GROUP 17- Cl, Br, I

By: Tea-bun Sam Thongss Huan Yih

GROUP 17 : Cl, Br, I

Known as the p-block elements or halogens(salt formers) exits as simple covalent molecule, X2

non-metallic elements. High reactivity, found in combine state in

nature. Astatine do not occur naturally. Unstable and radioactive. - synthetically made in laboratory. Commercially, - Chlorine is manufactured by the electrolysis of brine. - Bromine is manufactured by oxidation of Br- ions in seawater by chlorine. - Iodine is obtained by reducing sodium iodate(V) by sodium hydrogen sulphate(IV) Laboratory, Cl2, Br2, I2 are prepare by oxidising the appropriate halide ions using manganese(IV) oxide and concentrated sulphuric(VI) acid.

F

Cl

Br

I

At

Atomic radius increase Electronegativity decrease Electron affinity decrease M.P & B.P increase Enthalpy of vaporisation increase Volatility decrease Metallic property of elements increase Colour intensity increase Bond energy decrease

General Properties

Atomic radius increase

the nuclear charge and screening effect increase down the group

Screening effect > nuclear charge ... Effective nuclear charge decrease

Elements Cl Br I

Atomic radius 0.099 0.114 0.183

Ionic radius 0.181 0.196 0.216

Electronegativity

• Smaller size=higher nuclear charge=stronger the attraction of the covalent bond• Hence going down the group, the electronegativity decrease.

Element Cl Br I

Electronegativity 3.0 2.8 2.5

Cl Br I2.22.42.62.8

33.2

Column1

M.P, B.P and Enthalpy of Vaporisation

• the intermolecular increase as the total number of electron increase down the group. • hence increasing the m.p, b.p and enthalpy of vaporisation ( volatility decrease )• In I2 the van der Waals force are strong enough to sustain as solid, hence the metallic property increase as well as the colour intensity.

Element Cl Br I

Melting point -101.0 -7.2 114.0

Boiling point -34.7 58.8 184.0

∆H vaporisation/kJ mol-1 +10.2 +15.0 +30.0

Physical state (298k) Pale yellowgas

Reddish brown liquid

Black solid

Chemical properties

of the halogen

Recap !

• The colour intensity increases! • Volatility decreases !• E0 becomes less positive, more negative• Oxidising power

Going downThe

group from Cl2 Br2

I2

Reaction of Group 17

Group 17 Reactions

Reaction of Elements with hydrogen and the relative stability

of the halides

Reaction of halide ions

Reactions of chlorine with NaOH

Reaction of Cl2 , Br2 , I2 with hydrogen

General equation :

H2 + X2 2HX(g)

The order of reactivity between halogens and hydrogen descending the group

F

Cl

Br

I

At

•Reaction become less vigorous •Reaction rates become slower

Reaction of Cl2 , Br2 , I2 with hydrogen

H2(g) +F2(g) 2HF(g) Rapid reaction Fluorine explodes with in low temperature and in

the dark to form Hydrogen Fluoride

H2(g)+ Cl2(g) 2HCl(g) Explode when reaction exposed to sunlight /ultraviolet light

H2(g)+ Br2(g) 2HBr(g)

React only at high temp(200oc) and in presence of Pt catalyst

pt

200’c

Thermal stability of hydrogen halide

Reaction Condition

2HCl H2 + Cl2 20000c

2HBr H2 + Br2 6000c

2HI H2 + I2 2000c

The stability of HX towards decomposition decreases in the order The result shows the thermal stability of hydrogen halides decreases with increasing relative molecular mass.

HCL > HBR > HI

Thermal stability of hydrogen halide

Going downThe

group

•Atomic size of halogens •Bond length of H-X •Bond strength•Thermal stability

The decomposition of hydrogen halides involve bond breaking of the H-X bonds. The longer the bond length the weaker the bond the more easily the bond can be broken .

REACTIONS of hydrogen halide

1. Reaction with Aqueous Ag+ ions

Cl- (aq) AgNO3(aq) AgCl(s) dilute NH3(aq) [Ag(NH3)2]+(aq)

Aqueous Ag+ ions

Conc Sulphuric Acid

Br- (aq) AgNo3(aq) AgBr(s)

I- (aq) AgNo3(aq) AgI(s)

dilute NH3(aq)

dilute NH3(aq)

Conc NH3(aq)

Conc NH3(aq)

Insoluble

Insoluble

Insoluble

[Ag(NH3)2]+(aq)

White ppt

yellow ppt

Pale yellow

ppt

REACTIONS of hydrogen halide

1. Reaction with Concentration Sulphuric acid.

Hot, concentrated H2SO4

Concentrated H2SO4 act as an oxdising agent.

