Chapter%207%20Electrochemistry

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Chapter 7 Electrochemistry
7.7 Thermodynamics of reversible cell
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Self reading Ira N. Levine, Physical
Chemistry, 5th Ed., McGraw-Hill, 2002. pp.
294-310 Section 10.10 standard-state
thermodynamic properties of solution components
pp. 426 Section 14.6 thermodynamics of
galvanic cells Section 14.7 standard electrode
potentials Section 14.8 concentration cells
Section 14.9 liquid-junction potential
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7.7.1. Measurement of Electromotive forces
(emf's)
Can voltameter be used to measure electromotive
force?
Discussion
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1) Poggendorffs compensation method
i 0, thermodynamic reversibility.
EW working cell Ex test cell Es standard cell
Principle of potentiometer
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2) Weston standard cell
Commercial Weston Standard cell
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Weston standard cell
Temperature-dependence of emf
E(T) /V 1.01845 4.05? 10-5(T/K 293.15)
9.5? 10-7(T/K 293.15)2 1? 10-8 (T/K
293.15)3
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2. Nernst equation and standard EMF of cell
1889, Nernst empirical equation
cC dD gG hH
1920 Noble Prize Germany 1864/06/251941/11/18 Stu
dies on thermodynamics
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Theoretical deduction of Nernst Equation
For a general electrochemical reaction
cC dD gG hH
Vant Horff equation
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7.7.3. Standard electromotive forces
E? equals E when the activity of any chemical
species is unit.
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(No Transcript)
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Experimental determination of standard
electromotive force
Cf. Levine, p. 430
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7.7.4. Temperature-dependence of emf's
For Weston Standard Cell E/V 1.018646 -
4.05?10-5(T/?-20) - 9.5?10-7 (T/?-20)2
1?10-8(T/?-20)3
Temperature coefficient (?E/?T)p ? 10-5 VK-1.
By differentiating the equation - ?rGm
nFE with respect to temperature, we obtain
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By measuring E and (?E/?T)p, thermodynamic
quantities of the cell reaction can be
determined. Because E and (?E/?T)p can be easily
measured with high accuracy, historically, the
thermodynamic data usually measured using
electrochemical method other than thermal method.
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7.7.5. Thermodynamic quantities of ions
How to solve this deadlock?
The customary convention is to take the standard
free energy of formation of H(aq) at any
temperatures to be zero.
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By definition
Standard free energies of formation of aqueous
ions at 298.3 K
Ion / kJmol-1 Ion / kJmol-1
H 0.000 OH? -157.3
Li -298.3 Cl? -276.5
Na -261.87 Br? -131.2
K -282.3 SO42? -742.0
Ag 77.1 CO32? -528.1
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Exercise-1
At 298 K, for cell Ag(s)-AgCl(s)KCl(m)Hg2Cl2
(s)-Hg(l), E 0.0455V, (?E/?T)p 3.38 ? 10-4
VK-1. Write the cell reaction and calculate
?rGm, ?rSm, ?rHm, and Qre.
Exercise-2
At 198 K, for cell Pt(s), H2(g,
p?)KOH(aq)HgO(s)-Hg(l) E? 0.926 V, product
of water Kw10-14. Given ?fGm of HgO(s) is 58.5
kJ mol-1, calculate ?fGm of OH?.