# Balancing equations using oxidation numbers

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

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 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.

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

Steps to balancing equations
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 Compute 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.

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 1: Write equation: already done for us Step 2: Assign oxidation numbers 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 4: Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients

Practice Zn + HNO3  Zn(NO3)2 + NH4NO3 + H2O

PE 4c (pg. 453) 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 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here. 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 5: Compute the total change in oxidation number Step 3: Identify which atoms change ox. # Zn (0 to +2) and N (+5 to -3)

Assignment: PE 4b (pg. 453) & 12.21 (pg. 484)

PE 4b (pg. 453) Balance the following equation: -5 x 2 = -10 +2
+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

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Example 2 Balance the following equation: -2 x 3 = -6 +6 +6 +6 +4 1 7 -8 2 6 -8 +6 +18 -24 +4 -4 +2 -2 1 7 -2 1 6 -2 +3 +6 -2 +4 -2 +2 -2 KMnO4+ H2C2O4+ H2SO4 CO2+ K2SO4+ MnSO4+ H2O 2 3 3 3 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 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 2: Assign oxidation numbers Step 3: Identify which atoms change ox. # S (+6 to +4) and Al (0 to +3) Step 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here. Try PE 4 a, b (p 453). Try (p 484) For more lessons, visit