Chapter 21Main Group of Elements:
H
Li
Na Mg
K Ca
B C
Al Si
N
P
O
S
F
Cl
Br
Element Abundances
•
•
•
Note low abundance of Li, Be, and B
See also alternation of abundance with atomic number.
Even atomic number = more abundant
Ionic Compounds
• Elements form ions with electron configurations
that are the same as those for the previous or
following noble gases (ns2np6) are crystalline
solids with high melting points and conduct
electricity in the molten state.
• Metal + Nonmetal  Product / Name
Ba
+
Cl2  BaCl2 / Barium chloride
+
S8
 8 Na2S / Sodium sulfide
16 Na
+
Al
O2  Al2O3
Molecular Compounds
• Metalloids and nonmetals generally form molecular
compounds.
• Molecular compounds have covalent bonds.
• Nonmetal + Nonmetal  Product
/ Name
As
+
F2
 AsF5 / ________________
Ge
+
O2
 GeO2 / Germanium dioxide
+
P4
 4 PH3 / Phosphine
6 H2
Recognize the Incorrect Formula and give the
Correct Formula for each of the following:
PF5
CsSO4
MgO
Ba2SO4
CO3
CO
K2Cl
CaCH3CO2
Hydrogen - Properties
• The rocket engine in the Shuttle
itself is fueled by H2 + O2.
• 1H
1.007825 amu protium
• 2H = D 2.014102
deuterium
• 3H = T 3.016049
tritium
Half-life of tritium = 12.35 years
Hydrogen - Preparation
• Synthesis gas:
C + H2O --> H2 + CO
• Steam reforming process:
C3H8 + 3 H2O --> 3 CO + 7 H2
• Water gas shift reaction:
CO + H2O ---> CO2 + H2
• Electrolysis of water:
H2O ---> H2 + 1/2 O2
• Metal + Acid:
Mg + 2 HCl --> MgCl2 + H2
• Metal + KOH:
2 Al + 2 KOH + 6 H2O  2 KAl(OH)4 + H2
Hydrogen - Reactions
• Virtually every element (except Group
8) will form compounds with H.
H2 + Br2 --> 2 HBr.
• Haber Process: N2 + 3 H2 --> 2 NH3
• Fuel cell - reactants are supplied
continuously from an external source.
• Cars can use electricity generated by
H2/O2 fuel cells.
• One way to store H2 is to adsorb the
gas onto a metal or metal alloy (in
interstitial sites).
• Metal hydrides: Ca + H2  CaH2
Alkali Metals – Group IA
• Highly reactive.
• The important characteristic
of Group 1A elements is their
vigorous reaction with water.
• Solids are usually stored
under mineral oil.
• All of the Group 1A metals
are relatively soft and can be
cut with a knife.
http://video.google.com/videoplay?docid=-2134266654801392897
Sodium and Potassium - Properties
• 2 Na (s) + 2 H2O(l) -- > 2 Na+ (aq) + 2 OH- (aq) + H2
(g)
• 2 K (s) + Br2 (l) --> 2 KBr (s)
• 2 Na(s) + O2(g) ---> Na2O2 (s)
Sodium peroxide
• K(s) + O2(g) ---> KO2(s)
Potassium superoxide
• KO2 used in breathing apparatus:
4 KO2(g) + 2 CO2 (g) ---> 2 K2CO3(s) + 3 O2(g)
Sodium and Potassium - Preparation
• Na is prepared by the electrolysis
of molten Na in a “Downs cell”
Operating at 7-8 V with 25k-40kA at
T = 600 oC.
Na mp = 97.8oC.
• K is prepared from molten KCl:
Na(g) + KCl(l) --> K (g) + NaCl (l)
Li, Na, K – Important Compounds
• Chlor-alkali industry: electrolysis of NaCl(aq) - brine
2 NaCl(aq) + 2 H2O(l)  Cl2(g) + 2 NaOH(aq) + H2(g)
• Na2CO3: soda ash or washing soda, used as an industrial base, in
making soap, and in making glass. It is mined as trona:
Na2CO3•NaHCO3•2H2O
• Soda-lime process: to produce NaOH from inexpensive lime (CaO)
and soda.
