+2 UNIT 8 PAGE- 2

GROUP 14 ELEMENTS
The group 14 of the periodic table contains five elements carbon(C), silicon(Si ) , germanium (Ge) , tin (Sn) and lead (Pb). This group is known as carbon family.
Occurrence and uses
All the elements are well known except germanium. Carbon occurs both as free element ( graphite or diamond) and in the combined form (mainly as carbonates of Ca, Mg and other electropositive elements) . It also occurs as CO2, an important constituent of atmosphere. Carbon is seventeenth in the order of abundance in earth’s crust. Silicon is the second most abundant element in the earth’s crust after oxygen as silica (sand or quartz) and silicates. In silicates, [SiO4] unit may occur individual group or linked to form chains, rings , sheets or three-dimensional frameworks.
Germanium occurs rarely. Germanium minerals are extremely rare but the element is distributed in trace amounts in coal and zinc carbonates. Tin occurs as cassiterite, SnO2 and lead is found as galena, PbS. The abundance of the elements in the earth’s crust by weight is shown below :
Element Abundance in earth’s crust (ppm)
C 180
Si 2.72 x 105
Ge 1.5
Sn 2.1
Pb 13
Carbon is used extensively in its different forms. Coal is used as a fuel in boilers, engines, furnaces, etc. It is also used for the manufacture of coal gas, water gas, producer gas and synthetic petrol. Charcoal(activated) is used as an excellent adsorbent to purify and decoluorize sugar and other chemicals. It is also used to adsorb poisonous gases in gas masks and for removing offensive odour from the air conditioning processes. Graphite is a good conductor of electricity and is used for making electrodes and carbon rods and in covering moulds for electrodeposition of copper. It is also used in steel making , metal foundries for crucibles, as a lubricant and in pencils etc. It is also used as the moderator in the cores of gas cooled nuclear reactors to slow down neutrons. Diamonds (allotrope of carbon) are cut as gemstones and used in jewellery and other articles. It is also used for industrial purposes mainly for making drills or as an abrasive powder for cutting and polishing.
Silicon is used as n-type or p-type semiconductors when doped with Group 15 or Group 13 element respectively. Silicon and germanium are extensively used in very pure forms in semiconductor devices, which are the basis of the whole electronic industry including computer hardware. Silicon is added to steel or iron as such or more usually in the form of ferrosilicon to increase its resistance to attack by acids. Very pure silicon is used to make computer chips. Its alloys such as silicon bronze possess strength and hardness even greater than steel.

TABLE – 1
Atomic and Physical properties of Group 14 Elements

Property Carbon Silicon Germanium Tin Lead
Atomic number 6 14 32 50 82
Atomic mass 12.01 28.09 72.60 118.71 207.2
Electronic configuration [He]2s22p2 [Ne]3s23p2 [Ar]3d104s24p2 [Kr]4d105s25p2 [Xe]4f145d106s26p2
Covalent radius (pm) 77 118 122 140 146
Ionic radus M4+ (pm)
Ionic radus M2+ (pm) -
- 40
- 53
73 69
118 78
119
Ionisation enthalpy(kJ/mol) I
II
I I I
IV
1086
2352
4620
6220
786
1577
3228
4354
761
1537
3300
4409
708
1411
2942
3929
715
1450
3081
4082
Electronegativity 2.5 1.8 1.8 1.8 1.9
Density (g/cm3) 3.51 2.34 5.32 7.26 11.34
Melting point (K) 4373 1693 1218 505 600
Boiling point (K) - 3550 3123 2896 2024
Electrical resistivity (ohm cm) 1014 – 1016 50 50 105 2 x 105


Germanium has largest use in transistor technology, in making transistors and other semiconductor devices. It is transparent to infrared light and therefore is also used for making prisms and lenses and windows in infra red spectrometers and other scientific apparatus.
Because of low strength and high cost of tin, it is rarely used by itself but it used for electroplating and as alloys. Tin plates obtained by electroplating steel with tin are extensively used for making cans for food and drinks. It is used in the preparation of a number of important alloys such as solder (Sn/Pb) bronze(Pb/Cu/Sn), babbit (Sn/Pb or Cu/Pb/Sn/Sb), pewter (Pb/Sn/Sb/Cu) , type metal(Pb/Sn/Sb), etc. Lead is used for making lead sheets, lead pipes etc. It is also used for making telegraph and telephone wires which are to be burried in earth. It is used in storage batteries, making bullets. It is commonly used in making pigments like chrome yellow, chrome red, red lead, white lead , etc. Lead is also used for making important alloys such as type metal, solder, pewter etc.
General Characteristics of Group 14 Elements
Electronic configurations
The atoms of these elements have four electrons in the outermost shell , two in s and 2 in p-subshell. The general electronic configuration of this group may be expressed as ns2np2. The electronic configuration of the atoms of this group are given in TABLE.
Element Symbol Atomic number Electronic Configuration
Carbon C 6 [He]2s22p2
Silicon Si 14 [Ne]3s23p2
Germanium Ge 32 [Ar]3d104s24p2
Tin Sn 50 [Kr]4d105s25p2
Lead Pb 82 [Xe]4f145d106s26p2
Atomic and Physical Properties
The common physical constants of Group 14 elements are shown in TABLE 1.


1. ATOMIC RADII
The atomic radii of Group 14 elements are less than the corresponding elements of Group 13. However, the atomic radii increase down the family.
Explanation :
The decrease in atomic radii is due to increase in effective nuclear charge on going from Group 13 element to Group 14 element within the same period. As a result , the outermost electrons are attracted more strongly towards the nucleus and therefore , atomic radius decreases. Within the group, atomic radii increase on going down the group, due to the increase in the number of electron shells.
2. IONISATION ENTHALPIES
The first ionisation enthalpies of these elements are higher than the corresponding members of Group 13 elements.
Explanation : The higher ionisation enthalpies are due to the higher nuclear charge and smaller size of atoms of Group 14 elements. While moving down the group, the ionisation enthalpies decrease. This is due to the increase in atomic size and screening effect which overweigh the effect of increase in nuclear charge. Therefore, the outermost electron becomes less and less tightly held by the nucleus and ionisation enthalpy decreases.
3. Melting Points
The atoms of this group form covalent bonds with each other and therefore , there are strong binding forces between their atoms in both solid and liquid states. Consequently, the melting and boiling points of Group 14 elements are much higher in comparison to Group 13 elements. On moving down the group, the boiling points decrease.
4. Metallic character
Due to large ionisation enthalpies, the elements of this group are less metallic than the elements of Group 13. On moving down the group , the metallic character increases from carbon to lead. For example, carbon is typical non-metal , silicon also behaves as non-metal, but in certain physical properties , it behaves as semi-metal. Germanium is metalloid while tin and lead are typical metals.


