Review General Re action :( oxid )+ e −=( red ) K = ( red ) ( oxid )( e −) E = E o − 2.303 RT nF log Π( red ) Π( oxid ) ⎡ ⎣ ⎢ ⎤ ⎦ ⎥
Dec 19, 2015
Review
GeneralReaction: (oxid) +e−=(red)
K =(red)
(oxid)(e−)
E=E o −2.303RTnF
logΠ(red)Π(oxid)
⎡
⎣ ⎢
⎤
⎦ ⎥
Review IIpε =pεo−
1n
logΠ(red)Π(oxid) ⎡ ⎣ ⎢
⎤ ⎦ ⎥
Eo =−ΔGo
nF=RT
nFln(K ) =2.203RT
nFlog(K ) =2.203RT
Fpεo
where:F = Faraday constant=23,060cal/ molV=96,490J / molV
T = temperatureoK
n= number of electrons transfered
R =8.314J / mol K=0.082057 liter atm/ deg mol
and 2.303RT
F=0.05916 at25oC
peo =1n
log(Kredox) Notice the sign!
WHAT ABOUT NATURAL SYSTEMS?
peo Eo
O2 + 4H+ + 4e−⇔ 2H2 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? peo Eo
O2 + 4H+ + 4e−⇔ 2H2 O 20.75 1.227
Hg2+ + 2e−⇔ Hgo 13.35 0.789
Cu2+ + 2e−⇔ Cuo 5.71 0.337
2H+ +2e−⇔ H2 (g) 0 0
Pb2+ +2e−⇔ Pbo −2.31 −0.126
What about Natural systems?
This is a general equation for the oxidation/reduction of organic material
CO2 + 4e−+ 4H+ ⇔ CH2O+H2O
Relative to activity series peo Eo
O2 + 4H+ + 4e−⇔ 2H2 O 20.75 1.227
Hg2+ + 2e−⇔ Hgo 13.35 0.789
Cu2+ + 2e−⇔ Cuo 5.71 0.337
2H+ +2e−⇔ H2 (g) 0 0
CO2 + 4e−+ 4H+ ⇔ CH2O+H2O
−1.20 −0.071
Pb2+ +2e−⇔ Pbo −2.31 −0.126
Using the two half reactions
Eo = 1.227 + (0.07) = 1.297 V
O2 + 4H+ + 4e−⇔ 2H2 O 20.75 1.227
CO2 + 4e−+ 4H+ ⇔ CH2O+H2O
−1.20 −0.071
O2 +CH2O⇔ H2 O +CO2
O2 +CH2O⇔ H2 O +CO2Eo = 1.227 + (0.07) = 1.297 V
CH2O = 0.01 M, O2 = 0.2 atm CO2 = 1.0 atm
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E =1.297 −0.05916
4log
[1]
[0.2][0.01]
⎡
⎣ ⎢ ⎤
⎦ ⎥=1.26
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−nFE = ΔG = −4 * 23060 * (1.26) = −114Kcal
The negative ΔG 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!
Redox / Nutrient Cycling
(CH2O)106(NH3)16(H3PO4)
CO2
Fe(OH)2PO42-
Fe(II) + O2
PO43-
Fe(OH)3 (s)
Fe(OH)3 (s)
PO43-CO2 Fe(II) + PO4
3-
S2-
FeS iron sink.
Sediment
Bottom Water
Surface Water
Oxygen Demanding Substances
CH2O + O2 = CO2 + H2O
NH4+ + 2 O2 = 2H+ + NO3
- + H2O
4Fe2+ + O2 + 10H2O = 4Fe(OH)3 (s) + 8H+
2SO32- + O2 = 2SO4
2-
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