The elements of group 2 or IIA in the modern periodic table i.e. beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) are called alkaline earth metals.
Compounds of alkaline earth metals:
1. Quick lime (CaO)
Uses:
- Preparation of soda-lime.
- Drying agent.
- Manufacture of glass, bleaching powder, etc.
- Flux in metallurgy
- Production of the limelight.
2. Plaster of Paris (CaSO4.1/2H2O or 2CaSO4.H2O)
Uses:
- In surgery for setting broken or fractured bones.
- In making casts for toys, statues, etc.
- In making blackboard chalks.
- In making artificial marble and fireproof materials.
3. Bleaching powder (CaOCl2)
Uses:
- As bleaching agent.
- As germicides and disinfectants.
- In the manufacture of chloroform.
4. Magnesia (MgO)
Uses:
- As refractory material.
- To prepare crucible.
- To neutralize the acidity of the stomach.
5. Epsum salt (MgSO4.7H2O)
Uses:
- Purgative in medicine.
- To correct magnesium and sulphur deficiency in the soil.
- As desiccant.
Solubility of hydroxides, carbonates and sulphates of alkaline earth metals:
For any compound to be soluble in water, its lattice energy must be less than hydration energy.
The energy released when one mole of a compound is formed from its constituent ions under standard conditions is called lattice energy.
The energy released when one mole of an ionic compound is completely hydrated by water at standard conditions is called hydration energy.
solubility α Hydration energy
More is the difference in the size of cation and anion, more will be the solubility.
Solubility of hydroxides:
As we move down the group, the size of cation increases and the difference between the size of ions decreases which decreases Lattice energy and hence solubility increases.
Be(OH)2 < Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2
Increasing order of solubility.
Solubility of sulphates and carbonates:
The size of sulphates and carbonates is large. As we move down the group, the difference between the size of ions increase which increases lattice energy, and hence solubility decreases.
BeCO3 > MgCO3 > CaCO3 > SrCO3 > BaCO3
BeSO4 > MgSO4 > CaSO4 > SrSO4 > BaSO4
Decreasing order of solubility
Stability of carbonates and nitrates of alkaline earth metals:
As we move down the group, lattice energy of carbonates and nitrates of alkaline earth metals increases due to which their stability increases down the group.
BeCO3 < MgCO3 < CaCO3 < SrCO3 < BaCO3
Be(NO3)2 < Mg(NO3)2 < Ca(NO3)2 < Sr(NO3)2 < Ba(NO3)2
Increasing order of stability
Some Important Questions
1. Write the formula and any two uses of a. Quick lime b. Bleaching power c. Plaster of Paris d. Magnesia e. Epsum salt
2. Write the solubility trend of alkaline earth metal hydroxide and carbonates.
3. Write the stability trend of alkaline earth metal carbonates and nitrates.
Questions & discussion
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