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Halogens:
The elements of the group VIIA (17) are collectively called halogens
...
The word halogen is derived from the Greek words halos (means sea salt)
and gene ( means born) meaning sea salt producer because most of them are
present as soluble salt in sea water
...
So, not much known
about its chemistry
...
The electronic
configuration of halogens show that each of them has one electron less than the nearest
inert gas
...
They occur in nature in combined state
...
CaF2
Chlorine: Sea water (NaCl), rocksalt (KCl), carnalite KCl
...
6H2O
Bromine: sea water and salt lakes (NaBr, KBr, MgBr2),I mpurities in
carnallite (MgBr2)
Iodine: Alkali metal iodides in sea weeds, sodium iodate NaIO3 in Chile
saltpeter
...
Electronic configuration: From this electronic configuration, it is clear that all
these elements have seven electrons in their valence shell and have general outermost
electronic configuration ns2 np5
2
...
The atomic size and radius increases moving down the group
...
of shells)
...
Physical state: The different molecules of halogens are held together
by van der waals’ forces of attraction
...
Therefore, at ordinary condition, Fluorine and Chlorine exist as gas,
bromine as liquid while iodine in volatile solid
...
Melting point and boiling point: the melting point and boiling
point increases with increase of atomic number due to increase of van der
waal’s forces
...
Electronegativity: Due to small size and higher nuclear charge, each
halogen has the highest electronegativity in its period
...
As we move down the group, electronegativity
decreases due to increase in the size of atom
...
Ionization energy: Due to smaller size and greater nuclear charge,
the ionization energy of the members of halogen family is high
...
F > Cl > Br > I
7
...
As we move down the group,
the E
...
But E
...
of Fluorine is
lower than Chlorine
...
As
a result of which strong electron – electron repulsion are present in the
relatively compact 2p – orbitals of fluorine
...
Oxidation states: All the halogens uniformly show as oxidation
state of -1 since they have a strong tendency to gain electron to acquire the
nearest inert gas configuration
...
however, other
halogens i
...
chlorine, bromine iodine show positive oxidation states of
+1, +3, +5 and +7 due to the presence of vacant d- orbitals in their
respective valence shells
...
Bond dissociation energy: the bond dissociation energy of F – F
bond is lower than Cl –Cl bond and Br –Br bond is due to inter electronic
repulsion present in the small atom of fluorine
...
Oxidation is
carried out by electrolytic method as well as by chemical method
...
By electrolytic oxidation of halides:
By this method all halogens can be prepared
KHF2
KF + HF
KF
K + + FHF
H+ + F2H+ + 2e_
2F-
H2
F2 + 2e-
(at cathode)
( at anode)
Similarly, electrolysis of fused or aq
...
)
Electrolysis
2Na + Cl2
Electrolysis
2NaOH + Cl2 +H2
2
...
MnO2 + 2HCl
2HCl + [O]
MnO2 + 4HCl
MnCl2 + H2O + [O]
Cl2 + H2O
MnCl2 + Cl2 + H2O
Similarly, using K2Cr2O7 and KMnO4 as oxidizing agent
K2Cr2O7 + 14HCl
KCl + 2CrCl3 + 3Cl2 + 7H2O
2KMnO4 + 16HCl
2 KCl + 2MnCl2 + 5Cl2 + 8H2O
Similar, reactions are possible with HBr and HI but they are not
normally used as laboratory reagent since they are unstable
...
Laboratory preparation of halogens ( Cl2 , Br2 and I2 ) by application
of heat
When a mixture of metallic halide, conc
...
NaX +H2SO4
MnO2 + H2SO4
2HX + [O]
2NaX +3H2SO4 + MnO2
NaHSO4 + HX } x 2
MnSO4 + H2O + [O]
H 2O + X 2
Δ
MnSO4 + 2NaHSO4 + 2H2O + X2
Where
...
Laboratory preparation of chlorine ( Cl2) by application of heat
In laboratory chlorine gas is prepared by heating manganese dioxide with
concentrated hydrochloric acid
...
HCl + MnO2
MnCl2 + 2H2O + Cl2
Instead of HCl, a mixture of sodium chloride and concentrated Sulphuric
acid is heated with manganese dioxide
...
