CHAPTER 21
Chemistry of the Main-Group Elements
■ ANSWERS TO CONCEPT CHECKS
21.1. Given the high energy needs of animals to move and maintain body temperature, breaking the
very strong triple bond of N2 requires too much energy compared to the lower-energy double
bond of O2.
21.2. The only intermolecular forces in these materials are very weak van der Waals forces.
■ ANSWERS TO SELF-ASSESSMENT AND REVIEW QUESTIONS
21.1. An alloy is a material with metallic properties that is either a compound or a mixture. If the alloy
is a mixture, it may be homogeneous (a solution) or heterogeneous. Gold jewelry is made from an
alloy that is a solid solution of gold containing some silver.
21.2. A metal is a material that is lustrous (shiny), has high electrical and heat conductivities, and is
malleable and ductile.
21.3. A rock is a naturally occurring solid material composed of one or more minerals. A mineral is a
naturally occurring inorganic solid substance or solid solution with a definite crystalline structure.
An ore is a rock or mineral from which a metal or nonmetal can be economically produced.
Bauxite, the principal ore of aluminum, is a rock.
21.4. The basic steps in the production of a pure metal from a natural source follow. (1) Preliminary
treatment: separating the metal-containing mineral from the less desirable parts of the ore. The
mineral may also be transformed by chemical reaction to a metal compound that is more easily
reduced to the free metal. (2) Reduction: The metal compound is reduced to the free metal by
electrolysis or chemical reduction. (3) Refining: The free metal is purified.
(1) Preliminary treatment: Aluminum oxide is obtained from bauxite by the Bayer process.
Bauxite contains aluminum hydroxide, aluminum oxide hydroxide, and other worthless
constituents. It is mixed with hot, aqueous sodium hydroxide solution, which dissolves the
amphoteric aluminum minerals along with some silicates. When the solution is cooled, Al(OH)3
precipitates, leaving the silicates behind. The aluminum hydroxide is finally calcined (heated
strongly in a furnace) to produce purified aluminum oxide, Al2O3. (2) Reduction: Aluminum is
then obtained by reduction using the Hall-Héroult process, which is the electrolysis of a molten
mixture of aluminum oxide in cryolyte (Na3AlF6). (3) Refining: The aluminum can be further
refined and purified.
21.5. Lead is obtained from its sulfide ore by roasting, which forms lead(II) oxide. It is then heated
with coke in a blast furnace, where it is reduced to lead metal.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 21: Chemistry of the Main-Group Elements 853
21.6. (1) Iron oxide is reduced to iron using coke and limestone. The molten iron flows to the bottom of
the blast furnace while carbon dioxide gas escapes. Impurities in the iron react with the calcium
oxide from the limestone and produce slag, which floats to the top. (2) The Mond process is a
chemical procedure that depends on the formation, and later decomposition, of a volatile
compound of the metal (nickel tetracarbonyl). The compound decomposes over pellets of pure
nickel heated to 230C. (3) Copper is purified by electrolysis using a pure copper negative
electrode. Electrons from the impure copper positive electrode flow to the positive pole of the
battery, leaving copper(II) ions behind in the solution. These electrons replace the ones that plate
out on the pure copper electrode, enlarging it.
21.7. The Dow process use seashells (CaCO3) as a source of the base. When heated, calcium carbonate
decomposes to calcium oxide (CaO) and carbon dioxide (CO2).
CaCO3(s)
CaO(s) + CO2(g)
The calcium oxide reacts with water to produce calcium hydroxide, the base.
Ca(OH)2(aq)
CaO(s) + H2O(l)
The calcium hydroxide reacts with the magnesium ion in seawater to form a magnesium
hydroxide precipitate.
Ca(OH)2(aq) + Mg2+(aq)
Mg(OH)2(s) + Ca2+(aq)
The magnesium hydroxide is then treated with hydrochloric acid (HCl) to yield magnesium
chloride.
Mg(OH)2(s) + 2HCl(aq)
MgCl2(aq) + 2H2O(l)
The dry magnesium chloride is melted and electrolyzed at 700C to yield the metal.
MgCl2(l) Mg(l) + Cl2(g)
21.8. The flowchart for the preparation of aluminum from its ore (bauxite) follows.
(1) Preliminary treatment (the Bayer process): Mix the ore with hot NaOH(aq). Cool and filter
the Al(OH)3 precipitate. Heat the precipitate in a furnace to convert to Al2O3.
