- Group IIIA of the periodic table consists of Boron (B), Aluminium (Al), Gallium (Ga), Indium (In) and Thallium (Tl). Except boron all these elements are metals.
- The metallic or electropositive character increase from boron ( a non - metal ) to thallium ( a metal) on moving down . The metallic property in-group III A elements are less pronounced as compared to those in group IA and II A elements, since the elements of this group donate electrons with difficulty .
- The elements have two s-electrons and one p-electron in their valence shell. Since p-electron is held less tightly as compared to the s-electrons, the first ionisation energy of each element ( except boron) is rather low. The second and third ionistion energies are considerably higher. Ionisation energy decreases boron to aluminium, but does not changes much from Al to Tl . Although the valence shell configuration of all elements is same ( ns2p1) , the underlying core is different . The B and Al have inert gas core, Ga and In have noble gas plus d10 core and Tl has noble gas plus 4f143d10 core. Because of this difference , the atomic size and ionisation energy do not follow the expected periodic trend. For example , increase in atomic radii from B to Al is not followed by a further increase in Ga, because of higher effective nuclear charge for Ga, as the additional 3d10 electrons do not shield the nuclear charge effectively so that the outer electrons are more strongly held. The atoms with a d10 inner shell are, therefore, smaller and have higher ionisation energies . Tl, because of the inclusion of even more poorly shielding 4f electrons , in addition to d10 has a smaller size and higher ionisation energy . Thus first ionisation energies are low, comparable to those of s-block elements , but second and third are considerably high. The sum of the first three ionisation energies is very high and decreases from B to Al as expected, but increase for Ga and Tl, due to decrease in their size.
- Boron and Aluminium are expected to show an oxidation state of +3, because they don’t show this due to inert pair effect, while other members (Ga, In and Tl) show + 1 and +3 oxidation states due to inert pair effect. The inert pair effect increases from Ga to Tl, so the stability of +3 oxidation state decreases from Ga to Tl and stability of +1 state increases from Ga to Tl. Tl (III) compounds are less stable and tend to change to Tl (I) compounds. Hence Tl (III) compounds are oxidizing agents . –––––––––––––––
- The two s-electrons in the outermost orbit of Ga , In and Tl tend to remain paired and do not participate in bonding or compound formation . This is called inert pair effect. The inert pair effect means non-participation of the ns2 electrons in chemical bonding of p-block and is much more pronounced in Group IV A and V A . For example, Pb (II) is more stable than Pb (IV) state in Group IV A and Bi (III) is more stable in VA. The inert pair effect also indicates that the energy required to involve the ns2 electrons in bonding increases down the group.
- The first ionization energy for boron is about 800 kJ mole-1, which is lower than that for beryllium. This shows that monovalent boron, B+ should be obtainable . But it is not so. All known compounds of boron are trivalent. The balance of Energy required to produce an sp2 hybrid atomic orbitals, and (b) energy released in the formation of three covalent bonds in BX3 compounds leads to a much more stable system than would be obtained by the formation of a single B+X- ionic compound .
- All monomeric tri-covalent boron compounds are planar and in these systems, boron has an incomplete octet, as a result of which boron has electron acceptor properties. Since sum of the first three ionization energies for the elements are high, and that of boron is exceptionally high, the total energy required to produce B3+ ion is much more than would be compensated by lattice energies of ionic compounds or by hydration of B3+ ions in solution. Consequently boron has no cation (B3+ ) chemistry . The extremely high ionization energies associated with the formation of simple M3+ coupled with the small ionic radius for M3+ ions suggest a predominant covalent character for compounds of III A group elements. However, the large hydration energies of the ions of these elements compensate for the adverse ionization factor and hydrated ions of the type [ M(H2O)6 ] 3+ are known for all the elements except boron.
- The ionic radius of Tl+ is almost similar to that of bigger alkali ions. Hence Tl (I) compounds resemble those of alkali metals . For example, the carbonates , nitrates , sulphates, phosphates and cyanide and alums of K+ and Tl+ are often isomorhpous. The standard oxidation potentials of these elements are quite high except for boron, which does not exist as B3+ ions in aqueous solution. Aluminium is a strong reducing agent, even better than carbon. That’s why it is used as a reducing agent to obtain Cr, Mn and V etc. by Thermite process.
