0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more oxidized

species

more reduced

species

hydroxo speciesaquo species

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more oxidized

species

more reduced

species

hydroxo speciesaquo species

(H2O)6

0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more oxidized

species

more reduced

species

hydroxo speciesaquo species

+4

+2

0

(H2O)6

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more oxidized

species

more reduced

species

hydroxo speciesaquo species

= predominant

species over this

potential and pH

range

+4

+2

0

0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more reduced

species

hydroxo speciesaquo species

pH ! 3.8

+1.0V ! E1/2

! 0.23 V

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more reduced

species

hydroxo speciesaquo species

more oxidized

species

dominate

under more

basic

conditions

0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more reduced

species

hydroxo speciesaquo species

more reduced

species dominate

under more

acidic conditions

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more reduced

species

hydroxo speciesaquo species

pH ! 3.8

+1.0V ! E1/2

! 0.23 V

under acidic conditions

higher potentials are

required in order to maintain

the higher oxidation state

0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams What form of lead would you expect to

find at pH= 7, and a

potential of 0 V?

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams What form of lead would you expect to

find at pH= 7, and a

potential of 0 V?

the solution would

predominantly

consist of

Pb(OH)2, with a

small amount of

Pb2+

0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix DiagramsWhat does the solution

consist of at pH= 6.9, and

0.25 < E1/2 < +0.6 V

ie, on the dotted blue

line?

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix DiagramsWhat does the solution

consist of at pH= 6.9, and

0.25 < E1/2 < +0.6 V

ie, on the dotted blue

line?

the solution would

consist of exactly

50% Pb(OH)2, and

50% Pb2+

0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix DiagramsWhat about along this

dotted blue line?

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix DiagramsWhat about along this

dotted blue line?

the solution would

consist of exactly

50% PbO2, and

50% Pb2+

0+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more oxidized

species

more reduced

species

hydroxo speciesaquo species

as the potential

is raised above

this line Pb+2 is

oxidized to

Pb+4

+2

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pourbaix Diagrams

more oxidized

species

more reduced

species

hydroxo speciesaquo species

pH ! 3.8

as the pH is raised

above this line

Pb+2(H2O)6 is

converted to

Pb+2(OH)2

+2

1. Vertical lines:

separate species of the same oxidation state

pH-dependent acid/base rxns occur across

2. Horizonal lines:

separate species of different oxidation states

a redox rxn occurs as you cross this line

w/ a pH-independent redox potential

3. Diagonal lines:

separate species of different oxidation states

a pH-dependent redox rxn occurs as you cross

this line

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Is PbO2 a better oxidizing agent at pH= 5 or pH= 11?

PbO2 is reduced to Pb2+

PbO2 is reduced to Pb2+(OH)2

E(pH= 5)= ~ +0.88 V

E(pH= 11)= ~ +0.12 V

Thus, it is a better

oxidant at pH= 5

at pH= 5 PbO2 has a redox

potential of ~ +0.88 V

Is Pb0 a better reducing agent at pH= 0 or pH= 14?

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pb0 is oxidized to Pb2+

Pb0 is oxidized to Pb2+(OH)2

E(pH= 0)= ~ 0.25 V

E(pH= 11)= ~ 1.1V

Thus, it is a better

reductant at pH= 14

What is the approximate redox

potential of Pb2+ at pH= 4?

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pb2+ is reduced to Pb0

Pb2+ is oxidized to Pb4+O2

E(pH= 4)= ~ 0.25 V

Pb2+ is converted to Pb2+(OH)2

What is the approximate redox

potential of Pb2+ at pH= 11?

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pb2+(OH)2 is reduced to Pb0

Pb2+(OH)2 is oxidized to Pb4+O2

E(pH= 11)= ~ 0.6 V

Pb2+(OH)2 is reduced to Pb0

Pb2+(OH)2 is converted to Pb2+

What is the approximate redox

potential of Pb2+ at pH= 11?

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

E(pH= 11)= ~ + 0.12 V

Pb2+(OH)2 is oxidized to Pb4+O2

What reaction occurs across the line labeled a

at pH= 9? Write a balanced eqn.

0

+1.0

1.0

+0.5

0.5

0.75

0.25

+0.25

+0.75

7 14531 9 11

E (V)

pH

PbO2

Pb(OH)2

Pb2+

Pb

a.

Pb4+O2 is reduced to Pb2+(OH)2

Pb2+(OH)2 is oxidized to Pb4+O2

Pb4+O2 Pb2+(OH)2 + 2e

+ 2OH2H2O +