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Chlorine

Chlorine is usually made by the oxidation of conc. HCI. A number of oxidizing agents can be used to carry out the oxidation, e.g. lead(IV) oxide, PbO₂, Manganese(IV) oxide, MnO₂, trilead tetraoxide, Pb₃O₄ or potassium tetraoxomanganate(VII), KMnO₄

MnO_2(s) + 4HCI_(aq) → MnCl_2(aq) + 2H₂O_(l) + Cl_2(g)

In the course of preparation, the gas formed is passed into water - to remove hydrogen chloride, after which it is dried using conc. H₂SO₄.

Commercial Production of Chlorine

This is by the electrolysis of conc. sodium chloride solution (brine). Chlorine is produced at the anode - please see the lecture on electrolysis.

Properties of Chlorine

1. It is a greenish - yellow and very poisonous gas.
2. it has a choking, unpleasant and irritating smell.
3. It is very much denser than air - about 2¹/₂ times denser than air (density of air relative to hydrogen is about 14.4). It is thus collected by downward displacement of air, and if allowed to escape into the environment, it would roll along the ground without much of it escaping upward.
4. It bleaches a damp litmus paper and is highly reactive.
5. Chlorine combines explosively with hydro- gen in bright sunlight, although the reaction is slower in diffused light. In many of its reactions with hydrogen containing com- pounds, chlorine removes hydrogen from them and combines with it - this is due to its high affinity for hydrogen.

Example:

(a). Reaction of chlorine with water in the presence of sunlight (as catalyst). When chlorine is dissolved in water and the solution exposed to sunlight, the chlorine solution is converted to hydrochloric acid solution, while oxygen gas is given off.

The reaction goes through two stages:

i. Cl_2(g) + H₂O_(l) → 2HOCl_(aq) - unstable
                          oxochlorate(I)

ii. The decomposition of oxochlorate(I) acid to give HCl and O₂

2HOCl_(aq) → 2HCl_(aq) + O_2(g)

(b). Reaction of chlorine with warm turpentine. Turpentine is a hydrocarbon, i.e., It consists of hydrogen and carbon only. When warmed and reacted with chlorine, a violent reaction occurs, resulting in the formation of HCI and black clouds of solid particles of carbon.

The HCl formed can be confirmed by passing it into a vessel of ammonia - to give white fumes of ammonium chloride.

C₁₀H_16(l) + 8Cl_2(g) → 10C_(s) + 16HCl_(g)

(c). Reaction with burning taper. The reaction is similar to that with turpentine (the taper is wax, which is a hydrocarbon). Chlorine combines with the hydrogen, leaving a deposit of carbon.

(d). Reaction with hydrogen sulphide. Chlorine reacts with H₂S to give HCl and a yellow precipitate of sulphur.

H₂S_(g) + Cl_2(g) → HCl_(g) + S_(s)

Note: this reaction is an oxidation/reduction reaction - chlorine is the oxidizing agent, while H₂S the reducing agent.

6. Reaction with other elements - chlorine will combine very readily with other elements to form chlorides. Example: With Phosphorus to form Phosphorus(V) chloride; with copper and zinc to form copper(II) chloride and zinc(II) chloride respectively; with iron, to give iron(III) chloride.

7. As an oxidizing agent - chlorine accepts electrons from a reducing agent, thereby oxidizing it, while it is reduced. Example: Chlorine oxidizes iron(II) to Iron(III); and trioxosulphate(IV) ion in association with water to tetraoxosulphate(VI).

SO₃^2-_(aq) + H₂O_(l) → SO₄^2-_(aq) + 2H⁺_(aq) + 2e^-  - chlorine accepts the lost electrons.

8. Reaction with alkalis - the products of the reaction depends on the temperature and concentration of the alkalis.

- On cold dilute aqueous solution of alkalis (e.g., NaOH and KOH). The products are oxochlorate(I) and chloride of the metal.

I.e. CI_2(g) + 2NaOH_(aq) → NaOCl_(aq) + NaCl_(aq) + H₂O_(l)

- On hot concentrated aqueous solution of alkalis - a mixture of trioxochlorate(V) and the chloride of the metal are produced.

I.e. 6NaOH(aq) + 3Cl_2(g) → NaClO_3(aq) + 5NaCl_(aq) + 3H₂O_(l)

The Bleaching Action of Chlorine (on Dyes)

Bleaching by chlorine is an oxidation process. Chlorine bleaches damp dye by the following procedure:

1. Chlorine reacts with the water in the dye to form oxochlorate(l), also called hypochlorous acid.

Cl_2(g) + H₂O_(l)     →        HOCl_(aq)        +       HCl_(aq)
                                 oxochlorate(l)

2. The oxochlorate(I) acid is a very reactive compound and readily gives up its oxygen to the dye, resulting in the dye being colourless.

dye + HOCl       →       HCl      +     (dye + O)
                                  Acid            Colourless

Note:

* Hydrochloric acid is always produced after an article has been bleached with chlorine. Therefore, the article must be washed thoroughly to remove the HCl, or else it will be attacked by the acid.

*The bleached material does not change back to its original when exposed to air.

Test for Chlorine

It turns damp blue litmus paper red, before bleaching it eventually - this shows it to be acidic.

Uses of Chlorine

(1). Used as bleaching agent.

(2). In the manufacture of: sodium oxochlorate(I) solution, NaOCl - used as domestic antiseptic solution; trioxochlorate(V) -used as weed killers; and HCl - used to make a plastic products called polyvinyl chloride (PVC).

(3). For the manufacture of wide range of useful organic compounds. Example: tetrachloromethane, CCl₄, 1,1,2 - trichloro ethene C₂HCl₃ e.t.c. - these compounds are used as degreasing agents, dry - cleaning fluids, and as raw materials for other products, such as, DDT ( used as insecticide).

(4). For the sterilization of water.  

Bleaching Powder

Bleaching Powder is formed when chlorine is bubbled into solid calcium hydroxide for a long time. The chlorine becomes absorbed in the Ca(OH)₂

Ca(OH)₂ + Cl_2(g) → CaOCl₂. H₂O_(s) bleaching powder

Note: chlorine can be removed from the bleaching powder by dilute acid or by carbon(IV) oxide of the atmosphere.

CaOCl_2(s) + CO_2(g) → CaCO_3(s) + Cl_2(g)

Hence, exposing bleaching powder to the atmosphere causes it to gradually lose its bleaching power.