Physical and chemical properties of Group VIIA Elements
the term “halogen” comes from two Greek language “hals” and “genas” .
“Hals” means sea-salt and “Genas” means producer. Therefore halogen means sea-salt producer. Chlorine and bromine obtained from sea water and Iodine from sea weeds. So these are named as “Halogen”.
F(9)= 1S22S22P5
Cl(17)=1S22S22P63S23P5
Br(35)= 1S22S22P63S23P63d104S24P5
I(53)= 1S22S22P63S23P63d104S24P64d105S25P5
At(85)= 1S22S22P63S23P63d104S24P64d105S25P65d106S26P5
Some properties of halogens.
Electronegativity : Halogens are strong electronegative. The order of electronegativity of halogens is
F > C l> Br > I .
Valency : All halogen’s valency is one (1 ).
Nature : Halogens are di-atomic and nonmetal.
Oxidant : These are best oxidant.
Formation of halide : NaF, NaCl etc.
Formation of halogen acid : HF , HCl , HBr and HI.
Electronegativity :
The power of attraction of shared pair of electrons to one atom, in a covalent molecule is called electronegativity.
H δ+- Clδ-
Sources of halogens:
Sources of fluorine : Fluorine is obtained from (i) Fluorspar ( CaF2) (ii) Cryolite (Na3AlF6) etc. Besides these slight fluorine is present as compound in teeth and in kernel of oyster.
Sources of chlorine : The most important source of chlorine in sea water, 2.56% NaCl. Mineral sources are (i) Rock salt (NaCl) (ii) Silvine (KCl) (iii) Carnalite (KCl.MgCl2.6H2O ) etc.
Sources of Bromine : Bromine is found in small amounts in sea water as a salt of magnesium
Sources of Iodine : The main source of Iodine is sea weed, .5% NaI . Small amount of iodine is in mineral salt like
General methods for preparation of halogens :
Halogens can be produced in the laboratory on heating a mixture of metallic halide, concentrated H2SO4 and oxidizing agent like MnO2 or KMnO4 or K2MnO4.
2NaX + 2H2SO4 = 2NaHSO4 + 2HX
MnO2 + H2SO4 = MnSO4 + H2O + [O]
2HX + [O] = H2O + X2
---------------------------------------------------------
2NaX + 3H2SO4 + MnO2 = 2NaHSO4 + MnSO4 + H2O + X2
Why can’t fluorine be prepared in the general method ?
Fluorine is the most powerful oxidizing agent and there is no such type of oxidizing agent which can be used to oxidize metallic fluoride in order to produce fluorine.
Therefore fluorine has to be prepared through electrolysis of metallic fluorides like CaF2
Extraction of fluorine : Fluorine is prepared by the electrolysis of fluorspar ( CaF2) .
CaF2(l) → Ca2+(l) + 2F-(l)
Cathode reaction : Ca2+(l) + 2e- → Ca (s)
Anode reaction : 2F-(l) → F2 (g) + 2e-
Extraction or preparation of chlorine : Chlorine is prepared in laboratory by oxidizing different strong oxidizing agent. Again chlorine can be prepared in industry by the electrolysis of brine.
NaCl (aq) → Na+ (aq) + Cl- (aq)
Cathode reaction : 2Na+ (aq) +2e- → 2Na (s)
Anode reaction : 2Cl- - 2e- → Cl2
Preparation of bromine and iodine :
By exerting the relative oxidation capacities of halogens, bromine and iodine can be extracted from bromides and iodides. For example, if Cl2 gas is passed through sodium or potassium bromide, Br2 is produced .
2KBr (aq) + Cl2 (g) = 2KCl (aq) + Br2(g)
Similarly, iodine can be produced by adding Cl2 or Br2 to the aqueous solution of NaI or KI.
2KI (aq) + Br2 = 2KBr + I2
Industrial or commercial preparation of halogens:
Commercial preparation of chlorine :
The main source of chlorine is brine. If brine is electrolyzed then hydrogen is deposited in cathode and chlorine is deposited in anode and NaOH is produced in the solution.
NaCl (aq) Na+ (aq) + Cl- (aq)
H2O H+ +
Cathode reaction : 2H++2e- →H2(g)
Anode reaction : 2Cl- - 2e- → Cl2 (g)
On the other hand two remaining ions, Na+ and
NaOH + Cl2 = NaCl + NaOCl + H2O
To prohibit the waste of chlorine is used two special cell, i) Hg cathode cell ii) Nelson cell.