CaO(s) + Na2CO3(aq) + H2O(l)  2 NaOH(aq) + CaCO3(s)
• NaHCO3: baking soda, used in table salt:
MgCl2(s) + 2 NaHCO3(s)  MgCO3(s) + 2 NaCl(s) + H2O(l) + CO2(g)
• KNO3: used as oxidant in gunpower (mixed with C and S).
2 KNO3(s) + 4C(s)  K2CO3(s) + 3 CO(g) + N2(g)
2 KNO3(s) + 2S(s)  K2SO4(s) + SO2(g) + N2(g)
Alkaline Earth Metals – Group 2A
• Their compounds neutralize
acids, have a very high melting
point.
• Ca and Mg 5th and 7th in
abundance on Earth.
• Very reactive – only found in
compounds. Many of these of low
solubility: they are present as
minerals.
• Be is toxic.
Celestite: SrSO4
Calcium and Magnesium - Properties
• High melting, silvery metals, their ions have a 2+ charge.
• React with oxygen:
Ca + O2  CaO
halogens:
Ca + Cl2  CaCl2
water:
Mg + 2 H2O  Mg(OH)2 + H2
acids:
Mg + 2 HCl  MgCl2 + H2
• Ca:
limestone
gypsum
fluoride
CaCO3
CaSO4 2 H2O
CaF2
• Mg:
magnesite
talc
MgCO3
3 MgO . 4 SiO2 . H2O
asbestos
dolomite
3 MgO 4 SiO2 2 H2O
MgCa(CO3)2
.
.
Magnesium - Production
Ca, Mg - Applications
• Production of HF:
CaF2(s) + H2SO4(l)  2 HF(g) + CaSO4 (s)
• Production of steel.
• Production of H3PO4:
Ca5F(PO4)3(s) + 5 H2SO4(l)  5 CaSO4 +
fluoroapatite
3 H3PO4(aq) + HF(g)
• CaO (lime)
• CaCO3 (limestone)
• Mortar from slaked lime:
Ca(OH)2(s) + CO2 (g)  CaCO3(s) + H2O(l)
• Hard water:
CaCO3(s) + H2O(l) + CO2(g)
Ca2+(aq) + 2 HCO3-(aq)
CO2 can be removed by evaporation and CaCO3 precipitates.
• Very important biological elements: chlorophyll (contains Mg2+).
• Barium compounds have also medical applications.
Group 3A
• Boron: metalloid.
• Aluminum, Gallium, Indium, Thallium: metals.
• Al is the third most abundant element in the earth
crust. All other elements are very rare.
• Diagonal relationship: Li  Mg; Be  Al; B Si
• B2O3 boric oxide, B(OH)3 boric acid, in general borates
~ silicates.
• Be(OH)2 and Al(OH)3 are both amphoteric.
• Chlorides, bromides, iodides react vigorously with water
(same as silicon compounds).
• Hydrides of boron and silicon are volatile and
flammable.
• Hydrides of beryllium and aluminum are colorless,
nonvolatile solids extensively polymerized: Al-H-Al
bonds.
• Electronic configuration is ns2np1 (generally have an
oxidation number = 3+, but heavier elements have 1+).
Boron
• Borax: Na2B4O7 10. H2O
• Pure B can be obtained electrochemically
from the oxide or halide.
• Mg can be used for chemical reductions:
B2O3(s) + 3 Mg(s)  2 B(s) + 3 MgO(s)
• B has several allotropes, all have an
icosahedron (2-sided polyhedron) for 12
covalently linked boron atoms. Due to this
B is very hard, refractory, an a semiconductor.
• Al, Ga, In, and Tl are all relatively lowmelting, soft metals with high electrical
conductivity.
Boron - Therapy
•
10B
isotope (not 11B) has the ability to capture
slow neutrons
• In BNCT, tumor cells preferentially take up a
boron compound, and subsequent irradiation by
slow neutrons kills the cells via the energetic 10B
--> 7Li neutron capture reaction (that produces a
photon and an alpha particle)
•
10B
+ 1n ---> 7Li + 4He + photon
Boron - Compounds
• Borax: Na2B4O7 . 10 H2O is used to dissolve other metal oxides
– cleaning metallic surfaces.