Reddish brown

HCl(g)

HBr(g) + Br2(g)

HI(g) + I2(g)

Purple fumes

H2 SO4 reduced to SO2

NaCl(s) +H2 SO4 (l) NaHSO4 (s)+ HCl(g)

2Br-(s) +3H2SO4(l) HSO4(s)+ Br2(g) + SO2(g) + H2O(l)

NaBr(s) +H2SO4 (l) NaHSO4 (s)+ HBr(g) ……….

HBr(s) +H2SO4 (l) Br2(g) + H2O(l) + SO2(g) ……….

When sodium chloride is heated with concentrated sulphuric acid, HCl gas is produced.

In this reaction, Br- is oxidised to Br2 to H2SO4 is reduced SO2

NaI(s) +H2SO4 (l) NaHSO4 (s)+ HI(g) ……….

2HI(g) +H2SO4 (l) I2(g) + 2H2O(l) + SO2(g)

2I-(s) +3H2SO4(l) HSO4(s)+ I2(g) + SO2(g) + H2O(l)

or

2I-(s) +9H2SO4(l) 8HSO4(s)+H2S + 4I2(g) + SO2(g) + 4H2O(l)

2HI(g) +H2SO4 (l) 4I2(g) + 4H2O(l) + H2S(g) Further oxidation

In this reaction I- is oxidised to I2 andH2SO4 is reduced to H2S and SO2.

Cl-

Br-

I-

•Ionic size increases•Reducing power increases

REACTIONS of hydrogen Reaction of Chlorine with aqueous sodium Hydroxide.

Cl2

Cold dilute NAOH NaCl + NaClO+ H2O

NaCl + NaClO3 + H2OHot, concentrated NAOH

This reaction is a Disproportionation reaction. A Disproportionation reaction is a reaction in which a particular molecule is simultaneously oxidised and reduced .Example : Cl2

Cl2 (g)+ 2OH (Aq) Cl (aq)+ ClO (aq) + H2O(aq)

NaCl + NaClO3 + H2OCl2 (g)+ NaOH (Aq)

The following slides is by Huan Yih

28-29

The above slides is by Thongss

8-18

The halogen atoms with valence shell configuration of ns2np3,

short of one electron to achieve octet configuration.

The atoms can achieve octect configuration by:

 

 

The halogens are slightly soluble in water. But iodine is very soluble in aqueous potassium iodide due to the formation of the soluble I3

- ion.

In aqueous solution,

Chlorine is almost colourless

Bromine and iodine are yellowish/ orange In CCl4,

Bromine is reddish brown

Iodine is violet.

Halogen as Oxidising Agents

Element Cl Br I

Standard electrode potential/ V

+1.36 +1.09 +0.54

The S.E.P become less positive showing that the oxidising power of the halogens decrease down the Group.This is because the size of the element increases and the electron affinity as well as the hydration energy decreases.

Reaction of the halogen with hydrogen The halogens react with hydrogen to

produce the hydrogen halides, which are colourless gas but form white fumes in moist air.

H2(g)+X2(g)2HX(g) In the reaction, hydrogen is oxidised from

an oxidation state of zero to +1. Going down the Group, the oxidising power

of the element decreases hence the reactivity with hydrogen also decreases.

Reactions of Chlorine with Alkali

Reactions of aqueous halide ions with silver nitrate solution

 

Reactions of solid halides with concentrated sulphuric (VI) Acid

 

Oxoacids of chlorine All halogens (expect flourine) can exhibit variable oxidation states in their

compounds. Oxidation state Example

-1 NaCl , HCl

0 Cl2

+1 Cl2O , NaOCl

+3 HClO2 , NaClO2

+5 HClO3 , KClO3

+7 KClO4 , Cl2O7

The oxo-acids of chlorine are : HOCl , HClO2 , HClO3 , HClO4 All the oxo-acids dissociate in water. The strength of acids depends on the strength of the O --- H bond in molecule. oxygen is more electronegative than chlorine, the O --- H bond is weaken by the –I

inductive effect of the oxygen atoms bonded to the chlorine atoms. The inductive effect increase in the order HOCl < HClO2 < HClO3 < HClO4 because

the number of oxygen atoms bounded to the chlorine increase causing the strength of the oxo-acids to increase in the same order.