- When the elements of III A group form trihalides, trihydrides trialkyls, they have only six electrons in their valence shell and are still capable of accepting a pair of electrons. Thus they behave as Lewis acids. There are some notable distinctions between the acceptor behavior of boron and other elements. These are partly due to the reduced ability of the heavier elements to form multiple bonds and their ability to attain a co-ordination number greater than 4. The maximum covalency of boron is 4, while that of Al is 6. Boron halides are momomeric and other elements can make up this deficiency through dimersation. Hence the halides of Al, Ga and In are all dimeric. The shortening of B-X bonds in boron trihalides is because of the ability of three - co- ordinated boron to accept electrons from the fully filled pp orbitals of the halogens through back donation. The bond is formed using the vacant pp orbital of boron. This is most significant in BF3. The reduced tendency among the heavier elements to form p - p is probably due to
- An increase in size of the central atom
- Less effective overlapping of more diffused 3p , 4p etc. orbitals and subsequent weaker bonds
- Repulsion between filled inner orbitals of large M and X atoms.
- Boron is a high melting, covalently bonded, refractory material even harder than carborundum. The great hardness of the crystalline form indicates the powerful bonding forces between the boron atoms . The crystalline boron has low electrical conductivity. Other Group III A elements are silvery white and comparatively soft. They have relatively low melting points. Gallium has the lowest melting point 29.8°C and so may be considered as liquid at room temperature. The softness and low melting point is due to less rigid metallic bonding in their crystals.
- The non-metallic boron differs sharply from other elements (metals) of the group in its chemical reactivity which depends upon whether amorphous form or the crystalline form (Boron occurs in these two allotropic forms). Amorphous boron is very reactive, while crystalline form is quite unreactive. Conc. HNO3 and conc. H2SO4, both oxidize amorphous boron to boric acid, H3BO3 . Crystalline boron is hardly affected by these reagents, probably due to it’s structure.
- Aluminium is rather inactive than expected from its electrode potential. This is due to a thin film of oxide which normally covers its surface .
- The metals react with halogens and sulphur to yield compounds with an oxidation number of +3 . They react with water in different ways. Al reacts with water on heating, while in the absence of oxygen even boiling water does not react with Ga and In. Tl reacts with water in the presence of air to yield a strong hydroxide, TlOH and to evolve H2. Like alkali metals, thallium is stored in air tight containers under kerosene, glycerine or vaseline.
- Al, Ga and In react with dilute mineral acids such as HCl, H2SO4. Tl does not react with these acids due to the formation of sparingly soluble film of TlCl or Tl2SO4 (Salt passivation). Al readily dissolves in hot and conc. HCl or aqueous alkali at room temperature . Al also dissolves in hot H2SO4 .
Aluminium easily removes oxygen from the oxides of other metals. The reaction of aluminium with metal oxides is highly exothermic.
- The reduction of metals oxides by aluminium is called aluminothermy. This process is used for the preparation of those metals that are reduced with difficulty and have high melting points the examples are Ti, W, Zr, V.
- Hardest element of III A group is boron. All halides of boron are electron deficient compounds ( Lewis acids ) and triagonal planar in shape having sp2 hybridization . All halides of boron undergo hydrolysis to yield orthoboric acid (the stable oxy acid of boron, H3BO3) and hydrogen halide. Lewis acid nature of BF3 is less than expected because of back donation of electrons by F. Due to this back bonding and resonance the B-F bond in BF3 gets a bond order of 1.33. The tendency of accepting electron pair of boron halides follows the order BF3 < BCl3 < BBr3 > BI3. Expect the BI3, this order is reverse of what is expected on the basic of electronegativity. The trifluorides are all non-volatile ionic solids only slightly soluble in water. Other halides are largely covalent, relatively volatile and their vapour consist of dimeric molecules. They readily dissolve in non-polar solvents like benzene in which they are present as dimeric molecules. Anhydrous trichlorides fume in air (moist) because of hydrolysis. The tribromides and triiodides are also hydrolysed.
- Binary compounds of boron with hydrogen are known as boranes and all boranes are electron deficient compounds, spontaneously catch fire in air and undergo hydrolysis with water to yield boric acid. Hydrogen bridge in B2H6 is known as banana bond or tau bond. Borazine or Borazole or inorganic benzene ( B3N3H6) is isoelectronic with benzene. In it hybridization at B and N is sp2. Hybridization of boron atom in diborane (B2H6) is sp3. Borazine has a little aromatic character. Borazine nucleus undergoes addition reaction with molecules containing acidic hydrogen such as HCl, H2O, CH3OH, CH3COOH etc. The B - N distance in borazine is a shorter than expected for a single bond, which is probably due to delocalised pp - pp bonding. However decalisation is less than in benzene.
