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The Alkaline Earth Metals

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Chapter 12. The Alkaline Earth Metals. Alkaline Earth Properties ... Compound Properties. Sweet taste. extremely poisonous. Inhalation results in berylliosis ... – PowerPoint PPT presentation

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Title: The Alkaline Earth Metals


1
Chapter 12
  • The Alkaline Earth Metals

2
Alkaline Earth Properties
  • harder, denser, and less reactive than the alkali
    metals
  • less dense and more reactive than a transition
    metal
  • Beryllium acts as a semimetal
  • Radium is radioactive

3
Group Trends
  • silvery and of fairly low density
  • density increases down the group

4
Group Trends
  • greater enthalpies of atomization than the alkali
    metals
  • stronger metallic bonding
  • also harder and have higher melting points

5
Group Trends
  • Less chemically reactive than the alkalis
  • calcium, strontium, and barium react with water
  • Ba(s) 2H2O(l) ? Ba(OH)2(aq) H2(g)
  • magnesium reacts with hot water
  • Also will react with nonmetal diatomics
  • Ca(s) Cl2(g) ? CaCl2(s)
  • 3Mg(s) N2(g) ? Mg3N2(s)

6
Ionic Character
  • Oxidation number is always 2
  • Compounds are stable, colorless, ionic solids
  • unless with a colored anion
  • Bonds tend to be mainly ionic
  • exceptions found in beryllium and magnesium

7
Ion Hydration
  • Salts are almost always hydrated
  • As charge density decreases, so does the
    hydration number

8
Solubility
  • Many compounds are insoluble in water
  • Mononegative anions tend to be soluble
  • hydroxides
  • insoluble ? soluble
  • Dinegative and trinegative anions tend to be
    insoluble
  • sulfates
  • soluble ? insoluble

9
Solubility
  • Enthalpy considerations
  • electrostatic attractions are much more for a
    dipositive alkaline earth cation than the
    monopositive alkali metal cations
  • higher charge densities make the hydration
    enthalpy much more

10
Solubility
  • Entropy considerations
  • lattice entropy increases more than alkalis
  • two vs. three gaseous ions produced
  • hydration entropy is more negative due to charge
    density

11
Solubility
  • Free energy considerations
  • for mononegative anions, more soluble than the
    alkalis

12
Solubility
  • Mononegative vs. polynegative anions
  • polynegative anions have much higher lattice
    energies
  • polynegative anions have fewer total ions making
    the hydration enthalpy less
  • combinations of these two give a more positive
    free energy, thus lower solubility

13
Beryllium
  • Steel gray
  • Hard
  • High melting point
  • Low density
  • High resistance to corrosion
  • Nonmagnetic

14
Uses
  • gyroscopes
  • windows of X-ray tubes
  • transparency

15
Sources
  • Bertrandite
  • Be4Si2O7(OH)2
  • Beryl
  • Be3Al2Si6O18
  • aquamarine
  • emerald
  • contamination of Cr(III)

16
Compound Properties
  • Sweet taste
  • extremely poisonous
  • Inhalation results in berylliosis

17
Chemistry of Beryllium
  • Covalent bonds predominate
  • high charge density polarizes any anion causing
    overlap to occur
  • Simple ionic compounds are a mixture
  • BeCl24H2O
  • Be(OH2)422Cl-

18
Beryllium
  • Metallic, but can form oxyanions
  • amphoteric
  • weak metal
  • H2O(l) BeO(s) 2H3O(aq) ? Be(OH2)42(aq)
  • H2O(l) BeO(s) 2OH-(aq) ? Be(OH)42-(aq)

19
Magnesium
  • Found in many minerals in nature
  • carnallite
  • KMgCl36H2O
  • dolomite
  • MgCO3CaCO3
  • 3rd most common ion in seawater
  • 108 million tons

20
Isolation of Magnesium
  • Precipitation reaction
  • Ca(OH)2(s) Mg2(aq) ? Ca2(aq) Mg(OH)2(s)
  • Neutralization reaction
  • Mg(OH)2(s) 2HCl(aq) ? MgCl2(aq) 2H2O(l)
  • Placed in a Downs cell
  • Mg2(MgCl2) 2e- ? Mg(l)
  • 2Cl-(MgCl2) ? Cl2(g) 2e-

21
Magnesium Reactivity
  • E -2.37V
  • not as reactive in air because of the formation
    of a protective coating
  • 2Mg(s) O2(g) ? 2MgO(s)
  • 2Mg(s) CO2(g) ? 2MgO(s) C(s)
  • special fire exstinguishing measures

22
Magnesium Uses
  • 4 x 105 tons produced annually
  • ½ is used in aluminum-magnesium alloys
  • very low density (1.74 g/ml)
  • used in aircraft, railroads, ships, etc

23
Magnesium Chemistry
  • Decomposition upon heating
  • MgCl2H2O(s) heat ? Mg(OH)Cl(s) HCl(g)
  • Formation of covalent bonds
  • Grignard reagents
  • C2H5Br(ether) Mg(s) ? C2H5MgBr(ether)

24
Calcium and Barium
  • grayish metals
  • react with oxygen
  • 2Ca(s) O2(g) ? 2CaO(s)
  • 2Ba(s) O2(g) ? 2BaO(s)
  • Ba(s) O2(g) ? BaO2(g)

