The s-block elements: alkali & alkaline earth metals

Chapters 11, 12, and 6 in H&S

H&S, Fig 1.13, p. 23

Alkali Metals (s1)

Li Na

K

Rb Cs

General Properties of Metals ~lustrous surface (shiny)

~dense ~hard ~malleable (can be flattened, deformed) ~ductile (can be pulled into wires) ~ high thermal conductivity ~ high 3D electrical conductivity …even though two orders of magnitude difference between worst(plutonium) and best(silver) metal conductivities under ambient conditions, plutonium conductivity is 105 higher than best conducting nonmetallic element • Bonding in metals? ~ electrons are equally shared ~ in the solid state, the bulk structure consists of ordered arrays of atoms: these are crystalline materials with lattice structures.

Metallic Bonding ~ a sea of shared valence electrons within a crystal structure of cations

Metallic Bonding

a) In which direction would you expect the strength of the metallic bond to increase? b) In which direction would you expect the melting point to increase? A

B

C

D

Li Na

K

Rb Cs

Some chemical and physical properties of the alkali metals & ions

H&S, Fig 11.1, p. 287

Some chemical reactions of the alkali metals & ions element

Li

Na

K

Rb

Cs

ΔHrxn kJ/mol (M+H2OMOH + H2)

melting point (° C)

density (g/cm3)

Some chemical reactions of the alkali metals & ions element

ΔHrxn kJ/mol (M+H2OMOH + H2)

melting point (° C)

density (g/cm3)

Li

-403

180

0.53

Na

-284

98

0.97

K

-283

64

0.86

Rb

-272

39

1.53

Cs

-262

29

1.87

4 Li(s) + O2(g) –> 2Li2O The oxide in the above reaction is a:

a) Superoxide b) Peroxide c) Oxide

Provenance of alkali metals: salts!

H&S Fig. 11.1, p286: Electrolysis cell used in the Downs process to make sodium and chlorine from NaCl

Provenance of alkali metals: salts!

H&S Fig. 11.2a, p286: Uses of NaCl in the US in 2005

~1/2 world’s deposits of potash (KCl) found in ancient lakebeds under SK, MB and NB, cocrystallized with lots of other salts e.g. like carnalite and other Mg salts.

What is potash?

Common name

Chemical name

Formula

Caustic potash or potash lye

potassium hydroxide

KOH

Carbonate of potash, salts of tartar, or pearlash

potassium carbonate

K2CO3

Chlorate of potash

potassium chlorate

KClO3

Muriate of potash

potassium chloride

KCl

Nitrate of potash or saltpeter

potassium nitrate

KNO3

Sulfate of potash

potassium sulphate

K2SO4

Permanganate of potash

potassium permanganate

KMnO4

Let’s bring in group 2, the alkaline earth metals

Some properties of the alkaline earth elements

H&S, Table 12.1, p. 309

Comparing properties of the alkalis and alkaline earths

H&S, Table 12.3, p. 322

Oxides of the s-block elements-review of group one (alkali) • The alkali metals will also react with oxygen to give relatively simple ionic solids, However, when they are heated in (dry) air or an excess of O2(g), the main products vary depending on the metal. ~ The alkalis are unusual in this regard - most metals burn to give simple oxides, compounds of the O2– ion. • Li behaves “normally”

4 Li(s) + O2(g) –> 2Li2O (s) crystallizes with “antifluorite” structure

• Na combusts to form sodium peroxide (peroxide ion is O22–) 2Na(s) + O2(g) –> Na2O2 (s) • K, Rb, Cs react with oxygen to give “superoxides”

(O21–; paramagnetic)

e.g. K(s) + O2(g) –> KO2 (s) also called “potassium dioxide” • All of these products react vigorously with water to give the corresponding metal hydroxides. Li2O(s) + H2O(l) –> 2LiOH (aq) Na2O2(s) + 2H2O(l) –> 2NaOH (aq) + H2O2 (aq) Breathing masks!

2KO2(s) + 2H2O(l) –> 2KOH (aq) + H2O2 (aq) + O2(g)

KOH(aq) + CO2(g) –> KHCO3 (s) (HCO3– = hydrogencarbonate anion)

Oxides of the alkaline earth metals • The alkaline earth metals tarnishin air (can you write a balanced equation?) at RT, but combust (burn) when heated. • Mg-Ba form 1:1 metal oxides, MO, which crystallize with NaCl-type structures.

• MgO has a melting point of 2825°C; in its crystalline form is a great thermal conductor and a lousy electrical conductor

• CaO is called “quicklime”; produced in huge quantities from calcium carbonate (limestone): ~ quicklime exhibits “thermoluminescence”: glows with bright white light when flameheated. • MgO is pretty insoluble in water, but the others react with water to form the hydroxides, which tend to be insoluble.

Review of Bonding -intermolecular (between “molecules”) -Van der Waals aka London dispersion aka induced dipoleinduced dipole

-dipole-induced dipole

-dipole-dipole

-hydrogen bonding

Review of Bonding intramolecular (within molecules) -covalent

-polar covalent

-ionic*

intramolecular/intermolecular (crystal forming) -metallic

-ionic*

Induced Dipole

Ionic

Metallic