Chapter 19: Oxidation-Reduction Reactions

Section 2: Balancing Redox Equations

Objectives

• Explain what must be conserved in redox equations.

• Balance redox equations by using the half-reaction method.

Half-Reaction Method• The half-reaction method for

balancing redox equations consists of seven steps:

› 1. Write the formula equation if it is not given in the problem. Then write the ionic equation.

› 2. Assign oxidation numbers. Delete substances containing only elements that do not change oxidation state.

Half-Reaction Method, continued

› 3. Write the half-reaction for oxidation.• Balance the atoms.• Balance the charge.

› 4. Write the half-reaction for reduction.• Balance the atoms.• Balance the charge.

Half-Reaction Method, continued

› 5. Conserve charge by adjusting the coefficients in front of the electrons so that the number lost in oxidation equals the number gained in reduction.

› 6. Combine the half-reactions, and cancel out anything common to both sides of the equation.

› 7. Combine ions to form the compounds shown in the original formula equation. Check to ensure that all other ions balance.

Balancing Redox Equations Using the Half-Reaction Method

Balancing Redox Equations Using the Half-Reaction Method

Half-Reaction Method, continued

Sample Problem AA deep purple solution of potassium permanganate is titrated with a colorless solution of iron(II) sulfate and sulfuric acid. The products are iron(III) sulfate, manganese(II) sulfate, potassium sulfate, and water—all of which are colorless. Write a balanced equation for this reaction.

Half-Reaction Method, continued

Sample Problem A Solution 1. Write the formula equation if it is

not given in the problem. Then write the ionic equation.

4 4 2 4

2 4 3 4 2 4 2

KMnO + FeSO + H SO

Fe (SO ) + MnSO + K SO + H O

+ 2+ 2 + 24 4 4

3+ 2 2+ 2 + 24 4 4 2

K + MnO +Fe + SO + 2H + SO

2Fe + 3SO + Mn + SO + 2K + SO + H O

– – –

– – –

2. Assign oxidation numbers to each element and ion. Delete substances containing an element that does not change oxidation state.+1 +7 2 +2 +6 2 +1 +6 2

+3 +6

+ 2+ 2 + 2–

4 4 4

3+ 2– 2+ 2– + 2

4 4

2 +2 +6 2 +1 +6 2 +

4 2

1 2

K + MnO +Fe + SO + 2H + SO

2Fe + 3SO + Mn + SO + 2K + SO + H O

– – –

– ––

– –

– –

+7 2 +2 +3 +2 2+ 3+ 2+

4MnO + Fe Fe + Mn–

–

Sample Problem A Solution, continued

Only ions or molecules whose oxidation numbers change are retained.

3. Write the half-reaction for oxidation. The iron shows the increase in oxidation number. Therefore, it is oxidized.

• Balance the mass. • The mass is already balanced.

• Balance the charge.

+2 +2 3+

3+Fe Fe

2

+2+ 3+

+3

Fe Fe + e –

Sample Problem A Solution, continued

4. Write the half-reaction for reduction. Manganese is reduced.

+7 + 2

4 2

2+

+

MnO + 8H + 5 Mn + 4H Oe – –

+7

4

+22+MnO Mn–

+

+7 +2 2+

4 2MnO + 8H Mn + 4H O–

Balance the charge.

Balance the mass. Water and hydrogen ions must be added to balance the

oxygen atoms in the permanganate ion.

Sample Problem A Solution, continued

5. Adjust the coefficients to conserve charge.

+ 2+4 21(MnO + 8H + 5 Mn + 4H O)e – –

2+ 3+5(Fe Fe + )e –

lost in oxidation 1

gained in reduction 5

e

e

–

–

Sample Problem A Solution, continued

Sample Problem A Solution, continued

6. Combine the half-reactions and cancel.

+ 2+4 2MnO + 8H + 5 Mn + 4H Oe – –

2+ 3+Fe Fe + e –

2+ + 2+ 3+4 2MnO + 5Fe + 8H + 5 Mn + 5Fe + 4H O + 5e e– – –

Chapter 19Section 2 Balancing Redox Equations

Half-Reaction Method, continued

Sample Problem A Solution, continued

7. Combine ions to form compounds from the original equation.

2+ + 3+ 2+4 22(5Fe + MnO + 8H 5Fe + Mn + 4H O)–

2+ + 3+ 2+4 210Fe + 2MnO + 16H 10Fe + 2Mn + 8H O

4 4 2 4

2 4 3 4 2 4 2

10FeSO + 2KMnO + 8H SO

5Fe (SO ) + 2MnSO + K SO + 8H O

Chapter 19Section 2 Balancing Redox Equations

Half-Reaction Method, continued