Uses
The uses of group II elements are used widely in the commerce and industry. The bright white flame of magnesium is used in flares. It also has a very strong reducing power that it is widely used to protect steel objects, like ships for example. The strong reducing power also means that it can be used to extract less electropositive metals.
Oxidation States and ionisation Energies
In all their compounds these metals have an oxidation number of +2 and, with few exceptions, their compounds are ionic. The reason for this can be seen by examination of the electron configuration, which always has two electrons in an outer quantum level. These electrons are relatively easy to remove, but removing the third electron is much more difficult, as it is close to the nucleus and in a filled quantum shell. This results in the formation of M2+. The ionisation energies reflect this electron arrangement. The first two ionisation energies are relatively low, and the third very much higher.
Reaction with oxygen
All group II elements react in air to form an oxide layer which dulls the surface of the metal. Barium is so reactive it is stored under oil.The reactivity increases down the group, and they all forming a white oxide.
2Mg(s) + O2(g) 2MgO(s)
Reaction with water
Reactivity increases down the group. When magnesium and calcium react in water.
a) Virtually no reaction occurs between magnesium and cold water. However, in a reaction with steam it forms magnesium oxide and hydrogen.
Mg(s) + H2O(g) MgO(s) + H2(g)
b) Calcium is more reactive. It reacts with cold water to produce an alkaline solution of calcium hydroxide and hydrogen gas is released.
Ca(s) + H2O(l) Ca(OH)2(aq) + H2(g)
Hydroxides
Calcium, strontium and barium oxides react with water to form hydroxides:
CaO(s) + H2O(l) Ca(OH)2(s)
Calcium hydroxide is known as slaked lime. It is soluble in water and the resulting mildly alkaline solution is known as limewater, which is used to test for the acidic gas carbon dioxide.
Formation of salts with hydrochloric acid
The salts of group II are all white, crystalline compounds. They are prepared easily by reaction of the metal, metal oxide or metal carbonate with an acid. Magnesium, magnesium oxide or magnesium carbonate all dissolve in hydrochloric acid, to form colourless solutions containing (aq) magnesium chloride, MgCl2(aq).
This white crystalline compound may be obtained by partial evaporation of the solution.
The reaction of magnesium with hydrochloric acid is that the magnesium ribbon dissolves rapidly in cold dilute hydrochloric acid with rapid evolution of hydrogen gas.
Mg(s) + 2HC(aq) MgCL2(aq) + H2(g)
Chalk and lime
Chalk is made from the sedimentary remains of marine invertebrates, chemically they are composed of calcium carbonate CaCO3.
Limestone and chalk are used in large quantities to manufacture quick lime (calcium oxide) and cement.
Strong heating of calcium carbonate produces calcium oxide,
CaO(s) + CO2(g)
Calcium oxide reacts vigorously with water to produce calcium hydroxide Ca(Oh)2.