Page 98 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

176 Electrochemistry

by the presence of substrate atoms (rather than be from at much lower potentials [Eq. (73); at pH 0, +2.72 V vs
them). NHE]. At a silver electrode, water oxidation is facilitated
I
+
For example, with water at pH 0 the process of Eq. (73) via formation of an Ag –OH bond; the shift in oxidation
2
is shifted −4.82 V when hydrogen atoms are present, potential from +2.72 to +0.80 V is a measure of the bond-
formation energy (− G BF ),
−e −
+
+
[H·] + 2H 2 O (H 2 O)H O H O + H 2 O
3 3 −1
− G BF = (+2.72 − 0.80) 23.1 = 44.4 kcal mol . (78)
E , −2.10 V. (76)
◦
At pH 14 the anodic process is the oxidation of HO ,
−
e −
◦
HO − HO· E , +1.89 V
A. Oxidative Electrochemistry: Metals,
Metal Complexes, Lewis Bases (E ) pH 7 , +2.30 V, (79)
◦
I
1. Metals which is facilitated via formation of an Ag –OH bond
−1
[− G BF = (1.89 − 0.34) 23.1 = 35.8 kcal mol ]
The transformation of metal electrode surfaces via ele-
ctrooxidation to their metallo-oxides, solvated ions, and − −e − I
Ag (s) + HO Ag −OH (s)
metal complexes is fundamental to most anodic elec-
◦
trochemical processes (batteries, electrorefining, anodic E , +0.34 V
stripping analysis, and reference electrodes). Although ◦
(E ) pH 7 , +0.75 V. (80)
this is traditionally represented as the removal of one (or
more) valence electron from a metal atom at the electrode The data of Eqs. (77) and (79) can be combined to give a
+
◦
−
surface to give a metal ion [e.g., Ag (s) − e → Ag ; E , value for the solubility product (K sp ) for Ag–OH (s),
+0.80 V vs NHE], the gas-phase ionization potential [e.g., I + − I
Ag (OH 2 ) + HO Ag −OH (s) + 2H 2 O
2
Ag · (g) − e → Ag (g); IP, 7.6 eV] is far greater than the
+
−
I + −
observed oxidation potential. The difference is attributed Ag (OH 2 ) [HO ] = K sp ;
2
+
to the solvation energy for the metal ion [e.g., Ag + log K sp = (0.34 − 0.80)/0.059 =−7.8.
−1
nH 2 O → Ag (aq); − G(aq) ≈ 70–100 kcal mol ].
+
(81)
However, such a sequential path would not obviate the
7.6-V energy barrier for the initial step and is in conflict In the presence of chloride ion, metal electrodes facil-
◦
with the observed thermodynamic reversibility for many itate its oxidation [Eq. (75); E , +2.41 V vs NHE] via
metal/solvated-metal-ion redox couples. formation of metal-chlorine covalent bonds, e.g.,
All reactions, and particularly redox processes, occur −e −
◦
via the easiest and lowest energy pathway that is avail- Ag (s) + Cl − Ag−Cl (s) E , +0.22 V. (82)
able (mechanistically feasible) to the system. In the case Hence, the differential bond-formation energy
of a metal electrode/electrolyte interface undergoing an- [ (− G BF )] [Ag–Cl bond energy, minus the energy
odic transformations, the electrons can come from (a) sur- required to break an Ag–Ag bond at the Ag (s) surface)] is
face metal atoms (energy limit; first ionization potential), given by the difference in oxidation potentials [Eqs. (75)
(b) solvent molecules (energy limit; oxidation potential and (82)],
of solvent), (c) electrolyte anions (energy limit; oxidation −1
potential of anions), and (d) base ligands (energy limit; ox- (− G BF ) = (2.41 − 0.22) 23.1 = 50.6 kcal mol .
idation potential of ligand). All metal electrodes are elec- (83)
trochemically transformed via path b, c,or d and never
Because the escape energy for an Ag· atom from Ag (s) is
via path a. This general conclusion is illustrated for silver −1
68 kcal mol , a reasonable approximation for the break-
and copper electrodes in aqueous and acetonitrile (MeCN) −1
age of a single bond is 22.6 kcal mol [(1/3) 68]. When
−
solutions that contain inert electrolyte, chloride ion (Cl ), combined with Eq. (83), this gives a reasonable value for
or bipyridine (bpy).
− G BF ,
In aqueous solutions at pH 0, the silver electrode facil-
itates oxidation of water Ag·+ Cl· Ag–Cl − G BF = 50.6 + 22.6
−1
−e − = 73.2 kcal mol .
I
Ag (s) + 2H 2 O Ag (OH 2 ) +
2
(84)
◦
E , +0.80 V vs NHE. (77)
The literature value for the dissociative bond energy
−1
The gas-phase ionization potential for a silver atom is ( H DBE )ofAg–Cl (g) is 81.6 kcal mol , which is equiv-
7.6 eV. In contrast, water is oxidized (gives up an electron) alent to an estimated − G BF value of 73.8 kcal mol −1