Production of chlorineThe Mercury Cathode Cell

1. Chlorine is produced by the electrolysis of brine (conc. sodium chloride) using mercury as cathode and graphite as anode.

2. The electrolysis process are : Anode (graphite) Cathode (mercury) 2 Cl-(aq) Cl2(g) + 2 e 2 Na+

(aq) + 2e 2 Na(l) Na(l) + Hg(l) Na/Hg(l)

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

2 Na/Hg(l) + 2 H2O(l) 2 NaOH(aq) + H2(g) + 2 Hg(l) 4. Disadvantage – Indiscriminate disposal will lead to mercury poisoning of living

things.

The Diaphragm Cell

1. The brine solution enters the cell through the anode compartment.2. The chlorine ion is discharged as chlorine gas at the titanium anode and leaves

an excess of sodium ions in the anode compartment. 2 Cl-(aq) Cl2(g) + 2 e

3. Hydrogen gas is produced at the steel cathode and leaves an excess of hydroxide ions in the cathode compartment. 2 H+

(aq) + 2e H2(g) 4. The level of brine in the anode compartment is deliberately higher so that brine

floes through the asbestos diaphragm, carrying with it the sodium ions, into the cathode compartment where it combines with hydroxide ions to form sodium hydroxide.

5. Sodium hydroxide together with excess brine is drained out from the cell. 6. The anode and cathode compartment has to be separated by a porous partition to

prevent the chlorine formed at the anode from reacting with sodium hydroxide formed at the cathode. Cl2(g) + 2 NaOH(aq) NaCl(aq) + NaOCl(aq) + H2O(l)

Comparison Of the PropertiesChlorine Bromine Iodine

Physical State/ 298 K Yellowish green gas Reddish brown liquid Shiny black solid

Colour of vapour Yellowish green Reddish brown Violet

Reaction with hydrogen

Explosive in sunlight Combines on heating Reversibly at 450 ◦C and Pt as catalyst

Oxidising power Strong Mild Weak

Displacement reaction Displaces bromine and iodine

Displaces iodine Does not displace the other halogens

Reaction with S2O32- Oxidised to SO4

2- Oxidised to SO42- Oxidised to S4O6

2-

Silver salt White. Soluble in ammonia

Pale yellow. Soluble only in conc. ammonia

Yellow. Insoluble in ammonia

Action with concentrated sulphuric acid

HCl formed HBr and Br2 formed HI and I2 formed

UsesChlorine.-Extraction of bromine from seawater.-Purification of water-As bleaching agent in paper and fabric industry. Bromine-Manufacture of dyes and drugs.- 1,2-dibromoethane is added to leaded petrol to remove the lead oxide in the piston surface.-Silver bromide used to manufacture photograph film and photochromic lenses.

Iodine-Silver iodide is used photographic film - used in antiseptic.

Black and White Photography

AgCl(s) sunlight Ag(s) + ½ Cl2(g) AgBr(s) sunlight Ag(s) + ½ Br2(g) - slowly turn purple and finally dark grey

-The silver bromide that exposed to light get activated. AgBr(s) sunlight AgBr*

(s) -Then, the exposed film get treated with aqueous hydroquinone, activated silver is reduce to metallic silver. 2 AgBr*

(s) + C6H6O2(aq) 2 Ag(s) + 2 HBr(aq) + C6H4O2(aq) Amount of metallic silver formed is directly proportional to the amount of light fell on the film. Then the film is immersed in sodium thiosulphate (aq), where the unactive silver bromide is removed (negative)

AgBr(s) + 2 S2O32-

(aq) Ag(S2O3)23-

(aq) + Br-(aq)

PHOTOCHROMATIC LENS- Photochromic are sunglasses that darken when exposed to the sunlight, silver chloride and copper chloride crystal are added in the lens while its in the molten state.-When exposed to sunlight Cl- Cl + e Ag+ + e Ag

-Chloride and silver atoms are produce, silver atom block the transmission of light causing the lens to darken. degree of darkening proportional to the intensity of light. -When remove from sunlight Cl + Cu+ Cl- + Cu2+ Cu2+ + Ag Cu+ + Ag+ - Net result, lens become transparent again.