Chemical Thermodynamics of Tin - Volume 12 - OECD Nuclear ...

22 II Standards, conventions and contents of the tables
In addition, Δ
rG ο
m
(II.26) = 0, Δ
rH ο
m(II.26) = 0, Δ
rS ο
m
(II.26) = 0 by definition,
at all temperatures, and therefore Δ
rG ο
m(II.25) = Δ
rG ο
m(II.24). From Δ
rG ο
m(II.26) and
the values given at 298.15 K in selected auxiliary data for H 2 (g) and H + , the corresponding
values for e – can be calculated to be used in thermodynamic cycles involving half
cell reactions. The following equations describe the change in the redox potential of
Reaction (II.24), if and a are equal to unity (cf. Eq. (II.22)):
p
H +
2 H
⎛
E(II.24) = E ο 2+
Fe
(II.24) – RT ln a ⎞
⎜a
⎟
3+
⎝ Fe ⎠
(II.31)
For the standard hydrogen electrode a e
− = 1 (by the convention expressed in
Eq. (II.30)), while rearrangement of Eq. (II.29) for the half cell containing the iron perchlorates
in cell (II.23) gives:
⎛a
−log10 a =
e − log10
K ο 2+
⎞
Fe
(II.25) – log10
⎜
a ⎟
3+
⎝ Fe ⎠
and from Eq. (II.27):
⎛a
−log10 a =
e − log10
K ο 2+
⎞
Fe
(II.24) – log10
⎜
a ⎟
3+
⎝ Fe ⎠
(II.32)
and
F
−log 10
a − =
E (II.24)
e
RT ln(10)
(II.33)
which is a specific case of the general equation (II.28).
The splitting of redox reactions into two half cell reactions by introducing the
−
symbol “ e ”, which according to Eq. (II.27) is related to the standard electrode
potential, is arbitrary, but useful (this e – notation does not in any way refer to solvated
electrons). When calculating the equilibrium composition of a chemical system, both
+
“ e − ”, and H can be chosen as components and they can be treated numerically in a
similar way: equilibrium constants, mass balance, etc. may be defined for both.
+
However, while H represents the hydrated proton in aqueous solution, the above
equations use only the activity of “ e − −
”, and never the concentration of “ e ”.
Concentration to activity conversions (or activity coefficients) are never needed for the
electron (cf. Appendix B, Example B.3).
In the literature on geochemical modelling of natural waters, it is customary to
represent the “electron activity” of an aqueous solution with the symbol “pe” or “pε”
( =− log a 10 − ) by analogy with pH ( = − log +
e 10
a ), and the redox potential of an
H
aqueous solution relative to the standard hydrogen electrode is usually denoted by either
“Eh” or “ E H
” (see for example [1981STU/MOR], [1982DRE], [1984HOS],
[1986NOR/MUN]).
In this review, the symbol E ο ' is used to denote the so-called “formal
potential” [1974PAR]. The formal (or “conditional”) potential can be regarded as a
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012