Transcript Chapter 18 The Representative Elements: Groups 1A Through 4A Chapter 18: The Representative Elements: Groups 1A through 4A 18.1 A Survey of the Representative Elements 18.2

Chapter 18
The Representative
Elements: Groups 1A
Through 4A
Chapter 18: The Representative Elements:
Groups 1A through 4A
18.1 A Survey of the Representative Elements
18.2 The Group 1A Elements
18.3 The Chemistry of Hydrogen
18.4 The Group 2A Elements
18.5 The Group 3A Elements
18.6 The Group 4A Elements
The ignition of hydrogen soap bubbles.
Figure 18.1: The periodic table.
Figure 18.2:
Some atomic
radii
(in picometers).
Figure 18.3:
Structure of
quartz
Sand, such as that found in the massive sand
dunes bordering the desert plain near Namib,
Namibia, is composed of silicon and oxygen.
Source: Getty Images
Properties Based Upon ns1 Electron Configuration
1. Low heat of atomization (
Hatom)
(Energy needed to convert the solid to individual gaseous atoms)
M(s)
M(g)
Hatom ( L i > Na > K > Rb > Cs )
2. Low IE and High charge density
a great decrease in size occurs when the one valence electron is lost!
Vol of Li+ is < 13% of that of Li - thus M+ ions are small with
a great charge density.
3. High Lattice Energy
small cations pull very close to large anions, yielding large
quantities of energy liberated upon crystallization
Trend - as the cation becomes larger, the lattice energy becomes
smaller (less negative)
4. Hydration Energy
E+(g) + H2O(l)
E+(aq)
HHydr ( Li+ > Na+ > K+ > Rb+ > Cs+ )
The smaller ions attract water molecules strongly enough to form
larger hydrated ions: Li+(aq) is larger than Cs+(aq)
Sodium reacts violently with water
Reactions of the Alkali Metals with water
All of the alkali metals react with water to form hydrogen and the
hydroxide ion. Li, Na, K, Rb, and Cs react I order of reactivity, The
largest atom Cs is the most reactive, and the reactivity decreases as they
get smaller and smaller, with Li being the least reactive.
2 M(s) + 2 H2O(l)
H2 (g) + 2 OH-(aq) + 2 M+(aq)
Lithium and Sodium are less dense than water, and react on the water
surface, Potassium is so reactive it catches the hydrogen released catches
Fire from the exothermic reaction. Rubidium and Cesium literally
explode when they come in contact with water. They all produce
solutions of the alkali metal hydroxides, but the reactivity increases down
the group.
This reactivity increase is due to the increasing diameter of the atoms,
So that the attractive forces on the one valence electron is much less, and
It is free to leave.
Fig. 14.3 (P 547)
Reactions of Alkali Metals with Oxygen - I
All alkali metals will form oxides in the form of M2O, but only
lithium will form this oxide in excess oxygen:
4 Li(s) + O2 (g)
2 Li2O(s)
Sodium in an excess of oxygen will form the peroxide:
2 Na(s) + O2 (g)
Na2O2 (g)
The Peroxide anion reacts with water to from Hydrogen
peroxide and hydroxide anions.
Na2O2 (s) + 2 H2O(l)
2 Na+(aq) + 2 OH-(aq) + H2O2 (aq)
Reactions of Alkali Metals with Oxygen - II
Potassium, Rubidium and Cesium react with oxygen to form super-oxides
With the general formula of MO2 for example potassium:
K(s) + O2 (g)
____________
The superoxides release oxygen gas when they react with water or
carbon dioxide gas. For this reason they are very useful in emergency
breathers for fireman or emergency workers.
2 MO2 (s) + 2 H2O(l)
2 M+(aq) + 2 OH-(aq) + O2 (aq) + H2O2 (aq)
4 MO2 (s) + 2 CO2 (g)
2 MCO3 (s) + 3 O2 (g)
Important Reactions of the Alkali Metals - I
1. The alkali metals reduce hydrogen to form ionic (salt like) hydrides:
2 M(s) + H2 (g)
2 MH(s)
NaH is an industrial base and reducing agent that is used to prepare
other reducing agents, such as NaBH4.
