CCEA ADVANCED SUBSIDIARY

CHEMISTRY

 

MODULE 2

2.9    Group II (magnesium to barium, as indicated)

 

Trends within the group limited to colour, physical state, melting points, atomic and ionic radii, first ionisation energy.

Group trends

 

 

Magnesium

Calcium

Strontium

Barium

Colour

Silver

Silver/ grey

Silver

Silver

State at Ro

solid

solid

solid

solid

Melting pt. oC

650

850

768

714

Boiling pt. oC

1100

1487

1380

1640

Metallic radius nm.

0.160

0.197

0.215

0.224

Ionic radius M2+ nm.

0.072

0.100

0.113

0.136

First ionisation energy kJ mol-1

738

590

550

503

 

Group II comprises a series of reactive metals with low densities. They are a silver-grey colour when clean but tarnish quickly in air due to the formation of the metal oxide.

 

Where possible, the following should be investigated practically.

Chemical trends, including combustion, reaction with water and dilute acids. Flame colorations produced by the metal ions. Basic nature of oxides. Thermal stability of carbonates and hydroxides related to cation size and lattice energies. Solubility trends of sulphates and hydroxides explained only by enthalpy considerations.

 

Chemical Properties

 

Combustion

They burn in air or oxygen to form the solid white oxide MO.

            2M (s)  +  O2 (g)                                 2MO (s)

The metal oxides are ionic compounds containing the metal cation and oxide ion (M2+O2-).

 

Nature of the Oxides

The group II oxides all react with water to form hydroxides

            MO (s)  +  H2O (l)                              M(OH)2 (aq)

The metal hydroxides dissolve in water to give OH- (aq) and are thus basic.

            M(OH) (s)  +  aq                                M2+ (aq)  +  2OH- (aq)

 

Reaction of metals with water

The metals react vigorously with water giving an alkaline solution of the metal hydroxide and hydrogen gas.

            M (s)  +  2H2O(l)                               M(OH)2 (aq)  +  H2 (g)

Reactivity increases down the group due to the increasing ease of losing electrons.

Magnesium reacts very slowly with water, but quickly with steam forming MgO.

            Mg (s)  + H2O (g)                              MgO (s)  + H2 (g)

 

Reaction with dilute acids

Group II metals react even more vigorously with acids than with water forming the metal salt and hydrogen.

            M (s)  +  2H+ (aq)                              M2+ (aq)  +  H2 (g)

Again reactivity increases down the Group as the outer shell electrons are lost more readily.

 

 

Flame colourations

Some Group II metals produce characteristic flame colours when compounds containing their ions are heated. These colours can be used to identify their presence in unknown compounds (as well as making colourful firework displays !!).

 

METAL

FLAME COLOUR

Calcium

Brick red

Strontium

Crimson

Barium

Apple green

 

 

Thermal stability of the metal carbonates

The stability of a salt depends on two factors.

·        The greater the charges on the ions, the greater the attraction between them, and the greater the lattice enthalpy.

·        The smaller the ions, the more closely they can approach each other in the lattice, and the greater the lattice energy.

In group II carbonates we might expect that they would become easier to decompose as the group is descended.

In fact the ease of decomposition decreases down the group and this is related to the polarising power of the metal cation.

 

Thermal stability of the metal hydroxides

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solubility of metal sulphates

An ionic compound will be soluble in water if the energy evolved in the hydration of the cation and anion is greater than the lattice energy of the compound.

The sulphates of group II elements decrease in solubility as the atomic number of the metal increases (i.e. descending the group).

MgSO4         soluble

CaSO4                   sparingly soluble

SrSO4                   almost insoluble

BaSO4                   almost insoluble

 

Explanation

The lattice energies for the four are almost the same but

Ba2+ is larger than Mg2+. The hydration energy of Mg2+ (being of smaller ionic radius) will be larger than that of Ba2+. Therefore MgSO4 is soluble, BaSO4 almost insoluble.

 

 

Solubility of metal hydroxides

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analytical uses of the ions as exemplified by the use of barium ions to test for sulphate and sulphite, magnesium ions to distinguish between hydrogencarbonate and carbonate ions in solution; back titration to determine the purity of a group II metal, oxide or carbonate.

 

[See qualitative analysis notes.]