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Pengaruh Ph Poubaix

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Pengaruh pH pada laju reaksi
 Misalkan
hidrogen dibebaskan oleh reaksi

2H+ + 2e-  H2
 Konsentrasi ion hidrogen akan
mempengaruhi laju reaksi
 As the hydrogen ion concentration is
increased (i.e. the solution made more acid),
so the rate of the reaction increases
 Similarly the potential will influence the
reaction - the more negative the potential the
faster the reaction
Effect of pH and potential on
rate of hydrogen evolution
Slower
Potential
Faster
pH
Effect of pH on reaction rate
 On
platinum no metal dissolution will occur,
but to balance the charge a reaction which
creates electrons must occur
 If the solution contains dissolved hydrogen,
the reverse of the hydrogen evolution
reaction can occur:
H2 2H+ + 2e-
Effect of pH on reaction rate
 H2
2H+ + 2e-
 This
reaction will go faster in alkaline solution
(since H+ will be removed by H+ + OH-  H2O)
 This
reaction will go faster at more positive
potentials (because electrons will be removed
from metal)
Effect of pH and potential on
rate of hydrogen oxidation
Oxidation
Faster
Reduction
Slower
Potential
Oxidation
Slower Reduction Rates equal
Faster Electrochemical
Equilibrium
pH
Thermodynamic
Equilibrium
2H+ + 2e-  H2
 The potential at which it occurs for a given
solution composition is known as the
equilibrium potential.
 The concentrations of reactants controls the
rates of the forward and reverse reactions
and hence the equilibrium potential
Potential
The Pourbaix (E-pH)
Diagram
2.0
1.6
1.2
0.8
0.4
0.0
-0.4
-0.8
-1.2
-1.6
2H2O = O2 + 4H+ + 4eEquilibrium potential
as pH increases
Ofalls
is
stable
2
2H+ + 2e- = H2
Equilibrium
potential falls as
H2O is stable
pH increases
H2 is stable
pH = - log [H+]
0
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Pourbaix Diagram for Zinc
Potential
Equilibrium for
2.0 Zn(OH)2 + 2OH  ZnO22- + 2H2O
1.6
Equilibrium for
1.2
2+ + 2OH-  Zn(OH)
Zn
2
0.8
Zn(OH)
stable
0.4
ZnO22solid Equilibrium for
0.0 Zn2+ Equilibrium
stable
for
stable
in
- 
Zn
+
2OH
Zn(OH)
+
2e
2
-0.4
Zn  Zn2+ + 2e- solution
in for
solution
Equilibrium
-0.8 2Zn + 4OH  ZnO2 + 2H2O + 2e-1.2
Zn metal stable
-1.6
0
7
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2
2
Corrosion
2.0
1.6
1.2
0.8
0.4
0.0
-0.4
-0.8
-1.2
-1.6
possible with
oxygen
reduction
Corrosion Corrosion is
possible, but likely
possible with
to be stifled by solid
hydrogen
Zn(OH)
Corrosion
corrosion product
stable
evolution
ZnO22CorrosionCorrosion
requires
solid
is
2+
Zn stable
stable in
strong
oxidising
thermodynamically
solution
in solution agent
impossible
Passivity
Potential
Pourbaix Diagram
for
Zinc
Corrosion
Immunity
Zn metal
stable
0
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Potential
Pourbaix Diagram for Gold
2.0
1.6 C
1.2
0.8
0.4
0.0
-0.4
-0.8
-1.2
-1.6
0
Passivity
C
corrode
Gold Gold
metalcan’t
stable
with oxygen reduction
Immunity
or hydrogen evolution
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Pourbaix diagram for
Copper
Will copper
corrode in
acid?
Potential
Will copper
corrode in
neutral waters?
CuO22- stable in soln.
2.0
No - hydrogen
1.6
evolution only
Cu oxides
1.2
occurs below the
stable
0.8 Cu2+ stable
potential for copper
0.4
corrosion
in solution
Usually it will0.0
just
passivate,-0.4
but
corrosion can-0.8
occur
Cu metal stable
in slightly acid
-1.2
solutions
-1.6
0
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Pourbaix Diagram for Iron
Will iron
corrode in
acid?
2.0
1.6
Yes - there is a
1.2
reasonably wide
Fe3+
0.8
range of potentials
0.4
Fe oxides where hydrogen
Yes - although iron can
stable can be evolved and
0.0
form an oxide in neutral 2+
iron dissolved
Fe stable
-0.4
solution, it tends not to
-0.8
form directly
on
the
No - iron
forms a solid
Fe metal stable
-1.2
metal, as oxide
the potential
at all potentials,
is too low, therefore
is
and-1.6
will itpassivate
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not protective. 0
Potential
WillWill
ironiron
corrode
corrode
in
in alkaline
neutral
waters?
solution?
Potential
Pourbaix diagram for
Aluminium
1.2
0.8
0.4
0.0
-0.4
-0.8
-1.2
-1.6
-2.0
-2.4
Al3+
Al2O3
AlO2-
Al
0
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Limitations of Pourbaix
Diagrams
 Tell
us what can happen, not necessarily what
will happen
 No information on rate of reaction
 Can only be plotted for pure metals and
simple solutions, not for alloys
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