• A better formula for the ion of borax is: B4O5(OH)42• Can produce boric acid (a very weak acid):
Na2B4O7 10 H2O(s) + H2SO4(aq)  4 B(OH)3(aq) +
Na2SO4(aq) + 5 H2O(l)
O
HO
B
B OH
O
O B
HO
O
O
B OH
Boron - Compounds
• Boric acid has both Lewis and Brownsted acid behavior.
• Used as antiseptic.
• Boric acid is dehydrated to boric oxide:
2 B(OH)3(s)  B2O3(s) + 3 H2O(l)
• Borosilicate glass: 76% SiO2, 13% B2O3, Al2O3, Na2O.
Gives the glass higher melting T, better resistance to acids.
• B is less electronegative and H, in these hydrides H has a negative
charge. Boranes: BxHy:
• Diborane B2H6 is a colorless, gaseous compound.
Boron - Compounds
• Boranes are electron-deficient molecules, hydrogens have to two
bonds, to to B atoms.
• Diborane has a large endothermic DHof = +41 kJ/mol
• Considered a good fuel:
B2H6(g) + 3 O2(g)  B2O3(s) + 3 H2O(g) DHo = -2038 kJ
sp3
H
H
H
B
B
H
H
2e- spread over 3 orbitals
H
Boron - Compounds
• Synthesized from sodium borohydride (white, crystalline,
water-soluble solid made from NaH and borate).
2 NaBH4(s) + I2(s)  B2H6(g) + 2 NaI(s) + H2(g)
4 NaH(s) + B(OCH3)3(g)  NaBH4(s) + 3 NaOCH3(s)
• Sodium borohydride is used to bleach wood pulp.
• It is also used in the electrode-less plating of metals onto
plastics. It is a reducing agent used to reduce aldehydes,
carboxylic acids, and ketones.
Boron - Compounds
H
H
C
H
Element
Valence eElectroneg.
Radius
••
B
C
H
B
3
2.0
88
N
B—N š bonding
C
4
2.5
77
N
5
3.0
70 pm
Boron - Compounds
H
B2H6 + NH3
NaBH4
Borazine: Mol. wt. = 80.5
Mp = -57 ˚C; Bp = 55 ˚C
B—N = 144 pm
H
B
H
••
N
N
••
BCl3 + NH4Cl
B
H
N
••
B
H
H
Benzene: Mol. wt. = 78.1
Mp = 6 ˚C; Bp = 80 ˚C
C—C = 142 pm
Boron - Compounds
• Boron halides BX3 are monomeric
and volatile.
• Boron halides form Lewis acid
complexes.
BF3
BCl3
BBr3
BI3
Mp, ˚C
-127.1
-107
-46
49.9
Bp, ˚C
-99.9
12.5
91.3
210
Unused p orbital
F
B
F
F
Low p bonding
F
B
F
F
Order of Lewis acidity: BF3 < BCl3 < BBr3 < BI3
Aluminum
• Low cost, alloys with other metals,
pure
is soft and weak.
• Low density, strength, inert to corrosion.
• Aluminum foil, cans, parts of aircraft.
• Alloy: 4% copper and some Si, Mg, and Mn.
• Softer aluminum like for window frames
contain only Mn.
• Al is easily oxidized and forms a protective
coating of Al2O3 preventing further corrosion.
4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) ∆H˚ = – 3351 kJ
Aluminum - Production
• Al is obtained electrochemically:
Hall-Heroult method.
• In nature: aluminosilicates (Al, .
Si, O), can be break down in
aluminum oxides: Al2O3 . nH2O Al2 O3
Fe2O3
SiO2
Amphoteric
Basic
bauxite.
Acidic
• Al2O3 is separed from iron
190 ūC, 30% NaOH
and silicon by the Bayer
process (based on amphoteric
in solution
Na2Si(OH)6
NaAl(OH)4
Fe2O3, solid
properties):
add CO2 (acidic oxide)
Al2 O3
H2O + CO2 + 2 NaAl(OH)4
Na2CO3 + Al2O3 + 5 H2O
Aluminum - Compounds
Compound
Mp, ˚C
Subl. Temp.
