1. 2.6.2 Balancing equations using Oxidation Numbers

2. Chemical reactions are all about the movement of electrons
When an equation is balanced the electron movements must balance
Sometimes it is possible to balance an equation in more than one way
It is therefore possible to make the numbers of atoms balance but not the electron movement
Using oxidation numbers prevents this

3. Sequence of events
Write out the formulae of all reactants and products
Assign oxidation numbers to all atoms
Pick out the atoms whose oxidation numbers change
Write down the electron movements
Balance the electron movements
Enter the numbers from balanced electron movement into the overall equation
Balance equation by inspection - [do H last]

4. MnO4- + Fe2+ + H+ = Mn2+ + Fe3+ + H2O +7 -2 +2 +1 +2 +3 +1 -2 5 8 5 4
1. Assign oxidation numbers
(a) Assign Oxidation – increase in Ox. No
(b) Assign Reduction – decrease in Ox. No
2. Select those which change
MnO4- + Fe2+ + H+ = Mn2+ + Fe3+ + H2O 5 8 5 4 +7 +2 +2 +3
Oxidation
Reduction
5. Write electron gains and losses
6. Balance electron transfers 5
Fe = Fe3+
- 1 e- 5 5
Mn = Mn2+
+ 5 e-
7. Put numbers up into the equation
8. Balance equation by inspection - leave H+ till last

5. Sequence of events Write down reactants and products
Assign oxidation numbers
Identify oxidation and reduction
Write down changes and electrons involves
Balance e- change
Put changes into equation
Balance equation by inspection [H last]

6. x2 3 C2H5OH + Cr2O72-+ H+ = CH3COOH + Cr3++ H2O 2 16 3 4 11 -2 +6 +3
1. Assign oxidation numbers
(a) Assign Oxidation – increase in Ox. No
(b) Assign Reduction – decrease in Ox. No
2. Select those which change 3
C2H5OH + Cr2O H+ = CH3COOH + Cr H2O 2 16 3 4 11 -2 +6 +3
Reduction
Oxidation
5. Write electron gains and losses
6. Balance electron transfers 3 12 4 3 x3
C = C2 e- 2 x2
Cr2O = Cr3+ e- 12 6 2 4
7. Put numbers up into the equation
Balance equation by inspection - leave H+ till last :- O 3+(2*7) = (3*2) + x  x=11
H  3(6*1) + X = (3*4) + 11(2*1)  x= 16

7. Examples C2H5OH + MnO41- + H+ = CH3CHO + Mn2+ + H2O
C2H5OH + MnO41- + H+ = CH3COOH + Mn2+ + H2O
Cr2O72- + Fe2+ + H+ = Cr3+ + Fe3+ + H2O
MnO41- + C2O42- + H+ = CO2 + Mn2+ + H2O
Fe2+ + Cl2 = Fe Cl-
MnO41- + H2O2 + H+ = Mn2+ + O2 +H2O
Cr2O72- + CH3OH + H+ = Cr3+ + HCHO + H2O
Cr2O72- + CH3CHO + H+ = Cr3+ + CH3COOH + H2O

8. Answers 5C2H5OH + 2MnO41- + 6H+ = 5CH3CHO + 2Mn2+ + 8H2O
5C2H5OH + 4MnO H+ = 5CH3COOH + 4Mn H2O
Cr2O Fe H+ = 2Cr3+ + 6Fe3+ + 7H2O
2MnO C2O H+ = 10CO2 + 2Mn2+ + 8H2O
2Fe2+ + Cl2 = 2Fe Cl-
2MnO H2O2 + 8H+ = 2Mn2+ + 5O2 +8H2O
Cr2O72- + CH3OH + H+ = Cr3+ + HCHO + H2O
Cr2O72- + CH3CHO + H+ = Cr3+ + CH3COOH + H2O