Alkali metals
Cations occur in wide variety of important chemicals The metals are all very reactive when pure
– strongly reducing

Their cations (with the exception of lithium) have low charge densities
– often leads to good solubility and an ability to stabilize anions that can not be formed along with any other cation

Alkali metal physical properties
Element Li Na K Rb Cs Density (gcm-3) 0.53 0.97 0.86 1.53 1.87 Melting point (oC) 180 98 64 39 29 ∆Hatm (kJmol-1) 162 108 90 82 78

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Reactivity towards Bronsted acids
The alkali metals react even with weak Bronsted acids such as water
– The rate of reaction depends upon the acidity of the medium

2Na + 2H2O 2Na + 2EtOH 2Na + 2NH3

2NaOH + H2 2NaOEt + H2 2NaNH2 + H2

– very slow reaction in absence of catalyst

Alkali metals and liquid NH3
Alkali metals react very slowly with liquid ammonia
– however, they typically dissolve rapidly in ammonia to form highly colored solutions

Na(s) --(ammonia)--> Na(am) + e-(am)
– fast

2e-(am) + 2NH3

2NH2- + H2

– slow in absence of catalyst

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Sodide anions and electrides
2Na(s) + C12H24O6 [Na(C12H24O6)]+Na– complexing or chelating agent is used to stabilize the cation
» Na- is the sodide anion. It is the most stable alkalide ion

Other complexing agents have been used to form solid salts where e- is the anion
– electrides

Lithium
Lithium is the most reducing of all the alkali metals
– but not the most reactive

Unlike the other alkali metals it reacts with nitrogen to form a nitride Used in aerospace alloys, greases, batteries and antidepression medication Organometallics of lithium are important

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Sodium
From the electrolysis of a CaCl2/NaCl eutectic A very powerful reducing agent Used to produce metals like Ti and Zr
• TiCl4 + 4Na 4NaCl + Ti

Used in the production of antiknock additives
– tetraethyl lead etc.

Oxides of the alkali metals
Lithium forms a simple oxide on combustion Sodium forms a peroxide (O22-) on combustion Potassium forms a superoxide (O2-)
– useful for CO2 removal and oxygen replenishment

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Alkaline earth metals
The alkaline earth metals are reactive compared to most metals. However, they are less reactive (as a group) than the alkali metals They have relatively high melting points and densities when compared to the alkali metals

Physical properties
Element Mg Ca Sr Ba Density (gcm-3) 1.74 1.55 2.63 3.62 Melting point (oC) 649 839 768 727 ∆Hatm (kJmol-1) 149 177 164 175

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Common features
Most compounds of the alkaline earth metals are ionic and colorless
– Most beryllium compounds and several magnesium compounds are covalent

The simple salts of M2+ cations tend to be hydrates

Solubility
Unlike the alkali metals, many alkaline earth containing compounds are insoluble
– BaSO4, MgO etc.

Depends upon charge on anion and size match between anion and cation
– like the alkali metals solubility is determined by a balance between lattice and hydration thermodynamic terms
» for M2+ both lattice energies and hydration enthalpies tend to be larger

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Beryllium
Forms largely covalent compounds due to the large charge density of Be2+ Extracted from Beryl, Be3Al2Si6O18 Used in the manufacture of precision instruments
– non-magnetic, corrosion resistant, low density

Used in production of X-ray windows

Beryllium
Beryllium compounds are very toxic Beryllium is amphoteric (unlike the other alkaline earth metals)
– BeO dissolves in both acid and base – H2O + BeO + 2H3O+ ----> [Be(OH2)4]2+ – H2O + BeO + 2OH- ----> [Be(OH)4]2-

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Magnesium
Magnesium is widely used in low density Al/Mg alloys for mass critical applications Extracted by electrolysis from MgCl2
– obtained indirectly from sea water

Mg is thermodynamically very reactive
– however, oxide layer protects it

Forms organometallic with covalent bonds
– Grignards

Metal fires
Reactive metals like Na and Mg readily burn in air
– Mg + 0.5O2 MgO

They also react with common fire extinguishing agents such as water and carbon dioxide
– Mg + H2O MgO + 0.5H2 – Mg + 0.5CO2 MgO + 0.5C
» This makes metals fires difficult to put out!

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Oxides of the alkaline earths
All the alkaline earth metals react with O2 to forms simple oxides with the exception of barium - forms some peroxide Solubility varies widely
– MgO insoluble, highly refractory – CaO sparingly soluble, thermoluminescent – SrO and BaO readily soluble

Hydroxides
Mg(OH)2 - “Brucite” is insoluble. This makes it valuable in the preparation of antacids Ca(OH)2 is sparingly soluble. It reacts readily with CO2 to from a carbonate. At high CO2 levels in water a soluble hydrogen carbonate can be formed
– basis of cave formation and stalactite and stalagmite formation

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Calcium carbide
CaC2 has a rock salt (NaCl) structure with C22- ions on the chloride sites
– formed by high temperature reaction of coke and calcium oxide

The reaction of calcium carbide with water is an important source of acetylene
– CaC2 + 2H2O ---> C2H2 + Ca(OH)2

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