Transcript Powerpoint

Ch 14: Electrode Potentials
Hanford, WA
Na2S2O4 reduces
Fe3+ in soil to Fe0
Fe
CrO42-(aq) + Fe(s) + 4H2O(l)  Cr(OH)3(s) + Fe(OH)3(s) + 2OH-(aq)
Fe catalyst
ClHC=CCl2(aq)
H2C=CH2 + HCCH + Cl- (unbalanced)
Redox Reaction Review
8H+ + Cr2O72- + 3H2S  2Cr3+ + 7H2O + 3S
ox ½ rxn:
red ½ rxn:
reducing agent =
oxidizing agent =
Some Terminology
Current in Amp = C/s
Volt (emf) = 1 Joule/Coulomb (J/C)
Charge on electron (e) = 1.60 x 10-19 C
Faraday (F) = charge on a mole of e's
= NA x e
= 96,485 C/mol
Galvanic Cells (Sec 14-2)
Be able to:
•
draw the line diagram
and cell
•
identify cathode and
anode and 1/2 rxn's
•
determine electrode
polarities & direction of
electron flow
•
give the net cell reaction
•
Calculate Eocell, E, & K
Zn | ZnCl2(1.0M) || CuSO4(1.0M) | Cu(s)
Example p. 308: Write a line diagram for the given cell.
Write the half reactions and the net cell reaction.
Standard Potentials (Sec 14-3)
Standard Hydrogen
Electrode (SHE):
aH+ = 1.00 M
PH2 = 1.00 atm
Eo = 0.00 V (by
definition)
a more complete table -
Back to the Zn/Cu galvanic cell
in Section 14.2:
E ocell  Eocathode  Eoanode  E o  Eo
Zn | ZnCl2(1.0M) || CuSO4(1.0M) | Cu(s)
E > 0 spontaneous
E = 0 equilibrium
E < 0 spontaneous reverse
direction
The Nernst Equation (Sec 14-4)
calculating the non-standard cell potential
for aA + bB = cC + dD, from Thermodynamics we know c
d
[C]
[D]
ΔG  ΔGo  2.303RT log a b and Δ G  nFE
[A] [B]
c
d
[C]
[D]
 nFE  nFE o  2.303RT log a b
[A] [B]
2.303RT
[C] c [D] d
E E 
log a b
nF
[A] [B]
o
c
d
0.0592
[C]
[D]
E  Eo 
log a b at 25 o C
n
[A] [B]
Example p. 314: Find the voltage of the cell in Figure 14-4 if the right
half-cell contains 0.50M KCl(aq) and the left half-cell contains 0.010M
Cd(NO3)2(aq). Write the net cell reaction and state whether it is
spontaneous in the forward or reverse direction.
Eo and the Equilibrium Constant (Sec 14-5)
aA + bB = cC + dD
c
d
0.0592
[C]
[D]
E  Eo 
log a b at 25 o C
n
[A] [B]
at equilibrium -
c
d
0.0592
[C]
[D]
0  Eo 
log a b
n
[A] [B]
K
0.0592
E 
log K
n
o
K  10
nE o
0.0592
Example p. 318: Find the equilibrium constant for the reaction
Fe3+ + Ag(s) + Cl- = Fe2+ + AgCl(s), which is the net cell reaction
in Fig. 14-9.
Electrochemical Measurement of Kw
Reference Electrodes (Sec. 14-6)
The Standard Hydrogen Electrode (SHE) is inconvenient to
use, so other more practical electrodes are commonly used in
the lab and in the field. Their half-potentials have been
measured relative to the SHE.
Ag/AgCl Reference Electrode
AgCl(s) + e = Ag(s) + ClEo = +0.222 V (1.0 M KCl)
= +0.197 V (sat’d KCl)
Calomel Reference Electrode
½Hg2Cl2(s) + e = Hg(l) + Cl-
Eo = +0.268 V (1.0 M KCl)
= +0.241 V (sat’d KCl)
Voltage Conversions Between Different
Reference Scales