Chemical Properties of Alkaline Earth Metals
The alkaline earth metals are reactive metals, though less reactive than alkali metals. The reactivity increases from top to bottom in a group due to increase in electropositive character.
Reactivity and E° Values
The near constancy of the E° (M2+/M) values for group 2 metals is somewhat similar to that for group 1 metals. Therefore, these metals are electropositive and are strong reducing agents.
The less negative value for Be arises from, the large hydration energy associated with the small size of Be2+ being countered by relatively large value of the enthalpy of atomization of beryllium.
Oxides
The alkaline earth metals burn in oxygen forming the ionic oxides of the type MO where M stands for alkaline earth metals except Sr, Ba, and Ra which form peroxides. Peroxides are formed with increasing ease and increasing stability as the metal ions become larger.
2Mg + O2 → 2MgO
2Be + O2 → 2BeO
2Ca + O2 → 2CaO
Ba + O2 → 2BaO
Basic character of the oxides increases gradually from BeO to BaO. Beryllium oxide is amphoteric, MgO is weakly basic while CaO is more basic.
Hydrides
The alkaline earth metals combine with hydrogen to form hydrides of general formula MH2.
M + H2 → MH2 (M = Mg, Ca, Sr, Ba)
Reaction with Water
Usually the alkaline earth metals react with water to liberate hydrogen. Be does not react with water or steam even at red heat and does not get oxidized in air below 837 K.
Mg + H2O → MgO + H2
Ca, Sr, and Ba react with cold water with increasing vigour.
Ca + 2H2O → Ca(OH)2 + H2
Halides
All the alkaline earth metals combine directly with the halogens at appropriate temperature forming halides, MX2 where M stands for alkaline earth metals.
M + X2 → MX2
Solubility & Stability of Carbonates & Sulphates
Carbonates
The carbonates of alkaline earth metals are sparingly soluble in water. They decompose if heated strongly. Their thermal stability increases with increase in the size of the cation.
Sulphates
The sulphates of alkaline earth metals are white solids, stable to heat. The sulphates, BeSO4 and MgSO4 are readily soluble and the solubility decreases from CaSO4 to BaSO4. The greater hydration energies of Be2+ and Mg2+ ions overcome the lattice
energy factor and therefore, their sulphates are soluble.
The sulphates decompose on heating, giving the oxides.
MSO4 → MO + SO3
The thermal stability of sulphates increases with the increase in the size of cation.
Complex Compounds
Smaller ions of the group 2 elements form complexes. For example, chlorophyll is a complex compound of magnesium. Beryllium forms complexes like [BeF4]2–.