(2) Reduction (the Hall-Héroult process): Melt a mixture of Al2O3 with cryolyte produced from
Al(OH)3 by the reaction
Al(OH)3(s) + 3NaOH(aq) + 6HF(aq)
Na3AlF6(aq) + 6H2O(l)
The electrolytic cell has carbon electrodes and a molten cryolyte electrolyte, at about
1000C, into which some aluminum oxide is dissolved. The reaction is
2Al2O3(s) + 3C(sq)
electrolysis
4Al(l) + 3CO2(g)
21.9. In a metal, the outer orbits of an enormous number of metal atoms overlap to form an enormous
number of molecular orbitals that are delocalized over the metal. As a result, a large number of
energy levels are crowded together into bands. These bands are half-filled with electrons. When a
voltage is applied to the metal crystal, electrons are excited to the unoccupied orbitals and move
toward the positive pole of the voltage source. This is electrical conductivity.
21.10. The mineral source is given after the name of the metal: lithium–lithium aluminum silicate;
sodium–sodium chloride; magnesium–seawater (Dow process) and dolomite or magnesite;
calcium–calcium oxide; aluminum–bauxite; tin–cassiterite; and lead–galena.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, 854 Chapter 21: Chemistry of the Main-Group Elements
21.11. The reactions are
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
4Li(s) + O2(g) 2Li2O(s)
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
2Na(s) + O2(g) Na2O2(s)
21.12. The reaction is similar to that of lithium carbonate with calcium hydroxide:
Li2CO3(aq) + Ba(OH)2(aq) 2LiOH(aq) + BaCO3(s)
21.13. Potassium is expected to be more reactive than lithium because metals become more reactive
going down Group 1A. This is partly because potassium is much larger, so its 4s electron is lost
more readily than the 2s electron of lithium.
21.14. The reaction is 2Na(s) + 2C2H5OH(l) H2(g) + 2NaOC2H5(aq).
21.15. a. Cathode reaction: Na+(l) + e− Na(l)
Anode reaction: 2Cl−(l) Cl2(g) + 2e−
b. Cathode reaction: Na+(l) + e− Na(l)
Anode reaction: 4OH−(aq) O2(g) + 2H2O(g) + 4e−
21.16. Sodium hydroxide is manufactured by the electrolysis of aqueous sodium chloride, which also
produces chlorine gas as a major product.
21.17. The uses are given after each compound: sodium chloride–used for making sodium hydroxide
and in seasoning; sodium hydroxide–used in aluminum production and in producing sodium
compounds such as soap; and sodium carbonate–used to make glass and as washing soda with
many detergent preparations.
21.18. The main step in the Solvay process involves the reaction of carbon dioxide with ammonia and
sodium chloride to form sodium bicarbonate:
NH3(g) + H2O(l) + CO2(g) + NaCl(aq) NaHCO3(s) + NH4Cl(aq)
21.19. 2Mg(s) + O2(g) 2MgO(s)
Mg(s) + H2O(g) MgO(s) + H2(g)
2Mg(s) + CO2(g) 2MgO(s) + C(s)
21.20. a. Calcium oxide is prepared industrially from calcium carbonate:
CaCO3(s) CaO(s) + CO2(g)
b. Calcium hydroxide is prepared from the reaction of calcium oxide and water:
CaO(s) + H2O(l) Ca(OH)2(aq)
21.21. CaCO3(s) + 2HCl(aq) CO2(g) + CaCl2(aq) + H2O(l)
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 21: Chemistry of the Main-Group Elements 855
21.22. Ca(OH)2 + CO2(g) CaCO3(s) + H2O(l)
Ca(OH)2 + Na2CO3(aq) CaCO3(s) + 2NaOH(aq)
21.23. Fe2O3(s) + 2Al(s) 2Fe(l) + Al2O3(s)
21.24. Some major uses of aluminum oxide: making abrasives for grinding tools, fusing with small
amounts of other metal oxides to make synthetic sapphires and rubies, and making industrial
ceramics.
