Transcript Enthalpy - Career Launcher

Chemistry
Session objectives
General trends of group 17 elements
Chemical properties of group 17 elements
Oxoacids of group 17 elements
General properties of group 18 elements
Compounds of xenon
Occurrence
The halogens are very reactive, and
donot occur in free state.
All except At are found in combined
form in the earth’s crust.
Astatine is radioactive and has a short
half life.
Halogen
Fluorine
Chlorine
Main sources
Fluorite (CaF2 ), Fluorapatite Ca3 PO4 2 F  , cryolite Na3 AlF6 
Sea water, salt wells, salt beds (NaCl, KCl, MgCl2, CaCl2 )
Bromine Sea water, sea lakes (NaBr, KBr, MgBr2 )
Iodine
Brine wells, sea weeds (I ) Chile salt petre NaIO3 
Flourine
Flourine is found naturally in many minerals, such as flourspar.
There is no element that can be used to oxidize F- to F2
Flourine is produced by electrolysing a molten mixture of KF and HF
at a carbon anode.
KF  HF  K[HF2 ]
CaF2  H2SO4  CaSO4  2HF
electrolyse
HF  K[HF2 ] 
 H2  F2
Cathode :
2H  2e  H2 (g)
Anode :
2F  F2 (g) + 2e
Difficulties in obtaining flourine
HF is corrosive, and etches glass forming SiF6- ions and also causes very
painful skin wounds. Wounds are slow to heal because F- ions remove Ca2+
ions from the tissues.
Gaseous HF is very toxic (3 ppm) compared with HCN (10 ppm).
HF is poor conductor of electricity. Thus a mixture of KF and HF is
electrolysed to increase the conductivity.
Flourine oxidises water to dioxygen, so water should be excluded.
It catches fire with the traces of greases or with crystalline silicon.
Anode should be graphite free carbon otherwise flourine invade into the
sheet of the graphite causes an increase in the distance between sheets. It
reduces the conductivity of the graphite and loss of electricity as thermal
effect. Eventually a explosion may occur.
Properties and uses of flourine
Fluorine is a pale yellow, extremely reactive, corrosive gas. It has a
characteristic pungent odour.
It is used to produce the volatile UF6, used in processing nuclear fuel.
To make SF6; which is very inert gas ,used as dielectric for high voltage
equipment.
In making various of freons, which are chemically inert, not oxidised by
air, hot nitric acid or by concentrated sulfuric acid.
Traces of flouride ions in drinking water (~1 ppm) greatly reduce the
incidence of dental caries making an enamel on teeth.
F
[3Ca3 (PO4 )2.Ca(OH)2 ] [3Ca3 (PO4 )2.CaF2 ]
Chlorine
Prepared by electrolysis of molten salt or brine.
Reacts directly with nearly all elements (exceptions: C, N,
O and the noble gases)
Oxidizes metals to high oxidation state. For example, iron
reacts vigorously in Cl2 to form FeCl3, not FeCl2.
Uses : Manufacturing of plastics, solvents,
pesticides.
As bleach in paper and textile industries.
As disinfectant in water treatment
Bromine
Br2 is obtained from sea water and brine lakes. First H2SO4 is added to
adjust the pH to about 3.5. Then Cl2 gas is passed through the solution
to oxidize the Br– to Br2.
Cl2  2Br   2Cl  Br2
Br2 is removed by a stream of air, because Br2 is quite volatile. The gas is
passed through a solution of Na2CO3, when Br2 is absorbed, forming a
mixture of NaBr and NaBrO3. Finally, the solution is acidified and distilled
to give pure bromine.
3Br2  3Na2CO3  5NaBr  NaBrO3  3CO2
5NaBr  NaBrO3  3H2SO4  5HBr  HBrO3  3Na2SO4
5HBr  HBrO3  3Br2  3H2O
Bromine is corrosive, red brown fuming liquid .
Bromine compounds are used as a fire retardant in textiles
and as pesticides. AgBr is used in photographic emulsions.
ZnBr2(aq) has a very high density and is used to control
the escape of oil from wells.
Iodine
Can be obtained from seaweed
NaIO3 (aq) + 3NaHSO3 (aq)  NaI (aq) + 3NaHSO4 (aq)
NaIO3 (aq) + 5NaI (aq) + 3H2SO4 (aq)  3I2 (s) + 3H2O ( ) + 3Na2SO4 (aq)
It is blue black lustrous solid; sublimes easily to purple vapour.
Uses (1) Added to iodised salt.
(2) dissolves in alcohol to form tincture of iodine, an antiseptic.
(3) serves as indicator in presence of starch
From left,
Iodine dissolved in CCl4
Iodine dissolved in water.
Iodine dissolved in KI.
With a little starch added.
Illustrative Problem
Explain why fluorine does not undergo
disproportionation reaction but other halogens do.
Solution :
Fluorine being the most electronegative element undergoes only
reduction but not oxidation. As a result, it shows only –1 oxidation
state while other halogens show both negative (–1) and positive
(+1, +3, +5, +7) oxidation states. Thus, F does not show
disproportionation reactions while other halogens do.
Electron affinity
Electron affinity values are high in case of halogens. As we
move down the group, the electron affinities decrease as
the size of the halogen increases.
Electron affinity of fluorine is lower than that of chlorine.
This is due to small size of fluorine atom. As a result of
which strong electron-electron repulsions are present in
the relatively compact 2p-orbitals of fluorine.