AlF3
1290
1272
AlCl3
192.4
180
AlBr3
97.8
256
AlI3
189.4
382
Aluminum - Compounds
•
•
•
•
•
•
F
F
Al
F
F
AlF3 is a lattice of Al3+ and F- ions
F
F
F
Al
Octahedral Al3+
F
F
F
F bridges
F
Found in cryolite, Na2AlF6
Solid AlCl3 is a layer lattice of 6-coordinate Al3+ ions.
At mp the solid volume increases 85% and electrical conductivity
decreases.
221 pm
••
••
Cl
••
Cl
••
Cl
••
••
Cl
Al
101Þ
Al
Cl
118Þ
Cl
206 pm
In liquid and gas phase
AlCl3 is dimer.
AlBr3 and AlI3 are
dimers in all phases.
Aluminum - Compounds
•
Aluminum sulfate is the most
important Al compound after
Al(OH)3 and Al2O3.
•
Used in paper industry and as
a flocculant in water purification.
•
As pH increases, associated
species form. Their large
charge nucleates fine,
suspended dirt particles.
4+
H
O
Al(H2O)4
(OH2)4Al
O
H
• Aluminum hydroxide and
oxides have many different forms:
a-Al2O3
Corundum – Very
hard – used as abrasive in
sandpaper and toothpaste
a -AlO(OH) Diaspore
a -Al(OH)3 Bayerite
g -Al2O3
g -AlO(OH) Boehmite
g -Al(OH)3 Gibbsite
Group 4A
• C, Si, Ge, Sn, Pb
• General features:
Moving away from
metallic character
ns2np2
configurations
“inert pair” effect
leads to Ge2+, Sn2+,
Pb2+
Carbon
• Layers of 6-member carbon rings.
• sp2 C atoms.
• Extended π bonding throughout the layers.
Allotrope
Density
Hardness
∆H˚f
Graphite
2.266
<1
0
Diamond
3.514 g/cm3
10 Mohs
+1.90 kJ/mol
• Graphite has high electrical conductivity
• Diamond—has highest thermal
conductivity of any known material
Carbon
• Uses of natural graphite: Steelmaking, refractories, crucibles,
lubricants, brake linings, pencil lead (75,000 tons/year)
• Uses of artificial graphite: electrodes, crucibles, motor brushes,
fibers (350,000 tons/year).
SiO2 + C --> (SiC) + CO2
SiC (2500 ˚C) --> Si + C
• Heat coal in absence of air --> coke used in steelmaking (370 x 106
tons/year)
• Incomplete combustion of hydrocarbons - > carbon black used
93% in tires (~3 kg in car tire), 3% in printing ink (>10 million
tons/day).
• Burn carbon in high oxygen atmosphere - > activated charcoal
used in water and air filters (holes of 1-8 nm diameter -> high
surface area: 1000m2/g)
Carbon
• Allotrope: Fullerene
5- and 6-member carbon rings.
C atoms are bound into a sphere with 60 C atoms.
Silicon
• Quartz or sand + high purity coke --> Si
– SiO2 + 2 C --> Si + 2 CO
*Silicon is (oxidized or reduced)
*Carbon is (oxidized or reduced)
• Making very pure silicon:
– 1) Si + Cl2 --> SiCl4
– 2) SiCl4 + Mg --> MgCl2 + Si
* Identify the substances reduced and the
substances oxidized on these reactions.
• To purify the silicon, it is zone-refined
Tin
• Sn is relatively expensive, but used because it resists
corrosion.