2. The alkali metals reduce halogens to form ionic halides:
2 M(s) + X2(-)
2 MX(s)
( X = F, Cl, Br, I)
3. Sodium chloride is the most important alkali halide.
a) In the Downs process for the production of sodium metal, by the
electrolysis of molten NaCl:
2 NaCl(L)
electricity
2 Na(L) + Cl2 (g)
Important Reactions of the Alkali Metals - II
3. continued
b) In the chlor-alkali process, NaCl(aq) is electrolyzed to form several
key industrial chemicals:
electricity
2 NaCl(aq) + 2 H2O(l)
2 NaOH(aq) + H2 (g) + Cl2 (g)
c) In its reaction with sulfuric acid, NaCl forms two major products:
2 NaCl(s) + H2SO4 (aq)
Na2SO4 (aq) + 2 HCl(g)
Sodium sulfate is important in the paper industry; HCl is essential
in steel, plastics, textiles, and food production.
4. Sodium hydroxide is used in the formation of bleaching solutions:
2 NaOH(aq) + Cl2 (g)
NaClO(aq) + NaCl(aq) + H2O(l)
5. In an ion-exchange process water is “softened” when Na+ is displaced
by “hard-water” ions (M2+):
Mg2+(aq) + Na2(resin)(s)
Mg(resin)(s) + 2 Na+(aq)
Important Compounds of the Alkali Metals
1. Lithium chloride and lithium bromide, LiCl and LiBr. Because the Li+
ion is so small, Li salts have a affinity for H2O and yet a positive heat
of solution, so they are used in dehumidifiers and air-cooling units.
2. Lithium carbonate, Li2CO3. Used to make porcelain enamels and
toughened glasses and as a drug in the treatment of manic-depressive
disorders.
3. Sodium chloride, NaCl. Millions of tons used in the industrial
production of Na, NaOH, Na2CO3/NaHCO3, Na2SO4, HCl, and
purified for use as table salt.
4. Sodium carbonate and sodium bicarbonate, Na2CO3 and NaHCO3.
Carbonate used as an industrial base and to make glass. Bicarbonate,
which releases CO2 at low temperatures (500 to 1000), used in baking
powder and in fire extinguishers.
5. Sodium hydroxide, NaOH. Most important industrial base; used to
make bleach, sodium phosphates, and alcohols.
6. Potassium nitrate, KNO3. Powerful oxidizing agent used in gunpowder
and fireworks.
Where
Does
Hydrogen
Belong?
Hydrogen gas being used to blow
soap bubbles
As the bubbles float upward, they are lighted
by using a candle on a long pole.
Hydrides of Alkali Metals - I
Ionic Hydrides are formed with Group IA and IIA metals.
2 Li(s) + H2 (g)
2 LiH(s)
Ca(s) + H2 (g)
CaH2 (s)
These Hydrides react with water to release hydrogen gas.
LiH(s) + H2O(l)
CaH2 (s) + 2 H2O(l)
Li+(aq) + OH-(aq) + H2 (g)
Ca+2(aq) + 2 OH-(aq) + 2 H2 (g)
Covalent Hydrides are formed by the reaction of hydrogen with
non-metals.
Examples are: HCl, NH3, CH4, and H2O
Hydrides of Alkali Metals - II
Metallic or interstitial Hydrides These are hydrides formed with metals
and vary a great deal.
Hydrogen gas can occupy the interstitial holes in metals due to their
small size.
Palladium can absorb 900 times it’s volume of hydrogen gas
Some metallic Hydrides can be formed such as: UH3 and FeH6
Most metallic hydrides are interstitial or non-stoichiometric compositions.
such as:
LaH2.76 or VH0.56
Figure 18.5: Structure of ice, showing
the hydrogen bonding
Standing in Group 2A(2), Looking Backward
to 1A(1) and Forward to 3A(13)
Fig. 14.6 (P 551)
Important Reactions of the Alkaline Earth Metals - I
1. The metals reduce O2 to form the oxide:
2 M(s) + O2 (g)
2 MO(s)
Barium also forms the peroxide BaO (s).
2. The Metals of higher atomic weight reduce water to form hydrogen
gas:
M(s) + 2 H2O(l)
M(OH)2 (aq) + H2 (g)
M = Ca, Sr and Ba
Be and Mg form an adherent oxide coating that allows only slight
reaction.
3. The metals reduce halogens to form ionic halides:
M(s) + X2(-)
MX2 (s)
X = F, Cl, Br, I
4. Most of the metals reduce hydrogen to form ionic hydrides.
M(s) + H2 (g)
MH2 (s)
all except Be
Calcium metal reacting with water to form
bubbles of hydrogen gas.
Important Reactions of the Alkaline Earth Metals - II
5. Most of the metals reduce nitrogen to form ionic nitrides:
3 M(s) + N2 (g)
M3N2 (s)
all except Be
6. Except for amphoteric BeO, the oxides are basic:
MO(s) + H2O(l)
M(OH)2 (aq)
7. All carbonates undergo thermal decomposition to the oxide:
MCO3 (s)
____________________
This reaction is used to produce CaO (lime) in huge amounts from
naturally occurring limestone, and was the reaction used to generate
carbon dioxide to smother the graphite fire in the Chernobyl reactor.
