chapter 15 electro- chemistry 15.3 balancing redox equations

CHAPTER 15

Electro-chemistry

15.3 Balancing Redox Equations

2 15.3 Balancing Redox Equations

Redox reactions

Zn(s)

CuSO4(aq)

Cu(s) deposit

Zn(s) + CuSO4 → ZnSO4(aq) + Cu(s)

3 15.3 Balancing Redox Equations

Redox reactions

Zn(s) + CuSO4 → ZnSO4(aq) + Cu(s)

We saw that this is a redox reaction in which:some elements lose electrons; they are oxidized

other elements gain electrons; they are reduced

We learned how to determine oxidation numbers

4 15.3 Balancing Redox Equations

Redox reactions

Zn(s) + CuSO4 → ZnSO4(aq) + Cu(s)

We saw that this is a redox reaction in which:some elements lose electrons; they are oxidized

other elements gain electrons; they are reduced

We learned how to determine oxidation numbers

Now we look at how to balance redox reactions

5 15.3 Balancing Redox Equations

Two methods

At the end, both mass and charge have to be balanced

There are two methods:1) The oxidation number method

2) The half-reaction method

6 15.3 Balancing Redox Equations

Step 1 Assign oxidation numbers for all atoms

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Step 3 Adjust coefficients for atoms whose oxidation numbers change,

then make sure that the rule above is observed

Step 4 Check the overall mass balance

The oxidation number method

Increase in oxidation number for oxidized atoms

Decrease in oxidation number for reduced atoms

=

7 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

8 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 1 Assign oxidation numbers for all atoms

H N O3 + Cu2 O → Cu (N O3)2 + N O + H2 O

+1 +5 –2 +1 –2 +2 +5 –2 +2 –2 +1 –2

9 15.3 Balancing Redox Equations

oxidation

reduction

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

H N O3 + Cu2 O → Cu (N O3)2 + N O + H2 O

Oxidation numbers do not change

+1 +5 –2 +1 –2 +2 +5 –2 +2 –2 +1 –2

N is reduced its oxidation number goes from +5 to +2Cu is oxidized its oxidation number goes from +1 to +2

10 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 3 Balance all atoms whose oxidation numbers have changed

H N O3 + Cu2 O → 2Cu (N O3)2 + N O + H2 O

oxidation

reduction

+1 +5 –2 +1 –2 +2 +5 –2 +2 –2 +1 –2

N is already balanced: 1 atom on each sideCu needs to be adjusted with a coefficient of 2

N is reduced its oxidation number goes from +5 to +2Cu is oxidized its oxidation number goes from +1 to +2

11 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 3 Balance all atoms whose oxidation numbers have changedBalance the number of electrons (using oxidation numbers)

H N O3 + Cu2 O → 2Cu (N O3)2 + N O + H2 O

oxidation

reduction

N is reduced its oxidation number goes from +5 to +2; it gains 3 electronsCu is oxidized its oxidation number goes from +1 to +2; it loses 2 electrons

because of the coefficient

+1 +5 –2 +1 –2 +2 +5 –2 +2 –2 +1 –2

12 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 3 Balance all atoms whose oxidation numbers have changedBalance the number of electrons (using oxidation numbers)

H N O3 + Cu2 O → 2Cu (N O3)2 + N O + H2 O

–2 e–

+ 3 e–

N is reduced its oxidation number goes from +5 to +2; it gains 3 electronsCu is oxidized its oxidation number goes from +1 to +2; it loses 2 electrons

The number of electrons transferred must be the same

+1 +5 –2 +1 –2 +2 +5 –2 +2 –2 +1 –2

13 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 3 Balance all atoms whose oxidation numbers have changedBalance the number of electrons (using oxidation numbers)

2H N O3 + 3Cu2 O → 6Cu (N O3)2 + 2N O + H2 O

N is reduced its oxidation number goes from +5 to +2; it gains 3 electronsCu is oxidized its oxidation number goes from +1 to +2; it loses 2 electrons

The number of electrons transferred must be the same

(+ 3 e–) x 2 = +6 e–

+1 +5 –2 +1 –2 +2 +5 –2 +2

(–2 e–) x 3 = –6 e–

14 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 4 Check the overall mass balance

12HNO3 + 3Cu2O → 6Cu(NO3)2 + 2NO + 7H2O

H 2N 2Cu 6O 7

H 2N 14Cu 6O 39

Masses are not balanced yetStart balancing N

15 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 4 Check the overall mass balance

14HNO3 + 3Cu2O → 6Cu(NO3)2 + 2NO + 7H2O

H 14N 14Cu 6O 45

H 2N 14Cu 6O 39

Masses are not balanced yetNow balance O and H

16 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

Step 4 Check the overall mass balance

14HNO3 + 3Cu2O → 6Cu(NO3)2 + 2NO + 7H2O

H 14N 14Cu 6O 45

H 14N 14Cu 6O 45

Masses are now balanced

17 15.3 Balancing Redox Equations

Using the oxidation number method, balance the equation:

HNO3(aq) + Cu2O(s) → Cu(NO3)2(aq) + NO(g) + H2O(l)

14HNO3 + 3Cu2O → 6Cu(NO3)2 + 2NO + 7H2O

Answer:

18 15.3 Balancing Redox Equations

Two methods

At the end, both mass and charge have to be balanced

There are two methods:1) The oxidation number method

2) The half-reaction method

The redox reaction is split

into two half-reactions:

the oxidation reaction, and

the reduction reaction.

