The document provides steps for balancing chemical equations using oxidation numbers: 1. Write out reactants and products and assign oxidation numbers. 2. Identify atoms whose oxidation numbers change during oxidation and reduction reactions. 3. Write the electron transfers involved in oxidation and reduction. 4. Balance the overall electron transfers between reactants and products. 5. Insert the balanced electron transfer numbers into the chemical equation. 6. Balance the overall chemical equation by inspection, balancing hydrogen last.

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

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. 1. Assign oxidation numbers
2. Select those which change
3. (a) Assign Oxidation – increase in Ox. No
(b) Assign Reduction – decrease in Ox. No
3 C2H5OH + 2 Cr2O72-+16 H+ = 3 CH3COOH + 4 Cr3++11 H2O
-2+1 -2+1
-2
+6 -2
+6
+1
0 +1 0 -2-2+1
0
Oxidation
5. Write electron gains and losses
0
+3
+1-2
+3
Reduction
6. Balance electron transfers
2
3 C2 - 12 e- = 3 C2 x3
4
-2
0
4
2 Cr2O72- + 12 e- = 2 Cr3+ x2
6
3
7. Put numbers up into the equation
+6
+3
8. 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