GROUP 17

HALOGENSBY,

CHIA YENKEVIN LAI

SUGANRAAJWEI HONG

MELISSA TEH

GENERAL PROPERTIES•Non-metallic elements fluorine, chlorine, bromine, iodine and astatine.•Diatomic molecules , X 2 , with a single covalent bond.•As the proton number increases down the group:

- melting point increases- ionisation energy decreases - electronegativity decreases

•The strength of van der Waals forces of attraction increases, F2 <

Cl 2 < Br2 < I 2

•All halogens except iodine are slightly soluble in water to give coloured solutions.•Halogens dissolve in organic solvents to form coloured solutions.Halogen Colour in CCl4 solution

Cl ColourlessBr Reddish brownI Purple

•Have valence electronic configuration of ns2np5 .•Oxidising agents.

Element Chlorine(Cl) Bromine(Br) Iodine (I)

Proton number (Z) 17 35 53

Valence electron configuration 3s2 3p5 4s2 4p5 5s2 5p5

Relative atomic mass 35.5 79.9 127.0

State at room temperature Gas Liquid Solid

Colour Greenish yellow

Reddish brown Dark purple

Boiling point / 0 C -35 59 184

Covalent radius/ nm 0.099 0.114 0.133

Ionic radius/nm 0.181 0.196 0.216

Electronegativity 3.2 3.0 2.7

Electron affinity/ kJ mol-1 -348 -325 -295

Ionisation energy/kJ mol-1 1250 1141 1009

Standard electrode potential/V ( X2 + 2e 2X- )

+1.36 +1.07 +0.54

Atomic radius/ionic radius

Atomic radius Ionic radius

• Down Group 17, the nuclear charge and the screening effect increases.

• However, the increase in the screening effect is larger than the increase in the nuclear charge, causing a decrease in the effective nuclear charge.

Element Chlorine(Cl) Bromine(Br) Iodine (I)

Covalent radius/ nm 0.099 0.114 0.133

Ionic radius/nm 0.181 0.196 0.216

Electronegativity

• Is the measure of the relative strength of atoms to attract electrons in a covalent bond to which they are bonded.

• The smaller the size of the atom or/and the higher the nuclear charge, the stronger is the attraction for the electrons in a covalent bond

Element Chlorine(Cl) Bromine(Br) Iodine (I)

Electronegativity 3.2 3.0 2.7

Proton number

The Colour Intensity Of Group 17 Elements

The Volatility Of Group 17 Elements

• Decreases down the group.• The higher the melting point or boiling point,

the stronger the intermolecular forces of attraction.

• The van der Waals forces of attraction between the halogen molecules increase as the molecular size of the molecules increases.

The Relative Reactivity Of The Elements As Oxidising Agents

• An oxidising agent is an electron acceptor• X2 (aq) + 2e- 2X – (aq)

Cl2 (aq) + 2e- 2Cl- (aq) Eθ = + 1.36 VBr2 (aq) + 2e- 2Br -(aq)

Eθ = + 1.07 V

I2 (aq) + 2e- 2I- (aq) Eθ = + 0.54 V• The more positive the Eθ value, the stronger the

element acts as an oxidising agent.• The smaller is the atomic radius, the stronger the

element act as an oxidising agent. F2 < Cl 2 < Br2 < I 2

• The reactivity of halogens as oxidising agents can be shown by the following reactions

Cl2 (aq) + 2Br – (aq) 2Cl– (aq) + Br2 (aq) Eθ = + 0.27VCl2 (aq) + 2I – (aq) 2Cl– (aq) + I2 (aq) Eθ = + 0.82V

Br2(aq) + 2I – (aq) 2Br –(aq) + I2 (aq) Eθ = + 0.27V

2Fe2+(aq) + Cl2 (aq) 2Fe3+(aq) + 2Cl– (aq) Eθ = + 0.59V2Fe2+(aq) + Br2 (aq) 2Fe3+(aq) + 2Br– (aq) Eθ = + 0.30V

2Fe2+(aq) + I2 (aq) 2Fe3+(aq) + 2I – (aq) Eθ = - 0.23V

Reactions Of Chlorine, Bromine And Iodine With Hydrogen

• Hydrogen halides hydrides of halogens• H2 (g) + X2 2HX (g) , where X = F, Cl, Br, and

I.• The order of reactivity decreases down the group.Eg: a) Fluorine explodes with hydrogen at low

temperature and in the dark to form hydrogen fluoride.

