Transcript Enthalpy

Chemistry
Session Objectives
1. Chemical properties of group 14 elements
2. Extraction and properties of silicon
3. Extraction of tin
4. Extraction of lead
5. Silicates and silicones
6. Glass
7. General properties of group 14 elements
8. Oxoacids of phosphorous
Action of Acids on group 14 elements
Non-oxidising acids do not attack carbon and silicon. Ge is
not attacked by dilute HCl. However, when metal is heated in
a stream of HCl gas, germanium chloroform is formed.
Ge  3HCl  GeHCl3  H2
Tin dissolves slowly in dilute HCl but readily in Conc. HCl.
Sn  2HCl  SnCl2  H2
Lead dissolves in Conc. HCl formig chloro-plumbous acid.
Pb  2HCl  PbCl2  H2
PbCl2  2HCl 
H2PbCl4
(chloroplumbous acid)
Action of Alkalies
Carbon is unaffected by alkalies. Silicon reacts slowly
with cold aq. NaOH and readily with hot solution to
form silicate.
Si  2NaOH  H2O  Na2SiO3  2H2
Sn and Pb are slowly attacked by cold alkali but
readily by hot alkali giving stannates and plumbates.
Sn  2NaOH  H2O  Na2SnO3  2H2
Pb  2NaOH  H2O  Na2PbO3  2H2
Oxides of group 14 elements
Carbon forms the oxides CO, CO2, C3O2,
Si forms SiO2, which is solid at room temperature because it exists in the
form of a three-dimensional network due to lack of formation of p bonds
with oxygen. Three crystalline modifications of SiO2 are quartz,
cristobalite and tridymite
Pb forms a number of oxides like PbO, PbO2, Pb2O3, Pb3O4(red lead).
Pb3O4 is actually 2PbO.PbO2
2Pb(NO3)2 -2PbO + 4NO2 + O2
6PbO + O2  2Pb3O4
Pb3O4 + 4HNO3 Pb(NO3)2 + PbO2 + 2H2O
GeO2, SnO2 etc are also network solids. SnO2 is used as a polishing
powder and also in the manufacture of glass and pottery
Halides of group 14 elements
React with halogens directly to form tetrahedral
and covalent halides except C.
CCl4 does not undergo hydrolysis due to non availability of d orbital.
SiCl4 and the halides of heavier metals can undergo hydrolysis due to
availability of vacant d orbitals.
SiCl4 on hydrolysis gives silicic acid (H4SiO4).
PbBr4 and is PbI4 do not exist because Pb4 is a
strong oxidant and Br- and I- are strong reductants.
Apart from tetrahalides, germanium, tin and lead form
dihalides MX2. The stability of the dihalides increases
steadily in the sequence CX2 < SiX2 < GeX2 < SnX2 < PbX2
Illustrative Example
Explain why PbCl4 is less stable than SnCl4?
Solution
In the 14th group, the stability of +4 oxidation state decreases
down the group so Pb4+ is less stable than Sn4+.This is actually due
to the inert pair effect as s-electrons do not participate in bond
formation.
Extraction and properties of Si
By reduction of sand SiO2 with coke in an electric furnace(96-98% pure).
222732773 K
SiO2  2C 
 Si  2CO
Semiconductor grade silicon is prepared mainly by the
reduction of SiCl4/SiHCl3 with H2 or by the pyrolysis of SiH4
At room temperature Si is unreactive towards all elements except flourine.
Combines with halogens, N2 and O2 at high temperature.
Forms carborundum(SiC) with carbon; extremely hard; used as
abrasive and refractory material.
With hot aqueous alkali liberates hydrogen.
Si  4OH (aq)  SiO44 (aq)  2H2
Extraction of tin
Tin is commonly available as the mineral cassiterite, SnO2.
Ore is crushed and washed with water to remove
impurities such as arsenic and sulphur as volatile
oxides.
The roasted ore is heated with coal in a reverberatory furnace at 1500 K.
SnO2 + 2C  Sn + 2CO
Tin is remelted on inclined surface to remove the
impurities having higher melting point.
Properties of tin
Tin is a soft, silvery white metal. It is ductile and can
be rolled into thin foils.
Tin is not attacked by air or water at ordinary temperatures:
Heating with air or oxygen results in the formation of SnO2.