Commercial preparation of Iodine :
Iodine is prepared from kelp. The ash of burnt of see weeds is known as kelp. It contains 0.4- 1.3% iodine as iodides. The kelp is extracted with hot water and the water extract is concentrated and cooled, when crystals of K2SO4, KCl, Na2SO4 and NaCl separated out. The mother liquor containing iodides is mixed with manganese dioxide and conc. H2SO4 and the mixture is heated in an iron retort. Iodine is liberated which sublimes and condenses in earthen-ware condensers known as aludels.
Physical properties of halogens ( Color)
Color of F : Light yellow
Color of Cl : Yellowish greenish
Color of Br: Reddish brown
Color of I : Violet
Color of At : Metallic
Solubility of halogen
Except fluorine all halogens are slightly soluble in water. Fluorine is a strong oxidant so it releases oxygen due to the oxidation of water. Halogen is more soluble in organic solvent than water, because it is non polar like organic solvent.
Properties of halogens :
Elements F Cl Br I
ΔHfNaX(KJ) -573 -414 -361 -288
Bond dissociation enthalpy KJ/mol 158 242 224 213
Electro negativity KJ/mol 4.0 3.0 2.8 2.5
Oxidation number -1 -1,1,3,5,7 -1,1,3,5,7 -1,1,3,5,7
Std. Reduction potential X-/X2,Pt (volt)+2.87 +1.33 +1.07 +0.54
Ionization energy (KJ/mol) 1693 1264.6 1150.8 1015.7
Electron affinity KJ/mol -346.94 -361.36 -340.25 -308.32
Ionization potential of an element is the required to remove one mole electrons from it’s outer most shell of one mole isolated atoms to produced one mole uni-positive ions.
Na - e = Na+ ; ΔH= +Ve
Electron affinity:
Electron affinity may be defined as the amount of energy is released when one mole neutral isolated atoms received one mole electrons to form one mole negative ions.
Cl + e = Cl- ; ΔH= -Ve
Oxidizing power:
The element whose electron accepting power is as high that element’s oxidizing power is also high. The value of electron affinity of halogens are high so they are strong oxidizing agent . Though electron affinity of fluorine is less than chlorine but fluorine is strong oxidizing agent even among halogens . Because reduction electrode potential and electro negativity both are higher in fluorine than chlorine. For this reason fluorine oxidizes H2O to produce oxygen.
2F2 + 2H2O = 4HF + O2
Chemical reaction of halogens :
Reaction with metal
Halogens can produce ionic halide with metal. The order of reactivity of halogens is F> Cl> Br> I.
Again reactivity depends on the metal position in electrochemical series. So halogens reactivity decreases according to the order of metal
K>Na>Ca>Mg>Al>Zn>Fe>Sn>Pb>H>Cu>Hg>Pt>Au
For this reason all metals are formed fluride compounds with fluorine. But iodine can be produced compounds easiliy with the first position metals in electrochemical series , whereas iodine produce compounds very slowly with the last elements of the series in presence of high heat.
2Na +Cl2 = 2NaCl
Reaction of halogens with non-metal:
The order of reactivity of halogens ( F>Cl>Br>I) can be explained by reaction with hydrogen and different halogens. Fluorine react with hydrogen in a dark place or even very low temperature. Chlorine reacts with hydrogen in presence of light with sound. Only bromine reacts with H2 in presence of platinum catalyst with heat to produce HBr. Iodine produced HI with heat but the reaction is reversible.
H2 + F2 → 2HF (with explosion)
H2 + Cl2 → 2HCl (explosion)
H2 + Br2 → 2HBr
H2 + I2 → 2HI (very slow)
Reaction of sulphur with chlorine:
2S (s) +Cl2 (g) → S2Cl2 (l)
S2Cl2 (l) +Cl2 (g) → 2SCl2 (l)
SCl4 can be formed using similar process. But sulphur can form SF4 and SF6 both. In this reaction 3p electron of sulphur is shifted to the higher empty d- orbital. As a result valency of sulphur is expanded. In this way sulphur can exhibit 2, 4, 6 valency.
S*(16) = 1S22S22P63S13Px13Py13Pz13d13d1
In this case octate is expanded in sulphur.
Size of fluorine is small so it is possible to accommodate six fluorine atoms around the sulphur atom. It is possible to form SF6.