SILICON, TIN AND LEAD
Isolation of Silicon and its properties
Elemental silicon is commercially produced by the reaction of sand (which is largely SiO2) with coke in an electric furnace:

Silicon obtained by this method is generally 96 - 98% pure and is mostly used in metallurgical industry to produce ferrosilicon and other alloys. Semiconductor grade silicon is obtained by the reduction of highly pure SiCl4 / SiHCl3 with dihydrogen or by pyrolysis of SiH4. At room temperature silicon is unreactive towards all elements except fluorine. It combines with other halogens, nitrogen and elemental oxygen at high temperatures. Silicon forms carborundum (SiC) with carbon at high temperature. Carborundum which is extremely hard , is used as an abrasive and refractory material. Silicon dissolves in hot aqueous alkali to liberate hydrogen :
Si + 4 OH  SiO44 + 2 H2
Extraction of tin and its properties
The chief ore of tin is cassiterie or tin stone SnO2.
Tin is extracted by the following steps :
1. Concentration : The crushed ore is washed with water to remove lighter impurities such as sand mud etc. The heavier ore particles settle down. The magnetic impurity of wolfranite (FeWO4) is removed by magnetic separation.
2. Roasting : The ore is then roasted to remove arsenic and sulphur as volatile oxide.

3. Reduction : Tin is obtained by reducing the roasted ore with coke in a reverberatory furnace. Limestone is added to produce a slag with the impurities which can be removed.
SnO2 + 2 C  Sn + 2 CO
Lime acts as a flux and combines with acidic impurities of silica (SiO2) to form calcium silicate which is a slag.
CaO + SiO2  CaSiO3
slag
4. Purification : The crude tin obtained by this method is contaminated with iron and other metals. It is , therefore , remelted on an inclined surface. Tin which has a low melting point than other metallic impurities present, melts and flows down leaving behind the less fusible metals.
Properties
Tin is a soft silvery white metal. It is ductile and can be rolled into thin foils. Tin has two allotropes. The stable form at room temperature is the white tin, -Sn , which transforms to grey tin, -Sn , at 286 K. White tin is not attacked by air or water at ordinary temperatures, heating with air or oxygen results in the formation of SnO2. Tin readily reacts with halogens to form SnX4. It is not attacked by dilute HCl or H2SO4 to yield SnCl2 and SnSO4 respectively. It reacts with hot alkali to form stannates, M2[Sn(OH)6](aq)
Sn(s) + 2 KOH(aq) + 4 H2O(l)  K2[Sn(OH)6](aq) + 2 H2(g)
Extraction of Lead and its properties
Ores of lead
(i) Galena, PbS
(ii) Cerussite, PbCO3
(iii) Anglesite, PbSO4
Extraction
Lead is extracted from its main ore, galena by the following steps.
1. Concentration : The ore is broken into small pieces and concentrated by froth floatation process.
2. Roasting : The concentrated ore is roasted with air in a reverberatory furnace at a moderate temperature which is below the melting point of lead sulphide. As a result, a part of lead sulphide is oxidised to lead oxide while the rest to lead sulphate.
2 PbS + 3 O2  2 PbO + 2 SO2
PbS + 2 O2  2 PbSO4
3. Reduction : The supply of air is cut off for some time and the temperature is raised. Lead is formed as follows :
PbS + 2 PbO  3 Pb + 2 SO2
2 PbSO4 + PbS  2 Pb + 2 SO2
The molten lead is obtained.
There is an alternate method. In this method the concentrated ore is roasted with air at high temperature. Lead sulphide is oxidised to lead oxide.
2 PbS + 3 O2  2 PbO + 2 SO2
The roasted ore is then mixed with coke and lime stone and is smelted (reduced) in a blast furnace. Lead oxide is reduced by coke into lead.
PbO + C  Pb + CO
Lime stone changes into flux which combines with the impurity
of silica (SiO2 ) present in the ore to form calcium silicate(slag)

