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

2. Review: balancing chemical equations
Balance the following chemical reaction:
CuCl 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.

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 equations
Write the skeleton equation
Assign oxidation numbers to all atoms and identify the atom/ion whose oxidation numbers change.
Using the change in Oxidation numbers, write the number of electrons transferred per atom.
Using the chemical formulas, determine the number of electrons transferred per reactant.
Balance the electron transfer by adding coefficients to the reactants.
Balance the O atoms by adding water molecules, balance H atoms by adding H+
If the solution is basic, add OH- to both sides to neutralize the H+, combine to for water and reduce.

5. ClO3-1 + I2 → Cl-1 + IO3-1 in an acidic solution
Practice :
ClO3-1 + I2 → Cl-1 + IO3-1 in an acidic solution

6. Practice :
CH3OH + MnO4-1  CO3-2 + MnO4-2 In a basic solution

7. Homework: Page 613 #1-4