But it is not possible to form SCl6. Because the size of chlorine is bigger than fluorine and electro negativity is lower. As a result octate is expanded in limited range and bigger chlorine atoms are occurred steric hindrance . So it is not possible to form SCl6.
a) Addition of electronegative elements
2FeCl2 + Cl2 = 2FeCl3
SnBr2 + Br2 = SnBr4
b) Replacement of hydrogen :
H2S + Cl2 = 2HCl + S
2NH3 + 3Cl2 = N2 + 6HCl
c) Addition of nascent oxygen :
Cl2 + H2O = HCl + HOCl
HOCl = HCl + [O]
[O] + SO2 = SO3
SO3 + H2O = H2SO4
d) By removing electron :
X2 + 2e = 2X-
2Na - 2e = 2Na+
2Na+ + 2X- = 2NaX
Reaction of halogens with water:
The required standard electrode potential value is 1.23 volt for producing oxygen by the oxidation of water.
2H2O (l) = O2 (g) + 4H+ + 4e
From the electrode potential value it is seen that bromine and iodine do not oxidize H2O. But reducting potential value of chlorine is +1.36 and fluorine is +2.87. For this reason Cl2 and F2 oxidize H2O and liberated oxygen.
2F2 + 2H2O = 4HF + O2
Cl2 +H2O = H+ + Cl- + HClO
2HClO = 2H+ + 2Cl- + O2 .
Bromine reacts with water slowly in bright sunlight.
Br2 + H2O HBr + HOBr
Iodine does not react with water.
What is disproportion reaction? Discus disproportion reaction of halogen with base at different temperature.
Ans. When any substance undergoes oxidation and reduction simultaneously in a reaction, then the reaction is known as disproportion reaction.
Cl20 + NaOH (aq dill ) = NaCl-1 +NaOCl+1 + H2O ( At 150C )
Cl20 + NaOH ( aq dill ) = NaCl-1 + NaCl+5O3 + H2O ( at 700C)
Reaction with calcium hydroxide (lime):
Ca(OH)2 + Cl2 = Ca(OCl)Cl + H2O at 400C
Calcium chloro-
hypochlorite or
bleaching powder.
Bleaching activity of chlorine :
Cl2 + H2O = HCl + HOCl
HOCl = HCl + [O]
---------------------------------
Cl2 + H2O = 2HCl + [O]
Colored object + [O] = Colorless object
Identification of chloride ion : i) Dry test : Conc. H2SO4 and MnO2 is added with salt sample and heated then greenish gas is released which is chlorine.
NaCl + H2SO4 = NaHSO4 + HCl
4 HCl + MnO2 = MnCl2 + 2H2O + Cl2
ii) Wet test : NaCl (aq) +AgNO3 (aq) = AgCl + NaNO3 (aq)
white ppt.
AgCl + 2NH4OH = [Ag(NH3)2]Cl (aq) + H2O (soluble )
Diamine silver chloride
AgCl + HNO3 = X ( insoluble )
Identification of bromide ion:
i) Dry test : Conc. H2SO4 and MnSO4 is added with salt sample and heated the mixture then brown color vapour is released which is bromine.
2NaBr + 3H2SO4 + MnO4 = 2NaHSO4 + MnSO4 + 2 H2O + Br2
ii) Wet test:
NaBr (aq) + AgNO3 (aq) = AgBr + NaNO3 (aq)
Light yellow
AgBr + 2NH4OH = [Ag(NH3)2]Br (aq) ( partial soluble )
AgBr + HNO3 = X ( insoluble )
Identification of iodide ion:
i) Dry test : Conc. H2SO4 and MnO2 is added with salt sample and heated then violet color vapour will be produced. Violet coloured gas is iodine.
2KI + 3H2SO4 + MnO2 = 2KHSO4 + 2H2O + MnSO4 + I2
ii) Wet test:
NaI (aq) + AgNO3 (aq) = AgI + NaNO3 (aq)
yellow
Ppt. AgI is insoluble in nitric acid and ammonium hydroxide both.
Preparation of hydrogen halides:
i)Preparation of HF : Hydrogen fluoride is prepared by distilling a mixture of powdered fluorspar and 90% H2SO4 – the mixture being cautiously heated below 3000C on a sand bath.
CaF2 + H2SO4 = CaSO4 + 2HF
ii) Preparation of HCl: Hydrogen chloride is prepared by heating common salt with conc. H2SO4. The reaction takes place in two steps.
a) NaCl + H2SO4 = NaHSO4 + HCl
b) NaCl + NaHSO4 = Na2SO4
In the same way HBr and HI can not be produced. Because when bromide or iodide salt reacts with conc. H2SO4 then it produces HBr and HI but HBr or HI is reducing agent and conc. H2SO4 is an oxidizing acid. So the can react with each other producing Br2 or I2. As a result HBr or HI could not be produced.