- Diborane is used as rocket fuel. Boron nitride has a structure similar to graphite. Hybridization at B and N in it is sp2. Boron nitride is also known as white graphite. A hard substance known as borazone is obtained when boron nitride is heated at 3000 °C and 10,000 atm. pressure . The hardness of the four hardest known substances follows the order:
- Diamond > Borazone > Carborundum > Corundum.
- Aluminium is the most abundant metal in the earth crust. The principal ore of aluminium is bauxite, Al2O3.2H2O. Aluminium is bluish white metal with a brilliant lusture, which is soon destroyed by the formation of a layer of oxide on it. Aluminium is not affected by dry air but in moist air an oxide film is formed over its surface. It burns is oxygen forming brilliant light. The reaction is highly exothermic and used for reducing oxides of iron, chromium and manganese. The aluminium metal is praised for its lightness, high thermal and electrical conductivity, high ductility, high resistance to corrosion and high reflectivity for light and heat. Mixed with iron oxide, aluminium powder forms thermite, which is used for welding purposes. Aluminium has a great affinity for oxygen and this fact is used in the extraction of chromium and manganese from their oxides. Aluminium is mainly used in making alloys with light weight and good strength. Aluminum metal is soluble in both acids and alkalies with the liberation of H2 gas.
- The reduction potential of Al3+ ion is sufficiently negative (-1.66 V). All aluminium salts are soluble in water or in dilute acids. Aluminium forms a number of double salts with other cations. These are called alums.
- The common alloys of aluminium are aluminium bronze (88 - 90% Cu, 12-10% Al),
- Magnalium (5-30% Mg, 95 - 70% Al) and Duralumin (95 % Al, 4% Cu, 0.5% Mg and 0.5% Mn). Almasilicon and Alpax are the important alloys of aluminium with silicon. Gold containing 1% Al is green in colour; with 10% Al it is white as well as brittle. Gold containing 22% Al is deep purly. The m.p. of this alloy is 20°C higher than that of pure gold.
- The most stable compound of aluminium is Al2O3. A mixture of aluminium powder and aluminium nitrate is called ammonal. A mixture of aluminium powder and barium peroxide is called ignition mixtrue. Aluminium oxide is used for preparing artificial gems. Al2O3 is used as an absorbent in column chromatography. Both boron and aluminium form electron deficient compounds. Aluminium is usually extracted from bauxite by electrolytic reduction. Important minerals of aluminium are bauxite (Al2O3. 2H2O), diaspore (Al2O3. H2O), Corundum (Al2O3), Felspar (KAISi3O8) and Cryolite ( Na3AlF6). Bauxite is purified by Baeyer’s process, Hall’s process and Serpeck’s process. In the electrolytic reduction of alumina, cryolite and fluorspar are added to reduce the m.p. of alumina.
- Al2O3 is not a good conductor or electricity. It is dissolved in cryolite to make it a good conductor. Aluminium does not react with HNO3. A mixture of ferric oxide (Fe2O3) and Al powder in the ratio 3:1 is called thermite. Al is the metal which is present in silver paints. All double sulphates with similar composition and properties are called alums. Common alum is K2SO4. Al2(SO4)3.24H2O. Al2O3 is a while deliquescent crystalline compound which sublimes at 456 K. Al2O3 exists as dimer Al2Cl6 and is covalent in solid state but dissociates into ions in solution. It also acts as Lewis acid.
- Each metal ion in alums is surrounded by 6 water molecules.
- Artifical gems or precious stones are prepared by dissolving alumina with metal oxide in oxyhydrogen blow pipe flame. The formula of alum stone is K2SO4. Al2(SO4)3. 4Al(OH)3. An Alum shale is aluminium silicate containing iron pyrites (FeS2). Ruby (red) and Sapphire (blue) stones are the purest and natural form of alumina (Al2O3). Aluminium sulphate present in alum undergoes hydrolysis to form acidic solution. AlCl3 is used as a catayst in aa Friedel Crafts reaction and in cracking of petroleum.
group IV A
- The IV A group of periodic table contains C, Si, Ge, Sn and Pb. They have ns2 np2 as the general outer configuration. IV group is regarded as a transitional group between metals (electropositive) and non-metals (electronegative).
