Advanced A level inorganic chemistry exam revision notes for

Part 9: The chemistry of the Group 17 HALOGENS

9.6 The extraction of halogens from natural sources

9.7 Selected uses of  halogen elements and their compounds

9.6 The extraction of halogens from natural sources

The extraction of bromine from seawater

How are the halogens chlorine, bromine and iodine extracted?

What do we use halogens and halogen compounds for?

• Extracting bromine from the sea (e.g. from the Irish sea) which is done in 4 stages

• Stage 1 Oxidation of bromide ions Br^– to bromine Br₂:

  • Filtered sea water is acidified with sulphuric acid to minimise reaction of halogens with water, which leads to loss of bromine and wasted chlorine

  • X₂(aq) + H₂O(aq)   HX(aq) + HXO(aq) (X = Br or Cl)

  • Increasing the H⁺ concentration moves equilibrium to the left, 'suppresses' H⁺XO^– acid formation.

  • Excess chlorine added to displace the bromine

  • Cl₂(aq) + 2Br^–(aq) ==> 2Cl^–(aq) + Br₂(aq) 

  • In redox terms and oxidation state changes

    • Cl changes from 0 to –1, and Br changes from –1 to 0

• Stage 2 Removal of bromine vapour:

  • Air is blown through which extracts the easily vaporised bromine.

• Stage 3 Reduction of bromine Br₂ to hydrobromic acid HBr:

  • Sulphur dioxide gas is mixed with the air/bromine mixture and both are in contact with fresh water and the mixture condensed to form a concentrated solution of hydrobromic acid

  • This reduces the bromine to bromide (as hydrobromic acid) and also produces a much more concentrated solution of the bromide ion.

    • Br₂(aq) + SO₂(g) + 2H₂O(l) ==> 2HBr(aq) + H₂SO₄(aq)

    • or as an ionic redox equation

    • Br₂(aq) + SO₂(g) + 2H₂O(l) ==> 4H⁺(aq) + 2Br^–(aq) + SO₄^2–(aq)

    • oxidation state/number changes: S from +4 to +6, and Br from 0 to –1

  • Note: sulphur dioxide = sulfur dioxide = sulphur(IV) oxide = sulfur(IV) oxide!

• Stage 4 oxidation of hydrobromic acid HBr to bromine Br₂:

  • Steam and chlorine are blown through the acid solution to reform bromine by oxidation of the bromide ion.

  • Cl₂(aq) + 2HBr(aq) ==> 2HCl(aq) + Br₂(g)

  • or as an ionic redox equation

  • Cl₂(aq) + 2Br^–(aq) ==> 2Cl^–(aq) + Br₂(g)

  • The hot vapour mixture is condensed forming an upper aqueous layer and a lower more dense wet bromine layer

  • The bromine layer is dried with concentrated sulphuric acid (a powerful dehydrating agent).

  • The used sea water is treated with sulphur dioxide to remove traces of toxic chlorine or bromine (X = Cl or Br).

  • X₂(aq) + SO₂(g) + 2H₂O(l) ==> 2HX(aq) + H₂SO₄(aq)

    • Cl₂(aq) + SO₂(g) + 2H₂O(l) ==> 2HCl(aq) + H₂SO₄(aq)

    • Br₂(aq) + SO₂(g) + 2H₂O(l) ==> 2HBr(aq) + H₂SO₄(aq)

  • or as an ionic redox equation (X = Cl or Br)

  • X₂(aq) + SO₂(g) + 2H₂O(l) ==> 4H⁺(aq) + 2X^–(aq) + SO₄^2–(aq)

    • Cl₂(aq) + SO₂(g) + 2H₂O(l) ==> 4H⁺(aq) + 2Cl^–(aq) + SO₄^2–(aq)

    • Br₂(aq) + SO₂(g) + 2H₂O(l) ==> 4H⁺(aq) + 2Br^–(aq) + SO₄^2–(aq)

  • and the waste fluid containing residual chloride and bromide ions is discharged – although quite acidic, it is rapidly diluted in the sea.

• Consider +/– factors in make bromine from any seawater or Dead sea water in Israel

  • No evaporation needed for sea water BUT more (e.g. chlorination) stages needed in the process.

  • Dead Sea water is much more concentrated in bromide and one chlorine treatment sufficient but there is still the energy costs for evaporation and separation of the bromine.

• See below for 9.7 Uses of halogens and their compounds.

