Review
General Re action : (oxid) + e- = (red)
(red)
K=
(oxid )(e-)
é
ù
2.303RT
P(red)
o
ú
E=E logê
nF
ë P(oxid) û
Review II
1
é P(red ) ù
pe = pe - log ê
n
ë P(oxid ) úû
o
1
pe = log(Kredox ) Notice the sign!
n
o
DG o RT
2. 203RT
2. 203RT o
E ==
ln (K ) =
log(K ) =
pe
nF
nF
nF
F
o
where: F = Faraday constant = 23, 060cal / molV = 96, 490J / molV
T = temperature o K
n = number of electrons transfered
R = 8.314J / mol K = 0. 082057 liter atm / deg mol
2. 303RT
and
= 0.05916 at 25o C
F
WHAT ABOUT NATURAL
SYSTEMS?
o pe E o
+
-
O 2 + 4H + 4e Û 2H 2 O
20.75
1. 227
• Oxygen would like to be reduced.
• If we can combine oxygen with a compound that
wants to be oxidized we have an energy source!
What wants to be oxidized?
o pe E o
+
O 2 + 4H + 4e Û 2H 2 O
o
2+
Hg + 2e Û Hg
20.75
1. 227
13.35
0. 789
o
2+
Cu + 2e Û Cu
5. 71
0.337
0
0
+
-
2H + 2e Û H 2 (g)
o
2+
Pb + 2e Û Pb
- 2. 31 - 0.126
What about Natural systems?
-
CO 2 + 4e + 4H Û CH 2 O + H 2O

+
This is a general equation for the
oxidation/reduction of organic material
Relative to activity series
o
o
pe E
O 2 + 4H + + 4e - Û 2H 2 O
20.75
1. 227
Hg 2+ + 2e - Û Hg o
13.35
0. 789
Cu
2+
-
+ 2e Û Cu
o
+
2H + 2e Û H 2 (g)
5. 71
0.337
0
0
+
CO2 + 4e + 4H Û CH2 O + H2 O
-1. 20
Pb
2+
-
+ 2e Û Pb
o
- 0. 071
- 2. 31 - 0.126
Using the two half reactions
+
-
-
+
O 2 + 4H + 4e Û 2H 2 O
20.75
1.227
CO 2 + 4e + 4H Û CH 2 O + H 2O
-1. 20
- 0. 071
O 2 + CH 2 O Û H 2 O + CO 2

Eo = 1.227 + (0.07) = 1.297 V
O 2 + CH 2 O Û H 2 O + CO 2
Eo = 1.227 + (0.07) = 1.297 V
CH2O = 0.01 M, O2 = 0.2 atm CO2 = 1.0 atm
ù
0.05916 é
[1]
E = 1.297 logê
ú = 1.26
4
ë[0.2][0.01]û




The negative G indicates that this reaction
is spontaneous!
What happens when we run out of oxygen?
Other oxidants will take the place of oxygen.
Most sediments are anoxic!
Biologically Mediated reactions
O2 (g) + 4H + + 4e - = 2H2 O
peo = 20.8
MnO2(s) + H+ + e- = MnOOH(s)
peo = 18.3
3
+
-
2
NO + 2H + 2e = NO + H 2 O
peo = 14.2
NO2- + 8H + + 6e - = NH4+ + 2 H2 O
peo = 15.1
MnOOH(s) + 3H+ + e- = Mn2+ + 2H2O
peo = 25.4
+
-
2+
Fe(OH)3 (s) + 3H + e = Fe + 3H2 O
peo = 17.1
SO42 - + 8H + + 6e - = S(s) + 4H2 O
peo = 6.0
-
+
S(s) + 2e + 2H = H2 S (g)
peo = 2.9
CO2 + 4H + + 4e - = CH 2 O + H2 O
peo = -1.2
Sequence of Microbially
Mediated redox
processes
http://www.chem1.com/acad/webtext/pdf/c3redox.pdf
Redox / Nutrient Cycling
CO2
Surface Water
(CH2O)106(NH3) 16(H3PO4)
Fe(OH)2PO 4
PO43-
2-
Fe(II) + O2
Fe(OH)3 (s)
Bottom Water
Sediment
CO2
PO43-
Fe(II) + PO43Fe(OH)3 (s)
S2FeS
iron sink.
Oxygen Demanding Substances




CH2O + O2 = CO2 + H2O
NH4+ + 2 O2 = 2H+ + NO3- + H2O
4Fe2+ + O2 + 10H2O = 4Fe(OH)3 (s) + 8H+
2SO32- + O2 = 2SO42-
Biological Oxygen Demand


Quantity of oxygen utilized by a suitable
aquatic microorganism in five days.
Why five days?
Chemical Oxygen Demand (COD)


oxygen utilized by a specific chemical
titration.
3CH2O + 16H+ + 2 Cr2O72- = 4 Cr3+ + 3CO2
+ 11 H2O