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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. Balancing equations using oxidation numbers . Review: balancing chemical equations. In the past (e.g. in grade 11) we balanced equations by “inspection”.

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Balancing equations using oxidation numbers

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

  2. Review: balancing chemical equations • 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. • Balance the following chemical reaction: CuCl2 + Al  Cu + AlCl3

  3. +2 -2 0 0 +3 -3 +2 -1 0 0 +3 -1 Using Oxidation Numbers +2 -2 0 0 +3 -3 +2 -1 0 0 +3 -1 total oxidation # 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 • change • total • oxidation # • 3CuCl2 + 2Al  3Cu + 2AlCl3 +6 -6 +6 0 0 +6

  4. 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. Steps to balancing equations

  5. Example 1 -2 x 3 = -6 +6 Balance the following equation: change total ox. # H2SO4 + Al  Al2(SO4)3 + SO2 + H2O +6 0 +6 +4 +2 +6 -8 0 +6 +18 -24 +4 -4 +2 -2 +1 +6 -2 0 +3 +6 -2 +4 -2 +1 -2 H2SO4+ 3 2 6 3 3 Step 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here. 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 3: Identify which atoms change ox. # S (+6 to +4) and Al (0 to +3) Step 1: Write equation: already done for us Step 2: Assign oxidation numbers

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

  7. PE 4c (pg. 453) +2 x 4 = +8 -8 Balance the following equation: Zn+HNO3+ HNO3  Zn(NO3)2+NH4NO3+ H2O 0 +5 +2 -3 +1 +5 -6 +1 +5 -6 +2 +10 -12 -3 +4 +5 -6 +2 -2 0 +1 +5 -2 +1 +5 -2 +2 +5 -2 -3 +1 +5 -2 +1 -2 0 9 4 4 3 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) 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 4: Make the number of atoms that change oxidation number the same on both sides by inserting temporary coefficients

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

  9. PE 4b (pg. 453) -5 x 2 = -10 +2 x 5 = +10 Balance the following equation: +7 +2 +6 +4 2 -2 1 7 -8 2 6 -8 2 6 -8 2 6 -8 2 6 -8 6 18 -24 1 -2 1 7 -2 2 6 -2 1 6 -2 1 6 -2 2 6 -2 3 6 -2 2 2 KMnO4+ FeSO4+ H2SO4 K2SO4+ MnSO4+ Fe2(SO4)3+ H2O 8 2 5 8 10 Step 3: Identify which atoms change ox. # Mn (+7 to +2) and Fe (+2 to +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 1: Write equation: already done for us 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 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here.

  10. Example 2 -2 x 3 = -6 +6 Balance the following equation: +6 0 +6 +4 1 7 -8 0 +6 +18 -24 +4 -4 +2 -2 2 6 -8 1 7 -2 0 +3 +6 -2 +4 -2 +2 -2 1 6 -2 KMnO4+ H2C2O4+ H2SO4 CO2+ K2SO4+ MnSO4+ H2O 2 3 3 3 Step 2: Assign oxidation numbers Step 1: Write equation: already done for us Step 7: Balance the remainder by inspection. Note: only compounds that have not already been balanced need to be balanced here. 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 3: Identify which atoms change ox. # S (+6 to +4) and Al (0 to +3) Try PE 4 a, b (p 453). Try 12.21 (p 484) For more lessons, visit www.chalkbored.com

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