s-Block Elements : CHEMICAL PROPERTIES OF ALKALINE EARTH METALS

CHEMICAL PROPERTIES OF ALKALINE EARTH METALS

The chemical reactions of the alkaline earth metals are quite comparable to that of alkali metals. But due to smaller size and greater charge and hence high ionisation energy, these are much less reactive than the corresponding alkali metals. Further since their ionisation energies decrease with increase in atomic number, their reactivity increases from Be to Ba. Lastly, beryllium being extremely small in size has a unique chemical behaviours.

          

1. Action of air

They are less reactive than the alkali metals is evident by the fact that they are only slowly oxidised on exposure to air. When burnt in air they form ionic oxides of the type MO, however the higher members (Sr to Ra) form peroxides. Thus, the tendency of the metals to form higher oxides like peroxide increases on moving down the group.

                  2M + O2 → 2MO       (when M = Be, Mg or Ca)

                  M + O2 → MO2           (when M = Sr, Ba or Ra)

                                                         Metal peroxide

2. Action of water.

These metals react slowly with water liberating hydrogen and forming metal hydroxides, e.g.

                    Ca + 2H2O → Ca(OH)2 + H2

The reaction with water becomes increasingly vigorous on moving down the group. Beryllium does not react with water, magnesium reacts only with hot water while other metals react with cold water but, of course, not as energetically as the alkali metals.

                    Ba > Sr > Ca > Mg > Be                (Reactivity with water)

                    Mg + H2O \underrightarrow { \quad boil\quad } MgO + H2O          (Magnesium oxide)

 

3. Action of hydrogen.

All elements except beryllium, combine with hydrogen to form hydrides, MH2. Magnesium hydride (like BeH2) is covalent while other hydrides are ionic.

                             M + H2 → MH2    Metal hydride

 

4. Action of halogens

All these elements combine with halogens at elevated temperature forming halides, MX2. Beryllium halides are covalent while the rest are ionic and thus dissolve in water and conduct electricity in aqueous solution and in molten state. The solubility of halides (except fluorides) decreases on moving down the group: fluorides are almost insoluble in water. The fluorides are the most stable followed by chlorides, bromides, iodides.

The solubility of fluorides

                    (A)    BeF2 highly soluble

                    (B)    Other fluorides in soluble

                    (C)    Solubility of fluorides increases down the group.

 

5. Action with nitrogen

All these elements burn in nitrogen forming nitrides, M3N2, which react with water to liberate ammonia. For example,

                    3Ca + N2Ca3N2

                    Ca2N2 + 6H2O3Ca(OH)2 + 2NH3

The ease of formation of nitrides decreases on moving down the group

 

6. With dil Acids :

They readily liberate hydrogen from dilute acids. For example,

                    M + H2SO4 → MSO4 + H2

                    M + 2HCl → MCl2 + H2

The reactivity of alkaline earths increases on moving down the group. This could be related to the increase in electropositive character from Be to Ba. Thus beryllium reacts very slowly, Mg react very rapidly while Ca, Sr and Ba react explosively.

 

7. Formation of amalgam and alloys

They form amalgam with mercury and alloys with other metals.

 

8. Complex formation

As described earlier. complex formation is favoured by small size, highly charged ion and suitable empty orbitals, alkaline earth metal ions, not having these characteristics, do not have a significant tendency (although it is more than in the alkali metals by virtue of their double charge) to form complexes. e.g., [BeF4]2–, [Be(H2O4)]2+, etc.            

Note that the coordination number of Be is at the maximum 4 and hence, Be salt cannot have more than four molecules of water of crystallisation because in Be no vacant d orbital is present, while magnesium can have a coordination number of six by using 3d orbitals as well as 3s and 3p orbitals. Hence the salts of Mg and Ca etc. are hexahydrated.

 

9. Solubility in liquid ammonia

Like alkali metals, alkaline earth metals dissolve in liquid ammonia giving coloured solutions. When the metal ammonia solutions are evaporated, hexammoniates [M (NH3)6]2+ are formed. The ammoniates are good conductors of electricity and decompose at high temperature.

 

10. Oxides and Hydroxides

The alkaline earth metal oxides, MO are prepared either by heating the metals in oxygen or better by calcination (heating at high temperature) of carbonates.

                    2Ca + O2 \underrightarrow { \quad heat\quad }   2CaO

                    CaCO3 \underrightarrow { \quad heat\quad } CaO + CO2

These are extremely stable, white crystalline solids. Except BeO, all the alkaline earth oxides are ionic in which doubly charged ions are packed in a Na+Cl– type of lattice (6 : 6) leading to their high crystal lattice energy and hence high stability. However, beryllium oxide is covalent due to small size and relatively more charge on the beryllium ion. In BeO each beryllium atom is tetrahedrally coordinated by four oxygen atoms.

The oxides of beryllium and magnesium are so tightly held together in the solid state that they are insoluble in water. The oxides of Ca, Ba Sr are highly ionic and are soluble in water thus are strong bases.

                    MO + H2OM (OH)2 + Heat, where M = Ca2+, Ba2+ or Sr2+

The solubility of hydroxides of alkaline earth metals in water increases on moving down the group. The increasing solubility of the hydroxides on moving down the group is evident from their solubility products.

         

                    Metal hydroxide    Ksp            Metal hydroxide     Ksp

                    Be(OH)2             1.6 × 10-26           Sr(OH)2               3.2 × 10-4

                    Mg(OH)2             8.9 × 10-12          Ba(OH)2              5.4 × 10-3

                    Ca (OH)2             1.3 × 10-4

Owing to the small size, high electronegativity and high ionisation energy of beryllium, the oxides and hydroxides of beryllium are more covalent than ionic and thus are amphoteric.

