B.sc 1st year Book
Compounds of Alkaline earth metals
These are the Elements/compound of Alkaline Earth Metals beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
These are the factor mostly compounds of Alkaline Earth Metals burned.
All the elements of the IIA group except Be and Ra form two types of oxides such as monoxides and peroxides of the formula MO and MO2 respectively. Be does not form its peroxide whereas Mg, Ca, Sr and Br form their peroxides. The monoxides of these metals are known earlier than the metals themselves and are known as alkaline metals oxide. These oxides can be prepared by the direct action of O2 and metals or by heating their carbonates at high temperatures.
After heating these oxides in presence of carbon. Be forms Be2C whereas rests form MC2 where M=Mg, Ca, Sr, Ba.
The crystal lattice energy of alkaline earth metal oxides is large and it decreases from BeO to BaO. Due to very large values of lattice energy, BeO and MgO are almost insoluble in H2O. The other oxides react with water and form hydroxides which are strong and basic in nature. Be(OH)2 is amphoteric and forms beryllate BeO2power2-.
MO + H2O⟶ M(OH)
2BeO + 2NaOH⟶ Na2BeO2+H2O
The basic character of their oxides increases from top to bottom in a group gradually.
BeO < MgO < CaO <SrO <BaO
Be2+ is of the smallest size and hence forms covalent oxides while the oxides of other metal ions of this group are ionic. BeO exists in a polymeric form where each Be atom is tetrahedrally coordinated with four oxygen atoms.
Chemical properties :
1- The heavier elements like Ca, Sr and Ba form peroxides by heating monoxides with O2 at high temperatures.
2BaO + O2 ⟶ 2BaO2
Peroxides consist of[O−O]2–: peroxide ion and are ionic in nature 2- They form salts with acids.
2- They form salts with acid
BaO2+ H2SO4⟶ BaSO4 + H2O2
3- They react with H2O and form H2O2.
BaO2 + 2H2O⟶ Ba(OH)2 + H2O2
4- On heating, they lose O2 and form MO.
5-All the peroxides are less stable and sparingly soluble in H2O.
6- CaO2 is heated alone or with30%H2O2 a yellow powder of CaO4 is obtained.
The oxides, except BeO and MgO, dissolve in water to give their corresponding hydroxides and by evolving a large amount of heat. These hydroxides are less basic than tho hydroxides of alkali metals. The basic character in generating increases in the following order:
The solubility of these hydroxides also increases in the same order. Bo(OH)2 and Mg(OH)2 are practically insoluble in water.
The hydrides of Be are covalent in nature whereas that of MgH2 is partly ionic and partly covalent, Rest of the hydrides of alkaline earth metals are purely ionic in nature:
1-Hydrides are formed by heating metals in the current of H2.
M + H2 ⟶ MH2 (M=Ca,Sr,Ba)
2- By reduction withLiAiH4.
2MCl2 + LiAlH4 ⟶ 2MH2 + LiCl + AlCl3 (M=Be and Mg)
3- By Alexander’s Method: CaH2 is prepared by heating CaO in the current of H2 in the presence of Mg at 250∘ and 50 cm pressure ofH2.
CaO + H2 + Mg ⟶ CaH2 + MgO
1- All the ionic hydrides are white powder (CaH2, SrH2 and BaH2)
2- All the hydrides have high melting and boiling points and a good conductor of electricity.
3- The densities of hydrides are more than that of metals themselves.
4-BeH2 is covalent in nature, MgH2 is partly ionic and the remaining hydrides are ionic.
Thus, the ionic character of alkali metal hydrides increases on moving down a group.
5. Thermal stability of hydrides increases from top to bottom in a group whereas their activity decreases as follows :
All the halides of alkaline earth metals are salt-like ionic or semi-ionic crystals. The halides of alkaline earth metals are prepared by a direct combination of halogen at an appropriate temperature:
2M+X2⟶2MXM=Be to Ba
They can also be prepared by the action of halogen acids on the carbonates or oxides of metal ions.
MCO3 + 2HCl ⟶ MCl2 + H2O + CO2↑
MO + 2HF ⟶ MF2 + H2O
Due to the Ionic character and strong holding force between MX units they have high melting and boiling points.
All the fluorides are essentially ionic in nature. BeF2 is soluble in water but due to the larger value of lattice energy fluorides of Ca, Sr and Ba are insoluble in water. But the chlorides, bromides and iodides are soluble in water. They have water molecules as water of crystallizations e.g. MgCl2.6H2O, CaCl2⋅6H2O2and BaCl2⋅2H2O⋅CaCl2 is highly hygroscopic in nature whereas BaCl2 is the least.
Beryllium halides are covalent compounds due to their small size and relatively high charge of Be2+ ion causing high polarising power.
Due to covalent bonding BeCl2 has the following characteristics.
1- It has a low melting and boiling points.
2- It does not conduct electricity in the fused state.
3- It is electron-deficient and behaves as Lewis acid.
4- It is soluble in an organic solvent like ether.
5- It exhibits different structures in vapour and in solid states. In vapour, state BeCl2 exist as dimeric (BeCl2)2 in which each Be-atom is sp2 hybridised and the two Be-atoms are joined by Be−Cl−Be bridges. Apart from this, each Be-atom has two Be−Cl bonds as shown in figure 4.05. In the solid state, it exists as a polymeric solid, (BOCl2)n containing a chain structure in which each Be-atom is tetrahedrally surrounded by four Cl-atoms. Thus, each Be-atom, in this case, undergoes sp3 hybridizations.
The carbonates of these metals are invariably insoluble in water and therefore, occur as solid rock minerals in nature. Limestone is the most important mineral containing CaCO3 Their solubility in water decreases on moving down the group. However, the carbonates dissolve in water in the presence of CO2 to give bicarbonates.
The solubility of carbonates in water containing CO2 explains why several caves are found in limestone regions.
Reason: On moving down the group the lattice energy of carbonates does not decrease more while the degree of hydration of the M2+ ions decreases very much leading to increased insolubility.
Carbonates of alkaline earth metals decompose on heating to form corresponding oxides Ca and CO2-
The stability of carbonates increases with the rise in atomic number i.e. on moving down the group. This is illustrated by the temperature at which carbonate decomposes:
Metals from Mg to Ba or their oxides, when heated with carbon, form carbides with the formula MC2.
MO + CC ⟶ MC2 + CO↑
Beryllium under similar conditions gives beryllium carbide with the formulaBe2C.
The carbide MC2 reacts with water to give acetylene while Be2C gives methane.
CaC2 + 2H2O ⟶ Ca(OH)2 + HC≡CH
Be2C + 2H2O⟶ 2BeO + CH4
These can be prepared by dissolving the metal oxide inH2SO4.
MgO + H2SO4 ⟶ MgSO4 + H2O
Beryllium sulphate is highly soluble in water while barium and radium sulphates are practically insoluble in water. Thus, the actual order of solubility of sulphates of alkali earth metals is –
BaSO4 < BaSO4 < SrSO4 < CaSO4 < MgSO4 <BeSO4
Compounds of Alkaline Earth Metals.