Chapter 18: 2, 6, 8, 10, 30, 32, 38, 40, 58, 70
18.2 Balance each redox reaction occurring in acidic aqueous solution.
a. Zn(s) + Sn2+ (aq) →Zn2+ (aq) + Sn(s)
b. Mg(s) + Cr3+ (aq) →Mg2+ (aq) + Cr(s)
c. MnO4- (aq) + Al (s) →Mn2+ (aq) + Al3+ (aq)
SOLUTION:
a.
6-1/2 magic steps!
1. Separate into ½ reactions:
Zn(s) →Zn2+ (aq)
Sn2+ (aq) → Sn(s)
2. Balance everything but H and O
Zn(s) →Zn2+ (aq)
Sn2+ (aq) → Sn(s)
3. Balance O using H2O
Zn(s) →Zn2+ (aq)
Sn2+ (aq) → Sn(s)
4. Balance H using H+
Zn(s) →Zn2+ (aq)
Sn2+ (aq) → Sn(s)
5. Balance charge by adding electrons
Zn(s) →Zn2+ (aq) + 2 eSn2+ (aq) + 2e- → Sn(s)
6. Recombine half-reactions, making sure to cancel electrons
Zn(s) →Zn2+ (aq) + 2 eSn2+ (aq) + 2e- → Sn(s)
Zn(s) + Sn2+ (aq) → Zn2+ (aq) + Sn(s)
7. Only applies to basic solutions
Zn(s) + Sn2+ (aq) → Zn2+ (aq) + Sn(s)
b.
6-1/2 magic steps!
1. Separate into ½ reactions:
Mg(s) →Mg2+ (aq)
Cr3+ (aq) → Cr(s)
2. Balance everything but H and O
Mg(s) →Mg2+ (aq)
Cr3+ (aq) → Cr(s)
3. Balance O using H2O
Mg(s) →Mg2+ (aq)
Cr3+ (aq) → Cr(s)
4. Balance H using H+
Mg(s) →Mg2+ (aq)
Cr3+ (aq) → Cr(s)
5. Balance charge by adding electrons
Mg(s) →Mg2+ (aq) + 2 eCr3+ (aq) + 3e- → Cr(s)
6. Recombine half-reactions, making sure to cancel electrons
3x[Mg(s) →Mg2+ (aq) + 2 e-]
2x[Cr3+ (aq) + 3e- → Cr(s)]
3 Mg(s) + 2Cr3+ (aq) + 6e- → 3Mg2+ (aq) + 2Cr(s) + 6e3 Mg(s) + 2Cr3+ (aq) → 3Mg2+ (aq) + 2Cr(s)
c.
6-1/2 magic steps!
1. Separate into ½ reactions:
MnO4- (aq) →Mn2+ (aq)
Al (s) → Al3+ (aq)
2. Balance everything but H and O
MnO4- (aq) →Mn2+ (aq)
Al (s) → Al3+ (aq)
3. Balance O using H2O
MnO4- (aq) →Mn2+ (aq) + 4 H2O (l)
Al (s) → Al3+ (aq)
4. Balance H using H+
MnO4- (aq) + 8 H+ (aq) →Mn2+ (aq) + 4 H2O (l)
Al (s) → Al3+ (aq)
5. Balance charge by adding electrons
MnO4- (aq) + 8 H+ (aq) + 5 e- →Mn2+ (aq) + 4 H2O (l)
Al (s) → Al3+ (aq) + 3e6. Recombine half-reactions, making sure to cancel electrons
3x [MnO4- (aq) + 8 H+ (aq) + 5 e- →Mn2+ (aq) + 4 H2O (l)]
5x[Al (s) → Al3+ (aq) + 3e-]
3MnO4- (aq) + 24 H+ (aq) + 15 e- + 5Al(s)→3Mn2+ (aq) + 12 H2O (l) + 5Al3+ (aq) +
15e3MnO4- (aq) + 24 H+ (aq) + 5Al(s)→3Mn2+ (aq) + 12 H2O (l) + 5Al3+ (aq)
7. Only applies to basic solutions
3MnO4- (aq) + 24 H+ (aq) + 5Al(s)→3Mn2+ (aq) + 12 H2O (l) + 5Al3+ (aq)
18.6 Balance each redox reaction occurring in basic aqueous solution.
a. MnO4- (aq) + Br- (aq)→ MnO2(s) + BrO3- (aq)
b. Ag(s) + CN- (aq) + O2(g) → Ag(CN)2- (aq)
c. NO2- (aq) + Al (s) → NH3(g) + AlO2- (aq)
SOLUTION:
a.
