Chemistry Reference
In-Depth Information
Solution
Silver ion is above zinc ion in Table 17.2, so the zinc half-reaction is reversed:
2 Ag
£ Zn
2
Zn
2 Ag
ε
°
0.80 V
(
0.76 V)
1.56 V
a
[Zn
2
]
[Ag
]
2
a
0.0592
n
a
0.0592
2
c
0.0100
(0.500)
2
ε
ε
°
b
log
b
1.56 V
b
log
d
1.60 V
Two moles of electrons were transferred from zinc to silver ions. The positive
sign for the potential means that the reaction goes as written. The zinc is oxi-
dized and the silver ion is reduced.
Practice Problem 17.4
Calculate the potential of a lead/copper cell
in which the lead electrode is immersed in a 0.100 M lead(II) nitrate and the
copper electrode is immersed in 0.200 M copper(II) sulfate. Determine which
metal is reduced spontaneously.
Snapshot Review
❒
Note that the argument of the log in the Nernst equation is a ratio of
concentrations multiplied together, not a sum; that the concentrations
of the products are in the numerator and those of the reactants are in
the denominator.
ChemSkill Builder 22.5
A. Calculate the potential of the following half-cell with
H
2
gas at 1.000 atm
and hydrogen ion at 0.100 M:
2 H
(aq)
2 e
£ H
2
(g)
17.4
Electrolysis Cells
In Chapter 8, we found that certain decomposition reactions were carried out
by use of electricity. The production of a chemical reaction by means of elec-
tric current is called
electrolysis.
The very most reactive metals and fluorine are
usually produced in their elemental forms in
electrolysis cells.
The production
of aluminum is a prime example.
The requirements for an electrolysis reaction are
1. Ions capable of movement—that is, in the liquid state or in a liquid
solution—so that they can carry the charge.
2. An external source of electric energy with sufficient voltage to cause the
desired reaction to occur.
The conditions under which an electrolysis reaction is carried out are
extremely important. If a solution is used, the most easily oxidized substance
will be oxidized and the most easily reduced substance will be reduced. We can
