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!

Biologically Mediated reactions

Sequence of Microbially Mediated redox

processes

Stumm and Morgan 1996

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