HALOGEN AND ITS COMPOUND

 

HALOGEN AND ITS COMPOUND

The Swedish chemist Berzelius coined the term halogen from Greek "hal's means salt and ges meaning comes to be - for an element that produces a salt with a metal.

Halogen are group 17 or VIIA of the periodic table which comprises of the elements Flourine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). Each has seven electrons in their outermost shell. They are all just one electron shy of having a full shells. They have 7 valence electrons.

ELECTRONIC CONFIGURATION OF HALOGEN

Flourine (F) [9] - [He] 2S2 2P5                 2,7

Chlorine (Cl) [17] - [Ne] 3S23P5              2,8,7

Bromine (Br)  [35]- [Ar] 3d104s24p5     2,8,18,7

Iodine (I)   [53] - [Kr] 4d105s25p5           2,8,18,18,7

Astatine (At)  [85] - [Xe]4f145d106s26p5      2,8,18,18,32,7

PHYSICAL PROPERTIES OF HALOGEN AND GRADATION DOWN THE GROUP

1.       Physical state: All are non-metals and are diatomic in gaseous state. Under ordinary conditions Flourine and Chlorine are gases, Bromine is a liquid and Iodine is a solid. Non metallic character decreases from Flourine to Iodine. The colour change from flourine to iodine (Flourine- pale yellow; chlorine- Yellow; bromine- Red; iodine- Violet)

2.       Size of atom: They have smallest size in their period. Atomic size decreases down the group.

3.       Boiling and Melting point: it increases with increase in atomic number.

4.       Ionization potential: They have high value of ionization potential in their period. It decreases down the group.

CHEMICAL PROPERTIES OF HALOGEN AND GRADATION DOWN THE GROUP

1.       Reactivity : They are reactive non-metals. Their reactivity decreases down the group.

2.       Oxidizing properties: Oxidizing property decreases from flourine to iodine because the power to accept electron decreases. Thus, flourine is the strongest oxidizing agent.

3.       Reaction with halogen: They all form hydride with hydrogen. The reactivity of halogen acid increases from H2F2 to HI. HI is strong reducing agent and the strongest acid.

4.       Reaction with alkali and alkaline earth metals: They form ionic compounds with metals.

5.       Reaction with non-metal: They form covalent compound with non metal

CHLORINE

PREPARATION OF CHLORINE

Chlorine is prepared in the laboratory by the oxidation of concentrated hydrochloric acid with a strong oxidizing agent, such as manganese (iv)oxide or potassiumtetraoxomanganate(vii).

Using manganese(iv) oxide- Heat a mixture of manganese(iv) oxide and concentrated hydrochloric acid. The acid is oxidized to liberate chlorine.

MnO2(s)  +  4HCl(aq)                       MnCl2(aq) + 2H2O(l) + Cl2(g)

The gas is passed through a washed bottle containing water to dissolve any unoxidized hydrogen chloride gas. Chlorine is dried by passing it through concentrated tetraoxosulphate (vi) acid, and it is collected by upward displacement of air or downward delivery in a fumes cupboard.

Using tetraoxomanganate (vii)- without heating, KMnO4 oxidized Conc. HCl to liberate chlorine.

2KMnO4 +16HCl               2MnCl2 + 2KCl + 8H2O + 5Cl2 + Cl2

The same apparatus is used as in the first method, but if dry chlorine is not required, the gas could be collected over brine.

INDUSTRIAL PREPARATION OF CHLORINE

Chlorine is prepared industrially by the electrolysis of (i) brine (ii) the chloride of molten sodium, magnesium or calcium.

A specially designed cell, developed by castner, kellener and solvay  is used in  the process. The chlorine gas is then liquefied and stored under pressure in steel cylinder.

