Chlorine , bromine , iodine

Group 17

( Halogens )

General characteristics

1. All have 7 electrons in valence shell

Readily accepts 1 electron

2. Outermost electronic configuration :

ns2 np5

3. Reactive non metals

Exists as diatomic molecules

4. States at room temperature :

a. Cl2 : green/yellow gas

Br2 : reddish brown/red/

orange liquid

I2 : black solid

b.p increases

Volatility decreases

Less volatile

b. trend in volatility :

Halogens are non polar molecules

Weak VDW forces exist between molecules

Down the group, no of electrons increases

Strength of VDW increases

Boiling point increases ( less volatile )

Volatility decreases

Halogens as oxidising agents

1. Cl2 Br2 I2

Halogens are readily reduced

X2 + 2e 2X-

Functions as oxidising agents

From Cl2 I2 , size of X2 increases

Accepts e less readily , less readily reduced

Oxidising power decreases

Decreasing oxidising

power

2. Difference in oxidising power explains

their displacement reactions

Eg. Cl2 + 2I- 2Cl- + I2

Cl2 can oxidise I- to I2 ( Cl2 stronger

oxidising agent )

Chlorine displaces iodine

But I2 cannot oxidise Cl-

Chemical reactions

Reactions of halogens

1. With alkali : product depends on temp

a. cold dilute alkali ( temp 150 C )

Eg Cl2 + 2OH- Cl- + ClO- + H2O

chlorate (I) ion

0 -1 +1 reduced

oxidised

Disproportionation

reaction

b. hot concentrated alkali ( temp 700C )

Eg 3Cl2 + 6OH- 5Cl- + ClO3- + H2O

chlorate (V) ion

0 -1 +5

reduced

oxidised Disproportionation

reaction

Notes :

i)Products ( ClO- and ClO3- ) called oxoanions (

anions containing oxygen )

ii)Chlorine required can be formed by :

4HCl + MnO2 Cl2 + MnCl2 + 2H2O

conc

Similar reactions shown by bromine and iodine

eg BrO3- , bromate (V) and IO3

- , iodate (V) ion

2. With hydrogen hydrogen halides / halogen

hydrides

a. prepared by direct combination :

H2 (g) + X2 (g) 2HX (g)

b. solutions of HX (g) in water produces acidic

solutions :

HX + H2O H3O+ + X-

acidic H-X bond broken

Acidity of solution : HCl HBr HI

Size of halogen atom increases

HX bond length increases

HX bond strength decreases

H-X bond breaks more readily to form H3O+

Acidity increases

Acidity increases

From Data booklet :

Bond Bond energy/kJmol-1

H-Cl 432

H-Br 366

H-I 298

Bond energy decreases

Bond more readily

broken

Acidity increases

c. i) thermal stability of HX decreases down the group:

2H-X H2 + X2 ( H-X bond broken )

H-Cl

H-Br

H-I

Size of halogen atom increases

HX bond length increases

HX bond strength decreases

HX bond more readily broken (decomposes)

Thermal stability decreases

ii)Observations :

HCl : stable to heat

HBr : produces red brown fumes ( Br2 )

HI : decomposes readily on contact with a

hot glass rod (provides activation energy)

to form purple vapour ( I2 ) + black solid

Reactions of halide ions (X-)

1. Reaction with AgNO3(aq) followed by aqueous NH3 :

a)Used as a test for halide ions

Ag+ (aq) + X- (aq) AgX (s )

i)White ppt soluble in aq NH3 forming colourless solution: Cl-

ii)Cream ppt sparingly soluble in dilute NH3 but soluble in concentrated NH3 : Br-

iii) Yellow ppt insoluble in dilute or concentrated

ammonia : I-

b) Difference in solubility in NH3:

AgCl and AgBr dissolves due to complex ion

formation

Eg: Ag+ (aq) + Cl- (aq) AgCl (s) Equilibrium

Add aqueous NH3 : the ammonia acts as a

ligand and forms a complex ion with Ag+

Equation :

Ag+(aq) + 2NH3 (aq) [Ag(NH3)2]+ (aq)

complex ion

Complex ion : H3N Ag+ NH3

Formation of the complex ion causes [Ag+] in

equilibrium to decrease

Equilibrium shifts left

Observation : AgCl (s) dissolves

Or : AgCl(s) + 2NH3(aq) [Ag(NH3)2]+ Cl- (aq)

AgBr(s) + 2NH3(aq) [Ag(NH3)2]+ Br- (aq)

Add an excess of H+(aq)/acid ,

precipitate reappears

2. Reaction with concentrated H2SO4 :

a. concentrated H2SO4 + solid ionic halides

HX ( hydrogen halides )

H2SO4(l) + 2X- (s) 2HX (g) + SO42- (s)

acid

Eg H2SO4 + 2NaCl(s) 2HCl + Na2SO4

No further reaction occurs

Alternative method to prepare HCl only

b. However , conc H2SO4 is also an oxidising agent

powerful enough to oxidise HBr Br2

and HI I2.

But it is not powerful enough to oxidise HCl Cl2

i) with bromide ions :

Br- is oxidised to Br2 while SO42- is reduced to SO2

Eg H2SO4 + 2NaBr(s) 2HBr + Na2SO4

2HBr + H2SO4 Br2 + 2H2O + SO2 (*)

ii) with iodide ions :

I- oxidised to I2 , SO42- SO2 S H2S

Eg 2HI + H2SO4 I2 + 2H2O + SO2

8HI + H2SO4 4I2 + 4H2O + H2S

Note : Product usually contaminated by sulphur or sulphur compounds ( eg SO2

and H2S )

red

c. Observations :

i) Cl- : only steamy/white fumes of HCl

ii) Br- : Br2 (g) and HBr (g)

red brown fumes steamy/white fumes

iii) I- : I2 (g) and HI (g)

purple vapour steamy/white fumes

main product : I2 (g) + black solid

Other observations: yellow solid (S),

stinking gas (H2S)

HI most readily oxidised

Notes :

(1) comparing ease of oxidation of HX

HCl HBr HI

HX is more readily oxidised

reducing power of HX increases

Ease of oxidation

increases

(2) Concentrated H2SO4 is a stronger oxidising

agent than iodine / bromine but weaker than

chlorine

(3) Concentrated H3PO4 is a weaker oxidising

agent than concentrated H2SO4 or iodine

therefore not able to form iodine from reaction

with an ionic iodide

(4) Role of concentrated H2SO4

Compound As a strong acid As an oxidising

agent

Chloride √

Bromide √ √

Iodide √ √

Use of halogens and their

compounds

1. Chlorine :

a. in water purification : household and

swimming pool

Cl2 + H2O HCl + HClO

hypochlorous acid

(oxidising agent)

Acidic solutions formed kills bacteria or

microorganisms

b. manufacture of

bleach for textile and paper industry eg

NaClO,

degreasing solvent eg CH3CCl3 ,

CFC’s

polymers eg pvc

2. Bromine :

a. fire extinguisher/flame retardants (*) eg

CH2ClBr

b. petrol additive : eg C2H4Br2

c. photographic film : eg AgBr

3. Iodine :

a. dissolved in alcohol , used as mild antiseptic

for cuts and scratches

b. used in photographic film eg AgI