- Their Atomic size, Atomic Volume, Metallic character, Basic nature of the Oxides increases from Carbon to Lead. Carbon and Silicon are non- metals, Germanium is metalloid while Tin and Lead are metals.
- Ionization potential, Electron affinity, m.p. and b.p decreases from top to bottom (down the group). Carbon is most electronegative element of the group. Ionization energy of the Carbon is very high. With the exception of Carbon density increases from Silicon to Lead.
- Each element of this group has 4 electrons in the outermost shell of their atoms. The atoms of these elements could therefore accept or donate 4 electron each, exhibiting thereby the oxidation numbers of - 4 and + 4 respectively. All the elements of this group exhibit +2 and + 4 oxidation states. The +2 Oxidation State is more stable in the heavier elements due to inert pair effect.
- The oxidation number of - 4 and + 4 are characteristic of non metals Carbon and Silicon alone. They exhibit the maximum valence of 4 with respect to both hydrogen and oxygen. In addition they can also form a few unstable compounds with the oxidation number of +2 in oxides and sulphides.
- Tin and lead form stable compounds in +2 oxidation state which are more stable than those in oxidation state of + 4. Moreover, tin and lead can exist in the form of positive ions or cations.
- The elements of this group can utilize all the four valence shell electrons for bonding. They form molecules having 4 covalent bonds in which octet rule is obeyed.
- Elements of IV A group show Catenation. Carbon has maximum catenation power and then Silicon. The property of catenation is responsible for the existence of numerous compounds of Carbon. The highest catenation power of carbon is due to its small size and high bond energy. The property of catenation decreases rapidly for Silicon and Germanium, while it is very feeble in tin and almost nil in lead. In Tin and Lead, the linkage usually involves no more than two atoms. The decrease in catenation is due to decrease in strength of the M - M bonds. The polarity of C - H bonds is less than the polarity of Si - H bond.
- Compounds of Ge (II) are unstable and readily oxidized to Ge (IV) due to inert pair effect. Compounds of Ge(II) are thus powerful reducing agents. Sn (II) tends to be oxidized to Sn (IV) and hence Sn (II) is strongly reducing. Pb(II) is more stable than Pb(IV) which is strongly oxidizing.
- In all compounds carbon has a covalency of 4 and cases in which carbon has other than this covalency are rare. Ionic bonds of carbon are rare. Unstable carbonium ions and carbanions are, however known but such species do not have the charge localized on any carbon atom. The ion C4+ is unknown and C4 - is rare.
- Carbon is harder than silicon. Carbon in the from of graphite and gas carbon is a good conductor of electricity. Silicon is a bad conductor of electricity. Carbon and hydrogen have almost same electronegativities. The hydrides of silicon are called silanes. The latter are good reducing agents. Silianes undergo hydrolysis due to the polar nature of Si - H bond. Maximum covalency of Carbon is 4 and that of the Silicon is 6. This shows stability of carbon compounds and instability of silicon compounds. Carbon is limited to 4 electrons in its valence shell. So it exhibits a maximum covalency of 4. The valence shells of silicon and other elements have d-orbitals and can thus expand their shells beyond octet. Thus they exhibit covalency of greater than 4. While carbon forms no complex compounds, Silicon and other elements form complex compounds of the type SiF42-, SnCl62-, PbCl42- etc.
- IV group elements form the corresponding dioxides. They are CO2, SiO2, GeO2, SnO2 and PbO2. Out of these CO2 and SiO2 are acidic while GeO2, SnO2 and PbO2 are amphoteric.
- Carbon forms two oxides CO and CO2, which are gases at ordinary temperatures. Silicon forms solid SiO2. It has high m.p. and b.p. Purest form of SiO2 is quartz.
- The reaction of carbon and oxygen at higher temperature yielding CO and CO2 is highly exothermic. The relative yield of two oxides depends on temperature. The highest the temperature the greater is the proportion of CO.
- CO burns with blue flame and extremely poisonous due to its high affinity for oxygen . It combines with haemoglobin of the blood to give carboxy haemoglobin. It is neurtral to litmus. Exhaust pipes of autos contain CO. CO reacts with Cl2 in sunlight to form poisonous COCl2( Phosgene or carbonyl chloride). CO is a good reducing agent .
- CO also acts as an unsaturated compound because the four valencies of carbon are not satisfied in it. Thus it forms addition compounds with a number of substances. CO reacts with many metals to form metal carbonyls. In carbonyls the oxidation state of metals is always zero. The poisonous nature of CO is also due to its ability to form a bond with iron atom in the haemoglobin of blood.
