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p-BLOCK ELEMENTS - HALOGENS

GENERAL CHARACTERISTICS

The group 17 of periodic table consists of five elements fluorine, chlorine, bromine, iodine and astatine and are known as halogens (sea salt forming elements). Astatine is artificially prepared by radioactive element.

ELECTRONIC CONFIGURATION

Element
Symbol
At No.
Valence shell electronic configuration
Fluorine
F
  9
Chlorine
Cl
  17
Bromine
Br
  35
Iodine
I
  53
Astatine
At
  85

PHYSICAL STATE

Intermolecular forces in halogens are weak and increase down the group. Thus F2, Cl2 are gases, Br2 volatile liquid and I2 volatile solid.

ATOMICITY

All halogens are diatomic in nature.

ABUNDANCE

Being very reactive in nature they are not found free in nature. Their presence in earth’s crust follows the order.
F2 > Cl2 > Br2 > I2 > At

COLOUR

They absorb light in the visible range forming excited states and thus they are coloured in nature.
F2 (pale yellow)                   
Cl2 (yellowish green)
Br2 (reddish brown)       
I2 (deep violet)

METALLIC CHARACTER

All the elements are non metals and metallic character increases down the group.

OXIDATION STATE

F2 Cl2 Br2 I2 At
-1 -1 to + 6 -1 to +6 -1 to +7 -1,+1,+5

BOND ENERGY AND BOND LENGTH

The bond length increases from fluorine to iodine

F-F
Cl-Cl
Br-Br
I-I
Bond length (Ã…)
1.42
1.99
2.28
2.67
Bond energy (kJ mol-1)
158.8
242.6
192.8
151.1

Due to small size the interelectronic repulsions between non bonding electrons are high in case of fluorine which results in weakening of F-F bond.

DENSITY

It increases down the group in a regular fashion and follows the order
F > Cl > Br > I
  • Atomic radii
  • Ionic radii
  • Atomic volume
  • Density
  • Electronegativity
  • Oxidising power
  • Reactivity
  • Affinity for hydrogen
  • Reduction potential
  • Solubility, all follow the above given order

IONISATION POTENTIAL

The ionisation potential of halogens is very high and value decreases down the group. Thus iodine also forms I+ and I3+  and forms compounds like ICl, ICN, IPO4. In molten state these compounds conduct electricity and show ionic character.

ELECTRON AFFINITY

The halogens have the high values for electron affinity following the order.
Cl > F > Br > I
Due to small size of fluorine (hence high electron density) the extra electron to be added feels more electron-electron repulsion. Therefore fluorine has less value for electron affinity than chlorine.

SOLUBILITY

Halogens are soluble in water which follow the order  
F2 > Cl2 > Br2 > I2
The solubility of iodine in water is enhanced in presence of KI
KI + I2  KI3 K+ +
In organic solvents like CS2, CHCl3 and CCl4 the Cl2, Br2 and I2 are more soluble and give coloured solutions. Thus Cl2 gives yellow, Br2 gives brown and I2 gives violet solution.

PROPERTIES OF HALIDE IONS (X-)

Some properties of halide ions follow the following order
F > Cl> Br > I
Basic character
Reducing character
Heat of hydration

NATURE OF BONDS WITH OTHER ELEMENTS

Halogens with metals of low ionisation potentials form ionic bonds. The ionic character follows the order
M - F > M - Cl > M - Br > M - I
The more the ionic character, the more is the mp and bp of halides.
Halogens with metals of high ionisation potentials (transition metals) and nonmetals form covalent bond.

COMPOUNDS OF HALOGENS

HYDRACIDS (HX)

All halogens directly combine with hydrogen to form HX known as hydracid.
H2 + X2 2HX

HF and HCl can be obtained by heating fluorides and chlorides with conc. H2SO4
CaF2 + H2SO4 CaSO4 + 2HF
2 NaCl + H2SO4  Na2SO4 + 2HCl

HBr and HI are strong reducing agents in nature and reduce conc. H2SO4 and cannot be prepared by this method.
2HBr (or 2HI) + H2SO4 Br2 (or I2) + 2H2O + SO2
Hence they are prepared by hydrolysis of phosphorus halides
PX3 + 3H2O H3PO3 + 3HX (X = Br, I)

PROPERTIES
HF is a low boiling liquid due to intermolecular hydrogen bonding, while HCl, HBr, HI are gases. The bpt follows the trend  HF > HI > HBr > HCl

Some other properties observe the following trend : HI > HBr > HCl > HF
Acid Strength
Reducing character
Bond length

Trend for following properties : HI < HBr < HCl < HF
Thermal stability
Dipole moment
Bond strength
Stability

Itching of glass : Glass contains silica which reacts with HF.