Important Compounds of the Alkaline Earth Metals
1. Beryl, Be3Al2Si6O18. Beryl occurs as a gemstone with a variety of
colors. It is chemically identical to emerald, except for the trace of
Cr+3 that gives emerald its green color. Beryl is the industrial source
of Be metal.
2. Magnesium oxide, MgO. Because of its high melting point (28520C),
it is used as a refractory material for furnace brick and wire insulation.
3. Alkylmagnesium halides, RMgX (R = hydrocarbon group;
X = halogen). These compounds, called Grignard reagents, are used to
synthesize many organic compounds. Organotin agricultural
fungicides are made by treating RMgX with SnCl4.
3 RMgCl + SnCl4
3MgCl2 + R3SnCl
4. Calcium carbonate, CaCO3. Occurs as enormous natural deposits of
limestone, marble, chalk and coral. Used as a building material,
to make lime, and, in high purity, as toothpaste abrasive and antacid.
Figure 18.6: The structure of solid BeH2
Crystalline Beryl - BeCl2
Source: Photo Researchers, Inc.
Figure 18.7: Solid BeCl2 can be visualized as
consisting of many BeCl2 molecules
A) At high temperatures, BeCl2 occurs as a gaseous
molecule with only four electrons around Be.
B) In the solid state, BeCl2 occurs in long chains
with each Cl bridging two Be atoms, which gives
each Be an octet.
Figure 18.8: (a) A schematic representation of
a typical cation exchange resin.(b) and (c)
hard water
Gallium melts in the hand
Figure 18.9: The structure of B2H6
Diborane
A borane
The Two Types of Covalent Bonding
in Diborane
The Dimeric
Structure
of Gaseous
Aluminum
Chloride
Important Reactions of Boron Group
Elements - I
1. The elements react sluggishly, if at all, with water:
2 Ga(s) + 6 H2O(Hot)
2 Ga3+(aq) + 6 OH-(aq) + 3 H2 (g)
2 Tl(s) + 2 H2O(steam)
2 Tl+(aq) + 2 OH-(aq) + H2 (g)
2. When strongly heated in pure O2, all members form oxides:
4 M(s) + 3 O2 (g)
4 Tl(s) + O2 (g)
2 M2O3 (s)
M = B, Al, Ga, In
2 Tl2O(s)
Oxide acidity decreases down the group:
B2O3 (weakly acidic) > Al2O3 > Ga2O3 > In2O3 > Tl2O (strongly basic)
for Tl, the +1 oxide is more basic than the +3 oxide.
Important Reactions of Boron Group
Elements - II
3. All members reduce halogens (X2) :
2 M(s) + 3X2 (-)
2 Tl(s) + X2 (-)
2 MX3 (-)
(M = B, Al, Ga, In)
2 TlX(s)
BX3 are volatile covalent molecules. Trihalides of Al, Ga, and In are
(mostly) ionic solids but occur as covalent dimers in the gas phase;
in this way, the 3A atoms attains a filled outer level.
4. Acid treatment of Al2O3 is important in water purification:
2 Al2O3 (s) + 3 H2SO4 (l)
Al2(SO4)3 (s) + 4 H2O(l)
In water, Al2(SO4)3 and CaO form a colloid that aids in removing
suspended particles.
Important Reactions of Boron Group
Elements - III
5. The overall reaction in the production of aluminum metal is a
redox process:
2 Al2O3 (s) + 3 C(s)
4 Al(s) + 3 CO2 (g)
This electrochemical process is carried out in the presence of
cryolite (Na3AlF6), which lowers the melting point of the reactant
mixture and takes part in the change.
6. A displacement reaction produces gallium arsenide, GaAs:
(CH3)3Ga(g) + AsH3 (g)
_________________
Important Compounds of Boron Group
Elements - I
1. Boron oxide, B2O3. Used in the production of borosilicate glass.
.
2. Borax, Na [B O (OH) ] 8H O. Major mineral source of boron
2
4
5
4
2
compounds and B2O3. Used as a fireproof insulation material and as a
washing powder (20-Mule Team Borax).
3. Boric acid, H3BO3 [ or B(OH)3]. Used as external disinfectant,
eyewash, and insecticide.
4. Diborane, B2H6. A powerful reductant for possible use as a rocket
fuel. Used to synthesize higher boranes, compounds that led to new
theories of chemical bonding.