Molten lead is taken out from the bottom of the furnace.
4. Refining : The lead obtained from the bottom of the furnace is impure and it is purified by electrorefining. In this process , impure lead is made anode while a sheet of pure lead acts as cathode. The electrolyte used is lead silicofluoride PbSiF6 and hydrofluorosilicic acid (H2SiF6) with small amount of gelatin. On passing current , lead from anode gets dissolved in the electrolyte and an equivalent amount of pure lead gets deposited at the cathode.
Properties
Like tin, lead is a soft ductile metal which can be rolled into sheets and pipes. It has a bluish-grey appearance and a high density, nearly twice that of tin. When exposed to air , it gets covered with a thin layer of lead hydroxide Pb(OH)2 and PbCO3 and further reactivity is greatly decreased. Similarly, with concentrated H2SO4, an insoluble coating of PbSO4 is formed and this protects the lead from further reacting with the acid.
Oxidation States and Trends in Chemical Reactivity
The common formal oxidation states observed for group 14 elements are +4 for silicon and +2 for Ge, Sn and Pb. As in Group 13, this group also show inert pair effect. The stability of the the divalent state increases markedly in the sequence , Ge < Sn < Pb. In fact the divalent state becomes dominant for Pb. Some important Group trends are : (i) On account of high ionisation enthalpies , simple M4+ ions of the group are not known. (ii) Unlike carbon, the other elements of the group form compounds having co-ordination numbers higher than 4 like (SiF5), (SiF6)2 and (PbCl6)2. (iii) In the divalent state, stable compounds of the type M(II)X2 for carbon and silicon are rare. The stability of the divalent state increases in the sequence , Ge < Sn < Pb. (iv) The tendency for catenation decreases drastically on moving from carbon to silicon and is further diminished as we go down the group. The decreasing order of catenation is C >> Si > > Ge  Sn > Pb, which may be attributed to the steady decrease in MM bond strength.
(v) The ability to form p - p multiple bonds to itself and to other elements , particularly nitrogen and oxygen , decreases drastically as we descend the group from carbon to lead. The reluctance of silicon to form p - p bonds to itself is clearly shown by the following facts that :
(a) elemental silicon exists only in the diamond structure and no form of elemental silicon is comparable to graphite and
(b) while CO2 is a gas with two carbon-oxygen double bonds, SiO2 is a solid and consists of an infinite network of SiO single bonds.
(vi) Si, Ge, Sn and Pb form tetrahalides of the type MX4. They are tetrahedral and essentially covalent. The ionic character and thermal stability of the halides decreases with increasing atomic number of the halogen, with the result that PbI4 is virtually non-existent. They are hydrolysed readily by water , e.g.,
SiCl4 + 4 H2O  Si(OH)4 + 4 HCl
SnCl4 + 2 H2O  SnO2 + 4 HCl
Apart from tetrahalides, germanium , tin and lead form dihalides MX2. The stabilty of the dihalides increases steadily in the sequence
CX2 << SiX2 << GeX2 << SnX2 < PbX2 Thus , the divalent state becomes more stable as we decend the group. Tin(II) chloride is obtained by dissolving tin in concentrated HCl ; when the solution is cooled, crystals of tin(II)chloride dihydrate SnCl2 . 2 H2O separates out. Anhydrous SnCl2 is prepared by heating tin in a current of HCl vapour . SnCl2 is used as a reducing agent in acid solution. For example, it reduces iron(III) to iron(II)state : 2 F3+ + Sn2+  Sn4+ + 2 Fe2+ Lead (II) halides are formed by adding halide ions to a soluble lead salt : Pb2+ + 2 X  PbX2 ( X = F, Cl, Br or I) Pb(II)halides are colourless solids except PbI2, which is yellow. They are sparingly soluble in water. The formation of PbCl2 and PbI2 serves as a test for detection of Pb2+ in qualitative analysis. (vii) Silicon, germanium, tin and lead form oxides of the composition MO2 . Silica SiO2 is an infinite three-dimensional network solid of silicon and oxygen atoms connected by single covalent bonds. In SiO2, each silicon atom is bonded to four oxygen atoms in a tetrahedral arrangement. Three crystalline modifications of SiO2 are quartz, cristobalite and tridymite, of which quartz and crystobalite are important. Quartz is used as a piezoelectric material (crystal ocillators and transducers). Several amorphous forms of silica such as silica gel and fumed silica are known. Silica gel is made by acidification of sodium silicate and when dehydrated, is extensively used as drying agent in chromatographic and catalyst support. The dioxides GeO2 , SnO2 and PbO2 are all solids and exist in several modifications. While SiO2 is acidic, GeO2 and SnO2 are amphoteric and PbO2 is distinctly basic. Tin(IV) oxide can be prepared by heating elemental tin in oxygen or by treating it with concentrated HNO3. SnO2 is used as a polishing powder and also in the manufacture of glass and pottery. Lead(IV) oxide PbO2 is prepared by heating trilead tetroxide with dilute HNO3. Pb3O4 + 4 HNO3  Pb(NO3)2 + PbO2 + 2 H2O It is a powerful oxidising agent and liberates oxygen when treated with acids. Both Sn and Pb form stable monoxides MO. SnO is obtained by heating tin oxalate : SnC2O4  SnO + CO + CO2 PbO exists in red , orange or yellow forms depending on the method of preparation. It can be prepared by heating lead(II)carbonate or lead(II)nitrate. Further heating of PbO in air in a reverberatory furnace at around 750 K gives Pb3O4 known as red lead. Red lead is a combination of Pb(II) and Pb(IV)oxides , i.e., 2 PbO. PbO2. Problem SiF62 is known but SiCl62 is not. Why is it so ? Silicate minerals A large number of silicate minerals exist in nature.Some of these important minerals are : feldspar, e.g. Albite NaAlSi3O8, zeolites, e.g chabazite Ca2[AlO2)4(SiO2)8 H2O], micas (muscovite) [KAl2(Si3AlO10)(OH)2] and asbestos [Mg3(Si2O5)(OH)4]. The basic structural unit in silicates is the SiO4 tetrahedron. The SiO4 tetrahedra can be linked in several different ways. Depending on the number of corners (0, 1, 2, 3 or 4) of the SiO4 tetrahedra shared , various kinds of silicates, single or double chains, rings , sheets or three-dimensional networks are formed. Some of the structural units found in silicates are shown in Fig. Some typical silicate structures (a) (SiO4)4 (b) (Si2O7)6 (c) (Si3O9)6 (d) (Si6O18)12 (e) (SiO32)n chain and (f) (Si2O52)n sheet Simple orthosilicates ( e.g. Mg2SiO4) contain discrete SiO4 units. When two SiO4 tetrahedra share a corner (common oxygen atom), we get (Si2O7)6 unit and the silicates are called pyrosilicates. When SiO4 units share two oxygen atoms with each other, cyclic or linerar chain silicates having the empirical formula [(SiO3)2]n are formed . An example of a cyclic silicate is the mineral , beryl, Be3Al2Si6O18. Linear silicate chain is present in pyroxenes (e.g. MgCaSi2O6). If two chains are cross-linked , the resulting double stranded silicates have the composition [(Si4O11)6 ] and are called amphiboles. Asbestos belongs to this class. Two dimensional sheet structures are formed when three corners of each SiO4 tetrahedra are shared as found in clays which contain (Si2O5)2 units. When all the four corners of the SiO4 tetrahedra are shared, three-dimensional networks are formed leading to different forms of silica. If in this three-dimensional network , part of the silicon is replaced by aluminium (Al3+), this will require incorporation of other cations (Na+, K+ or Ca2+) for maintaining the charge balance. The resultant 3D frame works give aluminosilicates which include feldspars and zeolites. Zeolites Zeolites are microporous aluminosilicates of general formula Mx/n[AlO2]x[SiO2] y . m H2O and may be considered as open structures of silica in which aluminium has been substituted in a fraction x / ( x + y ) of the tetrahedral sites. The negative charge of the aluminosilicate frame work is neutralised by exchangeble cations of valence n. The void space which can be greater than 50% of the volume, is occupied by m molecules of water in the unit cell. The truncated octahedron (cubo-octahedron) (a) is the building block of zeolites. This is also called the b-cage or sodalite cage. Zeolites have high porosity due to the presence of one- two- or