SSS2: Chlorine and its compounds

Chlorine is the most important element in the halogen family. It does not occur as a free element in nature because it is too reactive. It is usually found in the combined state as chlorides e.g. NaCl. Which is found in the sea and as salt deposits.

Atomic mass =35.5 

Atomic number = 17 

Electronic configuration= 1s²2s²2p⁶3s²3p⁵ OR [Ne]3s²3p⁵

          Electronic shell configuration 

The nucleus contains 17 protons and 18 neutrons 

There are two stable isotopes of chlorine.

They are ³⁷Cl  and ³⁵Cl.

Chlorine has a naturally occurring radioactive isotope, the cosmogenic isotope, ³⁶Cl.

A Swedish chemist, Carl Wilhelm Scheele, discovered chlorine in 1774.

A. Laboratory preparation of chlorine 

The laboratory method for the preparation of chlorine:

1. Using manganese (IV) oxide.

Heat a mixture of Manganese(IV) oxide and concentrated HCl. The acid oxidizes to form chlorine
$Mn{O}^{}$_2(s) + 4HCl_(aq)→MnCl_2(aq) + 2H₂O_(l) + Cl_2(g)

2. Using potassium tetraoxomanganate(VII)

Potassium tetraoxomanganate(VII) oxidizes concentrated HCl to liberate chlorine

$\mathrm{2KM}n{O}^{}$_4(aq) + 16HCl_(aq) →  2MnCl_2(aq) + 2KCl_(aq)  +  8H₂O_(l) + 5Cl_2(g)

Note:

1. The water helps to remove the small amount of HCl that evolved from the acid.

2. The Concentrated H₂SO₄  dries the chlorine gas as it passes through it 

3. Chlorine is collected by downward delivery because it is denser than air 

  B.  Industrial preparation of chlorine 

Chlorine is manufactured industrially by:

1. Electrolysis of brine (sodium chloride solution)

2. Electrolysis of molten sodium, magnesium, or calcium.

Note: 

(I). The chlorine produced is liquefied and stored under pressure in steel cylinders.

(II). Liquid chlorine can cause skin burn

  

       Physical properties of chlorine 

1. Chlorine is a greenish-yellow gas with an unpleasant choking smell 

2. It is moderately soluble in water 

3. It is denser than air 

4. It can be liquefied under pressure 

5. It is poisonous 

       Chemical properties of chlorine 

1. Displacement of other halogens 

Chlorine can displace other halogens in a chemical reaction except fluorine because it is more reactive than them.

Cl_2(g) + 2NaBr_{(aq)  → 2NaCl(aq)} +  Br_2(l)

Cl_2(g) + 2HI_{(aq)  → 2HCl(aq)} + I_2(s)

2. Direct combination with other elements 

(A) reaction with metals 

  2Na_(s) + Cl_{2(g)→ 2NaCl(s)}

2Fe_(s) +  3Cl_{2(g)    →} 2FeCl_3(s)

(B) reaction with non-metals 

All the non-metals burn in chlorine except noble gases, oxygen, nitrogen, and carbon.

 H_2(g)  +   Cl_{2(g) → HCl(g)}

(C) reaction with water 

Chlorine reacts with water to form hydrochloric acid and oxo chlorate (I) acid (hypochlorous acid or chlorine water)

Chlorine water is formed by passing chlorine through water.

When chlorine water(HOCl or HClO) is exposed to sunlight, oxygen is given off.

(D) as a bleaching agent 

Chlorine is a common bleaching agent. The bleaching action of chlorine is due to its ability to react with water to form oxochlorate (I)acid 

(E) Reaction with alkali 

(I). reaction with dilute alkali solution 

When chlorine is passed through a cold dilute solution of sodium hydroxide, a pale-yellow mixture of the sodium oxochlorate(I) and chloride of the metal is formed.

Cl_2(g) + 2NaOH_{(aq)  → NaOCl(aq)} +  _NaCl(aq)  + H₂O_(l)

Note: 

Bleaching powder is produced by passing chlorine through slaked-lime 

Cl_2(g) + Ca(OH)_2(s)  → CaOCl₂.H₂O_(s)

(II). reaction with concentrated alkali solutions 

When chlorine is passed through a hot concentrated solution of sodium hydroxide, a mixture of the sodiumtrioxochlorate(V) and chloride of the metal is formed.

