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

Element Melting Point (C) ?Hatm (kJ/mol)
Mg 649 149
Ca 839 177
Sr 768 164
Ba 727 175
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

Element MCl2 M(NO3)2 MSO4
Mg 6 6 7
Ca 6 4 2
Sr 6 4 0
Ba 2 0 0
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

Compound Lattice Energy (kJ/mol) Hydration Enthalpy (kJ/mol) Net Enthalpy Change (kJ/mol)
MgCl2 2526 -2659 -133
NaCl 788 -784 4
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Solubility

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

Compound Lattice Entropy (kJ/mol) Hydration Entropy (kJ/mol) Net Entropy Change (kJ/mol)
MgCl2 109 -143 -34
NaCl 68 -55 13
11
Solubility

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

Compound Enthalpy Change (kJ/mol) Entropy Change (kJ/mol) Free Energy Change (kJ/mol)
MgCl2 -133 -34 -99
NaCl 4 13 -9
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)

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Oxides

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

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Oxides

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

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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)

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Hydroxides

• Solubility of hydroxides increase down the group

Hydroxide Solubility (g/L)
Mg 0.0001
Ca 1.2
Sr 10
Ba 47
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

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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

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Magnesium Reaction Flowchart