• Pure Sn can be obtained electrochemically from SnCl2
– About 40% used in “tin plate”
• “Tin cans” have 0.0004 - 0.025 mm layer of Sn on iron
• About 30 x 109 cans plated annually in US
• Alloys:
Solder: 1/3 Sn and 2/3 Pb
Bronze: 5-10% Sn + Cu
Pewter: 90-95% Sn, 1-8% Sb, and < 3% Cu
Bearing metal: 80-90% Sn, 5% Cu, and Pb
Lead
• Most abundant of the “heavy metals”
• Romans used it in “plumbing”
– the word comes from the Latin name for the
element
•
•
•
•
Main ore is galena, PbS
2 PbS + 3 O2 --> 2 PbO + 2 SO2(g)
PbO + C --> Pb + CO
About 60% of the batteries sold are Pb
storage batteries
ANODE:
Pb(s) + HSO4- --> PbSO4(s) + H+(aq) + 2eCATHODE:
PbO2(s) + 3 H+(aq) + HSO4-(aq) + 2e- --> PbSO4(s)
+ 2 H2 O
Carbon - Compounds
• CO2 — over 30 x 106 tons produced in
US/year.
– 1/2 used as refrigerant and propellant in
aerosols
– 1/4 used to “carbonate” soft drinks
• CO2 is a “greenhouse” gas.
Silicon - Compounds
SiO2 is not like CO2
• Reason is that 2 Si=O bonds are weaker (~640 kJ
each) than 4 Si-O bonds (464 kJ each)
• Also orbital overlap to form Si=O is not efficient.
• Most common form is alpha-quartz
• Less pure forms are rose quartz, smoky quartz,
amethyst, citrine.
• All silicon-oxygen compounds have corner-shared SiO4
tetrahedra
• a-Quartz has interlinked helical
chains of SiO4 tetrahedra.
• Helices can be right- or left-handed,
so crystals are optically active.
Silicon - Compounds
• Quartz is a key electronic material,
2nd only to Si in volume.
• Citrine and amethyst have Fe2+/Fe3+
impurities in quartz that give color.
• Quartz consists of interlinked chains
of SiO4 units.
Silicon - Compounds
• Quartz exhibits the property of piezoelectricity.
– The production of an electric dipole when the crystal is
deformed.
• Piezoelectric effect is used to control oscillators in
electric circuits such as watches and radios.
• Most quartz used commercially is synthetic.
• At 350-400 ˚C and 1-4 kilobars, SiO2
dissolves slightly in 1 M NaOH
3 SiO2 + 2 OH- ---> Si3O72- + H2O
• SiO2 crystallizes on quartz seed crystals.
Blue from Co2+ ions and
brown from Fe2+ ions.
mica
Silicon - Compounds
• Silicates have chains of SiO4
tetrahedra, often linked into a
sheet structure.
• Clays have sheets of SiO4
tetrahedra bound to sheets of
AlO6 octahedra (large variety of
clays). Used as remedies for
stomach upset because they
absorb toxins.
• The large disk is baked clay from
Africa; used medicinally.
Silicon - Compounds
• When quartz is melted, it forms silica glass.
• Mix: 60-75% silica, 12-18% soda (Na2CO3),
5-12% lime (CaO): most common type and
least expensive. Poor resistance to sudden
temperature changes and to corrosive
chemicals.
• Add about 20% PbO: glass is relatively soft
with a high refractive index gives it
brilliance, used for art glass and electrical
applications.
• Add at least 5% B2O3: high resistance to
temperature change and chemical
corrosion, used in pipelines, light bulbs,
photochromic glasses, sealed-beam
headlights, lab ware, baking ware.
Silicon - Compounds
• Add AgCl and CuCl2: obtain a photochromic glass
Cl- + light --> Cl + e• Darkening reaction: e- + Ag+ --> Ag (s)
• Reversing reactions:
Cl + Cu+ --> Cu2+ + ClCu2+ + Ag --> Cu+ + Ag+
• Mix impurities: colored glass
–
–
–
–
–
–
–
–
Blue-green: Fe2+
Yellow-green: Fe3+
Blue glass: Co2+
Purple: Mn2+
Fe2+ + Cr salts --> green wine bottles
Fe2+ + S --> brown
U2+: yellow
Se2-: red (as in traffic lights)
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www.chihuly.com
Silicon - Compounds
• 2 CH3Cl + Si --> SiCl2(CH3)2
• Also produces SiCl3(CH3), SiCl (CH3)3, and SiCl4
• Patented by E. G. Rochow of GE in 1945
• (CH3)3SiCl + 2 H2O --> (CH3)3Si—O—Si(CH3)3 + 2 HCl
• R2SiCl2 + 2 H2O --> HO—SiR2—OH + 2 HCl
• 2 HO—SiR2—OH ---> HO—SiR2—O—SiR2—OH + H2O
Units link to give polymers!