OXY ACIDS

All form oxyacids of the type HOX, HXO2, HXO3 and HXO4 as shown below
Name
O. N. of X
Fluorine
Chlorine
Bromine
Iodine
Salt’s name
Hypohalous acid, HXO
+1
HOF
HOCl
HOBr
HOI
Hypohalite
Halous acid, HXO2
+3
-
HClO2
-
-
Halite
Halic acid, HXO3
+5
-
HClO3
HBrO3
HIO3
Halate
Perhalic acid, HXO4
+7
-
HClO4
HBrO4
HIO4
Perhalate
Trend for following properties
HXO4 > HXO3 > HXO2 > HXO
Thermal stability
Acid strength
    Oxidising nature
The conjugate bases obtained from above acids are . Their  stability increases with the increase of oxygen atoms due to greater chances of dispersal of negative charge. The more is the stability of conjugate base, the more is the acid character.

HYPOHALOUS ACIDS (HOX)
They are formed in aqueous solution by disproportionation of halogens.
X2 + H2O    HOX + HX
Acid character and thermal stability follows the order
HClO > HBrO > HIO
Hypohalites disproportionate in aqs. solution to halides and halates.
3 OX    2X + XO3
Rate of disproportionation
The structure of HClO and HFO are    
Bleaching powder CaOCl2  is a mixed salt of HOCl and HCl

HALIC ACIDS
HClO3 and HBrO3 exist in aqueous solution and HIO3 is a white solid. The stability follows the order
They are strong oxidising in nature
Their salts, NaClO3 is a powerful weed killer and KClO3 (Berthelot’s salt) is used in fireworks and matches as oxidising agent.

PERHALIC ACIDS
Perhalates are obtained by electrolytic oxidation of halates
,
Perborate is obtained by oxidation with F2 in basic solution
They are strong oxidising agents following the order

STRUCTURE OF OXYACIDS
Hypochlorous acid (HOCl)
          

Chlorous acid (HClO2)

Chloric acid (HClO3)

Perchloric acid (HClO4)

OXIDES

All form oxides of different types. Halogens do not combine directly with oxygen hence they are prepared by indirect methods.

STRUCTURE OF SOME OXIDES
All oxides are powerful oxidising agents and decompose with explosion when heated or hit.

INTERHALOGEN COMPOUNDS

Halogens combine among themselves to form compounds known as interhalogens or interhalogen compounds of the type AX, AX3,  AX5 and AX7. Their structure and hybridisation is as follows:
AX type sp3 AX3 sp3d AX5 sp3d2 AX7 sp3d3
Linear            T-shaped Square pyramidal Pentagonal bipyramidal
A - X     
Examples - IBr, BrCl,ClF3, BrF3, ICl3BrF5, IF5,  IF7 ClF, BrF,  ICl

PROPERTIES
They are covalent, more reactive, strong oxidising and diamagnetic in nature.

PREPARATION
By direct combination of halogens
By the action of halogens on lower interhalogen
Polyhalide ions - The ions containing more than two halogen atoms are known as polyhalide ions eg.
Other examples are
Iodine Cation - Iodine also forms I+ and I3+ cations due to less ionisation energy as (CH3COO)3I, I (ClO4)3 and IPO4 have been isolated.

Pseudohalogens and Pseudohalides - The substances behaving like halogens are known as pseudohalides.

Some examples are-

FLUORINE (SUPER HALOGEN)

Discovered by Moisson 1886

OCCURRENCE

In combined state it occurs as
  • Fluorspar CaF2
  • Cryolite Na3AlF6
  • Fluorapatite CaF2.3Ca3(PO4)2
It is present in small amounts in soil, river water, bones and teeth of animals.

PREPARATION

  • Dennis method - By electrolysis of fused sodium or potassium hydrogen fluoride (dry) between graphite electrodes . (KHF2 is known as Fremy salt)
(At cathode)  
 (At anode)
  • Whytlaw Gray’s method - Electrolysis of fused KHF2 is carried out in Cu cell (electrically heated) which serves the purpose of cathode, Anode is of graphite
  • Modern method - Electrolysis of fused mixture of KF and HF is carried out in steel vessel (cathode). Anode is of graphite

PROPERTIES

  • It is pale greenish yellow gas can be condensed to pale yellow liquid and then pale yellow solid. It is very reactive.
  • Most active - It directly combines with metals and non metals eg Al, Mg, C, P, S, As, Sb, Br2, I2 etc to from fluorides, Cu and Hg form a protective coating of fluoride.
  • Reaction with Xe - With Xenon it forms three definite halides
XeF2, XeF4 and XeF6 having the following structure
XeF2  hybridisation sp3d Linear symmetrical
XeF4 hybridisation sp3d2 Square planar                                      
XeF6 sp3d3  hybridisation Distorted octahedral                                  
  • With hydrogen even in dark -
  • With water
  • Oxidising nature
  • With alkali
    Dil.
Conc.
  • With NH3
    (not explosive)
  • With H2S
  • With hydrocarbons -  It reacts with hydrocarbons violently eg. CH4
Hence fluorination is carried out in presence of N2 (it dilutes F2) and catalyst copper gauge.

