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.

3 2Ge 3HCl GeHCl H

2 2Sn 2HCl SnCl H

2 2Pb 2HCl PbCl H

2 2 4(chloroplumbous acid)

PbCl 2HCl H PbCl

Lead dissolves in Conc. HCl formig chloro-plumbous acid.

Tin dissolves slowly in dilute HCl but readily in Conc. HCl.

Action of Alkalies

Carbon is unaffected by alkalies. Silicon reacts slowly with cold aq. NaOH and readily with hot solution to form silicate.

2 2 3 2Si 2NaOH H O Na SiO 2H

Sn and Pb are slowly attacked by cold alkali but readily by hot alkali giving stannates and plumbates.

2 2 3 2Sn 2NaOH H O Na SnO 2H

2 2 3 2Pb 2NaOH H O Na PbO 2H

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 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

GeO2, SnO2 etc are also network solids. SnO2 is used as a polishing powder and also in the manufacture of glass and pottery

2Pb(NO3)2 -2PbO + 4NO2 + O2

6PbO + O2 2Pb3O4

Pb3O4 + 4HNO3 Pb(NO3)2 + PbO2 + 2H2O

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).

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

PbBr4 and is PbI4 do not exist because is a strong oxidant and Br- and I- are strong reductants.

4Pb

Illustrative Example

Explain why PbCl4 is less stable than SnCl4?

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.

Solution

Extraction and properties of Si

By reduction of sand SiO2 with coke in an electric furnace(96-98% pure).

22273 2773 K2SiO 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.

44 2Si 4OH (aq) SiO (aq) 2H

Extraction of tin

Tin is commonly available as the mineral cassiterite, SnO2.

SnO2 + 2C Sn + 2CO

The roasted ore is heated with coal in a reverberatory furnace at 1500 K.

Ore is crushed and washed with water to remove impurities such as arsenic and sulphur as volatile oxides.

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

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.

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

Toxicity of Lead

Pb3(OH)2(CO3)2

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.

O

O–

O–

–O

–O

Silicates

Orthosilicates : contain single discrete unit of SiO4

4– tetrahedra

Pyrosilicates

Basic unit is (Si2O7)-6

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

Small amounts of impurities impart beautiful colours.

•Fe2O3 green

•CoO blue

Glass is a mixture of sodium and calcium silicates.

Lead-potash glass has high refractive index and used in lenses.

Adding B2O3 gives, borosilicate glass (Pyrex) having low coefficient of thermal expansion and used in making laboratory glasswares.

Soda lime glass or Soft glass.

o1300 C2 3 2 2 3 3 2 2CaO Na CO 6SiO Na CO .CaSiO .4SiO CO

Glass is not a true solid and don’t have definite melting point.

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.

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.

Ionisation energy

Illustrative Problem

Why nitrogen exists as N2 whereas phosphorous exists as P4

Because d orbitals are not available in nitrogen.

Solution :

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 P4O6

+5 HNO3 (nitric acid) H3PO4 (phosphoric acid)

+5 NaNO3 (sodium nitrate) Na3PO4 (sodium phosphate)

+5 N2O5 P4O10

Down the group, covalent character,basicity and thermal stabilitydecrease while reducing character increases.

NH3 > PH3 > AsH3 > SbH3 > BiH3

107° 48’ 93° 48’ 91° 48’ 91° 18’ 90°

Hydrides: MH3

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 ?

NO2 has unpaired electrons,so it is coloured.

Solution :

Oxides

Oxides of phosphorous — P4O6 and P4O10

P, As, Sb and Bi form two types of oxides: M2O3 and M2O5 and exists as dimer due to reluctance for bonding.p p

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.

3 2 4NCl 4H O NH OH HOCl

3 2 3 3PCl 3H O H PO 3HCl

3 2 3 3AsCl 3H O H AsO 3HCl

3 2SbCl H O SbO 3Cl 2H

3 2BiCl H O BiO 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

Black phosohorus :

Inert and has layered structure.

470 K, high pr.White phosphorus Black phosphorus Most stable form

Red phosphorus:

570 KWhite phosphorus red phosphorus

Amorphous and polymeric structure.Less reactive and nontoxic

Oxy-acids of P

P P

O O

O

P

O

OH

OHO

O

OH

(HPO ) Cyclic phosphoric acid3 3

P

HOHH

O

HPO Hypophosphorus acid2

P

OHOHHO

O

H PO Ortho phosphoric acid

3 4

P

OHOHO

O

P

OHOH

O

Di phosphoric acidH P O4 2 7

P

OHOHH

O

H POPhosphoric acid

3 3

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.

3 4 2 2 4 2 4 2 4Ca (PO ) 2H SO Ca(H PO ) 2CaSO

Triple superphosphate Ca(H2PO4)2.H2O

5 4 3 3 4 2 2 4 2 2Ca (PO ) F 7H PO 5H O 5Ca(H PO ) .H O HF

Eutrophication

Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen or phosphorus.

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