3H2SO4 + MnO2
MnSO4 + 2NaHSO4 + 2H2O + Cl2
Laboratory preparation of chlorine ( Cl2) without application of heat
In laboratory chlorine gas can be prepared by dropping concentrated HCl
on potassium permanganate ( KMnO4)
...
16 HCl + 2KMnO4
2KCl + MnCl2 + 8H2O + 5Cl2
b
...
H2SO4 and MnO2
...
Δ
2NaBr +conc
...
Laboratory preparation of iodine ( I2) by application of heat
In the laboratory iodine is prepared by heating the mixture KI, conc
...
Δ
2KI +conc
...
Action with alkalies
i
...
With cold and dilute alkalies
Fluorine reacts with cold and dilute alkali solution to give oxygen difluoride
(OF2)
...
With hot and conc
...
alkali solution to give oxygen
...
4NaF + 2H2O + O2
ii
...
With cold and dilute alkalies
Chlorine reacts with cold and dilute alkali solution to give chloride and
hypochlorite
...
With hot and conc
...
alkali solution to chloride and chlorate
...
5NaCl + 3H2O + NaClO3
Sodium chlorate
ii
...
With cold and dilute alkalies
Bromine reacts with cold and dilute alkali solution to give sodium bromide and sodium
hypobromite
...
With hot and conc
...
alkali solution to bromide and bromate
...
Sodium bromate
iii
...
With cold and dilute alkalies
I2 + 2 NaOH
Cold and dil
b
...
alkalies
3I2 + 6 NaOH
hot and conc
...
Oxidizing nature of halogen:
All halogens are good oxidizing agents
...
Displacement reaction
Fluorine being strong oxidizing agent, it can displace other halogens (Cl2,
Br2, I2) from their aqueous salt
...
Cl2 + 2KBr
2KCl + Br2
And
ii
...
This is due to liberating nascent oxygen when halogens dissolved in
water
...
X2 oxidizes H2S to sulphur
X2 + H2O
H2S + [O]
H2S + X2
2HX + [O]
H2O + S
2HX + S
b
...
X2 oxidizes sulphite to sulphate
X2 + H2O
Na2SO3 + [O]
Na2SO3 + X2 +H2O
Sodium sulphite
2HX + [O]
Na2SO4
2HX + Na2SO4
Sodium sulphate
d
...
X2 oxidizes Arsenite to Arsenate
X2 + H2O
2HX + [O]
Na3AsO3 + [O]
Na3AsO4
Na3AsO3 + X2 +H2O
Sodium Arsenite
2HX + Na3AsO4
Sodium Arsenate
f
...
Oxidation of Thiosulphate to sulphur
Chlorine and bromine react with sodium thiosulphate to give sodium
sulphate and free sulphur
...
Na2S2O3 + I2
sodium
thiosulphate
2NaI + Na2S4O6
Sod
...
Bleaching Action:
F2 being most reactive and strong oxidizing agent destroy the substances to be bleached
in the presence of moisture
...
However, iodine
being weakest oxidizing agent has no bleaching property
...
Cl2 is a good bleaching agent
...
The process is called bleaching
...
Chlorine reacts with moisture in presence of sunlight and liberates nascent oxygen
which oxidizes coloring matter to colorless oxidation product
...
Both dry Cl2 and Br2 cannot show bleaching property
...
3
...
With fluorine: Fluorine reacts with ammonia violently to produce nitrogen
...
With chlorine: Chlorine reacts with excess of ammonia to produce nitrogen
...
NH3 + 3Cl2
excess
NCl3 + 3HCl
iii
...
8NH3 + 3Br2
6NH4Br + N2
excess
But excess of bromine gives nitrogen tribromine hexa ammoniate
...
6NH3
excess
iV
...
of
nitrogen tri – iodide ammonate
...
NH3
Nitrogen triodide
monoammonate
Nitrogen triodide ammonate explodes on rubbing in dry state
...
NH3
6NH4I + 9I2 + 5N2
3
...
2F2 + 2H2O
3F2 + 3H2O
4HF + O2
6HF + O3
• Chlorine and bromine dissolve in water to give chlorine water and
bromine water respectively
...
Cl2 + H2O
HClO + HCl
Bromine reacts with water to form hypobromous acid and hydrobromic acid
...
