Balance the equation for each of the following reactions using either the oxidat

Question

Balance the equation for each of the following reactions using either the oxidation number or the half-reaction method. Required conditions are indicated in brackets. State which reactant is the oxidizing agent and which is the reducing agent.

a) Au3+(aq) + SO2(aq) --> SO42-(aq) + Au(s) (acidic)

b) Ag(s) + NO3-(aq) --> Ag+(aq) + NO(g) (acidic)

c) Zn(s) + BrO4-(aq) --> Zn(OH)42-(aq) + Br-(aq) (basic)

d) ClO-(aq) --> ClO2-(aq) + Cl2(g) (basic)

e) S2O32-(aq) + OCl-(aq) --> S4O62-(aq) + Cl-(aq) (acidic)

Explanation / Answer

Part a

Au3+(aq) + SO2(aq) --> SO42-(aq) + Au(s)

Write the oxidation number of each atom.

Au(+3)3+ + S(+4)O(-2)2 S(+6)O(-2)4 2- + Au(0)

Write half reactions

Oxidation

S(+4)O(-2)2 S(+6)O(-2)4 2-

Reduction

Au(+3)3+ Au(0)

Balance the atoms except H and O

Oxidation

SO2 SO42-

Reduction

Au3+ Au

Balance O atoms by adding H2O

Oxidation :

SO2 + 2H2O SO4 2-

Reduction :

Au3+ Au

Balance H atoms by adding H+

Oxidation :

SO2 + 2H2O SO4 2- + 4H+

Reduction

Au3+ Au

Balance the charge

O:

SO2 + 2H2O SO4 2- + 4H+ + 2e-

R:

Au3+ + 3e- Au

Make electron balance

O: SO2 + 2H2O SO42- + 4H+ + 2e- multiply by *3

R: Au3+ + 3e- Au multiply by *2

O: 3SO2 + 6H2O 3 SO4 2- + 12H+ + 6e-

R: 2Au3+ + 6e- 2Au

Add the reactions

3SO2 + 2Au3+ + 6H2O + 6e- 3 SO4 2- + 2Au + 12H+ + 6e-

Simplify the equation

3SO2 + 2Au3+ + 6H2O 3 SO4 2- + 2Au + 12H+

Part b

Ag(s) + NO3-(aq) --> Ag+(aq) + NO(g)

Write the oxidation number of each atom.

Ag(0) + N(+5)O(-2)3- Ag(+1)+ + N(+2)O(-2)

Write half reactions

Oxidation

Ag(0) Ag(+1)+

Reduction

R:

N(+5)O(-2)3- N(+2)O(-2)

Balance the atoms except H and O

Oxidation

  Ag Ag+

Reduction

NO3- NO

Balance O atoms by adding H2O

Oxidation :

Ag Ag+

Reduction :

NO3- NO + 2H2O

Balance H atoms by adding H+

Oxidation :

Ag Ag+

Reduction

NO3- + 4H+ NO + 2H2O

Balance the charge

O:

Ag Ag+ + e-

R:

NO3- + 4H+ + 3e- NO + 2H2O

Make electron balance

O:

Ag Ag+ + e-

| *3

R:

NO3- + 4H+ + 3e- NO + 2H2O

| *1

O:

3Ag 3Ag+ + 3e-

R:

NO3- + 4H+ + 3e- NO + 2H2O

Add the reactions

3Ag + NO3- + 4H+ + 3e- 3Ag+ + NO + 3e-+ 2H2O

Simplify the equation

3Ag + NO3- + 4H+ 3Ag+ + NO + 2H2O

Part C

Zn(s) + BrO4-(aq) --> Zn(OH)42-(aq) + Br-(aq) (basic)

Write the oxidation number of each atom.

Zn(0) + Br(+7)O(-2)4- Zn(+2)[O(-2)H(+1)]4 2- + Br(-1)-

Write half reactions

O:

Zn(0) Zn(+2)[O(-2)H(+1)]4 2-

R:

Br(+7)O(-2)4- Br(-1)-

Balance the atoms except H and O

O:

Zn Zn(OH)4 2-

R:

BrO4- Br-

Balance O atoms by adding H2O

O:

Zn + 4H2O Zn(OH)4 2-

R:

BrO4- Br- + 4H2O

Balance H atoms by adding H+

O:

Zn + 4H2O Zn(OH)4 2- + 4H+

R:

BrO4- + 8H+ Br- + 4H2O

In basic medium, for every H+ ion, add OH- ion

O: Zn + 4H2O + 4OH- Zn(OH)4 2- + 4H2O

R: BrO4- + 8H2O Br- + 4H2O + 8OH-

Balance the charge

O:

Zn + 4H2O + 4OH- Zn(OH)4 2- + 4H2O + 2e-

R:

BrO4- + 8H2O + 8e- Br- + 4H2O + 8OH-

Make electron balance

O:

Zn + 4H2O + 4OH- Zn(OH)42- + 4H2O + 2e-

| *4

R:

BrO4- + 8H2O + 8e- Br- + 4H2O + 8OH-

| *1

O:

4Zn + 16H2O + 16OH- 4Zn(OH)42- + 16H2O + 8e-

R:

BrO4- + 8H2O + 8e- Br- + 4H2O + 8OH-

Add the reactions

4Zn + BrO4- + 24H2O + 16OH- + 8e- 4Zn(OH)42- + Br- + 20H2O + 8e- + 8OH-

Simplify the equation

4Zn + BrO4- + 4H2O + 8OH- 4Zn(OH)42-+ Br-

Part d

ClO-(aq) --> ClO2-(aq) + Cl2(g) (basic)

Write the oxidation number of each atom.

Cl(+1)O(-2)- Cl(+3)O(-2)2- + Cl(0)2

Write half reactions

O:

Cl(+1)O(-2)- Cl(+3)O(-2)2-

R:

Cl(+1)O(-2)- Cl(0)2

Balance the atoms except H and O

O:

ClO- ClO2-

R:

2ClO- Cl2

Balance O atoms by adding H2O

O:

ClO- + H2O ClO2-

R:

2ClO- Cl2 + 2H2O

Balance H atoms by adding H+

O:

ClO- + H2O ClO2- + 2H+

R:

2ClO- + 4H+ Cl2 + 2H2O

In basic medium, for every H+ ion, add OH- ion

O:

ClO- + H2O + 2OH- ClO2- + 2H2O

R:

2ClO- + 4H2O Cl2 + 2H2O + 4OH-

Balance the charge

O:

ClO- + H2O + 2OH- ClO2- + 2H2O + 2e-

R:

2ClO- + 4H2O + 2e- Cl2 + 2H2O + 4OH-

Make electron balance

O:

ClO- + H2O + 2OH- ClO2- + 2H2O + 2e-

| *1

R:

2ClO- + 4H2O + 2e- Cl2 + 2H2O + 4OH-

| *1

O:

ClO- + H2O + 2OH- ClO2- + 2H2O + 2e-

R:

2ClO- + 4H2O + 2e- Cl2 + 2H2O + 4OH-

Add the reactions

de.

3ClO- + 5H2O + 2OH- + 2e- ClO2- + Cl2 + 4H2O + 2e- + 4OH-

Step

Simplify the equation

.

3ClO- + H2O ClO2- + Cl2 + 2OH-

Finally, always

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