UNIT ONE MODULE ONE

CHEMISTRY OF THE ELEMENTS

• Explain the variations in properties of the elements in terms of structure and bonding.
• Describe the reactions of the elements with oxygen and water.
• Describe the behaviour of the oxides with water.
• Explain the variation in the solubility of the sulphates.
• Explain the variation in the thermal decomposition of the carbonates and nitrates.
• Discuss the uses of some of the compounds of magnesium and calcium.

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FAJAN’S RULE

An ionic compound will have a high degree of covalency if:

• The positive ion is small and highly charged.
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• The negative ion is large (highly polarisable)

Generally, covalency is promoted by small cations and large anions.

For example, the beryllium ion, Be²⁺, is so small that it manages to polarize any negative ion to such a degree that its compounds are predominantly covalent.

Group II elements are good reducing agents, they give ionic compounds, their oxides and hydroxides are basic and they give hydrogen with oxides. The exception is beryllium. One reason why beryllium is different is that its electrons are not strongly shielded from its nucleus. The radius of the Be²⁺ ion is extremely small, and it represents a very dense centre of positive charge. Beryllium also has a higher electronegativity than the other elements. Hence, the compounds it makes with non metals should have less ionic character. The solutions of beryllium compounds suffer hydrolysis and are amphoteric rather than completely basic.

The relationship between lattice energy, hydration energy and enthalpy change of solution

As the ionic radii of M+ and A- increases, the enthalpy change in both of the stages above decreases. The reverse lattice energy process becomes less endothermic and the hydration process exothermic. The cancellation effect of both processes is not always reliable to predict solubilities.

The general pattern and trends in the solubility group II salts are:

• Group II cations containing anions with a charge of -1 (e.g. Cl, NO₃^?) are generally soluble, except for the hydroxides.
• Group II cations containing anion with a charge of -2 (e.g. SO₄^2-,CO₃^2-) are generally insoluble, except for some magnesium and calcium salts.

The distinct trend of the sulphates: Beryllium and Magnesium (Very Soluble), Calcium (Sparingly Soluble), Strontium and Barium (Insoluble).

USES OF MAGNESIUM AND CALCIUM COMPOUNDS

Magnesium And Calcium

• Calcium Hydroxide (slaked lime). The solution is called lime water.
• Treatment of fields which are too acidic for healthy plant growth.
• Mortar – a mixture of slaked lime, sand and water.
• Manufacture of Calcium Hydrogen Sulphate Ca(HSO₄)₂. The paper industry needs this to remove lignin from woods and leave cellulose, ready to be made into paper.
• Reaction with chlorine to form bleaching powders, Ca(OCl)₂, CaCl_z. This is a useful source of chlorine, which it liberates readily when an acid is added.

Calcium Oxide

• Neutralizes excessive acidity.
• Used to dry ammonia.
• It is refractory, that is, it will not melt even when heated to a very high temperature.

Magnesium Oxide

• Use as refractory lining in furnaces.
• Calcium Carbonate
• Used in the Solvay process for manufacture of sodium carbonate.
• Used in the iron and steel industry.
• Used in the glass industry.
• Used in the cement industry.