PHYSICAL PROPERTIES OF CHLORINE

1.       Chlorine is a greenish-yellow gas with chocking, unpleasant, irritating smell.

2.       It is dense than air

3.       It is moderate soluble in air

4.       It is easily be liquefied under pressure of about 6 atm.

5.       It is a poisonous gas. If inhaled to a very small extent. It can damage the mucous lining of our lungs.

CHEMICAL PROPERTIES OF CHLORINE

1.       Displacement of other halogen- with exception of flourine, chlorine displaces other halogens from solution of their acids and salts.

Cl2 + 2NaBr        2NaCl + Br2;  Cl2 + 2HI               HCl + I2

2.        Combination with other elements- Chlorine combines directly with other elements to form chloride e.g.

1.       With metals: Metal react readily with chlorine, especially when heated to form their chloride.

Zn + Cl2              ZnCl2;     2Na + Cl2              2NaCl

2.       With non-metals: With the exception of oxygen, nitrogen, carbon, and noble gases, all other non-metal burns in chlorine to produce the corresponding chlorides e.g phosphorous burns in chlorine  to produce a mixture of phosphorous (iii) chloride and phosphorous (v) chloride.

P4  + 8Cl2                   PCl3 + PCl5

 because of its strong affinity for hydrogen, chlorine combines explosively with hydrogen in bright sunlight, although the reaction is slower in diffused light.

H2(g) + Cl2(g)            2HCl(g)

3.       As a reducing agent- chlorine is a powerful oxidizing agent because of its ability to remove hydrogen and readiness to accept electrons from reducing agents to form chlorides.

1.       With ammonia: 2NH₃ + 3Cl₂       N₂ + 6HCl. The hydrogen chloride produced, then combines with excess ammonia to form ammonium chloride.  6HCl + 6NH₃             6NH₄Cl

2.       With hydrogen sulphide: When hydrogen sulphide is mixed with chlorine, a yellow deposit of sulphur is formed as a result of the oxidation of the sulphide by chlorine. H₂S + Cl₂               2HCl + S.

3.       With water: Cl₂ + H₂O            HCl + HOCl (oxochlorate(i)acid  or chlorine water)

HOCl                                           2HCl + O2

4.       As a bleaching agent: chlorine is the common bleaching agent. It bleaches most dyes and inks in the presence of water, except those containing carbon.

The bleaching action of chlorine is due to its ability to react with water to form oxochlorate(i)acid, which is unstable and decomposes to release oxygen which oxidizes the dye to form a colourless compound.           HOCl + HCl                     [O]; dye + [O]                  [dye + O].

5.       Reaction with alkalis

1.       With dilute alkali solution: Chlorine forms a pale-yellowish mixture of the oxochlorate(i) and chloride  of metal, when bubbles into a cold solution of sodium hydroxide.

Cl2 + 2NaOH                    NaOCl + 5NaCl + 3H2O

2.       With hot concentrated alkali solution: When chlorine is bubble through a hot concentrated solution of sodium hydroxide, a mixture of the trioxochlorate (v) and the chloride of the metal is formed.

3Cl2  +  6NaOH                        NaClO3  + 5NaCl  + 3H2O.

3.       With slake lime: Bleaching powder is produced by bubbling  chlorine  through a freshly prepared solution of calcium hydroxide. Cl₂ + Ca(OH)₂                   CaOCl₂.H₂O (bleaching powder).

TEST FOR CHLORINE

1.       MOIST LITMUS PAPER TEST: Chlorine  readily bleaches moist litmus paper. If a moist blue litmus paper is used. It is first turned red before it finally bleached. This is because chlorine gas is acidic.

2.       STARCH IODIDE PAPER TEST: Chlorine turn moist starch-iodide paper blue because  it displaces iodine from the iodide. The iodine liberated then turns the starch blue.

2KI  + Cl2                                                  2KCl2  +   I2

USES OF CHLORINE

1.       Used as bleaching agent for cotton and wood.

2.       As a powerful germicide because of its oxidizing nature.

3.       Used in the manufacture of organic solvent

4.       Used in the manufacture of plastic e.g PVC

5.       Used in the manufacture of hydrochloric acid and domestic antiseptic.

6.       Used in aerosol propellants.