Ozone

1
Redox reactions in organic molecules
-Oxidation states of carbon counting owned
electrons provides a means of determining
whether a reaction oxidizes or reduces the
compound -The C in methane is least oxidized
that in carbon dioxide is most oxidized -Biomass
synthesis (carbon cycle) can also be considered a
redox reaction
2
Biological oxygen demand BOD
Biological oxygen demand of milligrams of
O2 required to oxidize the amount of organic
carbon in 1 L H2O Unpolluted water BOD 0.7
mg O2/liter Sewage - BOD 200 mg
O2/liter High BOD threatens aerobic aquatic
life Solubility of O2 in water 9 mg/L at 20C.
Set by Henrys Law, with KH 1.3 x 10-3
M/atm. PO2 0.21 atm. Solubility is higher
at lower temperature (thermal pollution). If
BOD exceeds solubility, then there may be no
remaining free O2 to sustain aerobic aquatic
life. Depends on the relative rates of oxygen
dissolution in to the water, and the speed of the
microbial metabolism that is consuming the O2.
3
Redox scale in biogeochemistry

O2 is the most powerful oxidizing agent
(oxidant) Creation of O2 by photosynthesis
greatly enabled biological metabolism in
cells, since the free energy change is very
high Without O2, though, redox reactions still
occur, to exploit anaerobic environments



The redox reactions, giving negative free-energy
changes, are mediated biologically, mainly by
single-celled bacteria and archaea
4
Energy-use classification of living organisms

Energy obtained using organic or inorganic
redox reactions under anaerobic conditions
5
Redox chemistry in natural waters
Aerobic organisms CH2O O2 ? CO2 H2O
epilimnion - oxidized
hypolimnion - reduced no contact with air O2
fully consumed
Anaerobic bacteria 2 CH2O ? CH4 CO2
6
Redox scale in biogeochemistry
0.81 V 0.75 V
O2 is the most powerful oxidizing agent
(oxidant) Creation of O2 by photosynthesis
greatly enabled biological metabolism in
cells, since the free energy change is very
high Without O2, though, redox reactions still
occur, to exploit anaerobic environments
0.53 V
-0.05 V
-0.22 V -0.24 V
The redox reactions, giving negative free-energy
changes, are mediated biologically, mainly by
single-celled bacteria and archaea
7
standard hydrogen electrode E 0.0 V
Oxidation (anode) Zn ? Zn2 2e- Reduction
(cathode) 2H 2e- ? H2 (g) Overall reaction
2H(aq) Zn(s) ? Zn2(aq) H2(g)
E 0.76 V E E - 2.3RT/nF logQ at T
25C, E E - (0.059/n) logQ DG -nFDE Redox
potentials are often pH-dependent
8
Acid mine drainage
FeS2 7/2 O2 H2O ? Fe2 2 HSO4- Sulfur
oxidation under aerobic conditions by
thiobacillus ferrooxidans, from disulfide S22-
FeS2 is pyrite, found in underground coal
seams Next reactions 4 Fe2 O2 2 H2O ? 4
Fe3 4 OH- (general to Fe2-laden
groundwater) Fe3 3 H2O ? Fe(OH)3 (s) 3 H
(orange-brown deposit acid runoff) Together, 1
mol FeS2 produces 2 mol H2SO4 and 1 mol
Fe(OH)3 pH gets as low as 3.0 this is a serious
source of acidification
9
3
-1
5
0
-3
3
2
1
3
4 reactions constitute the nitrogen
cycle Nitrogen Fixation (diazotrophy) Nitrificati
on Nitrate Assimilation Denitrification All are
catalyzed by enzymes in microorganisms Favorable
free energy changes are exploited for growth
and metabolism of the microorganism N2 fixation
nitrogenase is highly O2 sensitive (evolution,
root nodules) N2 fixation kinetically difficult,
requires ATP hydrolysis