H2 (g) + F2 (g) 2HF (g)

b) At room temperature, a mixture of chlorine and hydrogen will explode if exposed to sunlight or ultraviolet light. H2 (g) + Cl2 (g) 2HCl(g)

c) Bromine reacts with hydrogen only at a high temperature of 200 oC and in the presence of platinum catalyst to form hydrogen bromide.

H2 (g) + Br2 (g)

Pt

2HBr(g) 200 oC

d) At 400 oC and in the prensence of platinum catalyst, iodine and hydrogen react slowly to form hydrogen iodide.

H2 (g) + I 2 (g)

Pt 2HI(g)

400oC

Thermal Stability Of The Hydrogen Halides

• when heated , 2H−X (g) H2 (g) + X 2 (g)

• 2HCl (g) 2000 oC

H2 (g) + Cl2 (g)

• 2HBr (g) 600 oC

H2 (g) + Br2 (g)

• 2Hl (g) 200 oC

H2 (g) + l2 (g)

• HF > HCl > HBr > HI

On descending Group 17, the atomic size of the halogens

increases, and hence the H-X bond length increases.

H-F < H-Cl < H-Br < H-IBond length increases

The longer the bond length, the weaker the H-X bond and hence the more easily the bond can be broken.

BOND LENGTH(nm)

BOND ENERGY( kJ mol-1 )

H-Cl H-Br H-I0.127 0.141 0.161

H-Cl H-Br H-I432 366 298

Reactions Of Chlorine, Bromine And Iodine With Metal

• Halogens react with every metal in the Periodic Table forming halides.• examples:

2Na (s) + Cl 2 2NaCl (s)Mg (s) + Cl 2 MgCl2 (s)Fe (s) + Cl 2 FeCl2 (s)

• In the above reaction, reactivity of the halogen decreases down the group.

Reactions Of The Halides Ions With Aqueous Silver Ions

• Chloride ions react with aqueous silver nitrate to produce a white precipitate of silver chloride. Ag+ (aq) + Cl – (aq) AgCl (s)• Silver chloride turns violet in sunlight.• The white precipitate of silver chloride dissolves readily in dilute ammonia solution to form a colourless solution of silver complex. AgCl (s) + 2NH3 (aq) [Ag(NH3)2]+ (aq) + Cl- (aq) silver complex

• Bromide ions react with aqueous silver nitrate to form a cream precipitate of silver bromide. Ag+ (aq) + Br – (aq) AgBr (s)

• The cream coloured silver bromide is insoluble in dilute ammonia solution but dissolves in concentrated ammonia solution AgBr (s) + 2NH3 (aq) [Ag(NH3)2]+ (aq) + Br-(aq)

• Iodide ions react with aqueous silver nitrate to form a yellow precipitate of silver iodide.

Ag+ (aq) + I – (aq) AgI (s)

• The yellow precipitate of silver iodide is insoluble in both dilute ammonia solution ammonia solution and concentrated ammonia solution.

Reaction Of Halogens With Alkali

• Chlorine reacts with cold dilute sodium hydroxide to give a mixture of sodium chloride and sodium chlorate(I).

Cl2 (g) + 2NaOH (aq) 15oC NaCl (aq) + NaClO(aq) + H2O(l)

0 -1 +1

• In the above reaction, chlorine undergoes disproportionation to -1 in Cl- and +1 in ClO- .

• Heat ClO- from NaClO, it will undergoes disproportionation to ClO3

- and Cl- . 3ClO- (aq) ∆ 2Cl- (aq) + ClO3

- (aq) 3 NaClO (aq) ∆ 2NaCl(aq) + NaClO3 (aq)

+1 -1 +5• Reaction of chlorine with hot concentrated sodium

hydroxide. 70oC

3Cl2(aq) + 6NaOH(aq) ∆ 5NaCl(aq) + NaClO3(aq) + 3H2O(l)

Reactions Of Halides Ions With Concentrated Sulphuric Acid X– (aq) + H2SO4 (l) HX (g) + HSO4 –(aq)

• In these reactions, concentrated sulphuric acid acts as a strong acid to displace the acid, HX, from its salt.