Tin is used as a coating on metals and in making various
alloys like solder, bronze. It is also used for electroplating
steel to make tin-plate. Tin –plate is extensively used for
making cans for food and drinks.
Lead
The ore is concentrated by froth-floatation and then roasted in a
limited supply of air to give PbO which is reduced to the metal by
heating with coke and limestone in a blast furnace.
The molten lead is tapped from the bottom of the furnace.
(i)
2PbS + 3O2  2PbO + 2SO2
(ii)
PbO + C  Pb + CO
(iii)
PbO + CO  Pb + CO2
(iv)
PbS + 2PbO 3Pb + SO2
Lead is mostly used in storage batteries, in alloy
making and pigments/chemicals. PbCrO4 is used as
a strong yellow pigment for road signs and
markings. Lead compounds are also included in
crown glass and cut glass, and in ceramic glazes.
Toxicity of Lead
Large amounts of lead in a child's blood can cause brain
damage, mental retardation, behavior problems, anemia, liver
and kidney damage, hearing loss, hyperactivity, developmental
delays, other physical and mental problems, and in extreme
cases, death.
Pb3(OH)2(CO3)2
Silicates
Orthosilicates : contain single discrete unit of
SiO44– tetrahedra
Pyrosilicates
–
–
O
O
O
–
O
Basic unit is (Si2O7)-6
–
O
Cyclic structure
Basic unit is (Si6O18)-12 Example is beryl, Be3Al2Si6O18
Linear silicate chain
Continuous single chain units of tetrahedra each
sharing 2 oxygens. Basic unit is (SiO3)-2 or (Si2O6)-4.
e.g., pyroxenes; MgCaSi2O6.
Amphiboles
Continuous double chain units of tetrahedra
each sharing 2 and 3 oxygens alternately.
Basic unit is (Si4O11)-6 or (Si8O22)-12
e.g., asbestos; [Mg3(Si2O5)(OH)4]
Phyllosilicates
Continuous sheet units of tetrahedra each sharing 3 oxygens
Basic unit (Si2O5)-2 e.g.Mica
3 D framework
Continuous framework of tetrahedra each sharing
all 4 oxygen atoms.
Basic units can be (SiO2) e.g. zeolites,feldspar
Silanes and Silicones
The hydrides of silicon are called silanes having
general formula SinH2n+2
Polymeric organo-silicon compounds containing Si-O-Si bonds
are called silicones. These have the general formula (R2SiO)n.
Where R is CH3 group (majority cases) or C6H5 group.
Focus On Glassmaking
Glass is a mixture of sodium and calcium silicates.
o
1300 C
CaO  Na2CO3  6SiO2 
 Na2CO3 .CaSiO3 .4SiO2  CO2
Soda lime glass or Soft glass.
Glass is not a true solid and don’t have definite melting point.
Small amounts of impurities impart
beautiful colours.
•Fe2O3 green
•CoO blue
Adding B2O3 gives, borosilicate glass (Pyrex)
having low coefficient of thermal expansion and
used in making laboratory glasswares.
Lead-potash glass has high refractive index and
used in lenses.
Group 15 elements
Nitrogen
N
[He] 2s2p3
Phosphorus
P
[Ne] 3s23p3
Arsenic
As
[Ar]3d104s24p3
Antimony
Sb
[Kr]4d105s25p3
Bismuth
Bi
[Xe]4f145d106s26p3
General trends of group 15 elements
The covalent radius increases down the group.
All elements have nearly same and low
electronegativity except nitrogen.
P, As, Sb and Bi are solids under normal conditions.
Ionisation energy
The value of ionization energy is quite high for the members of
group 15 than the corresponding members of group 14. This is
due to smaller atomic radii, increased nuclear charge and
stable electronic configuration of half filled orbitals.
Illustrative Problem
Why nitrogen exists as N2 whereas
phosphorous exists as P4
Solution :
Because d orbitals are not available in nitrogen.
Oxidation state
Stability of +3 oxidation state increases down the
group while that of +5 oxidation state decreases
down the group.
Bi5+ salts are very rare and good oxidising agents
As3+ salts are good reducing agents.