• Properties of silicones
– Good thermal and oxidative stability, resistant to high and low Temp’s
– Water repellent, antistick and antifoam properties
– Resistant to UV radiation and weathering
– Physiologically inert (*see breast implant studies)
Silicon - Compounds
•
Can be made into oils, greases, emulsions, elastomers, and resins:
•
Production of >350,000 tons annually: 1000 different products
•
65-70% fluid silicones: Cosmetics — suntan lotion, lipstick, antifoams —
sewage treatment, antifroth — cooking oil, car polish, lubricants, release
agents.
•
25-30% elastomers: SiO2 added to linear dimethylpolysiloxane: Retains
inertness, flexibility, elasticity, and strength up to 250 ˚C and down to –100
˚C. Used in industrial sealants, belts and gaskets, medical tubing, space
suits, etc.
•
5-10% resins: Pure silicone resins are poly(organosiloxanes) with a large
proportion of branched siloxyl groups. Used as raw materials for paints,
binders and in building preservation. Electrical industry: insulating lacquers
and used as high temperature enamels.
Group 5A
• Nitrogen, phosphorus, arsenic, antimony,
and bismuth
• N exists as N2 molecules. Others have
more complex forms.
• N2 is quite unreactive owing to the NN
triple bond.
• Atmosphere is about 80% N2
• N2 easily liquefied. Boils at -196 0C.
• Used as a refrigerant
• Phosphorous originally prepared from
human waste.
• Now obtained from the reduction of Pcontaining minerals such as Ca3(PO4)2.
Nitrogen - Compounds
• NH4NO3 --> N2O + 2 H2O
• N2O, nitrous oxide (dinitrogen oxide), used
as an anesthetic. Soluble in fats. Used as
propellant in whipped cream cans.
• NO, nitrogen oxide, is present in polluted
air. Has 11 valence e-, is implicated in
biological processes (circulatory system).
Reacts readily with O2 to give NO2.
HNO3 --> 2 N2O + H2O + 1/2 O2
• NO2, nitrogen dioxide, is a brown gas in
equilibrium with N2O4, a colorless gas. Is a
common air pollutant
Nitrogen - Compounds
• Haber Process: N2 + 3 H2 --> 2 NH3
• HNO3 production:
– NH3 gas is oxidized on Pt surface in air to NO
and NO2. Pt wire catalyzes reaction. Heat of
reaction causes wire to glow.
4 NH3 + 5 O2 ---> 4 NO + 6 H2O
– NO2 in water gives HNO3 (and HNO2).
– Better prep’d from:
2 NaNO3 + H2SO4 --> 2 HNO3 + Na2SO4
• HNO3 readily reacts with almost
all metals - except Al - to give
metal nitrate and NO2.
Cu
Al
Phosphorus – Compounds
• White = P4 tetrahedron
• Red = polymer of P4 tetrahedra
• Yellow phosphorus spontaneously
burns in air.
P4(s) + 5 O2(g) -->P4O10(s)
• Product: tetraphosphorus decaoxide.
Phosphorus – Compounds
OXIDES:
Phosphorus – Compounds
• Phosphorus reacts readily (is oxidized) with chlorine to give
PCl3 and PCl5.
• A match head contains an oxidizing agent (KClO3) and P4S3.
• The striking strip on a match box contains red P.
• Redox reaction lights the match.
Group 6A
• Oxygen, Sulfur, Selenium, Tellurium, Polonium.
• O2 condenses to a pale blue liquid at -183 oC
• * List ways to obtain O2.
• Liquid O2 is paramagnetic and clings to a magnet.
• O2 allotrope: Ozone, O3, is made by passing O2
through electric discharge.
• The most stable allotrope of S is a crown-shaped
ring of 8 S atoms (S8).
• Heating S8 to the melting point causes the rings
to open and a polymeric allotrope forms.