three –dimensional network of interconnected channels and cavities of molecular dimensions. Accordingly, zeolite A shown as (b) is formed by linking sodalite cages through double four membered rings. Faujasite (zeolite X and Y ) is formed by linking the sodalite cages through double six –membered rings(c). Zeolites are used as molecular sieves and can separate molecules of different sizes. These are used extensively as catalyst. (a) (b) (c) Structure of zeolites SILICONES Silicones are polymeric compounds containing SiOSi linkages. These are polymers , which contain R2SiO repeating units. These have the general formula (R2SiO)n. These may be linear, cyclic or crossed linked. These have very high thermal stability and are called high temperature polymers ( R may be alkyl or phenyl group). The starting material for the manufacture of silicones is alkyl substituted chlorosilanes. These are obtained by the reaction of alkyl halides with silicon in the presence of metallic copper which acts as catalyst. The polymers are obtained by the hydrolysis of the above chloroderivative as : When two molecules of dialkyl silanol combine , we get a dimer with the elemination of a molecule of water. Since an active –OH group is left on each end of the chain, polymerisation reaction continues and length of the chain increases. It forms linear silicone as : The hydrolysis of monoalkyltrichloro silanes, RSiCl3 gives cross linked polymers. By regulating the conditions, the condensation can be stopped at any stage and the chains or rings of desired lengths can be obtained. Properties of silicone polymers 1. Silicone polymers are highly stable towards heat. 2. Low molecular weight silicone polymers are soluble in organic solvents like ether, carbon tetrachloride , benzene etc. 3. They are stable towards chemical reagents. They are not affected by weak acids, alkalies and salt solutions. 4. They are water repellants because of organic side chain. 5. They are good electrical insulators. 6. They are resistant to oxidation. However, when heated in air to 350C to 400C, silicones are rapidly oxidised and this leads to cross linking. Uses of silicon polymers The important uses of silicone polymers are : (i) Silicone polymers are used for high temperature oil baths, high vaccum pumps etc. (ii) Silicone polymers are used as greases, varnishes and these can be used even at low temperatures ( of the order of 40C). (iii) Because silicones are water repellants and good insulators, they are used for water proofing and in electrical condensers. (iv) They are used as lubricants at both high and low temperatures. (v) Silicone rubbers are very useful because they retain their elasticity at lower temperatures as compared to other rubbers. They are also mixed in paints to make them damp resistant. (vi) They are used as excellent insulators for electric motors and other electrical appliances. GROUP 15 ELEMENTS Group 15 of the periodic table includes the elements nitrogen, phosphorus , arsenic , antimony and bismuth. There is a transition from non-metallic to metallic character as we go down the group. Nitrogen and phosphorus are non-metals ; arsenic and antimony are semi-metals or metalloids ; bismuth is metallic. Besides nitrogen , the other important element of group 15 is phosphorus. Occurrence and uses The elements of Group 15, except phosphorus do not occur very abundantly in nature. Though nitrogen comprises about 78% of earth’s atmosphere, it is not very abundant in earth’s crust. Since nitrates are very soluble in water so these are not widespread in earth’s crust. The only major minerals are KNO3 (nitre, salt petre)and NaNO3 (soda nitre, chile salt petre). The major deposists of salt pettre (KNO3) occur in India. Nitrogen is an important constituent of proteins and amino acids. The continuous interchange of nitrogen between the atmosphere and biosphere is called nitrogen cycle. Phosphorus is the eleventh element in the order of abundance in crustal rocks of the earth. All its known minerals are orthophosphates. Major amounts of phosphorus occur in a single mineral family known as apatites , which have the general formula , 3Ca3(PO4)2CaX2 or Ca10(PO4)6X2. The common apatites are fluorapatite , Ca5(PO4)3F, chloroapatite Ca5(PO4)3Cl, hydroxyapatite Ca5(PO4)3(OH). Phosphorus is essential for life, both as a structural material in animals and plants. About 60% bones and teeth are Ca3(PO4)2 or [3{Ca3(PO4)2} . CaF2]. The elements As, Sb and Bi are not very abundant. Their important source is as sulphides occurring as traces in other ores. These are obtained as metallurgical by-products from roasting sulphide ores. The abundance of these elements in earth’s crust is given below. Element Abundance in earth’s crust (ppm) Nitrogen 19 Phosphorus 1120 Arsenic 1.8 Antimony 0.20 Bismuth 0.008 Nitrogen is used in large amounts as an inert atmosphere in laboratory and in industrial processes such as in iron and steel industry and in oil refineries. Liquid nitrogen is used as refrigerrant . Large amounts of nitrogen are used in the manufacture of ammonia, calcium cyanamide, etc. Vast amounts of phosphates are used in fertilizers. Phosphorus is used for the manufacture of matches, as a rat poison. The red variety of phosphorus is preferred to yellow variety for matches because of its non-poisonous nature. Phosphorus is also used for making phosphorus bronze which is hard and not corroded by water. It is used for the manufacture of tracer bullets, incendiary bombs and for producing smoke screens. Phosphorus is used for the preparation of other important compounds such as phosphoric acids, phosphorous chlorides, hypophosphites for their use in industry and medicines. Arsenic is used to form alloys with many metals. Compounds of arsenic are used for killing weeds and in medicines. Antimony metal is also used in alloys with tin and lead. It is also used to electroplate steel to prevent rusting. Antimony compounds are used as fire retardants, in foam fillings for furnitutre and mattresses. Bismuth is also largely used for making alloys of low melting points. Some of these alloys melt even below 100C and are called fusible alloys. For example, woods metal, rose metal, etc. These alloys are used mainly for making automatic electrical fuses, automatic fire alarms, automatic sprinklers, automatic safety plugs for boilers, etc. GENERAL CHARACTERISTICS OF GROUP 15 ELEMENTS Electronic configurations The atoms of Group 15 have five electrons in the outer shell , two in s and three in p-subshell. The general electronic configuration of this group may be expressed as ns2np3. Electronic configurations of group 15 elements Element Symbol Atomic number Electronic Configuration Nitrogen N 7 [He]2s22p3 Phosphorus P 15 [Ne]3s23p3 Arsenic As 33 [Ar]3d104s25p3 Antimony Sb 51 [Kr]4d105s25p3 Bismuth Bi 83 [Xe]4f145d106s26p3 Atomic and Physical properties The important physical constants of Group 15 elements are given in TABLE. Property Nitrogen Phosphorus Arsenic Antimony Bismuth Atomic number 7 15 33 51 83 Atomic mass 12.01 28.09 72.60 118.71 207.2 Electronic configuration [He]2s22p3 [Ne]3s23p3 [Ar]3d104s24p3 [Kr]4d105s25p3 [Xe]4f145d106s26p3 Covalent radius (pm) 70 110 120 140 150 Ionic radus M3 (pm) M3+ (pm) 171 - 212 - 222 - 76 103 Ionisation enthalpy(kJ/mol) I II I I I 1402 2856 1012 1903 947 1798 834 1595 703 1610 Electronegativity 3 2.1 2.0 1.9 1.9 Density (g/cm3) 0.879 1.823 5.77 6.697 9.808 Melting point (K) 63 317 (whiteP) 1089(grey form) 904 544 Boiling point (K) 77.2 554 (white P) 888 1860 1837 The important physical characteristics are discussed below : 1. Atomic and ionic radii : The atomic and ionic radii of Group 15 elements are smaller than the atomic radii of the corresponding group 14 elements. This is because of increased nuclear charge. On going down the group, the atomic radii increases due to increase in number of shells. 2. Melting and boiling points : Melting points (except for atimony and bismuth) and boiling points increases on going down the group from nitrogen to bismuth. 