Oxidation Number
Nitrogen Compound
Phosphorus Compound
0
N2
P4
+3
HNO2 (nitrous acid)
H3PO3 (phosphorous acid)
+3
N2O3
P 4O6
+5
HNO3 (nitric acid)
H3PO4 (phosphoric acid)
+5
NaNO3 (sodium nitrate)
Na3PO4 (sodium phosphate)
+5
N2O5
P4O10
Hydrides: MH3
Down the group, covalent character,
basicity and thermal stability
decrease while reducing character increases.
NH3
>
107° 48’
PH3
93° 48’
> AsH3
91° 48’
>
SbH3
91° 18’
>
BiH3
90°
Illustrative Problem
Explain why NH3 is a stronger base than PH3?
Solution
Since phosphorus is bigger in size as compared to nitrogen so,
availabilty of lone pair is less. Thus PH3 is a weaker base than NH3
Illustrative Problem
Give the order of basicity and reducing character
and stability for the following hydrides: NH3, PH3,
AsH3, SbH3
Solution :
Basicity
NH3 > PH3 > AsH3 > SbH3
Stability
NH3 > PH3 > AsH3 > SbH3
Reducing character
NH3 < PH3 < AsH3 < SbH3
Oxides of nitrogen
Oxides of nitrogen
Illustrative Problem
Which oxide of nitrogen is coloured ?
Solution :
NO2 has unpaired electrons,so it is coloured.
Oxides
P, As, Sb and Bi form two types of oxides: M2O3 and M2O5 and exists
as dimer due to reluctance for pp  pp bonding.
Oxides of phosphorous
— P4O6 and P4O10
Both P4O6 and P4O10 are acidic oxides which dissolve in water to give
phosphonic acid and phosphoric acid respectively.
Halides
Forms two series of halides;
MX3 (pyramidal)
MX5 (trigonal bipyramidal)
Trihalides readily hydrolyse with water.
NCl3  4H2O  NH4OH  HOCl
AsCl3  3H2O H3 AsO3  3HCl
BiCl3  H2O BiO  3Cl  2H
PCl3  3H2O H3PO3  3HCl
SbCl3  H2O  SbO  3Cl  2H
PCl5 is molecular in gas and liquid phases but exists
as [PCl4]+[PCl6]- in the solid state .
Illustrative Example
Explain why PCl5 exists but NCl5 does not?
Solution
NCl5 is not formed because nitrogen does
not have d-orbitals.
Illustrative Example
Solid phosphorous-pentachloride exhibits some
ionic character, why?
Solution
This is because PCl5 exists as [PCl4]+ [PCl6]- in solid
phase and hence exhibits ionic character.
Allotropy of P
White phosphorus:
Waxy solid, insoluble in water, highly soluble in CS2 and benzene,
highly reactive, highly toxic and glow in dark.
Stored in water because ignite spontaneously in air.
Consist of discreate molecules P4.
Allotropy of P
Red phosphorus:
570 K
White phosphorus 
red phosphorus
Amorphous and polymeric structure.
Less reactive and nontoxic
Black phosohorus :
470 K, high pr.
Most stable form White phosphorus 
 Black phosphorus
Inert and has layered structure.
Oxy-acids of P
HO
O
O
O
O
P
P
P
P
OH
OH
HO
H3PO4
Ortho phosphoric acid
OH
O
OH
OH
H
Di phosphoric acid
H4P2O7
H3PO3
Phosphoric acid
O
O
O
P
P
H
H
O
P
OH
OH
OH
OH
OH
O
O
P
HPO2 Hypophosphorus acid
O
OH
(HPO3)3 Cyclic phosphoric acid
Phosphatic fertilizers
Fertilizers usually contains N,P,K. Generally the ratio of N-P-K is written as
10-10-10 which indicates the percentage of N, P205, and K2O .
Most important phosphatic fertilizer is the
superphosphate of lime, Ca(H2PO4)2 which is produced
from the treatment of phosphatic rock with sulphuric
acid.
Ca3 (PO4 )2  2H2SO4  Ca(H2PO4 )2  2CaSO4
Triple superphosphate Ca(H2PO4)2.H2O
Ca5 (PO4 )3 F  7H3PO4  5H2O  5Ca(H2PO4 )2.H2O  HF
Eutrophication
Eutrophication is the enrichment of an ecosystem with
chemical nutrients, typically compounds containing nitrogen
or phosphorus.
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