                    Be(OH)2 +   2HCl → BeCl2 + 2H2O

                    Be(OH)2 + 2NaOH → Na2BeO2 + 2H2O

On the other hand, the oxides and hydroxides of the other metals are basic in character and their basicity increases on moving down the group.

                    Be(OH)2 < Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2

                    Weak base                                                    Strong base

Heavier metals of the group (Sr and Ba) are capable of forming peroxides which are obtained by heating the oxides with oxygen at high temperature

                    2BaO + O2 → 2BaO2

The Peroxides are white, ionic solids having the anion [O–O]2–. The peroxide anion is stable due to the low polarising power of the large cations like Ba2+ and Sr2+. These peroxides react with acids producing hydrogen peroxide.

                    BaO2 + H2SO4 → BaSO4 + H2O2

11. Halides

The alkaline earth metal halides are obtained

(i)      by heating the metal with halogens at high temperature

(ii)     by treating metal oxide, hydroxide or carbonates with dilute halogens acids.

                             M + 2HX → MX2 + H2

                             MO + 2HX → MX2 + H2O

                             MCO3 + 2HX → MX2 + CO2 + H2O

Beryllium chloride is preapred by heating its oxide with carbon and Cl2.

BeO + C + Cl2 \underrightarrow { \quad 870-1070\quad K\quad }  BeCl2 + H2O

Beryllium halides are covalent compounds due to small size and relatively high charge on Be2+ ion causing high polarising power. Due to covalent bonding, beryllium chloride, a typical member of beryllium halides, shows following anomalous characteristics.

                    (A)    It has low melting and boiling points

                    (B)    It does not conduct electricity in the fused state.

                    (C)    It is soluble in organic solvents such as ether.

                    (D)    It is hygroscopic and fumes in air due to hydrolysis.

                             BeCl2 + 2H2O → Be(OH)2 + 2HCl

Hygroscopic nature of beryllium halides is due to the fact that Be2+ ion has high value of hydration energy and therefore has a strong tendency to form hydrates.

                    (e)     It is electron-deficient and behaves as Lewis acid.

Beryllium chloride exhibits different structure in the vapour and in the solid states.

                             Cl– Be– Cl monomeric (vapour)

 

(A) Polymeric

                   

The polymeric structure (A) is based on an unusual three centre bond structure in which two electrons, one contributed by the chlorine atom and the other by one of the beryllium atoms cover three atoms in a banana shaped orbitals.

The chlorides, fluorides, bromides and iodides of other alkaline earth metals are ionic solids and thus possess the following characteristics.

(A)    The melting and boiling point are high.

(B)    They conduct electricity in the molten state. Further since the ionic character of the halides increases on moving down the group, the melting points and conductivity increase in the group from MgCl2 to BaCl2.

(C)    They are hygroscopic and readily form hydrates, e.g. MgCl2.6H2O, CaCl2.6H2O.BaCl2.2H2O. Calcium chloride has a strong affinity for water and is a popular dehydrating agent.

(D)    The halides (chlorides, bromides, iodides) of these alkaline earth metal are soluble in water and their solubility decreases with increasing atomic number of the metal due to decrease in the hydration energy with increasing size of the metal ions. BeCl2, BeBr2, BeI2 are insoluble.

The fluorides of alkaline earth metals, except beryllium fluoride, are insoluble in water because of large values of their lattice energy. The exceptional solubility of beryllium fluoride is due to great hydration energy of Be2+ ion which outweight the effect of high lattice energy.

 

12. Carbonates:

The carbonates are invariably insoluble and therefore, occur as solid rock minerals in nature. Limestone is the most important mineral containing calcium carbonate. However the carbonates dissolve in water in the presence of carbon dioxide to give bicarbonates.

                    CaCO3 + H2O + CO2  \rightleftarrows      Ca HCO3)2

                    Cal. carbonate             Cal. bicarbonate

                    (insoluble)                    (soluble)

                    Two characteristics of carbonates deserve attention.

(i)      Solubility in water. Their solubility decreases on moving down the group going down the group the lattice energies of carbonates do not decrease more readily while the hydration energy of the metal ions decrease very much leading to decreased solubility.

(ii)     Carbonates of alkaline earth metals decompose on heating to form corresponding oxide and carbon dioxide.

                   MCO3 \rightleftarrows          MO + CO2

                    BeCO3       298 K         SrCO3                  1562 K

                    MgCO3      813 K         BaCO3                 1633 K

                    CaCO3       1173 K

The increased stability of the carbonates with increasing atomic number is due to the small size of the resulting oxide ion. The increasing size of the cation that destabilizes the oxides and hence does not favour the decomposition of the heavier alkaline earth metal carbonates.

 

 13. Sulphates

These can be prepared by dissolving the metal oxide in H2SO4.

                    MgO + H2SO4  → MgSO4 + H2O

          Solubility is the most important aspect of sulphates of the alkaline earth metals. It decreases regularly on moving down the group which is evident from the solubility products of the various sulphates.

                    Sulphate Ksp                     Sulphate           Ksp

                    BeSO4       Very high           SrSO4                  7.6 × 10-7

                    MgSO4      10                        BaSO4                 1.5 × 10-9

                    CaSO4       2.4 × 10-5            RaSO4                 4.0 × 10-11

 

Solubility of Compounds of Alkaline Earth Metals

(i)      Beryllium and magnesium salts are generally very soluble in water. It is due to high hydration energies of these much smaller ions.

(ii)     The solubility of hydroxides, fluorides and oxalates increases from calcium to barium. It is due to decrease in lattice energy with the increasing size of the metal ions being more when compared to the hydration energy.

(iii)    The solubility of carbonates, sulphates, halides (except fluorides) and chromates decreases on moving down the group. It is due to decrease in hydration energy while the lattice energy doesn’t decrease much.