USES

  • Fluoro derivatives are solvents, lubricants, refrigerants, fire extinguishers, fungicides, germicides, dyes and plastics etc.
  • For separation of U235 by forming UF6 from natural uranium
  • Preparation of Teflon (C2F4)n
  • FREONS - Chlorofluoro compounds of methane and ethane are known as freons. They are extremely, unreactive, non corrosive, easily liquefiable compounds. Freon-12 (CCl2F2) is most common and prepared as
They are used as refrigerants and propellants .
  • Magic acid - FSO3H SbF5 is strongest acid and known as magic acid.

CHLORINE

It was discovered by Scheele by heating HCl acid (muriatic acid) with MnO2 and named it oxymuriatic gas. Davy established its nature and called it chlorine.

OCCURRENCE

Being very reactive does not occur free in nature. It is widely distributed as chlorides. Common salt NaCl is most important chloride present in sea water and as rock salt.

PREPARATION

By oxidation of Conc HCl
In place of HCl mixture of (NaCl + Conc.H2SO4) can be used

MANUFACTURE

  • Weldon’s process - By heating pyrolusite with Conc HCl
  • Deacon’s process - In this process HCl is oxidised by O2 in presence of CuCl2 as catalyst at 400° C
  • Electrolytic process -  By the electrolysis of brine solution in Nelson cell
NaCl    Na+ + Cl
  • Pure Chlorine - By heating AuCl3 or PtCl4 in hard glass tube

PROPERTIES

  • It is yellowish green gas. Collected by upward displacement of air, poisonous in nature, soluble in water. It’s aqueous solution is known as chlorine water which on careful cooling gives chlorine hydrate Cl2.8H2O
  • Action of water
Coloured matter + nascent O colourless matter
The bleaching action of chlorine is due to oxidation by nascent [O].
  • Action of Hydrogen
They combine explosively but in presence of charcoal catalyst combination is smooth at room temperature
  • Displacement reactions
  • Action of NaOH Cold
Aqueous solution of NaOCl is called Javelle water. It is used as bleaching agent
  • Action of H2S
         
  • Action of dry SO2
  • Action of CO
        
  • Oxidising properties
    In aqueous solution Cl2 acts as oxidising agent
  • Reaction with ammonia
When ammonia is in excess
When chlorine is in excess

USES

It is used as a
  • bleaching agent
  • disinfectant
  • in the manufacture of CHCl3, CCl4, DDT anti knock compounds, bleaching powder, poisonous gas phosgene (COCl2), tear gas CCl3NO2 and mustard gas ClC2H4SC2H4Cl.

Euchlorine
It is a mixture of chlorine and chlorine dioxide and obtained by heating KClO3 with conc. HCl

BROMINE

It was discovered by Balard (1826)

OCCURRENCE

In nature it occurs in combined state only as bromides of Na, K and Mg in sea water. Carnallite KCl.MgCl2.6H2O contains some bromo carnallite KBr.MgBr2.6H2O in Germany (Stassfurt). In Chile and Mexico in silver mines as AgBr.  

PREPARATION

  • By heating mixture of potassium bromide manganese dioxide and conc H2SO4
  • By passing chlorine through solution of a bromide

MANUFACTURE

  • From carnallite - The mother liquor left after the crystallisation of chlorides from carnallite (KCl.MgCl2.6H2O) contains bromides of Na, K and Mg and is known as bittern.
Uncondensed vapour passed through tower packed with moist iron fillings when these are absorbed and yield ferrosoferric bromide.
Alternative method -  By electrolysis of above mother liquor
Even if some MgCl2 is decomposed, the Cl2 evolved reacts with MgBr2 to liberate Br2
  • From sea water - The sea water is concentrated when salts separate as crystals. The mother liquor contains MgBr2 and is treated as above to get bromine.
In America sea water is acidified with 0.1%H2SO4 and chlorine is passed through it. The vapours liberated is passed in Na2CO3 solution.

PHYSICAL PROPERTIES

Bromine is a heavy dark brown liquid gives irritating vapour. Density 3.2g/ml bpt 58.5ºC and fpt- 7ºC. It is soluble in water and gives bromine water about 3.6% at 20º C.
Saturated solution of bromine on cooling gives bromine hydrate Br2.8H2O.