• When solid halides are heated with concentrated sulphuric acid, white fumes of hydrogen halides are liberated.• NaCl (s) + H2SO4 (aq) NaHSO4(aq) + HCl(g)• NaBr(s) + H2SO4 (aq) NaHSO4(aq) + HBr(g)• KI (s) + H3PO4 (aq) KHSO4 (aq) + HI(g)

• Concentrated sulphuric acid is also an oxidising agent.• Thus, it will further oxidise

~ HBr Bromine ( reddish brown vapour)~ HI Iodine (a violet vapour)~ HCl

To oxidise HCl to chlorine, a stronger oxidising agent such as manganese (IV) oxide is used in the presence of concentratedsulphuric acid.

4NaCl(s)+4H2SO4(aq)+MnO2(s) ∆

Cl2(g)+MnCl2(aq)+4NaHSO4(aq)+2H2O(l)

Because concentrated sulphuric acid is not strong enough to oxidise HCl.

• Concentrated sulphuric acid is also an oxidising agent

and will further oxidise HBr and HI to Br2 (g) and I2(g) respectively.

• NaBr(s)+ H2SO4(aq) NaHSO4(aq)+ HBr(g)• 2HBr(g) + H2SO4(aq)Br2(g)+2H2O(l)+SO2(g)• 2Br-(s) + 3H2SO4(aq)2HSO4

-(s)+Br2 (g)+SO2(g)+2H2O(l)

• NaI(s) + H2SO4(aq) NaHSO4(aq) + HI(g)• 2HI(g) +H2SO4(aq) I2(g)+ 2H2O(l)+ SO2(g)• 8HI(g) +H2SO4(aq) 4I2(g)+ 4H2O(l)+ H2S(g) Overall reaction:• 2I-(s)+ 3H2SO4(aq)2HSO4

-(aq)+I2(g)+SO2(g) +2H2O(l)•Or, 8I-(s)+ 9H2SO4(aq)8HSO4

-(aq)+H2S(g) +4I2(g) +4H2O(l)

FURTHER OXIDATION

Cl - < Br - < I -

• Ease of oxidation of halide ions.• Reducing power of halide ions.

To Obtain Just HBr And HIA non-oxidising acid such as excess concentrated phoshoric(v)

acid is used together with heat.

• KBr(s)+H3PO4(aq) ∆ KH2PO4(aq)+HBr(g)• KI(s)+ H3PO4(aq) ∆ KH2PO4(aq)+HI(g)

OXOACIDS OF CHLORINE

The important oxidations states of chlorine are -1,+1,+3,+5, and +7

•The oxo-acids are:a. HOCI : chloric(I) acidb. HCIO2 : chloric(III) acidc. HCIO3 : chloric(V) acid

d. HCIO4 : chloric(VII) acid

• All the oxo-acids dissociate in water:a. HOCI (aq) H+ (aq) + CIO - (aq) b. HOCI2 (aq) H+ (aq) + CIO2

- (aq) c. HOCI3 (aq) H+ (aq) + CIO3

- (aq) d. HOCI4 (aq) H+ (aq) + CIO4

- (aq)• The acid dissociation constant, Ka

ACID Ka /mol dm -3

HCIO 1.0×10⁻6

HCIO2 1.0×10⁻2

HCIO3 1.0×10

HCIO4 1.0×1010

As oxygen is more electronegative than chlorine, the O-H bond in the oxo-acids are weakened by the –I inductive effect (electron-withdrawing effect) of the oxygen atoms which are bonded to the chlorine atoms as shown below.

a. HCIO : H-O-CIb. HCIO2 : H-O-CI O

O

c. HCIO3 : H-O-CI O Od. HCIO4 : H-O-CI O O

HCIO < HCIO2 < HCIO3 < HCIO4

INDUCTIVE EFFECT INCREASES

AS THE NUMBER OF OXYGEN ATOMS BONDED TO THE CHLORINE INCREASES,THUS CAUSING

THE STRENGHT OF THE OXO-ACIDS TO INCREASE IN THE SAME ORDER

PREPARATION OF HALOGENSTHE MERCURY CATHODE CELL

Chlorine is produce by the electrolysis of brine(concentrated aqueous sodium chloride)

using mercury as cathode and graphite as anode.

