1

2

3

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

4

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

Zinc atoms enter the solution as Zn2+ ions.

5

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

Zinc atoms enter the solution as Zn2+ ions. Cu2+ ions convert to Cu atoms which deposit on the Zn metal.

6

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

Zinc atoms enter the solution as Zn2+ ions. Cu2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn(s) Zn2+

(aq) + 2 e-

7

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

Zinc atoms enter the solution as Zn2+ ions. Cu2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn(s) Zn2+

(aq) + 2 e-

reduction: Cu2+(aq) + 2 e- Cu(s)

8

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

Zinc atoms enter the solution as Zn2+ ions. Cu2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn(s) Zn2+

(aq) + 2 e-

reduction: Cu2+(aq) + 2 e- Cu(s)

Overall: Cu2+(aq) + Zn(s) Zn2+

(aq) + Cu(s)

9

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

Zinc atoms enter the solution as Zn2+ ions. Cu2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn(s) Zn2+

(aq) + 2 e-

reduction: Cu2+(aq) + 2 e- Cu(s)

Overall: Cu2+(aq) + Zn(s) Zn2+

(aq) + Cu(s)

When a piece of copper metal is placed in an aqueous ZnSO4 solution: nothing happens.

10

When a piece of zinc metal is placed in an aqueous CuSO4 solution:

Zinc atoms enter the solution as Zn2+ ions. Cu2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn(s) Zn2+

(aq) + 2 e-

reduction: Cu2+(aq) + 2 e- Cu(s)

Overall: Cu2+(aq) + Zn(s) Zn2+

(aq) + Cu(s)

When a piece of copper metal is placed in an aqueous ZnSO4 solution: nothing happens.

One conclusion: zinc has a greater tendency to be oxidized than copper.

11

12

It is possible, by a suitable arrangement, to make the electrons flow along an external circuit. This is carried out in a galvanic cell.

13

It is possible, by a suitable arrangement, to make the electrons flow along an external circuit. This is carried out in a galvanic cell.

A galvanic cell operates on the principle that the oxidation of Zn to Zn2+ and the reduction of Cu2+ to Cu can be made to take place separately (and simultaneously) with electron transfer taking place through a connecting wire between the two electrodes.

14

For the reaction Cu2+

(aq) + Zn(s) Zn2+(aq) + Cu(s)

15

For the reaction Cu2+

(aq) + Zn(s) Zn2+(aq) + Cu(s)

the two half-cell reactions are just the oxidation and reduction steps:

16

For the reaction Cu2+

(aq) + Zn(s) Zn2+(aq) + Cu(s)

the two half-cell reactions are just the oxidation and reduction steps:

oxidation: Zn(s) Zn2+(aq) + 2 e-

17

For the reaction Cu2+

(aq) + Zn(s) Zn2+(aq) + Cu(s)

the two half-cell reactions are just the oxidation and reduction steps:

oxidation: Zn(s) Zn2+(aq) + 2 e-

reduction: Cu2+(aq) + 2 e- Cu(s)

18

For the reaction Cu2+

(aq) + Zn(s) Zn2+(aq) + Cu(s)

the two half-cell reactions are just the oxidation and reduction steps:

oxidation: Zn(s) Zn2+(aq) + 2 e-

reduction: Cu2+(aq) + 2 e- Cu(s)

Note: It is essential to separate the two solutions, otherwise the Cu2+ ions will react directly with the zinc electrode, and there would be no electron flow through the external wire.

19

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium.

20

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium.

This is accomplished by using a salt bridge – which contains an inert electrolyte such as KCl.

21

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium.

This is accomplished by using a salt bridge – which contains an inert electrolyte such as KCl. The electrolyte is usually present dissolved in a gel (such as agar-agar).

22

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium.

This is accomplished by using a salt bridge – which contains an inert electrolyte such as KCl. The electrolyte is usually present dissolved in a gel (such as agar-agar).

During the reaction, electrons flow externally from the anode (the Zn electrode) to the cathode (the Cu electrode).

23

24

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes.

25

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes. This difference, called the electromotive force, or emf, can be measured by placing a voltmeter in the external circuit between the two electrodes.

26

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes. This difference, called the electromotive force, or emf, can be measured by placing a voltmeter in the external circuit between the two electrodes.

If electrons flow from the anode to the cathode, then the anode must be labeled negative with respect to the cathode, which is labeled positive.

27

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes. This difference, called the electromotive force, or emf, can be measured by placing a voltmeter in the external circuit between the two electrodes.

If electrons flow from the anode to the cathode, then the anode must be labeled negative with respect to the cathode, which is labeled positive.

In solution, negative ions move towards the anode, which means the region close to the anode must be positively charged (the Zn2+ are formed there).

28

Notation for Electrochemical Cells

29

Notation for Electrochemical Cells Zn(s)| Zn2+(aq)

30

Notation for Electrochemical Cells Zn(s)| Zn2+(aq) vertical line means a phase boundary

31

Notation for Electrochemical Cells Zn(s)| Zn2+(aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn2+(1 M)

32

Notation for Electrochemical Cells Zn(s)| Zn2+(aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn2+(1 M) Often ordered in the form: reactant|product

33

Notation for Electrochemical Cells Zn(s)| Zn2+(aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn2+(1 M) Often ordered in the form: reactant|product For example, Zn(s)| Zn2+(aq) would indicate

34

Notation for Electrochemical Cells Zn(s)| Zn2+(aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn2+(1 M) Often ordered in the form: reactant|product For example, Zn(s)| Zn2+(aq) would indicate Zn Zn2+

(aq)

35

For H+(aq)|H2(g)|Pt(s)

36

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode.

