1. 13.3b Balancing equations using oxidation numbers
C3H8O + CrO3 + H2SO4  Cr2(SO4)3 + C3H6O + H2O
13.3b Balancing equations using oxidation numbers

2. Balancing chemical equations
Balancing equations relied on having equal numbers of atoms on each side of the equation
We can balance equations using oxidation #s.
This relies on the idea that the number of electrons lost by an element must be equal to the number gained by a different element.
In other words the total gain in oxidation numbers must be equal to the total lost.

3. Using Oxidation Numbers
total
oxidation #
CuCl Al  Cu AlCl3 +2 -2 +3 -3 +2 -1 +3 -1
Notice: Cu has gained 2e– (oxidation #  by 2)
Notice: Al has lost 3e– (oxidation #  by 3)
But, number of e– gained must equal e– lost
Multiply Cu by 3, Al by 2: change is 6 for both
change
total
oxidation #
3CuCl2 + 2Al  3Cu + 2AlCl3 +6 -6 +6 +6 -1 -2 +2 +3 -3

4. Steps to balancing with ox #’s
Write the skeleton equation
Assign oxidation numbers to all atoms
Identify which atoms change oxidation number
Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients
Calculate the total change in oxidation number
Make the total increase in oxidation number equal the total decrease by multiplication using appropriate factors
Balance the remainder by inspection. Do not change what has been balanced. Compounds with elements that have changed in one case but not in another are considered twice.

5. Example 1 Balance the following equation: change total ox. #
H2SO4 + Al  Al2(SO4)3 + SO2+ H2O -2
x 3 = -6 +6 +6 +6 +4 +2 +6 -8 +6
+18
-24 +4 -4 +2 -2 +1 +6 -2 +3 +6 -2 +4 -2 +1 -2
H2SO4+ 3 3 2 3 6
Step 3: Identify which atoms change ox. #
S (+6 to +4) and Al (0 to +3)
Step 6: Make the total increase in oxidation number equal the total decrease by multiplication using appropriate factors
Step 2: Assign oxidation numbers
Step 4: Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients
Step 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here.
Step 5: Compute the total change in oxidation number
Step 1: Write equation:
already done for us

6. Example 2
Zn + HNO3  Zn(NO3)2 + NH4NO3 + H2O

7. Step 2: Assign oxidation numbers
Balance the following equation:
Zn+HNO3+ HNO3  Zn(NO3)2 + NH4NO3+ H2O +2
x 4 = +8 -8 +5 +2 -3 +1 +5 -6 +1 +5 -6 +2
+10
-12 -3 +4 +5 -6 +2 -2 +1 +5 -2 +1 +5 -2 +2 +5 -2 -3 +1 +5 -2 +1 -2 4 9 4 3
Step 2: Assign oxidation numbers
Step 6: Make the total increase in oxidation number equal the total decrease by multiplication using appropriate factors
Step 5: Compute the total change in oxidation number
Step 4: Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients
Step 1: Write equation:
already done for us
Step 3: Identify which atoms change ox. #
Zn (0 to +2) and N (+5 to -3)
Step 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here.

8. Example 3 Balance the following equation: -5 x 2 = -10 +2 x 5 = +10 +7+4 +2 +6 1 7 -8 2 6 -8 2 6 -8 2 6 -8 2 6 -8 6 18
-24 2 -2 1 7 -2 2 6 -2 1 6 -2 1 6 -2 2 6 -2 3 6 -2 1 -2 2
KMnO4+ FeSO4+ H2SO4 K2SO4+ MnSO4+ Fe2(SO4)3+ H2O 2 8 2 5 8 10
Step 4: Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients
Step 3: Identify which atoms change ox. #
Mn (+7 to +2) and Fe (+2 to +3)
Step 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here.
Step 2: Assign oxidation numbers
Step 6: Make the total increase in oxidation number equal the total decrease by multiplication using appropriate factors
Step 1: Write equation:
already done for us
Step 5: Compute the total change in oxidation number

9. Homework
LSM 13.3E