SS3 First Term Chemistry Lesson Note – Metals and Their Compounds II

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WEEK 6

TOPIC: METALS AND THEIR COMPONDS

 CONTENTS:

1. Calcium- extraction of calcium,
2. Properties of calcium.
3. Compounds of calcium and their uses.

PERIOD 1: CALCIUM

Calcium is too reactive to occur as free metals in nature. Instead, it occurs abundantly in the state as Calcium trioxocabonate (iv) (CaCO₃)_, in limestone marble, chalk, aragonite, calcite and coral; as calciumtetraoxosulphate (vi) (CaSO₄) in gympsum and anhydrite, as double trioxocarbonate (iv) CaCO_3. MgCO_3. In dolomite; as calcium fluoride CaF_2, in fluorspar and as various trioxosilicates (iv). Bones and teeth contain calcium tetraoxo-sulphate (v).

Extraction of calcium

Since calcium compounds are very stable, metallic calcium is commonly extracted electronically from fused calcium chloride, a fluoride is usually added to the fused calcium chloride to lower the melting point from 850^oC to about 650^oC. The mixture is place in a crucible, lined on the inside with graphite which serves as the anode of the cell. The cathode consists of an iron rod which just touches the surface of the electrolyte. As electrolysis proceeds, metallic calcium collects on the cathode which is gradually reused so that an irregular of calcium is formed on it.

Chlorine is liberated at the anode.

Chemistry of the reaction: Fused calcium chloride consists of calcium and chloride ions.

At the cathode: the calcium ions receive two electrons each to become reduced to the metal.

Ca²⁺ + 2e^–   → Ca [Reduction]

At the anode: Two chloride ions give up an electron each to become atomic chlorine. The two atoms the combine to become liberated as a gaseous molecule

Cl^–     →  Cl + e^– (oxidation)

Cl + Cl       →       Cl₂

Overall electrolytic reaction:

Ca²⁺_(l) + 2Cl^–   → Ca_(s) + Cl_2(g)

PERIOD 2: PHYSICAL PROPERTIES

Appearance	Silv Silvery-grey solid
Relative density	1.55
Malleability and ductility	Malleable and ductile
Tensile strength	Relatively low tensile strength
Melting point	85.10C
Conductivity	Good conductor of heat and electricity.

CHEMICAL PROPERTIES

Reaction with air: calcium is a very electropositive and reactive metal. On exposure to air, it rapidly tarnishes and loses its metallic lustre due to the formation of a white film of calcium oxide or quick lime on the surface of the metal. In the presence of atmospheric moisture and carbon (IV) oxide, some calcium hydroxide and calcium-trioxocarbonate (IV) may also be formed.

When calcium is heated in air, it burns with a brick-red flame calcium oxide.

2Ca_(s) + O_(g)    →    2Ca_(s)                

TEST FOR CALCIUM IONS

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