Page 159 - Thermodynamics of Biochemical Reactions

158 Chapter 9 Oxidation-Reduction Reactions

Table 9.1 Standard Reduction Potentials E” at 298.15 K, 1 bar,
and Zero Ionic Strength

Half-reaction E0 /v

F,(g) + 2e- = 2F- 2.87
O,(g) + 4Hf + 4e- = 2H,O 1.2288
Fe3+ + e- = Fe2+ 0.77 1
O,(g) + 2FI’ + 2e- = H,O, 0.70
N,(g) + 8H’ + 6e- = 2NHf 0.274
C,H,O(acetaldehyde) + 2H’ + 2e- = C,H,O(ethanol) 0.22 1
CU,+ + e- = Cuf 0.153
2H ‘ + 2e- = H,(g) 0
Fe2+ + 2e ~ = Fe(s) - 0.440
Li’ + e- = Li(s) - 3.045

This equation is the basis for the statement that knowledge of E: and E;: at a
specified pH for the two half-reactions determines the direction of spontaneous
reaction for the overall redox reaction.
I. H. Segel(1976) discussed oxidation-reduction reactions very clearly and has
written: “When any two of the half-reactions are coupled, the one with the greater
tendency to gain electrons (the one with the more positive reduction potential)
goes as written (as a reduction). Consequently, the other half-reaction (the one
with the lesser tendency to gain electrons as shown by the less positive reduction
potential) is driven backwards (as an oxidation). The reduced forms of those
substances with highly negative reduction potentials are good reducing agents
(and are easily oxidized). The oxidized forms of those substances with highly
positive reduction potentials are good oxidizing agents (and are easily reduced).”
A few standard reduction potentials of chemical half reactions at 298.1 5 K. 1
bar, and zero ionic strength are given in Table 9.1. Note that the half-reactions
balance atoms and charges and that the half-reactions are arbitrarily written in
such a way that there are no fractional stoichiometric numbers. It is interesting
to consider the extremes in this table. The fluorine molecule-fluoride ion electrode
has a very high affinity for electrons, and this is indicated by its very positive
standard reduction potential. Thus gaseous fluorine is a powerful oxidizing agent.
At the other extreme, the lithium ion-metallic lithium electrode has a very low
affinity for electrons, and this is indicated by its very negative standard reduction
potential. Thus lithium metal is a powerful reducing agent. Oxidizing agents with
reduction potentials above 1.2 V tend to oxidize H,O to O,(g), and metals with
negative reduction potentials tend to reduce H,O to H,(g). Note that half-
reactions with more positive reduction potentials than 1.229 V tend to produce
oxygen gas in aqueous solutions and half-reactions with negative reduction
potentials tend to produce molecular hydrogen.

9.2 OXIDATION-REDUCTION REACTIONS
INVOLVING SINGLE SPECIES AT SPECIFIED pH

When reactants in half-reactions involve only single species in a range of pH. the
dependence of E” on pH is linear in this range. The standard apparent reduction
potentials of a number of half-reactions have been calculated at 298.15 K at a
series of pHs and ionic strengths and are given in Table 9.2. This table gives both
the chemical form of the half reaction and the biochemical form of the half-
reaction. The chemical form is useful for understanding the pH and ionic strength
dependencies of the standard apparent reduction potentials, but the standard
apparent reduction potentials apply at a specified pH and ionic strength and