Therefore, the electron affinities decrease in the order:
Cl > F > Br > I
Stability of X-X bond
The bond energy in the X2 molecules would decrease as the
atoms become larger, since increased size results in less
effective overlap of orbitals.
The F — F bond energy is smaller than that of Cl — Cl. Due to
small size of fluorine atoms, there are high interelectronic
repulsions between non-bonding electrons in 2p-orbitals of
fluorine. As a result, F — F bond is weaker in comparison to Cl
— Cl and Br — Br bonds.
Illustrative Problem
Explain why F2 is a stronger oxidizing agent than Cl2
while electron affinity of fluorine is less than that of
chlorine.
Solution
F2 is a stronger oxidizing agent than Cl2 because F2 has higher
oxidation potential than Cl2. Electron affinity is the energy
released when a gaseous atom accepts an electron to form a
gaseous anion while oxidation potential is the sum of energy
changes taking place in the following steps:
1
X2 (g)  X (g),
2
1
H   D [Dissociation energy]
2
X (g)  e  X (g), H  E  A [Electron affinity]
X (g)  aq.  X (aq),
H  HE [Hydration Energy]
Some Important Inorganic
Compounds of Fluorine
Chemical properties
Hydrides : HX
HF is liquid
HCl, HBr and HI are gases.
Thermal stability : HF > HCl > HBr > HI
Reducing character: HF < HCl < HBr < HI
Acid strength : HF < HCl < HBr < HI
H — X bond strength : HF > HCl > HBr > HI
Illustrative Problem
A greenish yellow gas reacts with an alkali metal
hydroxide to form a halate which can be used in fire
works & safety matches.what will be the gas &
halate?
Solution:
The greenish yellow gas is Cl2 & halate used in fire
works & safety matches is KClO3
3Cl2  6KOH  KClO3  5KCl  3H2O
Oxo acids
Hypohalous acid, HOX
Halous acid, HXO2
Halic acid, HXO3
Acid strength: HClO3 < HBrO3 < HIO3
HClO4 > HClO3 > HClO2 > HOCl
Oxoanions of Chlorine
Illustrative Example
When NaBr is heated with Conc. H2SO4, Br2 is produced but when NaCl is
heated with Conc. H2SO4, HCl is produced. Why?
Solution :
When NaBr is heated with Conc. H2SO4, HBr is first produced
which being a reducing agent reduces H2SO4 to SO2 while itself
gets oxidised to Br2.
NaBr  H2SO4  NaHSO4  HBr
2HBr  H2SO4  2H2O  SO2  Br2
As a result, only Br2 is produced. Similarly, NaCl reacts with
Conc. H2SO4 to form HCl but since HCl does not act as a
reducing agent it does not get oxidised to Cl2.
NaCl  H2SO4  NaHSO4  HCl
HCl  H2SO4  No action
As a result, only HCl is evolved.
Pseudo halogens
Pseudohalides
Pseudohalogens
CN–
(CN)2 cyanogen
SCN–
(SCN)2 thiocynogen
OCN–
(OCN)2 oxy cyanogens
Illustrative example
Iodine is liberated when KI is added to solution of Cu2+ ions
but Cl2 is not liberated when KCl is added to a solution of
Cu2+ ions. Why?
Solution :
The I– ion, being a strong reducing agent, reduces Cu2+ to
Cu+ and itself gets oxidised to I2.
2Cu2  4KI  Cu2I2  I2  4K 
The Cl– ion is a week reducing agent.
Thus, it does not reduce Cu2+ ion.
Group 18 elements
Ionization energies are high but decrease
down the group.
The research leading to the first noblegas compound was prompted by the
observation that xenon lost an electron as
easily as oxygen.
Illustrative example
Liquids flow from a higher to a lower level .Give
the name of the liquid that can climb up the walls
of the glass vessel in which it is placed.
Solution:
HeII is liquid form of helium which is obtained
on cooling gaseous He to2.2K at 1 atm .It is able to flow
uphill like a gas.
Xenon and Fluorine
All three binary Xe-F compounds are crystalline solids
All are molecular in gas phase.
Solid XeF6 is ionic, being composed of XeF5+ cations and F- anions.
The binary Xe-F compound are powerful flourinating agents.
Compounds of xenon
F
F
F
F
Xe
F
F
Xe
F
(XeF2)
Linear
(XeF4)
Square planar
O
F
Xe
Xe
F
F
(XeO2F2)
Trigonal bipyramidal
(XeOF2)
T-shaped
F
F
(XeOF4)
Square pyramidal
(XeF6)
Distorted Octahedral
F
O
Xe
F
F
F
O
Xe
F
F
F
F
Xe
O
O
O
(XeO3)
Pyramidal
Illustrative example
Explain the following:
(i) Why do helium and neon not form compounds
with fluorine?
(ii) Neon is generally used for warning signal
illumination.
Solution
(a) He and Ne do not contain d-orbitals in their valence shell and
hence their electrons cannot be promoted to higher energy
levels like that in Xe to form bonds.
\ He and Ne do not form compounds with fluorine.
(b) Neon lights are visible from long distances. The lights are
even visible during fog and mist conditions.
Illustrative example
Which mixture of gases is used by sea divers to go deep
in the sea water ?
Solution:
Mixture of O2 &He (1:4) is used by deep sea divers
for repiration as He does not dissolve in blood at high
deep sea pressure
Illustrative example
Which gas is filled in electric bulbs/tubes?
Solution :
Argon is filled in electric bulbs/tubes.
Thank you