# ANNOUNCEMENTS
LECTURE 3
PAGE:
1 of 3
Half Reactions (continued from last lecture):
Example 1:
+
Pt(s) | H2 (g, 1 atm) | H
(aq, 1M) ||
-
Br2 (g, 1 atm) | Br (aq,1 M)
+
Anode (oxidation):
H2 (s) 2 H
(aq) +
Cathode (reduction):
Br2 (g) + 2e 2Br
-
2e
-
(aq)
Reference electrode: used to measure all voltage potentials by
comparing it to some standard
NOTETAKER: Courtney Tang
reference
also known as the Standard Hydrogen Electrode
(SHE), which is shown below:
+
-
2 H (aq) + 2e H2 (g)
= 0.000 V
E
Cell potential (electromotive force, emf): measures the ability of a
cell reaction to force ethrough a circuit
Ecell = Ecathode - Eanode
E for cathode and anode are reduction potentials, values straight
from Table 19.1
CLASS: CHE 2C
DATE: 4/6/12
DAY: Friday
Example 2:
Zn(s) | Zn
2+
(aq, 1 M)
|| Cu
2+
(aq, 1 M)
| Cu(s)
2+
+ 2e- Cu(s)
2+
+ 2e- Zn(s)
E for:
Cu
E for:
Zn
E = 0.340 V
E = -0.763 V
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Class: CHE 2C
Lecture: 3
Page 2 of 4
Ecell = Ecathode - Eanode
Ecell = +0.340 V (-0.763 V)
Ecell = +1.103 V
Cathode:
Cu
2+
+ 2e- Cu(s)
Anode:
V
Zn(s) Zn
Zn(s) + Cu
2+
2+
(aq)
E =
+ 2e-
Zn
2+
0.340 V
E =
(aq) +
Cu(s)
+0.763
Ecell = 1.103 V
Example down 3:
Write the cell reaction for the following voltaic cell diagram:
Zn(s) | Zn
2+
(aq)
|| Fe
3+
(aq)
, Fe
2+
(aq)
| Pt(s)
Write a balanced redox reaction for the cell and calculate the cell
potential:
Anode:
-0.762 V
Cathode:
Zn
2{ Fe
Zn
2+
2+
3+
(aq)
(aq)+
(aq)
+ 2e- Zn(s)
+ e- Fe
2Fe
2+
3+
(aq)
E =
(aq)}
Zn(s)+Fe
E = +0.771 V
2+
(aq)
Ecell =
Ecathode - Eanode
Ecell = 0.771 V (0.762 V)
Ecell = 1.53 V
Example 4:
+
Pt(s) | H2 (g, 1 atm) | H
(aq, 1M) ||
Zn
2+
(aq, 1 M) |
Zn(s)
Class: CHE 2C
Lecture: 3
Page 3 of 4
Cathode:
Zn
Anode:
2+
(aq) +
2e- Zn(s)
H2 (g) 2 H
H2 (g) + Zn
2+
(aq)
+
E = -0.763 V
(aq) +
2 H
+
2e-
(aq)
E = 0.000 V
+ Zn(s) Ecell = -0.763 V
If Ecell is negative, the reactions is non-favorable and the reverse
reaction is spontaneous
All metals with negative reduction potentials will react with acid to
form hydrogen gas
Thermodynamics of Electrochemistry
If a charged particle is moved across a potential, work is done
W = zFE;
where z = number of electrons (in moles, unitless)
F = Faradays constant
= 96, 485 Coulomb/mol
E = Electromotive force (V)
G = non-expansion work
= Welec
G = -zFE
The negative sign is the sign convention of work done by the system
Class: CHE 2C
Lecture: 3
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