3. Ionisation enthalpies : The first ionisation enthalpies of the group 15 elements are higher than the corresponding members of group 14 elements. Explanation : The larger ionisation enthalpy is due to greater nuclear charge, small size and stable configuration of the atoms of Group 15 elements. The electronic configuration of atoms of Group 15 are half-filled, n Px1, n p y1, n pz1 and are stable. Therefore they have high ionisation enthalpies. On going down the group, the ionisation enthalpies decrease. This is due to increase in atomic size and screening effect which outweigh the effect of increased nuclear charge. 4. Electronegativity : The electronegativity values of elements of group 15 are higher than the corresponding elements of group 14. Explanation :The elements of group 15 have smaller size and greater nuclear charge of atoms and therefore they have higher electronegativity values. On going down the group, the electronegativity values decreases. This is due to increase in size of atoms and shielding effect of inner electron shells on going down the group. 5. Metallic character : The elements of group 15 are less metallic. However, on going down the group, the metallic character increases from N to Bi. For example, N and P are non-metallic, As and Sb are partly non-metallic while Bi is a metal. 6. Catenation : The elements of group 15 also show a tendency to form bonds with itself known as catenation. All these elements show this property but to much smaller extent than carbon. For example, hydrazine (H2NNH2) has two nitrogen atoms bonded together, hydrazoic acid (N3H) has three N-atoms , azide ion , N3 has also three N atoms bonded together while diphosphine (P2H4) has two phosphorus atoms bonded together. The lesser tendency of elements of group 15 to show catenation in comparison to carbon is their low M – M bond dissociation energies. Bond C – C N – N P – P As–As Bond energy (kJ/mol) 353.3 163.8 201.6 147.4 7. Allotropy : Except nitrogen and bismuth , all other elements of this group show allotropy. For example, Phosphorus exists as : white, black or red phosphorous. Arsenic exists as : yellow , grey arsenic, black Antimony exists as : yellow metallic form, explosive antimony. OXIDATION STATES These elements have five electrons in the valence shell . The loss of five electrons is quite difficult because of energy considerations. Hence they do not form ionic compounds by the loss of 5 electrons. On the other hand these elements can also gain three electrons to complete their octets. But the gain of three electrons is also not energetically favourable. However , they do form N3 and P3 ions by gaining three electrons from highly electropositive elements, e.g. Mg3N2, Ca3P2. In addition to  3 oxidation state, the elements of group 15 exhibit + 3 and + 5 oxidation states. For example, phosphorus forms pentahalides such as PF5, PCl5 (+5 oxidation state) and trihalides PCl3, PF3 (+3 oxidation state). Nitrogen exhibits various oxidation states from  3 to +5 in its hydrides, oxides and oxoacids. For example, NH3 N2H4 N2 N2O NO N2O3 N2O4 N2O5  3  2 0 + 1 +2 + 3 + 4 + 5 TRENDS IN CHEMICAL REACTIVITY Nitrogen is a colourless gas and exists as diatomic. The two nitrogen atoms are held together by triple bond and have very high bond dissociation energy (945 kJ mol1). Due to the presence of triple bond, which has very high bond dissociation energy, nitrogen molecule has very little reactivity. However, the tendency to form multiple bonds (p-p) is limited only to nitogen . On the other hand, phosphorus, arsenic and antimony exist in various forms containing single bonded atoms. For example, phosphorus exists in tetrahedral P4 molecules as shown below. In this case , four P atoms lie at the corners of a regular tetrahedron. Each P is bonded to three P atoms by single P – P bonds . Therefore , nitrogen exists as a gas while phosphorus exists as solid. White phosphorus is more reactive than nitrogen. It catches fire when exposed to air, burning to form the oxide, P4O10. It is stored under water to prevent it. Red P is stable in air at room temperature but reacts on heating. Arsenic and antimony both occur in two forms. The most reactive is yellow form which contains M4 tetrahedral units and resembles white phosphorus. Arsenic is stable in dry air but tarnishes in moist air giving first a bronze and then a black tarnish. Antimony is less reactive and is stable towards water and air at room temperature. On heating in air , it forms Sb4O6, Sb4O8 or Sb4O10. Bismuth forms Bi2O3 on heating. Let us discuss some reactive trends of Group 15 elements. 1. Formation of hydrides The elements of Group 15 form trihydrides of the general formula MH3 such as : NH3 PH3 AsH3 SbH3 BiH3 Ammonia Phosphine Arsine Stibine Bismuthine The hydrides can be obtained by different chemical reactions : Ca3N2 + 6 H2O  3 Ca(OH)2 + 2 NH3 P4 + 3 KOH + 3 H2O  PH3 + 3 KH2PO4 Ca3P2 + 6 H2O  3 Ca(OH)2 + 2 PH3 Zn3M2 + 6 HCl  3 ZnCl2 + 2 MH3 ( M = As, Sb, Bi) Structure All the hydrides are covalent in nature and have pyramidal structure. These involve sp3 hybridisation of the central atom and one of the tetrahedral position is occupied by a lone pair. The structure of ammonia molecule is shown in Fig. Due to the presence of lone pair, the bond angle in NH3 is less than the normal tetrahedral angle. It has been found to be 107 . As we go down the group the bond angle decreases as : Molecule NH3 PH3 AsH3 SbH3 BiH3 Bond angle 107 94 92 91 90 Explanation : In all these hydrides , the central atom is surrounded by four electron pairs, three bond pairs and one lone pair. Now, as we move down the group from N to Bi, the size of the atom goes on increasing and its electronegativity decreases. Consequently, the position of bond pair shifts more and more away from the central atom in moving from NH3 to BiH3. For example, bond pair in ammonia is close to N in NH bond than the bond pair in PH bond in PH3. As a result, the force of repulsion between bonded electron pairs in NH3 is more than in PH3. In general , the force of repulsion between bonded pairs of electrons decreases as we move from NH3 to BiH3 and therefore , bond angle decreases in the order. Characteristics of hydrides : The important characteristics of these hydrides are : (i) Basic strength : All these hydrides have one lone pair of electrons on their central atom. Therefore they act as Lewis bases. They can donate electron pair to electron deficient species (Lewis acids). As we go down the group, the basic character of these hydrides decreases. For example, NH3 is distinctly basic ; PH3 is weakly basic ; AsH3 , SbH3 and BiH3 are weakly basic. Explanation : Nitrogen atom has the smallest size among the Group 15 elements. Therefore the lone pair is concentrated on a small region and electron density is the maximum. Consequently, its electron releasing tendency is maximum. As the size of the central atom increases down the family , the electron density decreases. As a result , the electron donor capacity or the basic strength decreases down the group. (ii) Thermal stability : The thermal stability of the hydrides of Group 15 elements decreases as we go down the group. Therefore, NH3 is most stable and BiH3 is least stable. The thermal stability of the hydrides of the Group 15 elements decreases in the order : NH3 > PH3 > AsH3 > SbH3 > BiH3
Explanation : This is due to the fact that on going down the group, the size of the central atom increases and therefore , its tendency to form stable covalent bond with small hydrogen atom decreases. As a result the M – H bond strength decreases and therefore thermal stability decreases.
(iii) Reducing character : The reducing character of the hydrides of group 15 elements increases from NH3 to BiH3. Thus, increasing order of reducing character is as follows :
NH3 < PH3 < AsH3 < SbH3 < BiH3
Explanation : The reducing character depends upon the stability of the hydride. The greater the unstability of the hydride, the greater is the reducing character. Since the stability of group 15 hydrides decreases from NH3 to BiH3 , hence the reducing character increases.
(iv) Boiling points : Ammonia ( 240 K) has a higher boiling pont than phosphine (190 K) and then the boiling point increases down the group because of increase in size.
Molecule NH3 PH3 AsH3 SbH3 BiH3
Boling point (K) 238.5 185.5 210.6 254.6 290
Explanation : The abnormally high boiling point of ammonia is due to its tendency to form hydrogen bonds.

