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Copyright © 2011 Pearson Canada Inc.General Chemistry: Chapter 21

Periodic Trends Group 1: The Alkali Metals

The mineral spodumene LiAl(SiO3)2

• Discoveries are recent.

–Sodium and potassium

(1807) by electrolysis.

–Cesium (1860) and

rubidium (1861) from

emission spectra.

–Francium (1939) from

actinium radioactive decay.

• Most salts are water soluble.

Natural brines are good sources.

Natural deposits allow mining of solids.

Physical Properties of the Alkali Metals

Flame Colors

Li Na589 nm Kcarmine violetyellow

Electrode Potentials

E°°°°cell = 2.212 V (for Li) 1.885 V (for Na) 2.096 V (for K)

2 M(s) + 2 H2O(l) 2 M+(aq) + 2 OH-(aq) + H2(g)

E°°°°cell = E°°°°H2O/H2- E°°°°M+/M

= -0.828 V - E°°°°M+/M

2.096 V (for Rb) 2.095 V (for Cs)

∆∆∆∆G° = -nFE°cell

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Production and Use of the Alkali Metals

2 NaCl(l) →→→→ 2 Na(l) + Cl2(g)

Electrolysis:

KCl(l) + Na(l) 2 NaCl(l) + K(g)

Sodium as a reducing agent:

TiCl4 + 4 Na →→→→ Ti + 4 NaCl

850 °CΔPreparation and reactions of sodium compounds

Lithium

• Alloys of Li-Al-Mg for aircraft

and space applications.

• Battery anodes.

Sodium

• Heat-transfer medium in

nuclear reactors.

• Sodium vapor lamps.

Halides

NaCl

• 50 Mtonne/year in U.S.

–Preservative, used on roads,

water softener regeneration,

feed stock for other

chemicals.

KCl

–from natural brines.

–Plant fertilizers, feed stock.

Sodium chloride stacks that have been

harvested by evaporation of seawater

Alkali Metal Hydrides

2 M(s) + H2(g) 2 MH(s)

MH(s) + H2O(l) MOH(aq) + H2(g)

4LiH + AlCl3 LiAlH4 + 3 LiCl(C2H5)2O

Oxides and Hydroxides

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Relative stabilities of M2O, M2O2, and 2 MO2 for the alkali metals

Carbonates and Sulfates

• Li2CO3 is unstable relative to the oxide.

– Used to treat manic depression (1-2 g/day).

• Na2CO3 primarily used to manufacture glass.

– Currently mined from rich U.S. resources, but

can be manufactured by the Solvay process.

Na+(aq) + Cl-(aq) + NH3(g) + CO2(g) + H2O(l)

NaHCO3 (s) + NH4+(aq) + Cl-(aq)

Sodium Sulfate

H2SO4(conc. aq) + NaCl(s) →→→→ NaHSO4(s) + HCl(g)

NaHSO4(s) + NaCl(s) →→→→ Na2SO4(s) + HCl(g)

Used in the Kraft Process for making paper:

Na2SO4(s) + 4 C(s) →→→→ Na2S(s) + 4 CO(g)

45 kg/ton paper

Alkali Metal Complexes

Three different crown ethers

•FIGURE 21-9“cryptands, cryptates”

Selectivity of 18-crown-6 towards M+

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Group 2: The Alkaline Earth Metals

Group 2 Compounds

• Alkaline earth metals are in +2 oxidation state.

• ns2 electrons are lost during compound formation.

• ionic compounds are primarily formed.

• higher lattice energies than alkali metal compounds.

• Principle forms:

– carbonates, sulfates and silicates.

• Oxides and hydroxides only sparingly soluble.

– Basic or “alkaline.”

• Compounds that do not decompose on heating.

– Therefore named “earths.”

• Heavier element compounds are more reactive and

are similar to Group I (also in other respects).

Beryllium

• Unreactive toward air and water.

• BeO does not react with water.

– All other Group 2 metal oxides form M(OH)2.

• BeO dissolves in strongly acidic or basic solutions.

– Therefore is an amphoteric oxide.

• BeCl2 and BeF2 melts are poor conductors:

– Therefore they are covalent rather than ionic

solids.

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Covalent bonds in BeCl2 Oxides and Hydroxides

Heavier elements form peroxides:

Thermal decomposition of carbonates

Carbonates and Sulfates

CaO + H2O→→→→ Ca(OH)2

slaked lime

In the lime slaker:

CaCO3 →→→→ CaO + CO2

burnt lime

or

quicklime

In the lime kiln:ΔThe decomposition (calcination)

of limestone is carried out in a

long rotary kiln

Slaked lime used in mortar:

Ca(OH)2 reacts with CO2 to form

CaCO3

Stalactites and stalagmites

•FIGURE 21-15CO2 + H2O →→→→ H3O

+ + HCO3-

Ka = 4.4××××10-7

HCO3- + H2O →→→→ H3O

+ + CO32-

Ka = 4.7××××10-11

CaCO3(s) + H2O(l) + CO2(g) →→→→ Ca(HCO3)2(aq)

• Gypsum, CaSO4·2H2O

– Plaster of Paris CaSO4·½H2O by heating gypsum.

– Used in drywall (gyproc) and casting.

• BaSO4 used in X-ray imaging .

Plaster of Paris castings