The sodium produced at the cathode combines with mercury to form unreactive sodium amalgam. The amalgam is directed into another steel tank where it reacts with water to produce sodium hydroxide and hydrogen gas.

2Na/Hg(l) + 2H2O (l) 2NaOH(aq)+ H2(g) +2Hg(l)

• The mercury is recycled.• Sodium hydroxide can be obtained by evaporating the

solution.• The disadvantage of this method is in the disposal of

used mercury. Indiscriminate disposal will lead to mercury poisoning of human, animals and fishes.

OTHER METHODS OF PREPARATION:

a. Heating a chloride with MnO2 and concentrated H2SO4.

2NaCl+3 H2SO4 +MnO2MnSO4+2NaHSO4+2H2O+Cl2

b. Action of acidified aqueous potassium manganate(VII) on a chloride.

10Cl- + 2MnO4- + 16H+ 5Cl2 + 2Mn2+ + H2O

•

BROMINE AND IODINE

Both bromine and iodine exist in large quantities in the sea water. Bromine is prepared commercially from sea water.

The sea water is treated with chlorine gas which oxidises bromide ions to bromine.

Cl2(g) + 2Br- (aq) Br2(l)+ 2Cl- (aq)

Certain marine plants, such as seaweeds, absorb and concentrate iodine ions(I-) selectively. Iodine is obtained in

small quantities from such plants.

USES OF HALOGENS AND THEIR COMPOUNDS

CHLORINE•used in the extraction of bromine from seaweed•used in the sterilising water supplies for home and industrial use, for swimming pools.

CI2 (g) + H2O (l) HCI (aq) + HOCI(aq)The OCI- ion which is formed kills bacteria by oxidising life-

sustaining compounds within them.• used as bleaching agent in the paper and fabric industry.• used as antiseptics and disinfectants eg: TCP( trichlorophenol)

Dettol (4-chloro-3,5-dimethylphenol)

Compounds of chlorine include:•Trichloromethane and tetrachloromethane are used as organic solvents.•Chloroethene is the monomer for PVC.•Sodium chloride is used as table salt.•D.D.T (dichlorodiphenyltrichloroethane) is used as insecticide.•Sodium chlorate(V), NaClO3 is a weed killer.•Sodium chlorate(I) ,NaCIO ,and calcium chlorate(I), Ca(OCI)2, is used as household bleach.•Freons (e.g. CCI2F2 , CFCI3 ) are used as refrigerants or as propellants in aerosol cans.

BROMINE• 1,2- dibromoethane, BrCH2CH2Br, is added to leaded petrol to remove the lead oxide sticking to the piston surface.•Silver bromide is used in the manufacturing of photographic film and photochromic lenses.•Used in the manufacture of dyes and drugs.

IODINE•A 50%( by mass) solution of iodine in ethanol (known as tincture) is used as an antiseptic.•Silver iodide is used in photographic film and in ‘cloud seeding’

BLACK-AND-WHITE PHOTOGRAPHY•Silver chloride and silver bromide slowly turn purple and finally dark grey when exposed to sunlight because of the following photochemical decomposition. AgCl (s) sunlight Ag(s) + ½ Cl2 (g)

AgBr (s) sunlight Ag(s) + ½ Br2(g)• the use of silver halides in black-and-white photography

depends on this photosensitive nature of silver salts.• Black-and-white photographic film is a clear, cellulose strip

coated with grains of silver bromide.• When the film is exposed, the light coming from the

subject passes through the lens of the camera and strikes the film. The silver bromide that was exposed gets activated: AgBr (s) sunlight AgBr *(s)

•The exposed film is then treated with aqueous hydroquinone (a reducing agent), where the activated silver bromide is preferentially reduces to metallic silver.

2AgBr(s)*+C6H6O2(aq)2Ag(s)+2HBr(aq)+C6H4O2(aq)

•The film is then immersed in an aqueous solution of thiosulphate (‘hypo’) where the unreactive silver bromide is removed: AgBr(s)+2S2O3

2-(aq)[Ag(S2O3) 2 ]3- (aq)+Br- (aq)

•The resulting film is now a ‘negative’.

•Where light shines through the ‘negative’ onto a piece of photographic paper, the dark areas containing precipitate or metallic silver that are opaque to light on the negative appear white on the photography paper while the white areas- the unactivated areas on the negative will appear dark.

•A print of the photograph is obtained.

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