19 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

The half-reaction method

20 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

The half-reaction method

21 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

Step 3 Write down the two unbalanced half-reactions

The half-reaction method

22 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

Step 3 Write down the two unbalanced half-reactions

Step 4 Balance mass with elements other than oxygen and hydrogen

Balance oxygen by adding H2O, then balance hydrogen with H+

The half-reaction method

23 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

Step 3 Write down the two unbalanced half-reactions

Step 4 Balance mass with elements other than oxygen and hydrogen

Balance oxygen by adding H2O, then balance hydrogen with H+

Step 5 Balance the charge for both half-reactions by adding electrons

The half-reaction method

24 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

Step 3 Write down the two unbalanced half-reactions

Step 4 Balance mass with elements other than oxygen and hydrogen

Balance oxygen by adding H2O, then balance hydrogen with H+

Step 5 Balance the charge for both half-reactions by adding electrons

Step 6 Adjust coefficients to balance the number of electrons transferred

The half-reaction method

25 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

Step 3 Write down the two unbalanced half-reactions

Step 4 Balance mass with elements other than oxygen and hydrogen

Balance oxygen by adding H2O, then balance hydrogen with H+

Step 5 Balance the charge for both half-reactions by adding electrons

Step 6 Adjust coefficients to balance the number of electrons transferred

Step 7 Combine the two half-reactions, and add the spectator ions

The half-reaction method

26 15.3 Balancing Redox Equations

Step 1 Write the unbalanced equation showing explicitly all ions

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

Step 3 Write down the two unbalanced half-reactions

Step 4 Balance mass with elements other than oxygen and hydrogen

Balance oxygen by adding H2O, then balance hydrogen with H+

Step 5 Balance the charge for both half-reactions by adding electrons

Step 6 Adjust coefficients to balance the number of electrons transferred

Step 7 Combine the two half-reactions, and add the spectator ions

Step 8 Simplify and check that both mass and charge are balanced

The half-reaction method

27 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 1 Write the unbalanced equation showing explicitly all ions

Zn(s) + Cu2+(aq) + SO4–2(aq) → Zn2+(aq) + SO4

–2(aq) + Cu(s)

28 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 2 Identify the atoms that are oxidized, and atoms that are reduced

Find spectator ions (unchanged oxidation numbers)

0 +2 –2 +2 –2 0

oxidationreduction

spectator

Zn(s) + Cu2+(aq) + SO4–2(aq) → Zn2+(aq) + SO4

–2(aq) + Cu(s)

29 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 3 Write down the unbalanced half-reactions

0 +2 –2 +2 –2 0

oxidationreduction

Zn(s) + Cu2+(aq) + SO4–2(aq) → Zn2+(aq) + SO4

–2(aq) + Cu(s)

Oxidation: Reduction:

Zn(s) → Zn2+(aq) Cu2+(aq) → Cu(s)

30 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 4 Balance mass with elements other than oxygen and hydrogen

Balance oxygen by adding H2O, then balance hydrogen with H+

Zn(s) → Zn2+(aq)

mass already balanced mass already balanced

Oxidation: Reduction:

Cu2+(aq) → Cu(s)

31 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 5 Balance the charge for both half-reactions by adding electrons

Zn(s) → Zn2+(aq) + 2e–

Oxidation: Reduction:

Cu2+(aq) + 2e– → Cu(s)

32 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 6 Adjust coefficients to balance the number of electrons transferred

Zn(s) → Zn2+(aq) + 2e–

already the same in both half-reactions

Oxidation: Reduction:

Cu2+(aq) + 2e– → Cu(s)

33 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 7 Combine the two half-reactions, and add the spectator ions

Zn(s) → Zn2+(aq) + 2e–

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Oxidation: Reduction:

Cu2+(aq) + 2e– → Cu(s)

34 15.3 Balancing Redox Equations

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Step 8 Simplify and check that both mass and charge are balanced

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Using the half-reaction method, we were able to determine

that 2 electrons were transferred

The full equation could not provide us with that information

35 15.3 Balancing Redox Equations

At the end, both mass and charge have to be balanced

There are two methods:1) The oxidation number method

2) The half-reaction method