NaBr + H2SO4 (conc.) = NaHSO4 + HBr
HBr + H2SO4 (conc.) = SO2 + 2H2O + Br2
Similarly , 2NaI + H2SO4 (conc.) = 2NaHSO4 + 2SO2 + 2H2O + I2
Preparation of HBr and HI
In laboratory HBr and HI are produced by mixing water with red phosphorous and bromine or iodine. Here PBr3 is produced first then it is react with water to produced HBr.
2P + 3Br2 = 2PBr3
PBr3 + 3H2O = 3HBr + H3PO3
Similarly,
2P + 3I2 = 2PI3
PI3 + 3H2O = 3HI + H3PO3
Solubility of halogen acids in water and acidic property of the aqueous solution:
The hydracids of halogens are volatile covalent compounds. Among HCl. HBr, and HI are gaseous in normal temperature and pressure. Only HF stays in liquid under 19.50C.
Solubility : All the hydracids of halogens are highly soluble in water. The reason of this solubility is that the hydracids decompose due to the polarity of H2O, producing hydronium ion. The H-X bond is decomposed and H+ is liberated which forms a H3O+ ion.
HX + H2O = H3O+ + X-
Acidic property:
The compounds which gives H+ ion as well as H3O+ ion are called acids. According to that hydrogen halides are acidic in nature.
According to the bond dissociation enthalpy and sequence of pKa , the sequence of strength of the hydrogen halides is :
HI > HBr > HCl > HF
HX + H2O = H3O+ + X-
Chemical reactions of halogen acids:
Reactions of halogen acids with metals
Zn + 2HCl = ZnCl2 + H2
Fe + 2HBr = FeBr2 + H2
Reactions of halogen acids with alkali
CaO + 2HCl = CaCl2 + H2O
NH3 + HCl = NH4Cl
Reactions of halogen acids with carbonate
Na2CO3 + 2 HCl = 2NaCl + CO2 + H2O
Prove that conc. HCl can act as a reducing agent.
When conc. HCl reacts with crystals of KMnO4 to produce greenish yellow gas having the smell of bleaching powder begins to evolve. A filter paper soaked in KI and starch solution is held in the evolved gas when the paper turns blue. Thus it is proved that the greenish yellow gas is chlorine which is produced by the oxidation of HCl by KMnO4. As HCl is oxidized, it has reduced KMnO4 to colorless manganese salt.
KMnO4 + HCl = KCl + MnCl2 + Cl2
KI + Cl2 = KCl + I2
I2 + starch = Blue
Metallic compound in presence of HCl creates characteristics color on Bunsen flame. Metals in the compound react with HCl producing volatile metal chloride which spread color in the flame. When it is heated on the flame then electrons from the atom of metals become excited and transfer to higher energy level . Again When this electrons back to the previous state then this absorbing energy is radiated as a visible light. This is why different metals give different characteristics color on the flame.
Uses of halogens:
Uses of Fluorine :
In the production of important fluorides, such as (a) UF6 which is used in the separation of U-235. (b) SF6 is used as an insulator in high voltage electrical works. ( c) Freon – a compound of fluorine , CF2Cl2 is used as a refrigerant. (d) SnF2 is used in tooth paste to prevent dental decay. (e) BF3 is used as a catalyst in the petroleum industry. CCl3F is used as insectides. (f) Poly tetra fluoro ethane ( teflon ) is resistant to the action of acids, alkalis and oxidizing agents. It is the best insulator used in a low temperature work. Other uses -------
Use of chlorine:
Chlorine is used for producing safe drinking water. Chlorinated compounds are used mostly for sanitation, pulp bleaching, disinfectants, and textile processing. Chlorine is also used for the manufacture of chlorates and it is important in organic chemistry, forming compounds such as chloroform, carbon tetrachloride, polyvinyl chloride, and synthetic rubber. Other uses of chlorine compounds include dyestuffs, petroleum products, medicines, antiseptics, insecticides, foodstuffs, solvents, paints and plastics
Other uses -------
Uses of bromine: Bromine compounds are used as pesticides, dyestuffs, water purification compounds, and as a flame-retardants in plastics. 1,2-dibromoethane is used as an anti-knock agent in gasoline. Potassium bromide is used in photography. Other uses -------
Use of Iodine:
i) Iodine is used to prepare iodoform , tincture iodine which are used as antiseptic in dressing wounds. (ii) It is used in the preparation of certain dyes , potassium iodide. (iii) It is used in the volumetric analysis and as a catalyst. It is also used as mild oxidizing agent. Other uses -------