21.25. To purify municipal water, aluminum sulfate and calcium hydroxide are added to waste water,
forming a gelatinous precipitate of aluminum hydroxide. Colloidal particles of clay (usually
present in the waste water) and other substances adhere to the aluminum hydroxide, whose
particles are large enough to be filtered from the water to purify it.
21.1. Lead(IV) oxide, PbO2, is formed by first packing a paste of PbO into the lead metal grids of the
storage battery. When the battery is charged at the factory, the PbO is oxidized by electrolysis to
PbO2. This gives the proper cathode for a new battery.
21.27. Lead pigments are no longer used for house paints because of the possibility of lead poisoning. If
chips of lead paint are eaten by children, or if the dust from lead paint that has been removed is
breathed by adults or children, lead(II) ion can enter the bloodstream. The lead(II) ion can
ultimately inhibit the production of red blood cells, causing anemia. (The Pb2+ ion can also be
absorbed in the brains of children and cause irreversible brain damage.)
21.28. Pb(NO3)2(aq) + CrO42−(aq) PbCrO4(s) + 2NO3−(aq)
21.29. In the steam-reforming process, steam and hydrocarbons from natural gas or petroleum react at
high temperature and pressure in the presence of a catalyst to form carbon monoxide and
hydrogen. A typical reaction involving propane (C3H8) is
C3H8(g) + 3H2O(g)
Ni
Δ
3CO(g) + 7H2(g)
The carbon monoxide is removed from the mixture by reacting with steam in the presence of a
catalyst to give carbon dioxide and more hydrogen.
CO(g) + H2O(g)
catalyst
Δ
CO2(g) + H2(g)
Finally, the carbon dioxide is removed by dissolving it in a basic aqueous solution.
21.30. The three isotopes of hydrogen and their symbols are as follows: protium, 11 H , or H, which is
most abundant; deuterium, 12 H , or D; and tritium, 13 H , or T. Tritium is radioactive, with a half-life
of 12.3 years.
21.31. The combustion of hydrogen produces more heat per gram than that of any other fuel
(120 kJ/g). Unlike hydrocarbons, it is a clean fuel since the product is environmentally benign
water. These features, in the face of a dwindling supply of hydrocarbons, indicate that hydrogen
gas may become the favorite fuel of the twenty-first century.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
Chemistry of the Main-Group Elements
■ ANSWERS TO CONCEPT CHECKS
21.1. Given the high energy needs of animals to move and maintain body temperature, breaking the
very strong triple bond of N2 requires too much energy compared to the lower-energy double
bond of O2.
21.2. The only intermolecular forces in these materials are very weak van der Waals forces.
■ ANSWERS TO SELF-ASSESSMENT AND REVIEW QUESTIONS
21.1. An alloy is a material with metallic properties that is either a compound or a mixture. If the alloy
is a mixture, it may be homogeneous (a solution) or heterogeneous. Gold jewelry is made from an
alloy that is a solid solution of gold containing some silver.
21.2. A metal is a material that is lustrous (shiny), has high electrical and heat conductivities, and is
malleable and ductile.
21.3. A rock is a naturally occurring solid material composed of one or more minerals. A mineral is a
naturally occurring inorganic solid substance or solid solution with a definite crystalline structure.
An ore is a rock or mineral from which a metal or nonmetal can be economically produced.
Bauxite, the principal ore of aluminum, is a rock.
21.4. The basic steps in the production of a pure metal from a natural source follow. (1) Preliminary
treatment: separating the metal-containing mineral from the less desirable parts of the ore. The
mineral may also be transformed by chemical reaction to a metal compound that is more easily
reduced to the free metal. (2) Reduction: The metal compound is reduced to the free metal by
electrolysis or chemical reduction. (3) Refining: The free metal is purified.
(1) Preliminary treatment: Aluminum oxide is obtained from bauxite by the Bayer process.
Bauxite contains aluminum hydroxide, aluminum oxide hydroxide, and other worthless
constituents. It is mixed with hot, aqueous sodium hydroxide solution, which dissolves the
amphoteric aluminum minerals along with some silicates. When the solution is cooled, Al(OH)3
precipitates, leaving the silicates behind. The aluminum hydroxide is finally calcined (heated
strongly in a furnace) to produce purified aluminum oxide, Al2O3. (2) Reduction: Aluminum is
then obtained by reduction using the Hall-Héroult process, which is the electrolysis of a molten
mixture of aluminum oxide in cryolyte (Na3AlF6). (3) Refining: The aluminum can be further
refined and purified.