1. Separate into ½ reactions:
MnO4- (aq) →MnO2 (s)
Br- (aq)→ BrO3- (aq)
2. Balance everything but H and O
MnO4- (aq) →MnO2 (s)
Br- (aq)→ BrO3- (aq)
3. Balance O using H2O
MnO4- (aq) →MnO2 (s) + 2 H2O (l)
Br- (aq) + 3 H2O (l) → BrO3- (aq)
4. Balance H using H+
MnO4- (aq) + 4 H+ (aq) →MnO2 (s) + 2 H2O (l)
Br- (aq) + 3 H2O (l) → BrO3- (aq) + 6 H+ (aq)
5. Balance charge by adding electrons
MnO4- (aq) + 4 H+ (aq) + 3 e- →MnO2 (s) + 2 H2O (l)
Br- (aq) + 3 H2O (l) → BrO3- (aq) + 6 H+ (aq) + 6e6. Recombine half-reactions, making sure to cancel electrons
2x [MnO4- (aq) + 4 H+ (aq) + 3 e- →MnO2 (s) + 2 H2O (l)]
Br- (aq) + 3 H2O (l) → BrO3- (aq) + 6 H+ (aq) + 6e2MnO4- (aq) + 8 H+ (aq) + 6 e- + Br- (aq) + 3 H2O (l) → 2MnO2 (s) + 4 H2O (l) +
BrO3- (aq) + 6 H+ (aq) + 6e2MnO4- (aq) + 2 H+ (aq) + Br- (aq) → 2MnO2 (s) + H2O (l) + BrO3- (aq)
7. In basic solution, add OH- to both sides to neutralize H+
2MnO4- (aq) + 2 H+ (aq) + 2 OH- (aq) + Br- (aq) → 2MnO2 (s) + H2O (l) + BrO3- (aq)
+ 2 OH- (aq)
2MnO4- (aq) + 2 H2O (l) + Br- (aq) → 2MnO2 (s) + H2O (l) + BrO3- (aq) + 2 OH- (aq)
2MnO4- (aq) + H2O (l) + Br- (aq) → 2MnO2 (s) + BrO3- (aq) + 2 OH- (aq)
b. (this one’s a little tricky to separate. The oxygen is going to end up as OH- but it isn’t
visible the way the book wrote the equation. They should have really put an OH- on the
right.)
1. Separate into ½ reactions:
Ag(s) + CN- (aq) → Ag(CN)2- (aq)
O2(g) →
2. Balance everything but H and O
Ag(s) + 2 CN- (aq) → Ag(CN)2- (aq)
O2(g) →
3. Balance O using H2O
Ag(s) + 2 CN- (aq) → Ag(CN)2- (aq)
O2(g) → 2 H2O (l)
4. Balance H using H+
Ag(s) + 2 CN- (aq) → Ag(CN)2- (aq)
O2(g) + 4 H+ (aq) → 2 H2O (l)
5. Balance charge by adding electrons
Ag(s) + 2 CN- (aq) → Ag(CN)2- (aq) + 1 eO2(g) + 4 H+ (aq) + 4 e- → 2 H2O (l)
6. Recombine half-reactions, making sure to cancel electrons
4x [Ag(s) + 2 CN- (aq) → Ag(CN)2- (aq) + 1 e-]
O2(g) + 4 H+ (aq) + 4 e- → 2 H2O (l)
4Ag(s) + 8 CN- (aq) + O2(g) + 4 H+ (aq) + 4 e- → 4Ag(CN)2- (aq) + 4 e- + 2 H2O (l)
4Ag(s) + 8 CN- (aq) + O2(g) + 4 H+ (aq) → 4Ag(CN)2- (aq) + 2 H2O (l)
7. In basic solution, add OH- to both sides to neutralize H+
4Ag(s) + 8 CN- (aq) + O2(g) + 4 H+ (aq) + 4 OH- (aq) → 4Ag(CN)2- (aq) + 2 H2O (l) + 4
OH- (aq)
4Ag(s) + 8 CN- (aq) + O2(g) + 4 H2O (l) → 4Ag(CN)2- (aq) + 2 H2O (l) + 4 OH- (aq)
4Ag(s) + 8 CN- (aq) + O2(g) + 2 H2O (l) → 4Ag(CN)2- (aq) + 4 OH- (aq)
c.
1. Separate into ½ reactions:
NO2- (aq) → NH3(g)
Al (s) → AlO2- (aq)
2. Balance everything but H and O
NO2- (aq) → NH3(g)
Al (s) → AlO2- (aq)
3. Balance O using H2O
NO2- (aq) → NH3(g) + 2 H2O (l)
Al (s) + 2 H2O (l) → AlO2- (aq)
4. Balance H using H+
NO2- (aq) + 7 H+ (aq) → NH3(g) + 2 H2O (l)
Al (s) + 2 H2O (l) → AlO2- (aq) + 4 H+ (aq)
5. Balance charge by adding electrons
NO2- (aq) + 7 H+ (aq) + 6 e- → NH3(g) + 2 H2O (l)
Al (s) + 2 H2O (l) → AlO2- (aq) + 4 H+ (aq) + 3 e-
6. Recombine half-reactions, making sure to cancel electrons
NO2- (aq) + 7 H+ (aq) + 6 e- → NH3(g) + 2 H2O (l)
2x[Al (s) + 2 H2O (l) → AlO2- (aq) + 4 H+ (aq) + 3 e-]
NO2- (aq) + 7 H+ (aq) + 6 e- + 2 Al (s) + 4 H2O (l) → NH3(g) + 2 H2O (l) + 2 AlO2- (aq) + 8
H+ (aq) + 6 e-
NO2- (aq) + 2 Al (s) + 2 H2O (l) → NH3(g) + 2 AlO2- (aq) + H+ (aq)
7. In basic solution, add OH- to both sides to neutralize H+
NO2- (aq) + 2 Al (s) + 2 H2O (l) + OH- (aq) → NH3(g) + 2 AlO2- (aq) + H+ (aq) +OH-(aq)
NO2- (aq) + 2 Al (s) + 2 H2O (l) + OH- (aq) → NH3(g) + 2 AlO2- (aq) + H2O (l)
NO2- (aq) + 2 Al (s) + H2O (l) + OH- (aq) → NH3(g) + 2 AlO2- (aq)