3Cl_2(g) + 6NaOH_(aq)  → NaClO_3(aq)  + 5NaCl_(aq) + 3H₂O_(l)

 

                 Test for Chlorine 

 Chlorine is a greenish-yellow gas with an unpleasant choking smell. The following materials can used to test for chlorine.

I. Damp litmus paper: the litmus paper turns pink and then bleached

II. Starch-iodide paper: starch iodide paper turns blue.

                  Uses of Chlorine 

1. It is used as a disinfectant 

2. It is used to treat sewage 

3. Chlorine is used in the treatment of drinking water to kill bacteria 

4. It is used to clean swimming pools 

5. It is used in the production of paper and paper products 

6. It is used as an antiseptic 

7. It is used to produce drugs 

8. It is used in the manufacture of dyes and plastics 

                   

                Hydrogen chloride (HCl)

Hydrogen chloride exists as a gas at s.t.p. When it dissolves in water, it forms Hydrochloric acid. Dry HCl readily dissolve in non-polar solvent e.g. Chloroform and toluene. It doesn't support combustion.

  

Laboratory preparation of HCl_(g)

Hydrogen chloride is prepared in the laboratory by reacting hot concentrated H₂SO₄  with sodium chloride (NaCl)

2NaCl_(s) + H₂SO_4(aq)  → Na₂SO_4(aq) + 2HCl_(g)

Note: The gas is passed through Conc. H₂SO₄ to dry it.

          Industrial preparation of HCl_(g)

Hydrogen chloride is prepared industrially by the direct combination of hydrogen and chlorine. It is dissolved in water to form hydrochloric acid.

  H_2(g)  +  Cl_{2(g)   →}  2HCl_(g)

            Fountain experiment 

The aim of the fountain experiment is to show that hydrogen chloride gas is extremely soluble in water. The round-bottom flask is filled with HCl_(g) and then inverted into a trough of water containing a few drops of blue litmus solution. A fountain of red water is observed.

Another gas used in the fountain experiment is ammonia gas (NH₃)

Note: fountain water turns red.

Test for hydrogen chloride 

1. The reaction between HCl_(g) and NH_3(g) forms a dense white fume of ammonium chloride (NH₄Cl)

HCl_(g)  +  NH_{3(g)   →}  NH₄Cl_(s)

2. A white precipitate of silver chloride, AgCl, is formed when HCl react with Silvertrioxonitrate(V), AgNO_3.

                   Uses of HCl

1. It is used to test, analyze, and prepare other compounds 

2. It is used in the synthesis of chloroethene 

3. It is used to remove oxides from metals before electroplating. This process is called PRICKLING. It is used in the steel-making industry.

4. It is used in the manufacture of textiles and dyes 

5. It is used in the manufacture of gelatin and glue from the tendons of animals.

                     CHLORIDES 

The chlorides are normal salts formed when metals replace the hydrogen in hydrochloric acid. e.g. CaCl₂, CuCl, KCl, NaCl, etc 

Chlorides are soluble in water EXCEPT CuCl, Hg₂Cl₂, and AgCl.

Lead(II) chloride, PbCl₂, is insoluble in cold water but dissolves gradually when warm.

Chlorides are not decomposed by heat. They can be recovered by evaporating their solutions to dryness and also by crystallization.

            Preparation of chlorides 

Chlorides are prepared by:

1. Action of hydrogen chloride or hydrochloric acid on metals 

2. Double Decomposition 

3. Direct combination of metals with chlorine 

4. Action of hydrochloric acid on oxides, hydroxides and trioxocarbonate (V)

                  The Halogens 

The halogen family are found in the group seven of the periodic table. The halogens are:

1. Flourine 

2. Chlorine 

3. Bromine 

4. Iodine 

5. Astatine (it's radioactive)

The halogens in decreasing order of reactivity: F₂ > Cl₂ > Br₂> I₂

Flourine is the most reactive element among the halogens and the most electronegative element known 

      Properties of the halogens 

1. They have high electronegativities

2. High oxidizing ability 

3. low melting and boiling points 

4. They are sparingly soluble in water

5. In chemical combinations, they attain stable octets by gaining an electron.