The industrial production of chlorine

• Making Chlorine:

  • Chlorine is manufactured by the electrolysis of brine (salt solution of sodium chloride)

  • The salt is readily obtained from mining rock salt, which is almost pure NaCl

  • At the negative electrode (cathode) hydrogen ions are reduced by electron gain to form hydrogen gas:

    • 2H⁺(aq) + 2e– ==> H₂(g)

  • At the positive electrode (anode) chloride ions are oxidised by electron loss to form chlorine gas:

    • 2Cl^–(aq) ==> Cl₂(g) + 2e^–

  • Left in solution is the useful alkali sodium hydroxide NaOH

    • from the residual Na⁺ and OH^– ions.

• More details and examples in section 9.7 Uses of halogens and their compounds.

9.7 USES of  HALOGENS and their COMPOUNDS

• Sodium chloride is an important raw material found as 'rock salt' or in seawater.
• Salt is used directly for food flavouring and food preservation.
• It is also used for de–icing roads because it lowers the freezing point of water (related to general effect of an impurity lowering the melting point of a substance).
• Most sodium chloride is mainly converted into other products by electrolysis (see above for the production of chlorine, hydrogen and sodium hydroxide by electrolysis of brine (brine = aqueous sodium chloride solution).

  • The hydrogen formed can be used to make hydrochloric acid via hydrogen chloride gas

  • H₂(g) + Cl₂(g) ==> HCl(g),

    • then, HCl(g) + aq ==> HCl(aq) hydrochloric acid

    • or ionically

    • HCl(g) + H₂O(l) ==> H⁺(aq) + Cl^–(aq)

  • Hydrochloric acid is a very important acid used in the chemical industry to make chloride salts.

  • or the hydrogen can be used to hydrogenate unsaturated fats (e.g. vegetable oils) to more saturated fats like margarine

    • >C=C< + H₂ => >CH–CH< (via Nickel Ni catalyst)

  • Haber Synthesis manufacture of ammonia for fertilisers

    • N₂(g) + 3H₂(g) ==> 2NH₃(g)

• The sodium hydroxide is used in the manufacture of paper, soluble sodium salts of acidic materials (eg soluble aspirin), ceramics, soaps and detergents

• All the Halogens are potentially harmful substances and chlorine in particular is highly toxic. It is dangerous to ingest halogens or breathe in any halogen gas or vapour.

• Chlorine has many 'positive' uses eg manufacturing hydrochloric acid, the useful hard wearing plastic PVC, chlorinated organic solvents like trichloroethane for cleaning, bleaches, disinfectants (TCP, Dettol etc.), water treatment to kill bacteria –

• BUT the use of  chlorinated organic pesticides is not so welcome due ecosystem damage increasing up the food chains.

• Chlorine is used to kill bacteria and so sterilise water for domestic supply or in in swimming pools, chlorine's bacteria 'killing' action is due to its powerful oxidising power which will disrupt any cell's chemistry including ours! However, traces in our drinking water should be insufficient to do us any harm and the benefits of water chlorination far outweigh the risk as long as the water treatment systems are properly managed.

• The sodium hydroxide and chlorine can be chemically combined at room temperature to make the bleach, sodium chlorate(I) NaClO. This is used in some domestic cleaning agents e.g. bleaching agents, it chemically 'scours' and chemically 'kills' germs!

  • 'cold' sodium hydroxide + chlorine ==> sodium chloride + sodium chlorate(I) + water

  • 2NaOH(aq) + Cl₂(aq) ==> NaCl(aq) + NaClO(aq) + H₂O(l)

• Organic phenolic chlorine compounds are used antiseptics and  disinfectants like 'Dettol' or 'TCP'
• Organic chlorine compounds are used as pesticides, including the now mainly banned DDT.
• Chlorine is used in making CFC refrigerant gases/liquids but their production and use are being reduced. They break down in the upper atmosphere and the chlorine atoms catalyse the destruction of ozone O₃ which absorbs harmful uv radiation.
• Liquid organic chlorine compounds are used as dry cleaning or de–greasing solvents.
• PVC: Chlorine from electrolysis of NaCl, and ethene from cracking oil fractions, are used to make a chemical called chloro(ethene), which used to be called vinyl chloride, this is then converted into the plastic–polymer poly(chloroethene) or PVC, because it is shorthand for the old name polyvinylchloride! (equation below)
  • 
  • Poly(chloroethene), old names PVC, from chloroethene (vinyl chloride) is much tougher than poly(ethene) and very hard wearing with good heat stability. so it is used for covering electrical wiring and plugs. It is also replacing metals for use as gas and water drain pipes and has found a use as artificial leather and readily dyed to bright colours! (old names : polyvinyl chloride, shortened to PVC)
• PVC is very tough hard wearing useful plastic and a good electrical insulator and is used for water piping, window frames, part of electrical fittings e.g. plug covers etc.
• Chlorine is used in the manufacture of potassium chlorate(V) weed killer, KClO₃.

