Balancing redox reactions in basic solution
Problems 11 - 25

Ten Examples      Problems 26-45      Balancing in acidic solution
Problems 1-10      Only the examples and problems      Return to Redox menu

Problem #11: ClO3¯ + N2H4 ---> NO + Cl¯

Solution:

1) Half-reactions:

ClO3¯ ---> Cl¯
N2H4 ---> NO

2) Balance in acidic solution first:

6e¯ + 6H+ + ClO3¯ ---> Cl¯ + 3H2O
2H2O + N2H4 ---> 2NO + 8H+ + 8e¯

4) Equalize electrons:

24e¯ + 24H+ + 4ClO3¯ ---> 4Cl¯ + 12H2O
6H2O + 3N2H4 ---> 6NO + 24H+ + 24e¯

5) Add:

4ClO3¯ + 3N2H4 ---> 4Cl¯ + 6NO + 6H2O

Note that there is no hydroxide in the final answer. Does that mean this reaction does not require a basic solution? The answer is no. The hydroxide is functioning as a catalyst to the reaction. Some hydroxide is used up to start the rection, but it is regenerated by the end of the reaction. So, while hydroxide is necessary for the reaction, it does not appear in the overall equation.


Problem #12: H2O2 + Cl2O7 ---> ClO2¯ + O2

Answered on Yahoo Answers.


Problem #13: HXeO4¯ + Pb ---> Xe + HPbO2¯

Solution:

1) Half-reactions:

HXeO4¯ ---> Xe
Pb ---> HPbO2¯

2) Balance in acid:

6e¯ + 7H+ + HXeO4¯ ---> Xe + 4H2O
2H2O + Pb ---> HPbO2¯ + 3H+ + 2e¯

3) Equalize electrons:

6e¯ + 7H+ + HXeO4¯ ---> Xe + 4H2O
6H2O + 3Pb ---> 3HPbO2¯ + 9H+ + 6e¯

4) Add:

2H2O + HXeO4¯ + 3Pb ---> Xe + 3HPbO2¯ + 2H+

5) Convert to basic with two hydroxides on each side:

2OH¯ + HXeO4¯ + 3Pb ---> Xe + 3HPbO2¯

Problem #14: HPbO2¯ + Cr(OH)3 ---> Pb + CrO42¯

Solution:

1) Half-reactions:

HPbO2¯ ---> Pb
Cr(OH)3 ---> CrO42¯

2) Balance in acidic solution:

2e¯ + 3H+ + HPbO2¯ ---> Pb + 2H2O
H2O + Cr(OH)3 ---> CrO42¯ + 5H+ + 3e¯

3) Equalize electrons:

6e¯ + 9H+ + 3HPbO2¯ ---> 3Pb + 6H2O
2H2O + 2Cr(OH)3 ---> 2CrO42¯ + 10H+ + 6e¯

4) Add:

3HPbO2¯ + 2Cr(OH)3 ---> 3Pb + 2CrO42¯ + H+ + 4H2O

5) Add one hydroxide to each side:

3HPbO2¯ + 2Cr(OH)3 + OH¯ ---> 3Pb + 2CrO42¯ + 5H2O

Problem #15: Basic solutions of thallium(III) oxide and hydroxylamine are mixed together to produce thallium(I) hydroxide and nitrogen gas

Solution:

1) The full equation followed by the half-reactions:

Tl2O3 + NH2OH ---> TlOH + N2

Tl2O3 ---> TlOH
NH2OH ---> N2

2) Balance in acidic solution:

4e¯ + 4H+ + Tl2O3 ---> 2TlOH + H2O
2NH2OH ---> N2 + 2H2O + 2H+ + 2e¯

3) Equalize electrons:

4e¯ + 4H+ + Tl2O3 ---> 2TlOH + H2O
4NH2OH ---> 2N2 + 4H2O + 4H+ + 4e¯

4) Add:

Tl2O3 + 4NH2OH ---> 2TlOH + 2N2 + 5H2O

Problem #16: Mg + NiO2 ---> Mg(OH)2 + Ni(OH)2

Solution:

1) Half-reactions:

Mg ---> Mg(OH)2
NiO2 ---> Ni(OH)2

2) Balance first in basic and second in acidic:

2OH¯ + Mg ---> Mg(OH)2 + 2e-
2e¯ + 2H+ + NiO2 ---> Ni(OH)2

3) Note that the electrons are already equalized. Note also that hydrogen ion and hydroxide ion (both on the left-hand side of the two half-reactions) react immediately to produce water. That leads to the final answer:

Mg + NiO2 + 2H2O ---> Mg(OH)2 + Ni(OH)2

Problem #17: Cr(OH)4¯ + H2O2 ---> CrO42¯ + H2O

Solution:

1) Half-reactions:

Cr(OH)4¯ ---> CrO42¯
H2O2 ---> H2O

2) Balance as if in acidic solution:

Cr(OH)4¯ ---> CrO42¯ + 4H+ + 3e¯
2e¯ + 2H+ + H2O2 ---> 2H2O

3) Equalize electrons:

2Cr(OH)4¯ ---> 2CrO42¯ + 8H+ + 6e¯
6e¯ + 6H+ + 3H2O2 ---> 6H2O

4) Add:

2Cr(OH)4¯ + 3H2O2 ---> 2CrO42¯ + 2H+ + 6H2O

5) Change to basic by adding two hydroxides to each side:

2OH¯ + 2Cr(OH)4¯ + 3H2O2 ---> 2CrO42¯ + 8H2O

The two H+ and the two OH¯ on the right combine to form two waters to make a total of eight waters.