So they decompose
in sunlight to give oxygen
...
The solubility of iodine is greatly enhanced
in presence of KI due to the formation of potassium tri-iodide
...
is obtained which is soluble in ammonia solution and reappears
on addition of dilute HNO₃
...
)
AgCl + 2NH₄OH ⟶ [Ag(NH₃)₂]Cl + 2H₂O
diammine silver chloride
(colourless)
[Ag(NH₃)₂]Cl + 2HNO₃ ⟶ AgCl↓ + 2NH₄OH
(white ppt
...
is obtained which is slightly soluble in ammonia solution
...
)
(c) Test for bromide ion ( I- )
yellow ppt
...
I - + AgNO₃ ⟶ AgI↓ + NO₃silver iodide
(yellow ppt
...
• to prepare AlF3 and synthetic cryolite which are used for the
electrolytic extraction of aluminium
...
• to prepare SF6 which is very inert used as insulator in high
voltage equipment
...
• Used as bleaching agent for cotton, fabric, paper, rayon, etc
...
• Used in the preparation of poisonous gases like phosgene,
mustard gas, tear gas etc
...
• Used as chlorinating agent in organic reactions
...
• Metal bromides such as NaBr, KBr, CaBr2 , etc
...
• It is used for the preparation of AgBr which is used in
making light sensitive eye glasses and photographic film
...
Uses of Iodine
• It is used as an antiseptic in the form of tincture of iodine (which is a
solution of iodine in alcohol) and as analgesic in the form of iodex
(iodine dissolved in petroleum jelly)
...
• Silver iodide is used to make photographic film and also used in cloud
seeding to cause artificial rain
...
02% KI which helps to prevent goiter
...
HALOACIDS
The halides of hydrogen are called hydrogen halides which are also
known as halogen hydrides
...
The
aqueous solution of these hydrogen halides are called hydrohalic acids
respectively
...
H2 + X2
2HX
All these hydrogen halides are colorless gases at ordinary temperature
and all are highly soluble in water
...
H2SO4
...
H2SO4 respectively
...
The order of reducing properties of
halogen acids is as HI>HBr > HCl > HF
...
H2SO4 to SO2 and themselves get oxidized to Br2 and I2 respectively
...
Why can not HBr and HI be prepared by treating conc H2SO4 with
bromide and iodide ?
Preparation of hydrogen chloride (HBr)
Laboratory preparation:
In laboratory, HBr gas is prepared by the action of bromine on moist red
phosphorus
...
P4 + 6I2
PI3+ 3 H2O
P4 + 6I2 + 12 H2O
4PI3
H3PO3 + 3HI
]x4
4H3PO3 + 12HI
Physical properties of hydrogen halides
( HCl, HBr and HI )
• All hydrogen halides are colourless gas with pungent
smell
...
• They can be liquified into a colorless liquid and can be
solidified into a white crystalline solid
...
Properties of halogen acid
1
...
However, in aqueous solution they ionizes to give
hydronium ion
HX + H2O
H3O+ + X-
so, in aqueous solution they behave as acids and are called hydrohalic acids
and can turn blue litmus to red
...
Reducing property : Since the thermal stability and bond
dissociation energy of hydrogen halides decreases from HF to HI, their
reducing property increases from HF to HI
...
e
...
HCl is a weak reducing agent
...
MnO2 + 4HCl
2KMnO4 + 10HCl
H2O2 + 2HCl
MnCl2 + 2H2O + Cl2
2KCl + MnCl2 + 8H2O + 6Cl2
2H2O + Cl2
• HBr and HI are stronger reducing agent and can reduce other compounds
besides above reaction shown by HCl
...
2CuSO4 + 4HI
2FeCl3 + HI
(yellow)
Cu2I2 ↓ + I2 + 2H2SO4
(brown ppt
...
• In the preparation of chlorine and chlorides
...
• In the manufacture of glucose by the hydrolysis of starch
...
• In the manufacture of textiles, dyes and chemicals
...
• In the preparation of bromides and bromine
...
• In organic chemistry as brominating agent
...
Uses of hydrogen iodide :
• As a reducing agent
...
Iodide salts like AgI is used
in photography and KI is used in medicine