Page 99 - 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

Electrochemistry 177

[− G BF = H DBE − T S = 81.6 − 7.8 (est) = 73.8kcal and Ce (OH 2 ) , each with one, two, and three M–OH +
III
3+
6
2
−1
mol ]. Thus, the proposition that metal electrode ox- covalent bonds, respectively],
idations are solvent or ligand centered with potentials
−e −
I
that reflect the metal-solvent/ligand bond-formation free Cu (OH 2 ) + H 2 O (H 2 O) Cu II
+
+

6
5
◦
energies (− G BF ) is supported by independent bond- (E ) pH 0 ,+0.16 V
II
energy data. The data of Eqs. (77) and (82) provide a meas- − OH (OH 2 ) (H 2 O) Cu −OH + H O
+
+
+
2
3
5
ure of the solubility product for AgCl (s). −1
− G BF , 59 kcal mol (90)
I
I
+
Ag (OH 2 ) + Cl − Ag −Cl (s) + 2H 2 O
2 −
II
I + Fe (OH 2 ) 2+ + H 2 O −e (H 2 O) 2+ Fe III
−
Ag (OH 2 ) [Cl ] = K sp ; 6 5
2 (E ) pH 0 ,+0.77 V
◦
log K sp = (0.22−0.80)/0.059 =−9.8. (85) III
2+
+
+
− OH (OH 2 ) (H 2 O) Fe −OH + H O
2 5 3
Another important example is the oxidation of Cl at a −1
−
− G BF , 45 kcal mol (91)
mercury electrode [Hg 2 (l)] to form calomel [mercurous
II
II
chloride, Hg 2 Cl 2 (s), Cl–Hg –Hg –Cl (s)]. III 3+ −e − 3+ IV
Ce (OH 2 ) + H 2 O (H 2 O) Ce
6 5
◦
(E ) pH 0 ,+1.60 V
−e −
Hg 2 (l) + Cl − [Cl−Hg−Hg·] + 3+ IV +
− OH (OH 2 ) (H 2 O) Ce −OH + H O
2 5 3
−e −
◦
− Cl−Hg−Hg−Cl (s) E , +0.27 V. (86) −1
Cl − G BF , 25 kcal mol . (92)
−
The potential shift for the Cl /Cl· couple from +2.41 In none of these examples has the potential for removal
[Eq. (75)] to +0.27 V in the presence of Hg 2 (l) is a mea- of an electron approached the ionization potentials of the
sure of the [Cl–HgHg] bond energy [− G BF = (2.41 − metals. Although traditional treatments attribute the po-
−1
0.27) 23.1 = 49.4 kcal mol ]. tentials of Eqs. (77), (87), and (89)–(92) to the removal of
Similar metal-facilitated oxidations of H 2 O and of Cl − electrons from the metals, coupled with large ionic solva-
occur for all metal electrodes. The respective potentials tion energies, this requires a pathway with the ionization
for the oxidation of each at a copper electrode are potentialasakineticbarrier.Furthermore,thespontaneous
reaction of iron with acidified water is driven by the for-
−e − I
◦
Cu (s) + 2H 2 O (H 2 O)Cu −OH + (E ) pH 0 , +0.52 V +
2 mation of Fe–OH and H–H covalent bonds that facilitate
2
−1 hydrogen-atom transfer from water (rather than electron
− G BF , 46 kcal mol (87)
transfer from iron),
and
H 2 O II
Fe (s) + 2H O (H 2 O) 4 Fe (OH 2 ) 2+ (93)
+
−e − I 3 2 + H 2
◦
Cu (s) + Cl − Cu −Cl E , +0.14 V
−3e −
3+
−1 {Note: To ionize a gas-phase iron atom (Fe Fe )
− G BF , 52 kcal mol . (88)
−1
requires 54.8 eV (1266 kcal mol ); in turn, this species
−
Additional redox data for oxidations of H 2 O/HO at Cu, reacts upon dissolution into liquid water {Fe (g) +
3+
Ag, and Au electrodes in aqueous and acetonitrile (MeCN) 7H 2 O(l) → [(H 2 O) ]Fe –OH + H O, − H ≈ 1000
III
2+
+
5 3
solutions are summarized in Table I. At pH 0 with an iron kcal mol −1 (1266 − 266)}; the net energy change often
electrode the water oxidation of Eq. (73) is shifted by is ascribed as the solvation energy for Fe 3+ (g) (heat of
−3.12 V, hydration)}.
Within an aprotic solvent (e.g., acetonitrile, MeCN)
−
−2e II 2+
Fe (s) + 6H 2 O (H 2 O) 4 Fe (OH 2 ) 2 oxidation of metals and metal complexes also is ligand
◦
(E ) pH 0 , −0.40 V, (89) centered with the potential determined by the oxidation
potential of the ligand and the metal-ligand covalent bond-
which indicates that the [H 2 O(H 2 O·) ] species is sta- formationfreeenergy(− G BF ).Forexample,thefreebpy
+
II
bilized by a strong [(H 2 O) 4 (H 2 O) ]Fe –OH + covalent
+
2 ligand in acetonitrile is oxidized near the solvent limit at
−1
bond (− G BF , ∼71 kcal mol ). a glassy-carbon electrode (GC) (Fig. 5),
◦
+
(bpy) MeCN bpy · E , +2.32VvsNHE, (94)
2. Metal Complexes
but at a copper electrode the oxidation occurs at a negative
In an analogous fashion, the removal of an electron (ox- potential,
idation) from water via Eq. (73) is aided by the presence −
I
II
I
of transition-metal ions [e.g., Cu (OH 2 ) ,Fe (OH 2 ) , Cu (s) + 2bpy −e Cu (bpy) + E , −0.16 V. (95)
2+
◦
+
6 6 2