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Eo values are given for the left to right direction in equations. Because E o is negative
in the Zn2+| Zn half-cell, the reaction which usually occurs is:
Zn2+(aq) + 2e –
A Gas and its Aqueous Ions
Gas / aqueous ion half-cells require an inert electrode to carry the current in or out of
the half-cell, because gases cannot form conducting electrodes. Platinum Pt, or
carbon C (in the form of graphite), are often used as electrodes.
The half-cell for a gas X2 (g) and its aqueous ions X- (aq) is written as:
X2 (g) , X –(aq)/ inert electrode. (The / is used to separate the conductor from the rest
of the half cell.)
For the chlorine/chloride half-cell, written as,
Cl2 (g), Cl –(aq) / Pt (or C):
 The half-reaction is Cl2 (g) + 2e2Cl-(aq) Eo = +1.36 V
 Cl2(g) is the oxidant
 Cl-(aq) is the reductant
Because Eo is positive in the Cl2, Cl-(aq) half-cell, the reaction which usually occurs
Cl2(g) + 2e2Cl-(aq)
Two Types of Aqueous Ions
Ions can be oxidised and reduced to form other ions, so aqueous ion / aqueous ion
half-cells exist. Ions, like gases cannot form conducting electrodes, so an inert
electrode is used in these half-cells.
The half-cell for the oxidised for of an aqueous ion (M2+(aq)) and the reduced form of
the aqueous ion (M+(aq)) is written as:
M2+(aq), M+(aq) / inert electrode. (The comma is used to separate the two reactants
in the same solution and the / indicates a phase separation between the solution and
the electrode.)
For the iron (III) | iron (II) half-cell, written as
Fe3+(aq), Fe2+(aq) / Pt (or C):
 The half-reaction is Fe3+(aq) + eFe2+(aq)
 Fe (aq) is the oxidant
 Fe2+(aq) is the reductant
Eo = +0.77 V
Because Eo is positive in the Fe3+(aq),Fe2+(aq) half-cell, the reaction that usually
occurs is:
Fe3+(aq) + eFe2+(aq)
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