CHEMICAL PROPERTIES

  • Combination with hydrogen
  • Oxidising nature
    Under ordinary conditions it does not react with water but in presence of an oxidisable substance it gives HBr.
Thus it oxidises SO2 to H2SO4
Sodium sulphite to sulphate
Thiosulphite to sulphate
Arsenite to arsenate
Hydrogen sulphide to sulphur
  • Reaction with alkali
In cold
In hot
  • Displacement reaction
  • Action of ammonia
  • Action of organic Compounds
  • or Br2 + moist starch  KI paper = violet

USES

  • In the manufacture of tetraethyl lead an important antiknock compound
  • NaBr and KBr are used as sedatives
  • AgBr is used in photography
  • Ethylene bromide increases efficiency of TEL
  • NaBr and KBr are used as sedatives

IODINE

Iodine was discovered by Courtois.

OCCURRENCE

It is not found in free state.
It occurs in seaweeds as NaI.
In Caliche (Chile saltpeter) as NaIO3.

PREPARATION

Lab method : By heating of a mixture of MnO2, H2SO4 and KI,

MANUFACTURE

  • From sea weeds
  • From Caliche
Exact amount of NaHSO3 is to be added since it reacts with I2, if present in excess.

PURIFICATION

It contains the impurities of Cl2, Br2 and H2O.
Cl2 and Br2 are removed by distillation with KI.
Water is removed by distillation over concentrated H2SO4.
Further it is purified by sublimation.

PHYSICAL PROPERTIES

It is black (dark violet) shining solid with metallic lustre. It sublimes on heating (mpt 114ºC).
It is metallic in character and forms I+ and I3+ ions.

CHEMICAL PROPERTIES

  • Solubility
    It is slightly soluble in water but dissolves in presence of KI
KI + I2   KI3
  • Combination with elements
  • Displacement reactions
  • Reaction with alkalies
Dilute and cold NaOH
Concentrated and hot NaOH
Hypoiodite decomposes even at room temperature
  • Action of ammonia
Explosive compound
  • Reaction with hypo
  • Action of strong oxidising agents
    eg conc HNO3, O3 and Cl2 They produce iodic acid.

USES

  • In medicines eg as tincture of iodine, iodex, iodoform
  • Solution of KI and I2 used in the treatment of goitre.

Tincture of iodine
It is mixture of  
pint rectified spirit.

Bleaching powder (CaOCl2.H2O)

The composition of bleaching powder is  

MANUFACTURE

It is manufactured by the action of chlorine on slaked lime Ca(OH)2
The manufacture is carried out in Hosenclever plant or Bachmann’s plant (Modern process)

PROPERTIES

It is yellowish powder with strong smell of chlorine.
  • By the action of dilute acids or carbon dioxide it loses its chlorine
The chlorine thus obtained as known as “available chlorine” A good sample of bleaching powder contains 35-40% available chlorine the value of which goes on decreasing on keeping the powder due to the following change
  • It give O2 in presence of cobalt chloride solution
  • In presence of slight amount of dilute acid it loses oxygen.
The evolution of nascent oxygen makes it oxidising and bleaching agent.
  • It reacts with ethyl alcohol or acetone to form chloroform
  • It does not form clear solution with water and aqueous solution contains ions.

USES

  • It is used as disinfectant and germicide
  • For the manufacture chloroform
  • For the sterilization of drinking water
  • For making unshrinkable wool
  • For bleaching cotton, wood pulp

STRUCTURE  

  • Due to Odling
  • According to Bunn, Clark and Clifford, it is a mixture of calcium hypochloriteand basic calcium chloride

ABNORMAL PROPERTIES OF HYDROFLUORIC ACID

  • It is highly poisonous and has corrosive action on skin
  • It is exist as H2F2 even in gaseous state and forms two series of salts KHF2 (FREMY’S SALT) and K2F2.
  • It is not oxidised even by strong oxidising agents.
  • It reacts with silica and glass
Hence it is used for etching glass
  • On heating with MnO2 and H2SO4 it does not give F2 while other hydrogen halides (HX) give X2
  • AgF and PbF2  are soluble in water while chlorides, bromides and iodides of silver (Ag) and lead (Pb) are insoluble in water.
  • CaF2 and SrF2 are insoluble in water while chlorides, bromides and iodides af Ca and Sr are soluble.
  • Azeotropes of hydracids
Hydrogen
Halide Composition
Boiling  point ºC
H2F2
36%
120
HCl
20.4%
110
HBr
47.0%
126
HI
57.0%
127

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