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Balancing equations using oxidation numbers C 3 H 8 O + CrO 3 + H 2 SO 4 Cr 2 (SO 4 ) 3 + C 3 H 6 O + H 2 O

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Review: balancing chemical equations Balance the following chemical reaction: CuCl 2 + Al Cu + AlCl 3 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.

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Using Oxidation Numbers total oxidation # CuCl 2 + Al Cu + AlCl 3 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 # 3CuCl 2 + 2Al 3Cu + 2AlCl

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Steps to balancing equations 1.Write the skeleton equation 2.Assign oxidation numbers to all atoms 3.Identify which atoms change oxidation number 4.Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients 5.Compute the total change in oxidation number 6.Make the total increase in oxidation number equal the total decrease by multiplication using appropriate factors 7.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.

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Step 2: Assign oxidation numbersStep 3: Identify which atoms change ox. # S (+6 to +4) and Al (0 to +3) Step 4: Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients Step 5: Compute the total change in oxidation number 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. Example 1 Balance the following equation: change total ox. # H 2 SO 4 + Al Al 2 (SO 4 ) 3 + SO 2 + H 2 O Step 1: Write equation: already done for us x 3 = H 2 SO 4 + 3

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Zn + HNO 3 Zn(NO 3 ) 2 + NH 4 NO 3 + H 2 O Practice

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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 2: Assign oxidation numbersStep 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here. Step 3: Identify which atoms change ox. # Zn (0 to +2) and N (+5 to -3) Step 5: Compute the total change in oxidation number Step 1: Write equation: already done for us PE4c (pg. 453) Balance the following equation: Zn + HNO 3 + HNO 3 Zn(NO 3 ) 2 + NH 4 NO 3 + H 2 O x 4 =

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

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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 5: Compute the total change in oxidation number Step 1: Write equation: already done for us Step 6: Make the total increase in oxidation number equal the total decrease by multiplication using appropriate factors KMnO 4 + FeSO 4 + H 2 SO 4 K 2 SO 4 + MnSO 4 + Fe 2 (SO 4 ) 3 + H 2 O Step 2: Assign oxidation numbersStep 3: Identify which atoms change ox. # Mn (+7 to +2) and Fe (+2 to +3) PE4b (pg. 453) Balance the following equation: x 2 = x 5 =

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Step 2: Assign oxidation numbersStep 3: Identify which atoms change ox. # S (+6 to +4) and Al (0 to +3) Step 4: Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients Step 5: Compute the total change in oxidation number 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. Example 2 Balance the following equation: Step 1: Write equation: already done for us x 3 = Try PE 4 a, b (p 453). Try (p 484) KMnO 4 + H 2 C 2 O 4 + H 2 SO 4 CO 2 + K 2 SO 4 + MnSO 4 + H 2 O For more lessons, visit

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