Problem #18: CrO42¯ + HSnO2¯ + H2O ---> CrO2(s) + H2SnO3(aq) + OH¯(aq)

1) Balanced half-reactions:

4H+ + CrO42¯ + 2e¯ ---> CrO2 + 2H2O
HSnO2¯ + H2O ---> H2SnO3 + H+ + 2e¯

2) Add, then simplify:

4H+ + CrO42¯ + HSnO2¯ + H2O ---> CrO2 + H2SnO3 + H+ + 2H2O

3H+ + CrO42¯ + HSnO2¯ ---> CrO2 + H2SnO3 + H2O

3) Add 3OH¯ to each side to neutralize H+ since the reaction is in basic solution, then simplify:

3H2O + CrO42¯ + HSnO2¯ ---> CrO2 + H2SnO3 + H2O + 3OH¯

2H2O + CrO42¯ + HSnO2¯ ---> CrO2 + H2SnO3 + 3OH¯ <--- that's the balanced equation


Problem #19: HXeO4¯(aq) ---> XeO64¯(aq) + Xe(g)

Solution:

1) Half-reactions:

HXeO4¯(aq) ---> XeO64¯(aq)
HXeO4¯(aq) ---> Xe(g)

2) Balance in acidic:

2H2O + HXeO4¯(aq) ---> XeO64¯(aq) + 5H+ + 2e¯
6e¯ + 7H+ + HXeO4¯(aq) ---> Xe(g) + 4H2O

3) Equalize electrons:

6H2O + 3HXeO4¯(aq) ---> 3XeO64¯(aq) + 15H+ + 6e¯
6e¯ + 7H+ + HXeO4¯(aq) ---> Xe(g) + 4H2O

4) Add:

2H2O + 4HXeO4¯(aq) ---> 3XeO64¯(aq) + Xe(g) + 8H+

5) Add eight hydroxides to each side:

8OH¯ + 2H2O + 4HXeO4¯(aq) ---> 3XeO64¯(aq) + Xe(g) + 8H2O

6) Remove water for the final answer (with state symbols for everything):

8OH¯(aq) + 4HXeO4¯(aq) ---> 3XeO64¯(aq) + Xe(g) + 6H2O(ℓ)

Problem #20: S2O42¯ + O2(g) ---> SO42¯

Solution:

1) Half-reactions:

S2O42¯ ---> SO42¯
O2(g) ---> OH¯

No hydroxide is indicated in the problem but, since we know the reaction to be in basic solution, we can add it in.

2) Balance:

4H2O + S2O42¯ ---> 2SO42¯ + 8H+ + 6e¯
4e¯ + 2H+ + O2(g) ---> 2OH¯ <--- note the hydrogen ion on one side and the hydroxide on the other. Unusual.

3) Equalize electrons:

8H2O + 2S2O42¯ ---> 4SO42¯ + 16H+ + 12e¯
12e¯ + 6H+ + 3O2(g) ---> 6OH¯

4) Add:

8H2O + 2S2O42¯ + 3O2(g) ---> 4SO42¯ + 10H+ + 6OH¯

5) Add four hydroxides:

4OH¯ + 8H2O + 2S2O42¯ + 3O2(g) ---> 4SO42¯ + 10H2O

Note that I combined the hydrogen ion and hydroxide ion on the right into water.

6) Remove water for the final answer:

4OH¯ + 2S2O42¯ + 3O2(g) ---> 4SO42¯ + 2H2O

Here's an incorrect answer to this problem. The answerer failed to reduce the O2 properly and wound up with an answer that is not balanced for charge or for oxygen


Problem #21: SO32- + Cr2O72- ---> Cr3+ + SO42-

Solution:

1) Half-reactions:

SO32- ---> SO42-
Cr2O72- ---> Cr3+

2) Balance in acidic solution first:

H2O + SO32- ---> SO42- + 2H+ + 2e¯
6e¯ + 14H+ + Cr2O72- ---> 2Cr3+ + 7H2O

3) Equalize electrons:

3H2O + 3SO32- ---> 3SO42- + 6H+ + 6e¯
6e¯ + 14H+ + Cr2O72- ---> 2Cr3+ + 7H2O

4) Add:

8H+ + 3SO32- + Cr2O72- ---> 2Cr3+ + 3SO42- + 4H2O

5) Convert to basic:

4H2O + 3SO32- + Cr2O72- ---> 2Cr3+ + 3SO42- + 8OH¯

6) Since chromium(III) hydroxide precipitates, this may the desired answer:

4H2O + 3SO32- + Cr2O72- ---> 2Cr(OH)3 + 3SO42- + 2OH¯

Problem #22: Sb2O3(s) + NO3¯(aq) ---> H3SbO4(aq) + NO(g)

Solution:

1) Half-reactions:

Sb2O3(s) ---> H3SbO4(aq)
NO3¯(aq) ---> NO(g)

2) Balance in acidic:

5H2O + Sb2O3(s) ---> 2H3SbO4(aq) + 4H+ + 4e¯
3e¯ + 4H+ + NO3¯(aq) ---> NO(g) + 2H2O

3) Equalize electrons:

15H2O + 3Sb2O3(s) ---> 6H3SbO4(aq) + 12H+ + 12e¯
12e¯ + 16H+ + 4NO3¯(aq) ---> 4NO(g) + 8H2O

4) Add:

4H+ + 7H2O + 3Sb2O3(s) + 4NO3¯(aq) ---> 6H3SbO4(aq) + 4NO(g)

5) Convert to basic:

11H2O + 3Sb2O3(s) + 4NO3¯(aq) ---> 6H3SbO4(aq) + 4NO(g) + 4OH¯

Problem #23: Al(s) + NO3¯ ---> Al(OH)4¯ + NH3(g)

Solution:

I propose to balance one half-reaction in basic, the other in acidic. I'll then add and change over to basic at the end.

1) Half-reactions:

Al(s) ---> Al(OH)4¯
NO3¯ ---> NH3(g)

2) Balance:

Al(s) + 4OH¯ ---> Al(OH)4¯ + 3e¯
8e¯ + 9H+ + NO3¯ ---> NH3(g) + 3H2O

3) Equalize electrons:

8Al(s) + 32OH¯ ---> 8Al(OH)4¯ + 24e¯
24e¯ + 27H+ + 3NO3¯ ---> 3NH3(g) + 9H2O

4) Add:

8Al(s) + 32OH¯ + 27H+ + 3NO3¯ ---> 8Al(OH)4¯ + 3NH3(g) + 9H2O

5) Make twenty-seven waters from 27 hydrogen ions and 27 hydroxide ions:

8Al(s) + 5OH¯ + 27H2O + 3NO3¯ ---> 8Al(OH)4¯ + 3NH3(g) + 9H2O

6) Eliminate nine waters for the final answer:

8Al(s) + 3NO3¯ + 5OH¯ + 18H2O ---> 8Al(OH)4¯ + 3NH3(g)

I rearranged the left-hand side a bit, simply because it looks better to me. Also, note how I never had to change over to basic, it just happened naturally.


Problem #24: MnO4¯ + I¯ ----> MnO42¯ + IO3¯

Solution:

1) Half-reactions:

MnO4¯ ---> MnO42¯
I¯ ---> IO3¯

2) Balance in acidic solution:

e¯ + MnO4¯ ---> MnO42¯
3H2O + I¯ ---> IO3¯ + 6H+ + 6e¯

3) Equalize electrons:

6e¯ + 6MnO4¯ ---> 6MnO42¯
3H2O + I¯ ---> IO3¯ + 6H+ + 6e¯

4) Add:

3H2O + I¯ + 6MnO4¯ ---> 6MnO42¯ + IO3¯ + 6H+

5) Change to basic by adding six hydroxides to each side:

6OH¯ + 3H2O + I¯ + 6MnO4¯ ---> 6MnO42¯ + IO3¯ + 6H2O

6) Remove three H2O:

6OH¯ + I¯ + 6MnO4¯ ---> 6MnO42¯ + IO3¯ + 3H2O

Problem #25: Cu2+ + SO32¯ ---> SO42¯ + Cu

Solution:

1) Half-reactions:

SO32¯ ---> SO42¯
Cu2+ ---> Cu

2) Balance them:

H2O + SO32¯ ---> SO42¯ + 2H+ + 2e¯
2e¯ + Cu2+ ---> Cu

3) Note that I balanced the first half-reaction in acidic solution. I'll change it to basic in a moment. Since there are already an equal number of electrons on each side, we may add the two half-reactions:

H2O + SO32¯ + Cu2+ ---> SO42¯ + Cu + 2H+

4) Add two hydroxide ions to each side to make the conversion from acidic to basic:

2OH¯ + H2O + SO32¯ + Cu2+ ---> SO42¯ + Cu + 2H2O

Note that the hydrogen ions and hydroxide ions form water.

5) Cancel out one water:

2OH¯ + SO32¯ + Cu2+ ---> SO42¯ + Cu + H2O

Ten Examples      Problems 26-45      Balancing in acidic solution
Problems 1-10      Only the examples and problems      Return to Redox menu