.
5. Aluminum sulfate (alum), Al2(SO4)3 18H2O. Used in water
purification, tanning leather, and as an antiperspirant.
Important Compounds of Boron Group
Elements - II
6. Aluminum oxide, Al2O3. Major compound in natural source
(bauxite) of Al metal. Used as abrasive in sandpaper, sanding and
cutting tools, and toothpaste. Large crystals with metal ion
impurities often of gemstone quality. Inert support for
chromatography. In fibrous forms, woven into heat-resistant fabrics;
also used to strengthen ceramics and metals.
7. Tl2Ba2Ca2Cu3O10. Becomes a high-temperature superconductor at
125 K, which is readily attained with liquid Nitrogen. (77K)
C-C : ethane(C2H6), benzene (C6H6), graphite, diamond
B-N : amine-borane(BNH6), borazine (B3N3H6), boron nitride,
borazon
Standing in Group 4A(14), Looking Backward
to 3A(13) and Forward to 5A(15)
Important Reactions of the Carbon Family - I
1. The elements are oxidized by halogens:
M(s) + 2 X2(-)
MX4(-)
(M = C, Si, Ge)
The +2 halides are more stable for tin and lead, SnX2 and PbX2.
2. The elements are oxidized by O2:
M(s) + O2(g)
MO2(-)
(M = C, Si, Ge, Sn)
Pb forms the +2 oxide, PbO. Oxides become more basic down the
group. The reaction of CO2and water provides the weak acidity of
natural unpolluted waters:
CO2(g) + H2O(l)
[H2CO3(aq)]
carbonic acid
H+(aq) + HCO3-(aq)
Important Reactions of the Carbon Family - II
3. Air and steam passed over hot coke produce gaseous fuel mixtures
(producer gas and water gas):
C(s) + air(g) + H2O(g)
CO(g) + CO2(g) + N2(g) + H2(g)
(not balanced)
4. Hydrocarbons react with O2 to form CO2 and H2O. the reaction for
methane can be adopted to yield heat or electricity:
CH4(g) + 2 O2(g)
__________________
The gas is used to make other organic compounds and as a fuel in
welding.
Important Reactions of the Carbon Family- III
6. Freons (chlorofluorocarbons) are formed by fluorinating carbon
tetrachloride:
CCl4(l) + HF(g)
CFCl3 (g) + HCl(g)
Production of trichlorofluoromethane (Freon-11), the major
refrigerant in the world, is being eliminated because of its severe
effects on the environment, the ozone destruction in the stratosphere.
7. Silica is reduced to form elemental silicon:
SiO2(s) + 2 C(s)
Si(s) + 2 CO(g)
This crude silicon is made ultrapure through zone refining for the
manufacture of computer chips.
Important Compounds of the Carbon Family - I
1. Carbon monoxide, CO. Used as a gaseous fuel as a precursor for onecarbon organic compounds, and as a reactant in the purification of
nickel. Formed in internal combustion engines and released as a toxic
air pollutant.
2. Carbon dioxide, CO2. Atmospheric component used by photosynthetic
plants to make carbohydrates and O2. The final oxidation product of
all C - based fuels; its increase in the atmosphere is leading to global
warming. Used industrially as a refrigerant gas, blanketing gas in fire
extinguishers, and effervescent gas in beverages. Combined with NH3
to form urea for fertilizers and plastics manufacture.
3. Methane, CH4. Used as a fuel and in the production of many organic
compounds. Major component of natural gas. Formed by anaerobic
decomposition of plants (swamp gas) and by microbes in termites and
certain mammals. May contribute to global warming.
Important Compounds of the Carbon Family - II
4. Silicon dioxide, SiO2. Occurs in many amorphous (glassy) and
crystalline forms, quartz being the most common. Used to make
glass and as an inert chromatography support material.
5. Silicon carbide, SiC. Known as carborundum, a major industrial
abrasive and a highly refractory ceramic for tough, high-temperature
uses. Can be doped to form a high-temperature semiconductor.
6. Organotin compounds, R4Sn. Used to stabilize PVC (polyvinyl
chloride) plastics and to cure silicone rubbers. Agricultural biocide
for insects, fungi, and weeds.
7. Tetraethyl lead, (C2H5)4Pb. Once used as a gasoline additive to
improve fuel efficiency, but now removed because of its inactivation
of auto catalytic converters. Major source of lead as a toxic air
pollutant.
Roman baths such as these in
Bath, England, used lead pipes for water
Source: Getty Images
Lead (II) oxide