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1.
Oxide
The binary compounds formed by alkaline earth metals and oxygen include ordinary oxides, peroxides, and superoxides
.
Alkaline earth metals burn in sufficient air, Be, Mg, Ca, and Sr all produce common oxides BeO, MgO, CaO, SrO; Ba produces peroxide BaO2
.
Alkaline earth metal elements can form oxides, and the oxides are all white
.
It can be obtained by thermal decomposition of its carbonate , hydroxide, nitrate or sulfate
The melting point of alkaline earth metal oxides is much higher than that of alkali metal oxides of the same period
.
This is mainly because the radius of alkaline earth metal ions is relatively small, and the positive charge is high, resulting in a relatively large lattice energy of its oxide
The calcined BeO and MgO are extremely difficult to react with water and have a high melting point
.
Therefore, they are very good refractory materials
Among alkaline earth metal peroxides, BeO 2 does not exist, and anhydrous MgO 2 can only be obtained from liquid ammonia
.
SrO 2 can be obtained by direct combination of Sr and high-pressure oxygen
Anhydrous calcium peroxide CaO 2 can be prepared by indirect methods
.
Under low temperature and alkaline conditions, the reaction of calcium chloride and hydrogen peroxide can produce nearly white CaO 2 ·8H 2 O containing crystal water
The thermal stability of alkaline earth metal peroxides gradually increases from MgO 2 to BaO 2 , but the stability is not as high as that of alkali metal peroxides
.
Alkaline earth metal peroxides can react with water or dilute acids to generate hydrogen peroxide H 2 O 2
.
H 2 O 2 is unstable and easily decomposes to release oxygen
Most oxygen bars on the market use CaO 2 as an oxygen generator and MnO 2 as a catalyst
.
Ca, Sr, and Ba can generate yellow superoxide M(O 3 ) 2
.
2.
Hydroxide
Most alkaline earth metal oxides react with water to form corresponding hydroxides and release heat
.
The alkalinity of alkaline earth metal hydroxide can be determined by the ion potential value of the metal cation
In the formula, Z is the charge of the ion; r is the radius of the ion
.
The larger the value of Φ, the stronger the polarization ability of the metal ion
.
(1) If the Φ value is large, the hydroxide adopts an acidic dissociation method, and the hydroxide appears acidic
.
→ the MO of MOH - + H +
(2) If the Φ value is small, the hydroxide adopts the basic dissociation method, and the hydroxide appears alkaline
.
→ M of MOH + + OH -
(3) If the Φ value is moderate, the two dissociation modes of the hydroxide are equivalent, and the hydroxide is amphiphilic
.
If the unit of the ionic radius r is pm, the empirical formula for judging the acidity and alkalinity of the metal hydroxide is
According to the empirical formula, Be(OH) 2 is amphoteric hydroxide (=0.
27), and Mg(OH) 2 , Ca(OH) 2 , Sr(OH) 2 and Ba(OH) 2 are all alkaline hydroxides Things
.
The solubility of alkaline earth metal hydroxides in water is much smaller than that of alkali metal hydroxides, and increases with the increase in the number of alkaline earth metal element groups
.
Be(OH) 2 and Mg(OH) 2 are hard to dissolve in water, and the solubility of other alkaline earth metal hydroxides is not large
.
Be(OH) 2 is a typical amphoteric hydroxide, which can be dissolved in a strong base
.
Mg(OH) 2 is a medium strong base, and Ca(OH) 2 , Sr(OH) 2 and Ba(OH) 2 are strong bases
.
Related Links: Properties of Alkaline Earth Metals
