Chapter 17 Electrochemistry

1
Chapter 17Electrochemistry

• Electrochemistry deals with the relationships
  between electricity and chemical reactions.
• Oxidation-reduction (redox) reactions were
  introduced in Chapter 4
• Can be simple displacement reactions
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)
• Cu(s) 2Ag(aq) ? Cu2(aq) 2Ag(s)

2
Zn Sn2 ? Zn2 Sn

3
Zn Cu2 ? Zn2 Cu

4
Oxidation-Reduction Reactions

• Redox reactions can also be more complex, with
  structural and composition changes as well as an
  exchange of electrons.
• 5VO2(aq) MnO4-(aq) H2O(l) ?
• 5VO2(aq) Mn2(aq) 2H(aq)
• 5Fe2(aq) MnO4-(aq) 8H(aq) ?
• 5Fe3(aq) Mn2(aq) 4H2O(l)

21m03vd1
5
Oxidation and ReductionYou cant have one
without the other!

• Oxidation
• Decrease in number of electrons
• (loss of electrons)
• Increase in oxidation number
• Oxidation number -3 -2 -1 0 1 2 3
• Increase in number of electrons
• (gain of electrons)
• Decrease in oxidation number
• Reduction

6
Half-Reactions

• Oxidation-reduction reactions can be written as
  separate oxidation and reduction reactions,
  called half-reactions.
• Oxidation
• VO2(aq) H2O(l) ? VO2(aq) 2H(aq) e-
• VO2 is called the reducing agent (or reductant),
  because it causes the reduction of another
  substance the reducing agent is oxidized in the
  process

7
Half-Reactions

• Reduction
• MnO4-(aq) 8H(aq) 5e- ? Mn2(aq) 4H2O(l)
• MnO4- is called the oxidizing agent (or oxidant),
  because it causes the oxidation of another
  substance the oxidizing agent is reduced in the
  process
• Oxidation-reduction can be considered to be the
  competition between two substances for electrons.
  The one with the greater attraction for
  additional electrons becomes the oxidizing agent
  the one with the least is the reducing agent.

8
Spontaneous Oxidation-Reduction Reactions

• If we place a metal in a solution of another
  metal ion, sometimes we get metal deposition,
  sometimes not. How do we decide?
• Zn Sn2 ? Zn2 Sn
• Sn Zn2 ? no rxn

9
Spontaneous Redox

• How do we know which substances will act as
  oxidizing agents or as reducing agents?
• An activity series gives the relative activity of
  substances as oxidizing or reducing agents.
  Review from Chapter 4.
• Determine an activity series in several ways
• activity in displacing H2 from water
• activity in displacing metals from metal ion
  solutions (more active metal displaces a less
  active metal from solution)
• generation of an electrochemical potential or
  voltage

10
Spontaneous Redox

• Na displaces H2 from H2O
• Zn displaces Pb from a
• solution of Pb2

11
Spontaneous Redox

• Pb 2Ag ? Pb2 2Ag
• Ag Pb2 ? no rxn

12
Spontaneous Redox

• The reverse reactions can be made to occur with
  electric current, but they are not spontaneous.
• General rule, using an activity series
• stronger stronger weaker weaker
• oxidizing reducing ? reducing oxidizing
• agent agent agent
  agent
• Depending on the relative strengths, the reaction
  can go to completion, or reach a state of
  equilibrium.

13
Activity Series
14
Group Work

• Predict which of the following combinations will
  undergo an oxidation-reduction reaction
• Mg K
• Mg Zn2
• H2 Ni2
• H2 Cu2

15
Group Quiz

• Predict which of the following combinations will
  undergo an oxidation-reduction reaction
• Sn Pb2
• Na H2O
• Pb H2O
• Co H(aq)
• Ag H(aq)

16
20.2 Balancing Oxidation-Reduction Equations

• Some redox equations can be balanced by
  inspection, just like other types of reactions.
• Zn CuCl2 ? ZnCl2 Cu
• Net ionic equation
• Zn Cu2 ? Zn2 Cu
• Need to balance atoms and charge
• What is wrong with the following?
• Al Cu2 ? Al3 Cu
• (Al-can demo)

17
Balancing Equations

• 5Cr3 3MnO4- 8H2O ? 5CrO42- 3Mn2 16H
• How do we balance an equation as complex as this?
• Two methods
• half-reaction method
• oxidation number change method
• Will focus on the half-reaction method since it
  is useful in more circumstances.

