CHEMISTRY
TOPIC: CHLORINE & ITS COMPOUNDS - 1

By Kingsley Idiagbor, B.Sc(Hons), PGDCs, NCE, MNSM

Chlorine is a member of Group VII in the periodic table of elements with the electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁵. It is therefore one of the halogens ("salt formers") and was first isolated by Carl Scheele in the 18th century. The name, however. was given to it by Humphrey Davy who was the first to recognize it as an element and not a compound.

Occurrence: Chlorine does not occur in nature, but its compounds are common minerals like Rock Salt. It is the 20th most abundant element in the Earth's crust. Chlorine melts at -101° C (-149.8° F), boils at -34.05° C (-29.29° F) at one atmosphere pressure, and has a relative density of 1.41 at -35° C (-31° F); the relative atomic mass of the element is 35.45.

Laboratory Preparation

Chlorine is prepared in the laboratory by the action of hydrochloric acid (HCl) on strong oxidizing agent like manganese (IV) oxide (MnO₂) and potassium tetraoxomanganate (VII) (KMnO₄) crystals. The gas produced is led through a series of three Erlenmeyer flasks. The first flask is empty so as to prevent suck back phenomenon; the second contains ordinary water and this absorbs phosphine and other impurities from the gas while the last of the three flasks is filled with tetaoxosulphate (VI) acid (H₂SO₄) to dry the gas. The dried gas is collected in a gas jar by downward delivery.

Equation for the reaction:
MnO₂ + 4HCl ------------>MnCl₂ + 2H₂O + Cl₂
2KMnO₄ + 16HCl ----------> 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂

test for gas

(i) Colour: Chlorine has a characteristic greenish-yellow colour;
(ii)Smell : The unknown gas is confirmed as chlorine if it has a strong choking smell. It is advisable to prepare the gas in fume cupboard or better still outdoors.
(iii)Litmus: Chlorine turns damp blue litmus paper red, and then bleaches it.

PROPERTIES
Physical:
(i) Chlorine is a greenish-yellow gas with choking smell;
(ii) It is soluble in water;
(iii) The gas is about 1.25 times denser than air; hence it is collected by downward delivery.
(iv) Chlorine turns damp litmus paper to red then bleaches it.

Chemical
(i) Reaction with water: Chlorine reacts with water to form two acids: hydrochloric acid (HCl) and hypochlorous or oxochlorate (I) acid (HOCl).

Cl₂(g) + H₂O (l) -----------> HCl(aq) + HOCl (aq)

(ii) Bleaching Powder Chlorine is absorbed by calcium

hydroxide to form bleaching powder:

Cl₂(g) + Ca(OH)₂(aq)-------->CaOCl₂,H₂O(s)

Bleaching powder combines with trioxocarbonate(IV) acid present in the atmosphere to release chlorine gas. This explains why bleaching powder is considered a convenient medium of transporting chlorine.

CaOCl₂(s)+ H₂CO₃-------->CaCO₃+H₂O + Cl₂

(iii)Bleaching action: Chlorine owes its bleaching action to the oxochlorate (I) acid, HOCl, formed when it is dissolved in water. First, the HOCl is broken down in the presence of sunlight to form an oxidizing agent, [O], and hydrochloric acid. Thereafter, the oxidizing agent, [O] binds to the dye in the material (such as litmus or fabric) to form a colourless complex, Dye-[O]. This is illustrated by the following set of equations:

HOCl(aq) -------------> HCl(aq) + [O]
         Sunlight
Dye + [O]-------------> Dye-[O]
Coloured    Colourless complex