Exercises 893Exercises
Redox Review and General Concepts
*001.(a) Define oxidation and reduction in terms of electron
gain or loss. (b) What is the relationship between the
numbers of electrons gained and lost in a redox reaction?
(c) Do all electrochemical cells involve redox reactions?
*002.Define and illustrate (a) oxidizing agent and (b) reducing
agent.
*003.For each of the following unbalanced equations, (i) write
the half-reactions for oxidation and for reduction, and
(ii) balance the overall equation in acidic solution using
the half-reaction method.
(a) Hg^2 Cu88nHgCu^2
(b) MnO 2 Cl88nMn^2 Cl 2
(c) Sn^2 O 2 88nSn^4 H 2 O
*004.For each of the following unbalanced equations, (i) write
the half-reactions for oxidation and reduction, and
(ii) balance the overall equation using the half-reaction
method.
(a) FeSNO 3 88n
NOSO 42 Fe^2 (acidic solution)
(b) Cr 2 O 72 Sn^2 88n
Cr^3 Sn^4 (acidic solution)
(c) S^2 Cl 2 OH88n
SO 42 ClH 2 O (basic solution)
*005.(a) Compare and contrast ionic conduction and metallic
conduction. (b) What is an electrode? (c) What is an inert
electrode?
*006.Support or refute each of the following statements: (a) In
any electrochemical cell the positive electrode is the one
toward which the electrons flow through the wire. (b) The
cathode is the negative electrode in any electrochemical
cell.
*007.For each of the following unbalanced equations, (i) write
the half-reactions for oxidation and reduction, (ii) iden-
tify the species that lose and the species that gain
electrons, and (iii) write the balanced net ionic equation
for the overall reaction.
(a) Zn(s)Cu^2 (aq)88nZn^2 (aq)Cu(s)
(b) NO 2 (aq)Cr 2 O 72 (aq)88n
NO 3 (aq)Cr^3 (aq) (acidic solution)
(c) N 2 O 4 (aq)Br(aq)88n
NO 2 (aq)Br 3 (aq) (basic solution)
*008.For each of the following unbalanced equations, (i) write
the half-reactions for oxidation and reduction, (ii) iden-
tify the species that lose and the species that gain
electrons, and (iii) write the balanced net ionic equation
for the overall reaction.
(a) Ni(s)Cd^2 (aq)88nNi^2 (aq)Cd(s)
(b) MnO 4 (aq)Cu(s)88n
Cu^2 (aq)Mn^2 (aq) (acidic solution)
(c) Cr(OH) 3 (s)IO 3 (aq)88n
I(aq)CrO 42 (aq) (basic solution)Electrolytic Cells: General Concepts
*009.(a) Solids such as potassium bromide, KBr, and sodium
nitrate, NaNO 3 , do not conduct electric current even
though they are ionic. Why? Can these substances be
electrolyzed as solids? (b) Support or refute the statement
that the Gibbs free energy change, G,is positive for any
electrolysis reaction.
*010.(a) Metallic magnesium cannot be obtained by elec-
trolysis of aqueous magnesium chloride, MgCl 2.
Why? (b) There are no sodium ions in the overall cell
reaction for the electrolysis of aqueous sodium chloride.
Why?
*011.Consider the electrolysis of molten aluminum oxide,
Al 2 O 3 , dissolved in cryolite, Na 3 AlF 6 , with inert elec-
trodes. This is the Hall–Héroult process for commercial
production of aluminum (Section 22-6). The following
experimental observations can be made when current is
supplied:
(i) Silvery metallic aluminum is produced at one elec-
trode.
(ii) Oxygen, O 2 , bubbles off at the other electrode.
Diagram the cell, indicating the anode, the cathode, the
positive and negative electrodes, the half-reaction occur-
ring at each electrode, the overall cell reaction, and the
direction of electron flow through the wire.
*012.Do the same as in Exercise 11 for the electrolysis of
molten calcium chloride with inert electrodes. The obser-
vations are
(i) Bubbles of pale green chlorine gas, Cl 2 , are produced
at one electrode.
(ii) Silvery white molten metallic calcium is produced at
the other electrode.
*013.Do the same as in Exercise 11 for the electrolysis of aque-
ous potassium sulfate, K 2 SO 4. The observations are
(i) Bubbles of gaseous hydrogen are produced at one elec-
trode, and the solution becomes more basic around that
electrode.
(ii) Bubbles of gaseous oxygen are produced at the other
electrode, and the solution becomes more acidic around
that electrode.
*014.Do the same as in Exercise 11 for the electrolysis of an
aqueous solution of copper(II) bromide, CuBr 2. The
observations are
(i) One electrode becomes coated with copper metal, and
the color of the solution around that electrode fades.
(ii) Around the other electrode, the solution turns brown,
as bromine is formed and dissolves in water.
*015.(a) Write the equation for the half-reaction when H 2 O is
reduced in an electrochemical cell. (b) Write the equa-
tion for the half-reaction when H 2 O is oxidized in an
electrochemical cell.