CHAPTER 15 Electro- chemistry 15.3 Balancing Redox Equations.
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Transcript of 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