Chemistry Reference
In-Depth Information
or:
ΔE = cathode potential - anode potential
(3.24)
or:
Δ
E = E
cathode
- E
anode
(3.25)
If all substances involved in the reaction are in their standard states (all concentra-
tions are specified to be 1 M and the H
2
gas is 1-atm pressure, temperature is 25°C),
the cell is referred to as a standard cell, its voltage being denoted as superscript
zero, or ΔE
0
.
In order to determine the standard electrode potential for a metal, the galvanic
cell is designed so that a half-cell is formed by a piece of metal immersed in a
solution that contains 1.00 M of ions of that metal, and one half-cell with potential
convention defined to be exactly zero volts. This electrode is called the
standard
hydrogen electrode
(SHE) and it consists of a platinum electrode over which H
2
gas at 1-atm of pressure is bubbled, immersed in a solution that contains 1.00 M of
hydronium ion at 25°C.
The notation of the cell formed by the half-cells described above can be:
M(s) |M
n+
(1M) || H
3
O
+
(1M) |H
2
(1atm) |Pt(s)
(3.26)
or
Pt(s) |H
2
(1atm)|H
3
O
+
(1M) || M
n+
(1M) |M(s)
(3.27)
where a single vertical bar represents a boundary between two phases; a double
vertical bar represents a salt bridge or some other device used to maintain electrical
contact between two different solutions that cannot be allowed to mix. The situations
described by Relations (3.26) and (3.27) correspond to two main groups of metals:
1. Metals with an oxidation tendency higher than that of H
2
. These metals oxi-
dize before H
2
and represent the anode in a coupling with SHE. For these
metals:
Δ
E
0
= E
o
H
+
|
H2
|
Pt
- E
o
M
n+
|
M
⇒ E
o
M
n+
|
M
= − ΔE
0
(3.28)
2. Metals with an oxidation tendency lower than that of H
2
. These metals oxi-
dize after H
2
and represent the cathode in a coupling with SHE. For these
metals:
Δ
E
0
= E
o
M
n+
|
M
- E
o
H
+
|
H2
|
Pt
⇒ E
o
M
n+
|
M
= ΔE
0
(3.29)
According to their standard electrode potentials, all metals are arranged in an order
called an
electrochemical series
(
Figure 3.5
)
.