37

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode. So the reaction is 2 H+

(aq) H2(g)

38

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode. So the reaction is 2 H+

(aq) H2(g) (The Pt is not involved in the chemistry.)

39

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode. So the reaction is 2 H+

(aq) H2(g) (The Pt is not involved in the chemistry.)

Carbon is also often used as an inert electrode.

40

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode. So the reaction is 2 H+

(aq) H2(g) (The Pt is not involved in the chemistry.)

Carbon is also often used as an inert electrode.

For Pt(s)|H2(g)|H+(aq), the reaction is

H2(g) 2 H+(aq)

41

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode. So the reaction is 2 H+

(aq) H2(g) (The Pt is not involved in the chemistry.)

Carbon is also often used as an inert electrode.

For Pt(s)|H2(g)|H+(aq), the reaction is

H2(g) 2 H+(aq)

An electrode consisting of a platinum wire dipping into a solution of iron (II) and iron (III) is denoted by

42

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode. So the reaction is 2 H+

(aq) H2(g) (The Pt is not involved in the chemistry.)

Carbon is also often used as an inert electrode.

For Pt(s)|H2(g)|H+(aq), the reaction is

H2(g) 2 H+(aq)

An electrode consisting of a platinum wire dipping into a solution of iron (II) and iron (III) is denoted by

Fe2+(aq), Fe3+(aq)|Pt(s)

43

For H+(aq)|H2(g)|Pt(s) The Pt is employed as an

inert metal electrode. So the reaction is 2 H+

(aq) H2(g) (The Pt is not involved in the chemistry.)

Carbon is also often used as an inert electrode.

For Pt(s)|H2(g)|H+(aq), the reaction is

H2(g) 2 H+(aq)

An electrode consisting of a platinum wire dipping into a solution of iron (II) and iron (III) is denoted by

Fe2+(aq), Fe3+(aq)|Pt(s) There is no phase boundary between the Fe2+ and Fe3+ solutions.

44

A comma is used to separate half-cell components that are in the same phase.

45

A cell is represented as: Zn(s)| Zn2+(aq)||Cu2+(aq)|Cu(s)

46

A cell is represented as: Zn(s)| Zn2+(aq)||Cu2+(aq)|Cu(s) The || represents a salt bridge.

47

A cell is represented as: Zn(s)| Zn2+(aq)||Cu2+(aq)|Cu(s) The || represents a salt bridge. It also indicates the

divide between the two half cells.

48

A cell is represented as: Zn(s)| Zn2+(aq)||Cu2+(aq)|Cu(s) The || represents a salt bridge. It also indicates the

divide between the two half cells. Often ordered so that it is in the form: anode compartment|| cathode compartment

49

A cell is represented as: Zn(s)| Zn2+(aq)||Cu2+(aq)|Cu(s) The || represents a salt bridge. It also indicates the

divide between the two half cells. Often ordered so that it is in the form: anode compartment|| cathode compartment

The contents of the salt bridge are usually omitted – they are not part of the chemistry of the cell.

50

Example: Write the balanced reactions occurring at the anode and cathode for the cell:

C|I-(aq)|I2(s)||MnO4-(aq), H+(aq), Mn2+(aq)|C

51

Example: Write the balanced reactions occurring at the anode and cathode for the cell:

C|I-(aq)|I2(s)||MnO4-(aq), H+(aq), Mn2+(aq)|C

anode 2 I-(aq) I2(s) + 2e-

52

Example: Write the balanced reactions occurring at the anode and cathode for the cell:

C|I-(aq)|I2(s)||MnO4-(aq), H+(aq), Mn2+(aq)|C

anode 2 I-(aq) I2(s) + 2e-

cathode MnO4

-(aq) + 8H+

(aq) + 5e- Mn2+(aq) + 4H2O

53

Example: Write the balanced reactions occurring at the anode and cathode for the cell:

C|I-(aq)|I2(s)||MnO4-(aq), H+(aq), Mn2+(aq)|C

anode 2 I-(aq) I2(s) + 2e-

cathode MnO4

-(aq) + 8H+

(aq) + 5e- Mn2+(aq) + 4H2O

Note that the C serves as an inert electrode, and is not involved in the chemistry.

54

55

Example: The following chemistry is made to occur in an electrochemical cell:

56

Example: The following chemistry is made to occur in an electrochemical cell:

14H+(aq) + Cr2O7

2-(aq) + 6e- 2 Cr3+

(aq) + 7H2O

57

Example: The following chemistry is made to occur in an electrochemical cell:

14H+(aq) + Cr2O7

2-(aq) + 6e- 2 Cr3+

(aq) + 7H2O

Fe2+(aq) Fe3+

(aq) + e-

58

Example: The following chemistry is made to occur in an electrochemical cell:

14H+(aq) + Cr2O7

2-(aq) + 6e- 2 Cr3+

(aq) + 7H2O

Fe2+(aq) Fe3+

(aq) + e-

Write the cell notation that describe this cell.

59

Example: The following chemistry is made to occur in an electrochemical cell:

14H+(aq) + Cr2O7

2-(aq) + 6e- 2 Cr3+

(aq) + 7H2O

Fe2+(aq) Fe3+

(aq) + e-

Write the cell notation that describe this cell.

It is a solution reaction, so use an inert electrode such as C or Pt.

60

Example: The following chemistry is made to occur in an electrochemical cell:

14H+(aq) + Cr2O7

2-(aq) + 6e- 2 Cr3+

(aq) + 7H2O

Fe2+(aq) Fe3+

(aq) + e-

Write the cell notation that describe this cell.

It is a solution reaction, so use an inert electrode such as C or Pt.

C|Fe2+ (aq),Fe3+ (aq)||Cr2O72-(aq), H+(aq), Cr3+(aq)|C