In PH3 and other hydrides, the intermolecular forces are van der Waal’s forces. These van der Waal’s forces increases with increase in molecular size and therefore , boiling points increase on moving from PH3 to BiH3. The main points are summarised as under :

2. Formation of halides
Group 15 elements form two series of halides of the type MX3 (trihalides) and MX5(pentahalides). The trihalides are formed by all the elements while pentahalides are formed by all the elements except nitrogen. Nitrogen cannot form pentahalides due to the absence of vacant d-orbitals in its outermost shell. Similarly the last element, Bi has little tendency to form pentahalides because +5 oxidation state of Bi is less stable than +3 oxidation state due to inert pair effect.
(a) Trihalides : The trihalides are mainly covalent with the exception of BiF3 which is ionic. The trihalides are easily hydrolysed by water. However, the products are different in hydrolysis of different chlorides.
NCl3 + 3 H2O  NH3 + 3 HClO
Hypochlorous acid
PCl3 + 3 H2O  H3PO3+ 3 HCl
Phosphorus acid
2 AsCl3 + 3 H2O  As2O3 + 6 HCl
Antimony and bismuth trichlorides are only partly hydrolysed to form oxychlorides
SbCl3 + H2O  SbOCl + 2 HCl
Antimony
oxychloride
BiCl3 + H2O  BiOCl + 2 HCl
Bismuth
oxychloride
The trihalides of P, As and Sb ( especially fluorides and chlorides) act as Lewis acids and combine with Lewis bases.
PF3 + F2  PF5
SbF3 + 2 F  [SbF5]2
(b) Pentahalides
The pentahalides are thermally less stable than trihalides . For example, PCl5 exists as molecules in gas phase but exists as [PCl4]+[PCl6] in the crystalline state. PBr5 and PI5 also exists in the ionic form [PBr4]+[PBr6] and [PI4]+I respectively in the solid state.
3. Oxides
The elements of group 15 combine with oxygen either directly or indirectly to form oxides. The important oxides of group 15 elements along with their oxidation states are listed in the TABLE.
Oxides of Group 15 elements in different oxidation states
Element 
Oxidation state N P As Sb Bi
+1 N2O
+2 NO
+3 N2O3 P4O6 As2O3 Sb2O3 Bi2O3
+4 N2O4 P4O8
+5 N2O5 P4O10 As2O5 Sb2O5 Bi2O5
Thus, it is nitrogen which forms all oxides having oxidation states +1 to +5. All oxides of nitrogen (except N2O and NO) and phosphorus are strongly acidic ; oxides of arsenic are weakly acidic ; oxides of antimony are amphoteric while those of bismuth are weakly basic.
There is an important differnce between oxides of nitrogen and other group congeners in their structures. The nitrogen has the ability to form p - p multiple bonds and this is present in its structures. On the other hand the reluctance of P, As, Sb and Bi to form p - p multiple bonds leads to the cage structures for their oxides.
General preparative routes
The general preparative routes of oxides of nitrogen are :

