Oxidation and Reduction Definitions




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Oxidation and Reduction
Definitions

  • Oxidation

  • Reduction

  • Half reactions (electrode reactions)

  • Oxidation potentials (electrode potentials).


Electrode potentials

  • When manipulating half reactions, keep in mind that the voltages are not multiplied by coefficients (unlike G, H, …. etc.)!

  • E° for the reaction: ½ H2  H+ + e- is set at 0 (standard state condition)

  • All electrode potentials are given at 25°C!


Electromotive series (cf. Table 1)

The electromotive series is a means by which we rank the elements according to their oxidizing powers. At the top of the list are most reducing elements with the smallest standard electrode potential, and at the bottom are the most oxidizing with the highest electrode portentials.


Balancing Redox Reactions

  1. Determine the oxidation state of each species in solution.

  2. For each reduced ionic species, write a balanced reaction with its oxidized form. Charge balance it with electrons based on step 1.

  3. For each oxidized ionic species, write a balanced reaction with its reduced form. Charge balance it with electrons.

  4. Add or subtract the reactions obtained in 1 and 2 in order to get rid of all free electrons.

  5. Mass balance for any remaining elements which have not yet been balanced out.

  6. Charge balance using H+ or (OH-), depending on whether the solution is basic or acidic.

  7. Balance the H+ or (OH-) ions with H2O.

  8. Double check your balancing by counting the Oxygens on both sides of the reaction.


Relationship between DG and E

DG = n F E

where n is the # of electrons exchanged, F is the Faraday constant and is equal to 96485 Coulombs, E is the electromotive force (emf) in volts, Þ DG will be in joules.
Redox Potentials "Eh" and The Nernst equation

· Potential difference between Pt and a H electrode or any other electrode of known potential

· Redox Potentials of sea water range from + 0.3 to -0.5 volts.

· Nernst equation: Eh = E° + (2.303RT/nF) log K


Electron activity "pe"

· pe = -log ae-

· Eh = (2.303RT/F).pe

· Eh and pe are proportional, but the proportionality constant changes with changing T.


Eh - pH diagrams: (Fig. 1)

· pH ranges in nature: 4 - 9 (although higher and lower pH's are known)

· Maximum Eh controlled by the equilibrium:

H2O = 0.5 O2 + 2 H+ + 2 e- E° = +1.23 volts

Eh = 1.23 + (0.059/2) log (aO2)0.5 . (aH+)2

substituting the fugacity of atmospheric O2 (0.2) for aO2 in the above equation yields

Eh = 1.22 - 0.059 pH (1)

· Minimum pH controlled by the equilibrium:

H2 = 2 H+ + 2 e-

Eh = 0 + (0.059/2) log (ae-)2 + (0.059/2) log (aH+)2 – (0.059/2) log (aH2)

Since the pressure of hydrogen gas cannot possibly exceed 1 atm in near atmospheric conditions, the maximum possible reducing potential in the presence of water would be met at conditions of aH2 = 1, the above relation reduces to:

Eh = (0.059/2) log 1 - 0.059 pH -(0.059/2) log 1 = - 0.059 pH (2)



These two equations allow us to construct Eh vs. pH diagrams covering all “reasonable” natural environments (Fig. 1b).

  • Natural (measured) vs. computed conditions on Eh – pH diagrams.

  • Kinetics of redox reactions.

  • PO2 can be contoured on Eh-pH diagrams (Fig. 2).

  • The slopes of lines on Eh-pH diagrams vary as a function of T.

· Eh - pH diagrams for various minerals, ions, or other aqueous species are constructed for a given solution or system (Fig. 3a) Smilar diagrams can be constructed with log fO2 vs. pH, and at different T (Figs. 3b & c).

  • A simple example: relations between Hematite and magnetite (Fig. 4)

  • Details of the construction of Eh-pH diagrams: Fig. 5 + text (p. 227ff).

  • Adding more species/ minerals: Figs. 6 & 7.

  • Applications of Eh – pH diagrams to natural Fe bearing solutions (Figs. 4 – 7).

  • Stability fields of oxides and sulfides (Fig. 8): Note conditions of possible stable coexistence of pyrite and hematite. Compare this to Fig. 7!

  • Atmospheric O2 is the ultimate oxidizing agent in natural waters, whereas organic compounds are the ultimate reducing agent.

  • Most organic matter is unstable in water; its stability lying below the H2O = H2(g) boundary. Exceptions include a handful of acids and alcohols.

  • Redox Capacity “” is defined as the the quantity of strong reductant in equivalents which must be added to 1 litre of sample to lower its Eh by 1 volt.

 = -dCr/d(Eh)

  • Redox Classification: (cf. Table 2).


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