.R R.H.R
2Cl^- (aq) ------------> Cl2(g) Cr2O7^2- (aq) ----------> 2Cr^3+ (aq)
2Cl^- (aq) ------------> Cl2(g) Cr2O7^2- (aq) ----------> 2Cr^3+ (aq)+
7H2O(l)
2Cl^- (aq) ------------> Cl2(g) Cr2O7^2- (aq) +14H^+ (aq)----------> 2Cr^3+
(aq)+ 7H2O(l)
2Cl^- (aq) ------------> Cl2(g)+2e^- Cr2O7^2- (aq) +14H^+ (aq)+6e^- ---------->
2Cr^3+ (aq)+ 7H2O(l)
6Cl^- (aq) ------------> 3Cl2(g)+6e^-
Cr2O7^2- (aq) +14H^+ (aq)+6e^- ----------> 2Cr^3+ (aq)+ 7H2O(l)
-----------------------------------------------------------------------------------------------------------
Cr2O7^2- (aq) +6Cl^- (aq) +14H^+ (aq)- ----------> 2Cr^3+ (aq)+3Cl2(g) + 7H2O(l)
b) Al(s) + MnO4-(aq) ? MnO2(s) + Al(OH)4-(aq) (basic medium)
O. H.R R.H.R
Al(s) --------> [Al(OH)4]^- MnO4^- (aq) --------------> MnO2(s)

2.
Al(s) +4H2O(l)--------> [Al(OH)4]^- MnO4^- (aq) -------------->
MnO2(s)+2H2O(l)
Al(s) +4H2O(l) +4OH^- (aq) -----> [Al(OH)4]^- + 4H2O MnO4^- (aq) +4H2O(l)------>
MnO2(s)+2H2O(l)+ 4OH^-(aq)
Al(s) +4H2O(l) +4OH^-(aq) ---> [Al(OH)4]^- +4H2O+3e^- MnO4^- (aq) +4H2O(l)+3e^- -->
MnO2(s)+2H2O+ 4OH^-(aq)
Al(s) +4H2O(l) +4OH^-(aq) ---> [Al(OH)4]^- +4H2O+3e^-
MnO4^- (aq) +4H2O(l)+3e^- --> MnO2(s)+2H2O+ 4OH^-(aq)
---------------------------------------------------------------------------------------
Al(s) + MnO4^- (aq) +2H2O(l) ------------> [Al(OH)4]^- (aq) + MnO2(s)