Nitrous oxide is known as laughing gas because it produces hysterical laughter.
2 NaNO2 + 2 FeSO4 + 2 H2SO4  Fe2(SO4)3 + 2 NaHSO4
+ 2 H2O + 2 NO


Structures of oxides of nitrogen
The strtures of different oxides of nitrogen are given in Fig.


Structure of oxides of nitrogen
It may be noted that nitric oxide has eleven valence electrons. Since the molecule contains odd number of electrons , it is paramagnetic in gaseous state. However, in liquid and solid states it forms a loose dimer in such a way that the magnetic effects of two unpaired electrons are cancelled out. The molecule is diamagnetic.

Oxides of phosphorus
The two common oxides of phosphorus are phosphorus(III) oxide and phosphorus(V)oxide. Phosphorus trioxide , P4O6 is a dimer of P2O3 and is prepared by heating white phosphorus in limited supply of oxygen.

Phosphorus (V) oxide is a dimer of P2O5 and is prepared by heating white phosphorus in excess of air or oxygen.

As4O6 and Sb4O6 are obtained by burning the metals in air or oxygen. Heating the sulphide minerals in air also give As4O6. Both As4O6 and Sb4O6 are very poisonous. Bi2O3 is not dimeric like others. It is ionic. Though the pentoxides, As4O10 and Sb4O10 are formed but pentoxide of bismuth is not formed showing that the stability of higher oxidation state decreases on descending the group.
5. Formation of oxoacids
The elements of group 15 form a large number of oxoacids.
(i) Oxoacids of nitrogen : The important oxoacids of nitrogen are given below :







Name

Formula
O.S of N
Nature
Nitroxylic acid H4N2O4 +2 Highly explosive and difficult to get in pure state.
Nitrous acid HNO2 +3 Weak acid and unstable
Nitric acid HNO3 +5 Strong acid and stable.
Out of the oxoacids of nitrogen, nitric acid is most important. It is very strong oxidising agent and is quite useful.
(ii) Oxoacids of phosphorus
Phosphorus forms a number of oxoacids as given in the TABLE.
Name Formula Oxidation state of P
Hypophosphorus acid
(Phosphinic acid) H3PO2 +1
Phosphorus acid
(Phosphonic acid) H3PO3 +3
Hypophosphoric acid H4P2O6 +4
Orthophosphoric acid H3PO4 +5
Diphosphoric acid
(Pyrophosphoric acid) H4P2O7 +5
Metaphosphoric acid HPO3 +5
Peroxophosphoric acid H3PO5 +7
Among the oxoacids of phosphorus, orthophosphoric acid is the most important and is used in the manufacture of phosphate fertilizers. It is prepaed by buring phosphorus in oxygen followed by hydrolysis.

It is a tribasic acid and ionises as :

Therefore , it forms three types of salts e.g. NaH2PO4, Na2HPO4 and Na3PO4.
Production of phosphorus
Phosphorus is widely distributed in nature. It occurs mainly in the form of phosphate minerals in the crust of earth. Some of these are :
(i) Phosphorite : Ca3(PO4)2
(ii) Fluorapatite : CaF2 3 Ca3(PO4)2
(iv) Chlorapatite : 3 Ca3(PO4)2 CaCl2
(v) Hydroxyapatite : Ca5(PO4)3OH
Phosphates are also essential constituents of plants and animals. It is mainly present in bones which contain about 85% calcium phosphate. Vast deposites of phosphate rocks are located in North africa and North America. In India, the phosphate rocks are found mainly in Rajasthan.

Production of phosphorus
Phosphorus is produced by heating bone ash or phosphate rock [phosphorite, Ca3(PO4)2] with silica (SiO2) and coke in an electric furnace.
Bone ash is obtained from bones which contain about 58% of calcium phosphate, the remaining being organic matter. The bones are crushed and finely powdered. The powder is washed with benzene or ether (organic solvents) to remove the fatty matter. The powder is heated in iron retorts in absence of air (destructive distillation) . Bone oil or dipple oil is obtained as the distillate, while bone charcoal is obtained as the residue. Bone charcoal also known as animal charcoal is used for decolourising sugar. When it becomes useless , it is burnt to get bone ash which contains about 15% calcium phosphate, remaining
Electrochemical process for manufacture of phosphorus
Details of the process
The process used for the manufacture of phosphorus is known as elctrochemical process and was suggested by Wholer. A mixture of calcium phosphate (from phosphorite or bone ash), sand and coke is introduced into a closed electric furnace through a hopper by means of a worm or screw conveyer as shown in the Figure (above). The electric furnace is an iron tank lined inside with refractory bricks.
There is an outlet pipe at the upper part which is connected to a condensing system. An electric arc is struck between two carbon electrodes in order to maintain a temperature of about 1775 K. At this high temperature, calcium phosphate reacts with sand to form calcium silicate and phosphorus pentoxide. The latter is reduced by coke to give phosphorus and carbon monoxide.

Calcium silicate (slag) is removed from the bottom while, phosphorus vapours are taken out from the top exit along with carbon monoxide. The vapour of phosphorus are condensed under water (not shown) while carbon monoxide being a gas escapes out.
Purification : The phosphorus thus obtained is impure. This is purified by melting in chromic acid (mixture of potassium dichromate and con. Sulphuric acid) when the impurities are oxidised and form a scum over the surface. This scum is removed. The remaining mass is filtered through chamois leather to remove the insoluble impurities and is cooled to get sticks of pure phosphorus. It is stored under water, because it is inflammable.
ALLOTROPIC FORMS OF PHOSPHORUS
Phosphorus exists in three main allotropic forms :
(i) White phosphorus
(ii) Red phosphorus
(iii) Black phosphorus
1. White phosphorus : It is the common variety of phosphorus and is obtained from phosphorite rock with coke and sand in an electric furnace at 1775 K as explained above. It consists of P4 units as shown in Fig.

Structure of white phosphorus
In this case, the four P-atoms lie at the corners of a regular tetrahedron. Each phosphorus is bonded to each of the other three P-atoms by covalent bonds , so that each P-atoms completes its valence shell.
The main characteristics of white phosphorus are :
(i) It is soft, waxy and white solid with garlic smell.
(ii) It can be cut with a knife.
(iii) It melts at 317 K and boils at 553 K.
(iv) It is insoluble in water but soluble in benzene, carbon disulphide, liquid ammonia.
(v) It is very poisonous. The vapours of phosphorus , if continuously inhaled, may prove fatal.
(vi) It is very reactive and catches fire in air. Therefore, it cannot be kept in air. It is generally stored under water.
P¬4 + 5 O2  P4O10
(vii) It combines with metals forming their phosphides such as Na3P, Ag3P, Cu3P2, etc.

(viii) It is a weak reducing agent and reduces sulphuric acid to sulphur dioxide, nitric acid to nitrogen peroxide etc.
P4 + 10 H2SO4  10 SO2 + 4 H3PO4 + 4 H2O
Phosphoric acid
P4 + 20 HNO3  4 H3PO4 + 20 NO2 + 3 H2O
(ix) It readily combines with halogens to form trihalides (PX3) and on prolonged treatment forms pentahalides (PX5).
P4 + 6 Cl2  4 PCl3
P4 + 10 Cl2  4 PCl5
(excess)
(x) On heating with caustic soda solution , it forms phosphine.
P4 + 3 NaOH + 3 H2O  PH3 + 3 NaH2PO2
Phosphine sodium hypophosphite



2. Red phosphorus
It is obtained by heating white phosphorus out of contact of air to 540 K for several hours. Red phosphorus has a polymeric structure as shown in Fig.

Structure of red phosphorus
In this structure, P4 molecules are linked by covalent bonds. The important characteristics of red phosphorus are :
(i) It is a hard crystalline solid without any smell.
(ii) It is non-poisonous in nature.
(iii) It is insoluble in water as well as carbondisulphide.
(iv) It is denser than white phosphorus.
(v) Red phosphorus is quite stable and its ignition temperature is quite high (543 K) . It , therefore , does not catch fire easily.
(vi) It is less reactive than white phosphorus.
(vii) It is a bad conductor of electricity.
(viii) It burns with oxygen at 565 K to form phosphorus pentoxide.