Sulfur - Compounds
• Sulfur is found in pure form in
underground deposits along the coast
of the U.S. It is recovered by pumping
superheated steam into the beds to melt
the S.
• Sulfur is burned in air to give SO2 and
then SO3.
• SO3 reacts with water ---> H2SO4
• H2SO4 is the chemical produced in the
largest amount in the U.S.
• Vents -- BLACK SMOKERS -- in the
bottom of the world’s oceans are a
source of metal sulfides.
Sulfur - Compounds
• Orpiment, As2S3
• Pyrite (fool’s gold), FeS
• Stibnite, Sb2S3
• Lead sulfide (galena), PbS
Sulfur - Compounds
• SO2 is produced by burning sulfur in
oxygen.
• SO2 is produced by treatment of
metal sulfides with O2.
• 2 ZnS + 3 O2 --> 2 ZnO + 2 SO2
• Also produced by burning fossil
fuels.
• About 2 x 108 tons of sulfur oxides
are released into the atmosphere by
human activities annually.
Halogens – Group 7A
• Diatomic elements: Cl2 gas, liquid Br2, and solid I2
• Cl is the most abundant in Group 7A.
• Occurs in the sea and in salt (NaCl) deposits.
Chlorine - Preparation
• Commercial: Electrolysis of NaCl(aq) in a membrane cell.
Anode: 2 Cl-(aq) --> Cl2(g) + 2eCathode: 2 H2O(liq) + 2e- --> H2(g) + 2 OH-(aq)
• Oxidation of NaCl with strong oxidant (K2Cr2O7). Cl2 gas
bubbles into water.
Iodine - Preparation
2 I- + 4 H+ + MnO2 --> Mn2+ + I2 + 2 H2O
Mixture of
NaI and
MnSO4
Add H2SO4;
reaction
produces I2.
Halogens Reactions
• Halogens react with
nonmetals and metals to
give covalent or ionic
halides.
Chlorine - Reactions
• Forms ionic compounds:
Bromine - Reactions
• With metals give metal bromides.
• Aluminum reaction:
End of Chapter
• Go over all the contents of your
textbook.
• Practice with examples and with
problems at the end of the chapter.
• Practice with OWL tutor.
• Practice with the quiz on CD of
Chemistry Now.
• Work on your OWL assignment for
Chapter 21.
21.12. Place the following oxides in order of
increasing basicity: Na2O, Al2O3, SiO2, SO3.
21.14. Complete and balance the equations for
the following reactions.
K(s) + I2(g)
Ba(s) + O2(g)
Al(s) + S8(s)
Si(s) + Cl2(g)
21.24. (a) Write equations for the half-reactions that occur at the cathode and
the anode when an aqueous solution of KCl is electrolyzed. Which chemical
species is oxidized, and which chemical species is reduced in this reaction?
(b) Predict the products formed when an aqueous solution of CsI is electrolyzed.
21.26. Calcium reacts with hydrogen gas at 300−400 °C to form a hydride. This
compound reacts readily with water, so it is an excellent drying agent for
organic solvents. (a) Write a balanced equation showing the formation of
calcium hydride from Ca and H2. (b) Write a balanced equation for the reaction
of calcium hydride with water (Figure 21.6).
21.36. (a) Write an equation for the reaction of Al and H2O to produce
H2 and Al2O3. (b) Using thermodynamic data in Appendix L, calculate
DH°, DS°, and DG° for this reaction. Do these data indicate that the
reaction should favor the products? (c) Why is aluminum metal
unaffected by water?
21.42. "Aerated" concrete bricks are widely used building materials. They are obtained
by mixing gas-forming additives with a moist mixture of lime, sand, and possibly cement.
Industrially, the following reaction is important:
2 Al(s) + 3 Ca(OH)2(s)+ 6 H2O(l) [3 CaO . Al2O3 . 6 H2O](s) + 3 H2(g)
Assume that the mixture of reactants contains 0.56 g of Al for each brick. What volume
of hydrogen gas do you expect at 26 °C and atmospheric pressure (745 mm Hg)?
21.54. Unlike carbon, which can form extended chains of atoms,
nitrogen can form chains of very limited length. Draw the Lewis
electron dot structure of the azide ion,N3-.