21.5. Lead is obtained from its sulfide ore by roasting, which forms lead(II) oxide. It is then heated
with coke in a blast furnace, where it is reduced to lead metal.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 21: Chemistry of the Main-Group Elements 853
21.6. (1) Iron oxide is reduced to iron using coke and limestone. The molten iron flows to the bottom of
the blast furnace while carbon dioxide gas escapes. Impurities in the iron react with the calcium
oxide from the limestone and produce slag, which floats to the top. (2) The Mond process is a
chemical procedure that depends on the formation, and later decomposition, of a volatile
compound of the metal (nickel tetracarbonyl). The compound decomposes over pellets of pure
nickel heated to 230C. (3) Copper is purified by electrolysis using a pure copper negative
electrode. Electrons from the impure copper positive electrode flow to the positive pole of the
battery, leaving copper(II) ions behind in the solution. These electrons replace the ones that plate
out on the pure copper electrode, enlarging it.
21.7. The Dow process use seashells (CaCO3) as a source of the base. When heated, calcium carbonate
decomposes to calcium oxide (CaO) and carbon dioxide (CO2).
CaCO3(s)
CaO(s) + CO2(g)
The calcium oxide reacts with water to produce calcium hydroxide, the base.
Ca(OH)2(aq)
CaO(s) + H2O(l)
The calcium hydroxide reacts with the magnesium ion in seawater to form a magnesium
hydroxide precipitate.
Ca(OH)2(aq) + Mg2+(aq)
Mg(OH)2(s) + Ca2+(aq)
The magnesium hydroxide is then treated with hydrochloric acid (HCl) to yield magnesium
chloride.
Mg(OH)2(s) + 2HCl(aq)
MgCl2(aq) + 2H2O(l)
The dry magnesium chloride is melted and electrolyzed at 700C to yield the metal.
MgCl2(l) Mg(l) + Cl2(g)
21.8. The flowchart for the preparation of aluminum from its ore (bauxite) follows.
(1) Preliminary treatment (the Bayer process): Mix the ore with hot NaOH(aq). Cool and filter
the Al(OH)3 precipitate. Heat the precipitate in a furnace to convert to Al2O3.
(2) Reduction (the Hall-Héroult process): Melt a mixture of Al2O3 with cryolyte produced from
Al(OH)3 by the reaction
Al(OH)3(s) + 3NaOH(aq) + 6HF(aq)
Na3AlF6(aq) + 6H2O(l)
The electrolytic cell has carbon electrodes and a molten cryolyte electrolyte, at about
1000C, into which some aluminum oxide is dissolved. The reaction is
2Al2O3(s) + 3C(sq)
electrolysis
4Al(l) + 3CO2(g)
21.9. In a metal, the outer orbits of an enormous number of metal atoms overlap to form an enormous
number of molecular orbitals that are delocalized over the metal. As a result, a large number of
energy levels are crowded together into bands. These bands are half-filled with electrons. When a
voltage is applied to the metal crystal, electrons are excited to the unoccupied orbitals and move
toward the positive pole of the voltage source. This is electrical conductivity.
21.10. The mineral source is given after the name of the metal: lithium–lithium aluminum silicate;
sodium–sodium chloride; magnesium–seawater (Dow process) and dolomite or magnesite;
calcium–calcium oxide; aluminum–bauxite; tin–cassiterite; and lead–galena.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, 854 Chapter 21: Chemistry of the Main-Group Elements
21.11. The reactions are
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
4Li(s) + O2(g) 2Li2O(s)
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
2Na(s) + O2(g) Na2O2(s)
21.12. The reaction is similar to that of lithium carbonate with calcium hydroxide:
Li2CO3(aq) + Ba(OH)2(aq) 2LiOH(aq) + BaCO3(s)
21.13. Potassium is expected to be more reactive than lithium because metals become more reactive
going down Group 1A. This is partly because potassium is much larger, so its 4s electron is lost
more readily than the 2s electron of lithium.