25
Calcium and Barium
  • strong absorbers of x-rays
  • bones
  • dyes
  • CaCl22H2O(s) heat ? CaCl2(s) 2H2O(g)
  • BaCl22H2O(s) heat ? BaCl2(s) 2H2O(g)

26
Oxides
  • Formed upon reaction with air or heating of the
    carbonate
  • 2Mg(s) O2(g)? 2MgO(s)
  • CaCO3(s) heat ? CaO(s) CO2(g)

27
Oxides
  • MgO
  • very high melting point
  • used as a refractory compound in furnaces
  • crystalline MgO is a good conductor of heat, but
    not electricity

28
Oxides
  • CaO
  • called quicklime
  • undergoes thermoluminescence

29
Oxides
  • CaO
  • reacts with water to form slaked lime
  • CaO(s) H2O(l) ? Ca(OH)2(s)
  • used as a neutralizing agent in gardening, along
    with CaCO3
  • Ca(OH)2(s) 2H(aq) ? Ca2(aq) 2H2O(l)
  • CaCO3(s) 2H(aq) ? Ca2(aq) H2O(l)

30
Hydroxides
  • Solubility of hydroxides increase down the group

31
Hydroxides
  • Limewater
  • a solution of calcium hydroxide
  • Ca(OH)2(aq) CO2(g) ? CaCO3(s) H2O(l)
  • CaCO3(s) H2O(l) CO2(g) ? Ca2(aq)
    2HCO3-(aq)
  • leads to deterioration of marble

32
Calcium Carbonate
  • Two naturally occurring crystalline forms
  • calcite
  • Iceland spar
  • two refractive indices
  • aragonite

33
Calcium Carbonate
  • Chemistry involved in the formation of caves,
    stalagmites, and stalactites
  • CaCO3(s) CO2(aq) H2O(l) ? Ca2(aq)
    2HCO3-(aq)
  • Ca(HCO3)2(aq) ? CaCO3(s) CO2(g) H2O(l)
  • See animation at
  • http//www.classzone.com/books/earth_science/terc/
    content/visualizations/es1405/es1405page01.cfm?cha
    pter_novisualization

34
Calcium Carbonate
  • Biological importance
  • dietary supplement
  • helps reduce osteoporosis
  • antacid
  • constipative

35
Cement
  • Originally a paste of calcium hydroxide and sand
  • process perfected by the Romans
  • One of the largest chemical industries
  • 700 million tons annually

36
Production of Cement
  • Grind limestone and shales together and heat to
    1500C
  • calcium carbonate and aluminosilicates
  • carbon dioxide is released and the melt is called
    clinker
  • The clinker is ground with a small amount of
    calcium sulfate
  • Portland cement

37
Portland cement
  • 26 dicalcium silicate
  • Ca2SiO4
  • 51 tricalcium silicate
  • Ca3SiO4
  • 11 tricalcium aluminate
  • Ca3Al2O6
  • 2Ca2SiO4(s) 4H2O(l) ? Ca3Si2O73H2O(s)
    Ca(OH)2(s)

38
Calcium Chloride
  • White, deliquescent solid
  • Formation of CaCl26H2O is very exothermic (?H
    -82 kJ/mol)
  • used in hot packs

39
Calcium Chloride
  • Concentrated solutions
  • used to melt ice
  • lowers m.p. to -55C
  • used to coat unpaved roads
  • minimizes dust
  • used to fill tires of earth-moving equipment
  • better traction

40
Calcium Sulfate
  • Found naturally as a dihydrate
  • CaSO42H2O
  • gypsum or alabaster
  • Upon heating, forms the hemihydrate
  • CaSO41/2H2O
  • Plaster of Paris

41
Calcium Sulfate
  • Uses
  • fire-resistant wallboard
  • endothermic process to form the hemihydrate
  • releases water

42
Calcium Carbide
  • CaC2
  • contains the acetylide ion or dicarbide(2-) ion,
    C22-
  • adopts the sodium chloride crystal structure

43
Calcium Carbide
  • Preparation
  • CaO(s) 3C(s) heat ? CaC2(s) CO(g)
  • 5 million tons produced annually
  • China is the main producer

44
Calcium Carbide
  • Uses
  • production of acetylene
  • CaC2(s) 2H2O(l) ? Ca(OH)2(s) C2H2(g)
  • 2C2H2(g) 5O2(g) ? 4CO2(g) 2H2O(g)
  • production of cyanamide ion
  • CaC2(s) N2(g) heat ? CaCN2(s) C(s)
  • CaCN2(s) 3H2O(l) ? CaCO3(s) 2NH3(aq)

45
Biological Aspects
  • Photosynthesis
  • magnesium is contained in chlorophyll and keeps
    the molecule in a specific configuration
  • 6CO2(g) 6H2O(l) ? C6H12O6(aq) 6O2(g)

46
Biological Aspects
  • Magnesium is concentrated inside cells
  • triggers the relaxation of muscles
  • Calcium is concentrated outside cells
  • important in blot-clotting
  • trigger the contractions of muscles

47
Magnesium Reaction Flowchart
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