18
Half-Reaction Method

• Write an unbalanced half-reaction for either
  oxidation or reduction.
• Balance the half-reaction
• a. Balance all atoms other than H and O.
• b. Balance O by adding H2O to the equation.
• c. Balance H by adding H to the equation.
• d. Balance ionic charges by adding electrons to
  the equation.
• e. If in basic solution, add OH- to each side of
  the equation to neutralize all the H.
• f. Cancel any H2O occurring on both sides.

19
Half-Reaction Method continued

• Write an unbalanced half-reaction for the other
  process.
• Balance by the same procedure.
• Equalize the number of electrons lost and gained
  by multiplying each coefficient in each
  half-reaction by the appropriate constant.
• Add the two half-reactions and cancel equal
  amounts of anything occurring on both sides.
• Make a final check of atom and charge balances.

20
Balancing Equations

• Cr3 MnO4- ? CrO42- Mn2
• How do we balance this equation in acidic
  solution?
• 1. Cr3 ? CrO42-
• 2a. Cr already balanced
• 2b. Cr3 4H2O ? CrO42-
• 2c. Cr3 4H2O ? CrO42- 8H
• 2d. Cr3 4H2O ? CrO42- 8H 3e-

21
Balancing Equations

• 3. MnO4- ? Mn2
• 4a. Mn already balanced
• 4b. MnO4- ? Mn2 4H2O
• 4c. MnO4- 8H ? Mn2 4H2O
• 4d. MnO4- 8H 5e- ? Mn2 4H2O
• 5. 3 e- lost, but 5 e- gained. Equalize by
  multiplying half-reactions
• 5(Cr3 4H2O ? CrO42- 8H 3e-)
• 3(MnO4- 8H 5e- ? Mn2 4H2O)

22
Balancing Equations

• 5Cr3 20H2O ? 5CrO42- 40H 15e-
• 3MnO4- 24H 15e- ? 3Mn2 12H2O
• 6. Add the equations
• 5Cr3 3MnO4- 20H2O 24H 15e- ? 5CrO42-
  3Mn2 12H2O 40H 15e-
• Cancel any substances on both sides of the
  equation
• 5Cr3 3MnO4- 8H2O ? 5CrO42- 3Mn2 16H
• 7. Check atom and charge balance
• 5 Cr, 3 Mn, 20 O, 16 H, 12 charges on each side

23
Balancing Equations in Basic Solution

• Neutralize any H with OH- to form H2O and add
  the same number of OH- to the other side of the
  equation.
• Cr3 4H2O ? CrO42- 8H 3e-
• Add 8OH- to each side of the equation
• Cr3 4H2O 8OH- ? CrO42- 8H 8OH- 3e-
• Form water by neutralization
• Cr3 4H2O 8OH- ? CrO42- 8H2O 3e-
• Cancel any water occurring on both sides
• Cr3 8OH- ? CrO42- 4H2O 3e-

24
Group Work

• Balance the following equation in acidic
  solution
• MnO4- Cl- ? Mn2 ClO3-
• Answer
• MnO4- 8H 5e- ? Mn2 4H2O
• Cl- 3H2O ? ClO3- 6H 6e-
• 6MnO4- 5Cl- 18H ?
• 6Mn2 5ClO3- 9H2O

25
Group Quiz

• Balance the following chemical equation in acidic
  solution
• VO2 CrO42- ? VO2 Cr3
• 3VO2 CrO42- 2H ? 3VO2 Cr3 H2O
• 3V, 1Cr, 7O, 2H, 6 charges on each side