21.14. The reaction is 2Na(s) + 2C2H5OH(l) H2(g) + 2NaOC2H5(aq).
21.15. a. Cathode reaction: Na+(l) + e− Na(l)
Anode reaction: 2Cl−(l) Cl2(g) + 2e−
b. Cathode reaction: Na+(l) + e− Na(l)
Anode reaction: 4OH−(aq) O2(g) + 2H2O(g) + 4e−
21.16. Sodium hydroxide is manufactured by the electrolysis of aqueous sodium chloride, which also
produces chlorine gas as a major product.
21.17. The uses are given after each compound: sodium chloride–used for making sodium hydroxide
and in seasoning; sodium hydroxide–used in aluminum production and in producing sodium
compounds such as soap; and sodium carbonate–used to make glass and as washing soda with
many detergent preparations.
21.18. The main step in the Solvay process involves the reaction of carbon dioxide with ammonia and
sodium chloride to form sodium bicarbonate:
NH3(g) + H2O(l) + CO2(g) + NaCl(aq) NaHCO3(s) + NH4Cl(aq)
21.19. 2Mg(s) + O2(g) 2MgO(s)
Mg(s) + H2O(g) MgO(s) + H2(g)
2Mg(s) + CO2(g) 2MgO(s) + C(s)
21.20. a. Calcium oxide is prepared industrially from calcium carbonate:
CaCO3(s) CaO(s) + CO2(g)
b. Calcium hydroxide is prepared from the reaction of calcium oxide and water:
CaO(s) + H2O(l) Ca(OH)2(aq)
21.21. CaCO3(s) + 2HCl(aq) CO2(g) + CaCl2(aq) + H2O(l)
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 21: Chemistry of the Main-Group Elements 855
21.22. Ca(OH)2 + CO2(g) CaCO3(s) + H2O(l)
Ca(OH)2 + Na2CO3(aq) CaCO3(s) + 2NaOH(aq)
21.23. Fe2O3(s) + 2Al(s) 2Fe(l) + Al2O3(s)
21.24. Some major uses of aluminum oxide: making abrasives for grinding tools, fusing with small
amounts of other metal oxides to make synthetic sapphires and rubies, and making industrial
ceramics.
21.25. To purify municipal water, aluminum sulfate and calcium hydroxide are added to waste water,
forming a gelatinous precipitate of aluminum hydroxide. Colloidal particles of clay (usually
present in the waste water) and other substances adhere to the aluminum hydroxide, whose
particles are large enough to be filtered from the water to purify it.
21.1. Lead(IV) oxide, PbO2, is formed by first packing a paste of PbO into the lead metal grids of the
storage battery. When the battery is charged at the factory, the PbO is oxidized by electrolysis to
PbO2. This gives the proper cathode for a new battery.
21.27. Lead pigments are no longer used for house paints because of the possibility of lead poisoning. If
chips of lead paint are eaten by children, or if the dust from lead paint that has been removed is
breathed by adults or children, lead(II) ion can enter the bloodstream. The lead(II) ion can
ultimately inhibit the production of red blood cells, causing anemia. (The Pb2+ ion can also be
absorbed in the brains of children and cause irreversible brain damage.)
21.28. Pb(NO3)2(aq) + CrO42−(aq) PbCrO4(s) + 2NO3−(aq)
21.29. In the steam-reforming process, steam and hydrocarbons from natural gas or petroleum react at
high temperature and pressure in the presence of a catalyst to form carbon monoxide and
hydrogen. A typical reaction involving propane (C3H8) is
C3H8(g) + 3H2O(g)
Ni
Δ
3CO(g) + 7H2(g)
The carbon monoxide is removed from the mixture by reacting with steam in the presence of a
catalyst to give carbon dioxide and more hydrogen.
CO(g) + H2O(g)
catalyst
Δ
CO2(g) + H2(g)
Finally, the carbon dioxide is removed by dissolving it in a basic aqueous solution.
21.30. The three isotopes of hydrogen and their symbols are as follows: protium, 11 H , or H, which is
most abundant; deuterium, 12 H , or D; and tritium, 13 H , or T. Tritium is radioactive, with a half-life
of 12.3 years.
21.31. The combustion of hydrogen produces more heat per gram than that of any other fuel
(120 kJ/g). Unlike hydrocarbons, it is a clean fuel since the product is environmentally benign
water. These features, in the face of a dwindling supply of hydrocarbons, indicate that hydrogen
gas may become the favorite fuel of the twenty-first century.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