• The precipitation of silver salts is important and know the observations eg the colours of the precipitate for halide ion identification (AgX salts are used in photography, see CS M1)

  • AgNO₃(aq) + NaX(aq) ==> AgX(s) + NaNO₃(aq)

  • where X is the halogen Cl, Br and I, the latter is better expressed as the ionic (NOT redox) equation

  • Ag⁺(aq) + X^–(aq) ==> AgX(s) 

  • When silver salts are exposed to light, silver is formed – the basis of using silver salts in photography.

• Silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI) are all sensitive to light ('photosensitive'), and all three are used in the production of various types of photographic film used to detect visible light and beta and gamma radiation from radioactive materials.

  • Each silver halide salt has a different sensitivity to light.

  • When radiation hits the film the silver ions in the salt are reduced by electron gain to silver

  • Ag⁺ + e^– ==> Ag

  • (the halide ion is oxidised to the halogen molecule 2X^– ==> X₂ + 2e^–)

  • AgI is the least sensitive and used in X–ray radiography.

  • AgCl is the most sensitive and used in 'fast' film for cameras

  • AgBr is used in most standard photographic films.

  • BUT, much of their use is being superseded by digital cameras!

• Fluorine is used as fluoride salts in toothpaste or added to domestic water supplies to strengthen teeth enamel helping to minimise tooth decay. (e.g. sodium fluoride or potassium fluoride).

  • The addition of sodium fluoride to domestic water supplies is controversial e.g. fluoride ions are potentially poisonous, though only traces are added, and some people think you shouldn't have 'treatment'' forced upon you, and would like to make up their own mind as to the validity of fluoride treatment of water.

• Fluorine is used in the manufacture of the tough non–stick plastic PTFE coating of cooking pans.

• Fluorine is used in manufacture of aerosol propellants and refrigerant gases.

• Bromine is a dense, dark and dangerous liquid BUT useful:

• Apart from its silver salt use in photography, bromine is used to manufacture organic

• Organic bromine compounds are used as ...

  • flame inhibitor chemicals (flame retardants like TBBA) for plastic products to reduce their flammability

  • petrol additives like 1,2–dibromoethane, to reduce the build–up of lead oxide in car engines (a use decreasing as 'green' unleaded fuels are becoming more popular),

  • agri–chemicals like the pest fumigant bromomethane, organobromo compounds are used as pesticides and fungicides because of their poisonous nature, so they are not good for us either!

  • but bromine is an ozone 'destroyer' so its use is hoped to be phased out.

• silver halide salts are used in photographic film – when visible light, uv, X–rays or gamma rays hit silver halide crystals metallic silver is formed. An electron is knocked off the halide ion and reduces the silver ion to silver: Ag⁺X^– + hv ==> Ag + X (Assignment 5 tests you on the ideas)

• Bromine and iodine are both used in 'halogen' car headlamps.

• Iodine is used in hospitals in the mild antiseptic solution 'tincture of iodine'.

• Sodium hydroxide is used in the manufacture of soaps, detergents, paper, ceramics and to make soluble salts of organic acids with low solubility in water (e.g. soluble Aspirin).
• It isn't a halogen compound, but it is made from the electrolysis of salt solution.

• The sodium hydroxide and chlorine can be chemically combined at room temperature to make the bleach, sodium chlorate(I) NaClO. This is used in some domestic cleaning agents, it chemically 'scours' and chemically 'kills' germs!

• sodium hydroxide + chlorine ==> sodium chloride + sodium chlorate(I) + water

  • 2NaOH(aq) + Cl₂(aq) ==> NaCl(aq) + NaClO(aq) + H₂O(l)

  • For more details see section 9.8 More on redox reactions of halogens & halide ions

• Halide salts are formed by neutralising the corresponding acid and alkali ...

  • eg NaOH(aq) + HCl(aq) ==> NaCl(aq) + H₂O(l)

  • but this is a ridiculously expensive method of production, evaporation of seawater is much simpler and cost effective!