3. Black Phosphorus
It is obtained by heating white phosphorus at 470 K under very high pressure (12000 atm) in an inert atmosphere.

It has a double layered crystal lattice. Each layer is made up of zig-zag chains with P – P – P bond angles of 99 (fig.). Since it is highly polymeric , therefore, it has high density.

Structure of black phosphorus

The important characteristics of black phosphorus are :
(i) It has a black metallic lustre.
(ii) It has a highly polymeric layered structure and exists in three crystalline and one amorphous forms. Therefore it has high density.
(iii) It is a moderate conductor of heat and electricity .
(iv) It is most inactive form of phosphorus.
Thus, the three allotropics of phosphorus differs in their chemical reactivity. White phosphorus is the most reactive form while black phosphorus is the least reactive form. Therefore , white phosphorus is stored under water to protect it from air while red and black phosphorus are stable in air


The main characteristics of three forms of phosphorus are summed up in the following TABLE.
Properties White phosphorus Red phosphorus Black phosphorus
Colour White but turns yellow Dark red Black
State Waxy solid Brittle powder Crystalline
Density 1.84 g/cm3 2.1 g/cm3 2.69 g/cm3
Ignition temperature 307 K 543 K 673 K
Stability Less stable at ordinary temp. More stable at ordinary temp. Most stable
Chemical reactivity Very reactive Less reactive Least reactive
PHOSPHINE
It is the hydride of phosphorus, PH3.
Preparation
1. From phosphides : By the action of water or dilute mineral acid on metallic phosphides (Na3P, Ca3P2, AℓP, etc.)
Na3P + 3 H2O  3 NaOH + PH3
Ca3P2 + 6 H2O  3 Ca(OH)2 + 2 PH3
AℓP + 3 HCl  AlCl3 + PH3
2. From phosphorus acid : Pure phosphine can be prepared by heating phosphorus acid at 205 – 210 C.

3. From phosphonium salts : Pure phosphine can also be obtained by heating phosphonium iodide with caustic soda solution.
PH4I + NaOH  NaI + H2O + PH3
Phosphonium iodide is obtained from phosphorus.
P4 + 2 I2 + 8 H2O  2 PH4I + 2 HI + 2 H3PO4
4. Laboratory preparation : Phosphine is prepared in the laboratory by heating white phosphorus with 40% solution of caustic soda in an inert atmosphere of carbon dioxide or coal gas.
4 P + 3 NaOH + 3 H2O  NaH2PO2 + PH3
Sodium hypophosphite
It may be noted that pure phosphine is not inflammable but the gas may catch fire in air. This may be due to the presence of impurity P2H4 which is spontaneously inflammable. Therefore, a current of carbon dioxide or coal gas is passed through the flask to displace air.
To get pure phosphine, the gas is passed through a U-tube placed in a freezing mixture. The liquid P2H4 gets condensed while PH3 remains unaffected. Impure gas also be purified by treating it with hydrogen iodide followed by heating with caustic soda solution.
PH3 + HI  PH4I
PH4I + NaOH  PH3 + NaI + H2O
The impurity P2H4 may also be controlled by using alcoholic solution of potassium hydroxide in place of aqueous solution of caustic soda.
Structure
Like ammonia, phosphine has pyramidal structure. Phosphorus involves sp3 hybridisation . Three bonds are formed by the overlap of sp3 hybrid orbitals of phosphorus with 1s-orbital of hydrogen. One of the orbitals is occupied by a lone pair of electrons . The HPH bond angle in PH3 is 93.6 and P – H bond distance 124 pm.

Structure of phosphine
Structures of some compounds of phosphorus
The structures of some compounds of phosphorus , halides , oxides and oxyacids are discussed below :
1. Phosphorus trichloride : PCl3
It has a pyramidal structure and has one lone pair as shown in Figure.

Structure of PCl3
In this case, the central phosphorus atom involves sp3 hybridisation.
2. Phosphorus pentachloride , PCl5 :
In PCl5 , the phosphorus undergoes sp3d hybridisation and has trigonal bipyramidal structure as shown below :

Structure of PCl5
3. Phosphorus trioxide , P4O6
In P4O6 , phosphorus atoms lie at all the corners of tetrahedron and six oxygen atoms are present along the edges between the P atoms as shown in Fig :




4. Phosphorus pentoxide, P4O10
P4O10 has a structure similar to P4O6 except that it has additional oxygen atom linked to each P atom by double bond as shown above.
5. Oxoacids of phosphorus
The structures of different oxoacids of phosphorus acids are given below .





Structures of oxoacids of phosphorus


Note :
The formula of oxoacids of P can be remembered as :
Prefix :
• meta acid is used for the acid obtained by the loss of one water molecule.
• Pyro acid is used for the acid obtained by heating two molecules with loss of one water molecule.
• Hypo is generally used for the acid having lower oxygen content than the parent acid.
• It may be noted that metaphosphoric acid does not exist as simple monomer, rather it exists as cyclometaphosphoric acid or polymetaphosphoric acid.


Problem
03. Are all the five bonds in PCl5 molecule equivalent ? Justify your
answer.
Phosphatic Fertilisers
Fertility of soil can be enhanced by using chemical fertilisers which provide the essential plant nutrients , potassium, nitrogen and phosphorus.
The most important phosphatic fertiliser is the superphosphate of lime, Ca(H2PO4)2. This is produced directly from phosphate rocks by treatment with concentrated sulphuric acid. In this way, insoluble phosphate rock is rendered soluble in water to improve the release of phosphorus to the soil for the uptake by the plants.
Ca3(PO4)2 + 2 H2SO4  Ca(H2PO4)2 + 2 CaSO4
Phosphate rock generally contains fluoride which reacts with H2SO4 to give hydrogen fluoride, which in turn generates other side products. The gaseous side products are removed by washing with water in a srubber. Almost 90% of the phosphate rock mined goes into the production of phosphatic fertilisers ; the remaining 10% is used for the production of elemental phosphorus.
Treatment of phosphate rock with phosphoric acid yields triple superphosphate , Ca(H2PO4)2 H2O which is free from calcium sulphate and hence contains a greater percentage of phosphorus.
Ca5(PO4)3F + 7 H3PO4 + 5 H2O 5 Ca(H2PO4)2 H2O + HF
Phosphate Esters
Phosphate esters play a very important role in life processes. The most important of these biomolecules are DNA, RNA, adenosine mono-, di-, tri-phosphates (AMP, ADP and ATP). Hydolysis of PO P link in ADP releases energy which can be put into useful work.