Chemistry Reference
In-Depth Information
Symbols
e
(charge on the electron) (17.2)
(electron) (17.1)
ε
(potential) (17.3)
ε°
(standard potential) (17.2)
e
Summary
V
oltaic cells produce electric current by means of oxida-
tion and reduction reactions occurring at different loca-
tions. A complete circuit is necessary, including a wire to
transport the electrons and a path for ions to migrate to
maintain charge neutrality at each electrode. To allow for
ion mobility, the reactants must be in a solution or a
molten salt. (Section 17.1)
The inherent tendencies of redox half-reactions to
proceed are measured with standard half-cell potentials.
The larger the (positive) potential, the greater the tendency
for a reduction to occur. When combining half-cells into a
complete cell, reversing an equation requires changing the
sign of the potential, but altering the coefficients in an
equation does not affect the potential. (Section 17.2)
The potential of a half-cell or cell is dependent
on the concentrations of the reagents involved. The
potential can be calculated using the Nernst equation.
(Section 17.3)
Electrolysis reactions use direct current to produce
redox reactions. Production of very active elemental
metals and very active elemental nonmetals is often
done using electrolysis. The conditions under which
an electrolysis is carried out often make a great deal
of difference as to which products are obtained.
(Section 17.4)
Calculation of the quantity of reaction from the
quantity of charge that passes, or vice versa, can be done
using the techniques of Sections 10.1 and 10.3, as long as
we know how to measure the quantity of charge. Current
in amperes times time in seconds gives the charge in
coulombs, and coulombs can easily be converted to fara-
days (moles of electrons). (Section 17.5)
Items for Special Attention
If for a cell is positive, it is an
indication
that the cell
reaction is spontaneous as written. If for a cell is positive,
it is
certain
that the cell reaction is spontaneous as written.
ε
°
in the denominator. The exponents are the corresponding
coefficients from the balanced equation. The terms are
multiplied and/or divided, not added or subtracted.
■
ε
In the Nernst equation, the concentration terms of the
products
are in the numerator and those of the
reactants
are
■
Answers to Snapshot Reviews
2 Ag
(aq) £
17.1 A. (a) and (b)
Ni(s)
Cu
2
H
Ni
2
17.4 A. (a)
(b)
(d)
Ni
2
(aq)
2 Ag(s)
Ag
(aq)
e
£ Ag(s)
17.5 A.
B. Because electrons
must
be in every half-reaction equa-
tion, there must be at least one ionic species included
in every equation to balance the charge.
a
1 mol e
1 mol Ag
a
1 mol Ag
108 g Ag
a
96,500 C
1 mol e
1 s
2.50 C
6.00 g Ag
b
b
b
a
b
2140 s
35.7 min
Sn
4
(aq)
2 e
£ Sn
2
(aq)
17.2 A.
ε°
0.13 V
Pb(s) £ Pb
2
(aq)
2 e
ε°
0.13 V
Sn
4
(aq)
Pb(s) £
Pb
2
(aq)
Sn
2
(aq)
ε°
0.26 V
2 H
(aq)
2 e
£ H
2
(g)
17.3 A.
ε°
0.000 V
P
H
2
[H
]
2
0.0592
2
ε
ε°
log
1
(0.100)
2
0.000
0.0296
log
0.592 V
