Alkali and alkaline earth metals, Lecture notes of Inorganic Chemistry

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The s-block elements:

Alkali and alkaline earth metals

oThe alkali family is found in the first column of the periodic table include; Li, Na, K, Rb, Cs, Fr. o Metallic elements. o Among the alkali metals;  sodium and potassium are abundant and  lithium, rubidium and caesium have much lower abundances.  Francium is highly radioactive; its longest- lived isotope 223 Fr has a half-life of only 21 minutes.

Alkali metals

The metals are all very reactive* when pure.

• strongly reducing (very powerful reducing agents)  In nature, these elements only ever appear as their + 1 cations.  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. Cations occur in wide variety of important chemicals.  Form ionic compounds with highly electronegative (nonmetals) found in Groups 6 (O, S, Se and 7 (F, Cl, Br, I, At) , as well as P and N from Group 5.

Alkali metals

• Most alkali metals are stored under oil. But lithium is so light that it floats on oil. So a coat of petroleum jelly is applied to lithium before it is stored.

Physical Properties

• Lowest ionization energies and electronegativities in periodic table
  • easily ionized to form ions with + 1 charge/+ 1 oxidation state.

Flame tests

o Alkali metal ions are identied by flame tests. o In flame ions in excited states return to their ground states  emit light. Element Li Na K Rb Cs Flame colour crimson yellow lilac purple blue o The emission spectrum depends on a number of factors;

• the flame temperature;
• but is most strongly is the energy levels of the emitter, leading to characteristic colours.

o Overall reactions: LiCl(l)  Li(l) + ½ Cl 2 (g) NaCl(l)  Na(l) + ½ Cl 2 (g)

Production of alkali metals

o Lithium sodium and potassium metal are produced by electrolysis of the molten chlorides.

Oxides of the alkali metals

oAlkali metal on combustion in Oxygen M + O 2 → depends on the alkali metal Li 2 O (oxide) Na 2 O 2 (peroxide) KO 2 , RbO 2 , CsO 2 (superoxide) o On combustion in excess of air, lithium forms mainly the oxide, Li 2 O (plus some peroxide Li 2 O 2 ), sodium forms the peroxide, Na 2 O 2 (and some superoxide NaO 2 ) whilst potassium, rubidium and caesium form the superoxides, MO 2. oThe increasing stability of the peroxide or superoxide, as the size of the metal ion increases, is due to the stabilisation of large anions by larger cations through lattice energy effects. The oxidation state of O?

Oxides of the alkali metals

o These oxides and peroxides are colourless when pure, but the superoxides are yellow or orange in colour. The superoxides are also paramagnetic. Sodium peroxide is widely used as an oxidising agent in inorganic chemistry. oAll oxides compounds react violently with H 2 O (hydrolysis) under formation of hydroxide Li 2 O(s) + H 2 O(l)  2 LiOH(s) Na 2 O 2 (s) + 2 H 2 O(l)  2 NaOH(s) + H 2 O 2 (l) 2 KO 2 (s) + 2 H 2 O(l)  2 KOH(s) + H 2 O 2 (l) + O 2 (g)

Reactivity of Alkali Metals with Group 5 , Elements (N, P)

o With Phosphorus 12 M + P 4 → 4 M 3 P (phosphides) o With Nitrogen 6 Li + N 2 → 2 Li 3 N (nitride) Only Li forms a nitride. The lattice energy for nitrides with larger alkali metal cations is not sufficient to make ΔGrxn< 0.

Reactivity of Alkali Metals with Group 7 Elements “Halogens”

o The reactivity of alkali metals increases as the ionization energy decreases Cs > Rb > K> Na > Li o Reactivity increases moving down the group. oExample: o The resulting compounds are colorless, crystalline ionic salts called halides.

Some typical reactions

Reaction with water: All alkali metals react with water to produce hydrogen and the metal hydroxide. M(s) + H 2 O(l)  M

(aq) +^ OH

• (aq) +^ ½^ H 2 (g) The metals are called alkali metals because the resulting solution is alkaline owing to the presence of the hydroxide ion. Reaction of lithium with oxygen: 2 Li(s) + ½ O 2 (g)  Li 2 O(s)  Alkali metal compounds are almost ionic.

Some typical reactions

Reaction with halogens (group 17 : F 2 , Cl 2 , Br 2 , I 2 )

• All alkali metals react with halogens to form ionic halides. M(s) + 1 / 2 X 2  MX(s) Ex. 2 Na(s) + Cl 2 (g)  2 NaCl(s)
• The most vigorous reaction occurs between the elements which are furthest apart on the Periodic Table:
  • the most reactive alkali metal, francium (Fr), at the bottom of Group 1 , with the most reactive halogen, fluorine, at the top of Group 7.

Basicity of oxides, sulfides nitrides

Salts containing high charge density anions like O 2 - , S 2 - , N 3 - , P 3 - with low charge density cations (alkali metal and most alkaline earth metals) are basic

• S 2 - + H 2 O  HS - + OH -
• N 3 - + 3 H 2 O  NH 3 + 3 OH -

Hydration

o An ion in solution is surrounded by water molecules. o Hydration enthalpy ( hydrH): Enthalpy change for the transfer of an ion from the gas phase to solution M

(g) ^ M

(aq)