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

II.1 Symbols, terminology and nomenclature 21
and the potential, E, is related to E ο by:
E
ο
1 ο RT
= − ΔrGm
= ln K ο (II.21)
nF nF
E = E ο −( RT/ nF) ∑ ν ln a.
For example, for the hypothetical galvanic cell:
i
i
(II.22)
Pt H 2 (g, p = 1 bar) HCl(aq, a H + = 1,
f
H 2
= 1)
Fe(ClO 4 ) 2 (aq, a = 1)
Fe
2+
Fe(ClO 4 ) 3 (aq, a = 1)
Fe
3+
Pt
(II.23)
where denotes a liquid junction and a phase boundary, the reaction is:
Fe 3+ + ½ H 2 (g) Fe 2+ + H +
(II.24)
Formally Reaction (II.24) can be represented by two half cell reactions, each
involving an equal number of electrons, (designated “ e − ”), as shown in the following
equations:
3+ −
2+
Fe + e Fe
(II.25)
½ H 2 (g) H + + e –
(II.26)
The terminology is useful, although it must be emphasised “ e − ” here does not
represent the hydrated electron.
Equilibrium (II.26) and Nernst law can be used to introduce a −
e
:
RT
E = E ο (II.26) + ln( f / ( H
a ))
+ a
(II.27)
2
F
H e−
According to the SHE convention
RT
E = − ln
F
a e
−
E
ο (II.26) = 0, f
H
= 1, + = 1, hence
2
This equation is used to calculate a numerical value of a e − from potentialdifference
measurements vs. the SHE; hence, as for the value of E (V vs. the SHE), the
numerical value of a e depends on the SHE convention. Equilibrium constants may be
−
written for these half cell reactions in the following way:
a
K ο 2+
Fe
(II.25) =
(II.29)
a ⋅ a
(II.26) =
a
3+
Fe
+
H
⋅ a
f
H2
−
e
−
e
a H
(II.28)
= 1 (by definition) (II.30)
K ο CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012