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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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review balancing chemical equations
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

using oxidation numbers

+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

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.
Steps to balancing equations
example 1
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

practice
Practice

Zn + HNO3 Zn(NO3)2 + NH4NO3 + H2O

pe 4c pg 453
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

pe 4b pg 453
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.

example 2
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