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C3H8O + CrO3 + H2SO4 
Cr2(SO4)3 + C3H6O + H2O
Balancing equations using
oxidation numbers
Review: balancing chemical equations
• Balance the following chemical reaction:
CuCl2 + Al  Cu + AlCl3
• In the past (e.g. in grade 11) we balanced
equations by “inspection”.
• Balancing equations relied on having equal
numbers of atoms on each side of the equation
• We now also concerned with transfer of
electrons or the balance of charge
• 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.
Using Oxidation Numbers
+2 -2
0
0
+3 -3
+2 -1
0
0
+3 -1
CuCl2 + Al  Cu + AlCl3
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
+6
-6
change
+6
0
0
+6
total
+2 -2
0
0
+3 -3
0
0
+3 -1
oxidation # +2 -1
3CuCl2 + 2Al  3Cu + 2AlCl3
total
oxidation #
•
•
•
•
Steps to balancing equations
1. Write the skeleton equation
2. Assign oxidation numbers to all atoms and identify
the atom/ion whose oxidation numbers change.
3. Using the change in Oxidation numbers, write the
number of electrons transferred per atom.
4. Using the chemical formulas, determine the number
of electrons transferred per reactant.
5. Balance the electron transfer by adding coefficients
to the reactants.
6. Balance the O atoms by adding water molecules,
balance H atoms by adding H+
7. If the solution is basic, add OH- to both sides to
neutralize the H+, combine to for water and reduce.
Practice :
ClO3-1 + I2 → Cl-1 + IO3-1 in an acidic solution
Practice :
CH3OH + MnO4-1  CO3-2 + MnO4-2 In a basic solution
Homework:
Page 613 #1-4