10.3 Alkaline Earth Metals
🎯 Learning outcomes
Learners should be able to:
- Give general characteristics of alkaline earth metals.
- Write the molecular formula and uses of (quick lime, bleaching powder, magnesia, Plaster of Paris, and Epsom salt).
- Explain the solubility of hydroxides, carbonates, and sulphates of alkaline earth metals (General trend).
- Describe the stability of carbonates and nitrates of alkaline earth metals (general trend).
- Elements like Beryllium, Magnesium, Calcium, Strontium, etc. are kept in the IIA or 2 group of the periodic table.
- The compounds of these elements, mainly found in the earth with high melting points.
- Found to dissolve in water forming an alkaline solution, hence, called Alkaline earth metals.
- The valence cell electronic configuration = ns²
- Due to the presence of 2 electrons in the last cell, their valency is 2 and the oxidation number is +2.
- Highly electropositive, but less electropositive than alkali metals, being reactive metals, never found in the free state in nature.
- Atomic sizes relatively high but lower than those of the corresponding alkali metals.
- Possess metallic properties like malleability, ductility, sonorous, good conductivity of heat and electricity, and metallic lustre (silvery white).
- Relatively soft metals with low densities. But slightly harder than alkali metals.
- Like alkali metals, alkaline earth metals, except Be and Mg, impart a characteristic colour to the Bunsen flame. This is used for the identification of alkaline earth metals by the flame test.
- Get tarnished when exposed to air due to the formation of a layer of oxides.
- Be does not react with water, Mg reacts with hot water and the rest of the metals react with cold water, forming hydroxides (alkali) and liberating hydrogen gas.
- React with dilute acids, giving hydrogen gas.
Magnesium oxide (MgO) is called Magnesia.
Uses
- Due to its high Melting point, used as refractory material in the lining of furnaces, bricks etc.
- To make crucibles, especially in metallurgical processes.
- Its aqueous suspension, called milk of magnesia, is used to neutralise the acidity of the stomach (as an antacid).
- To produce magnesia-based cement in place of CaO-based cement.
Heptahydrate magnesium sulphate is commonly known as Epsom salt because of its previous source i.e., the water of Epsom Springs in England.
Molecular formula — MgSO₄.7H₂O
Uses
- In medicine as a purgative
- Source of Mg in fertilizers
- Dyeing and sizing of textiles
- Manufacture of paints, soaps etc.
- Cosmetic lotions.
- In tanning
Calcium oxide (CaO) is also called quicklime, burnt lime, common lime, or caustic lime.
Uses
- Quick lime is used in the manufacture of slaked lime, bleaching powder, cement, mortar etc.
- In preparation of soda lime (CaO + NaOH)
- For whitewashing
- In agriculture, to decrease the acidity of the soil
- In softening water
- As germicide and fungicide
- As a basic flux in metallurgy to remove acidic impurities
- As a cheap drying agent in desiccators in laboratories
Plaster of Paris is calcium sulphate hemihydrate.
(CaSO₄)₂. H₂O or CaSO₄.1/2H₂O or 2 CaSO₄.H₂O
Uses
- Making casts, molds, statues etc.
- In surgical bandages for plastering fractured parts of the bones
- To make blackboard chalk etc.
It is also called chloride of lime or chlorinated lime it has the following formula. CaOCl₂
Uses
- For bleaching clothes and textiles
- As a disinfectant and germicide, especially in the sterilization of water
- In the manufacture of chloroform
- As an oxidizing agent
- Solubility of a substance is defined as the amount of substance (mass in grams) that is dissolved in 100 g of water (solvent) to make a saturated solution at a given temperature.
- Solubility of a solid depends on the lattice energy of the solid and the hydration energy of the ions.
- Lattice energy is a measure of the energy contained in the crystal lattice of a compound, equal to the energy that would be released if the component ions were brought together from infinity.
- Hydration energy can be defined as the amount of energy released when one mole of ions is hydrated.
- For a substance to dissolve, the hydration energy must exceed the lattice energy.
- An increase in lattice energy decreases solubility, and an increase in hydration energy increases solubility. The extent of dissolution of a solute in water is decided by the competition between these two energies.
Hydroxides of alkaline earth metals
The hydroxides of group II metals have relatively low solubility in water, and the solubility increases down the group. This is because the lattice energy decreases down the group in the solid because of the increasing size of the positively charged ions.
Sulphates of II group metals
The solubility of sulphates of alkaline earth metals decreases down the group. The increase in the size of the metal ions decreases both the lattice energy and hydration energy, but the decrease in hydration energy is higher.
Carbonates of group second metals
The alkaline metal carbonates are not very soluble in water. Their solubility decreases down the group. This is mainly due to a decrease in the hydration energy.
This is due to their insolubility in water; these metal carbonates occur as solid rock minerals in nature. For example, CaCO₃ is an abundant mineral of Calcium.
When alkaline earth metal carbonates and nitrates are heated, they decompose to give metal oxides.
Stability of carbonates
The group 2 metal carbonates are fairly stable to heat. The thermal stability increases from top to bottom in the group.
Stability of nitrates
The nitrates also thermally decompose on heating to give metal oxides. Similar to carbonates the stability increases top to bottom in the group.
Test your understanding
Take a short quiz on alkaline earth metals.