Balancing Redox Equations

Balancing Redox Equations

In balancing redox equations, the # of electrons lost in oxidation (the increase in ox. #) must equal the # of electrons gained in reduction (the decrease in ox. #)

There are 2 methods for balancing redox equations:1. Change in Oxidation-Number Method2. The Half-Reaction Method

1. Change in Oxidation-Number Method:

based on equal total increases and decreases in oxidation #’s

Steps: page 645 in textbook

1. Write equation and assign oxidation #’s.

2. Determine which element is oxidized and which is reduced, and determine the change in oxidation # for each.

3. Connect the atoms that change ox. #’s using a bracket; write the change in ox. # at the midpoint of each bracket.

4. Choose coefficients that make the total increase in ox. # = the total decrease in ox. #.

5. Balance the remaining elements by inspection.

Example

S + HNO3 SO2 + NO + H2O

Step 1:

Oxidation numbers

0 +1 +5 -2 +4 -2 +2 -2 +1 -2

Step 2:

Which was oxidized?

S. By how much? 0 to +4 = change of +4

Which was reduced? N. By how much? +5 to +2 = change of -3

+4

-3

3 4 3 4 2

So, 3 coefficient x +4 = +12

So, 4 coefficient x -3 = -12

If needed, reactions that take place in acidic or basic solutions can be balanced as follows:

Acidic: Basic:

add H2O to the side needing oxygen

balance as if in acidic sol’n

then add H+ to balance the hydrogen

add enough OH- to both sides to cancel out each H+ (making H2O) & then cancel out water as appropriate

Example: Balance the following equation, assuming it takes place in acidic solution. Page 648 in textbook

ClO4- + I- Cl- + I2

Step 1: Oxidation numbers

+7 -2 -1 -1 0

Step 2: Which was oxidized? Iodine, -1 to 0 = +1

Which was reduced? Chlorine, +7 to -1 = -8

+1

-8

8 4

Step 5: Balance acid soln with water…

+ H2O +8 H+ 4

Example: Balance the following equation, assuming it takes place in basic solution.

ClO4- + I- Cl- + I2

Cancel H+ by adding OH- to both sides.

H+ + OH- = H2O

Cancel H2O

2. The Half-Reaction Method:

separate and balance the oxidation and reduction half-reactions.

Steps:1. Write equation and assign oxidation #’s.2. Determine which element is oxidized and which is reduced,

and determine the change in oxidation # for each.3. Construct unbalanced oxidation and reduction half reactions.4. Balance the elements and the charges (by adding electrons

as reactants or products) in each half-reaction.5. Balance the electron transfer by multiplying the balanced half-

reaction by appropriate integers.6. Add the resulting half-reaction and eliminate any common

terms to obtain the balanced equation.

Example: Balance the following using the half-reaction method:

HNO3 + H2S NO + S +H2O

Example: Balance the following using the half-reaction method:

HNO3 + H2S NO + S +H2O

Step 1: oxidation numbers

+1 +5 +2+1 +1-2 -2 -2-2 0

Step 2: Which was oxidized?

S. -2 to 0 = +2

Which was reduced?

N. +5 to +2 = -3

Step 3: unbalanced half-rxns

S2- S

N5+ N2+

Step 4: balance the half – rxns by adding electrons

+ 2 e-

+ 3 e-

Step 5: balance electron transfer by multiplying by appropriate integers

x3

x2

3S2- 3S + 6e-

2 N5+ + 6e- 2 N2+

Step 6: Add half-rxns and cancel any common terms to obtain a balanced eq.

----------------------------------

3S2- + 2N5+ 3S + 2N2+

Now, balance the eq. w/coefficients

2 3 2 3 4

If needed, reactions that take place in acidic solutions can be balanced as follows:

Acidic: 1. Write separate eq’ns for the oxidation & reduction half-rxns2. For each half-rxn:

a) Balance all the elements except H and Ob) Balance O using H2O

c) Balance H using H+

d) Balance charge using elections3. If necessary, multiply one or both balanced half-rxns by an integer

to equalize the number of electrons transferred in the two half-rxns.4. Add the half-reactions and cancel the identical species (those

appearing in reactants and products)5. Check that the elements and charges are balanced

If needed, reactions that take place in basic solutions can be balanced as follows:

Basic:

1. Balance as if in acidic sol’n (follow ALL steps for acidic redox)

2. Add enough OH- to both sides to cancel out each H+ (making H2O) & then cancel out water as appropriate

3. Check that the elements and charges are balanced

HOMEWORK:

Balance the following using the

half-rxn method…

In acidic sol’n:

a) Cu + NO3- Cu2+ + NO

b) Cr2O72- + Cl- Cr3+ + Cl2

c) Pb + PbO2 + H2SO4 PbSO4

In basic sol’n:

a) Al + MnO4- MnO2 + Al(OH)4

-

b) Cl2 Cl- + OCl-

c) NO2- + Al NH3 + AlO2

-