- CO2 is product of combustion of carbon and its compounds, CO2 is not poisonous, but it does not support life. Solid CO2 is known as dry ice. The latter mixed with ether is used as refrigerant. CO2 is neither combustible nor supporter of combustion. Air contains 0.03% of CO2 by volume . CO2 is about 1.5 times heavier than air .
- Aerated water contains CO2 under pressure . Dry ice undergoes sublimation at 78° C. A mixture of 90% O2 and 10% CO2 is called carbogen which is used for artificial respiration . CO2 turns lime water milky. CO2 is acidic to litmus. Presence of about 15% CO2 in air acts as fire extinguisher. Oxygen gas is liberated during photosynthesis.
- Silicon is relatively unreactive except at high temperatures. It is unaffected by oxygen, water and steam probably due to the formation of protective surface layer of SiO2. Germanium is more reactive than silicon. It reacts with water and dilute acids and alkalies only in the presence of an oxidizing agents such as H2O2 or NaOCl. Tin and lead are reactive as well as electropositive.
- All the elements of VIA group form tetra halides of the type MX4. The stability decreases from CX4 to PbX4. All the tetra halides are liquid at room temperature and covalent in nature. With the exception of carbon all the elements of IVA group react with alkalies to form H2 gas.
- The existence of an element in different forms is called allotropy. Carbon exists in different allotropic forms. Diamond and graphite are the crystalline forms while coal, coke, wood charcoal, animal charcoal, gas carbon, lampblack, petroleum coke and sugar charcoal are amorphous forms of carbon. Graphite and gas carbon are good conductor of electricity. Diamond is the hardest form of carbon. Lamp black is the softest. Sugar charcoal is the purest form of carbon. Diamond has tetrahedral structure (sp3 hybridization) and graphite has hexagonal structure (sp2 hybridization). Diamond is big three- dimensional polymer with a large number of carbon atoms arranged tetrahedrally. That is why it is hardest substance known. The density of diamond is 3.51 g/ cc. The refractive index of diamond is 2.45. Diamond is very hard due to strong sigma bonds. Graphite is soft due to the presence of weak vander Walls forces between various hexagonal layers. Diamond is insoluble in any solvent.
- The C - C covalent distance in graphite is 1.42 A°. The layer structure of graphite is the cause of softness and lubricating properties of graphite. Lead pencil is a mixture of graphite with variable quantities of clay depending upon the hardness required. Because of the presence of free electrons graphite is good conductor of, heat and electricity. Graphite forms a yellow mass known as graphitic acid (C11H4O5) when heated with conc. HNO3. Diamond glitter because of high refractive index. Diamond is used as precious stones for jewellery. The famous Kohinoor diamond (106 carats) occurs in India. A carat is equal to 0.2 gm (= 200mg). Diamond on heating at 1800-2000°C in vacuum gives graphite.
- The similarity in electronegativity between carbon and hydrogen shows that C- H bond would normally have low polarity . Thus they exert no strong inductive effects on the other bonds involving carbon The C-C bonds are also not weakened by electron withdrawal by hydrogen . Thus carbon atoms form stable links through chains and rings of infinite length.
- The ability of carbon to be more than divalent gives it characteristic to form long chains and branched chains permitting a wide variety of structures . The smaller size of carbon (77 pm) allows effective sideways overlapping of orbitals giving p bonds . Carbons can form stable multiple bonds of the pp- pp type. Thus carbon forms the largest number of hydrides known as hydrocarbons .
- Silianes are much more reactive than hydrocarbons because of larger size of silicon atom (117 pm) which facilitates attack by nucleophiles. Greater plarity of Si-X bond ( X = H, C,O,N, F, Cl , Br , I ) and availability of low lying d-orbitals, decreases the activation energy of the reaction. In general , C-X bond energy is less than Si-X bond energy ( X = O, N, F, Cl, Br, I ) . All silanes are spontaneously inflammable in air and react vigorously with aqueous alkali forming silicate and evolving H2.
- The decrease in thermal stability of the hydrides is due to the fact that M-H bond energies decrease from carbon to lead. C-H> Si-H> Ge-H > Pb-H. The difference in the stability of the hydrides of carbon and silicon is, however, not due to the weaker Si-H bond, but rather a result of inductively weakened Si-Si bond due to electron withdrawal by hydrogen. The decreasing stability of the germanes, stannanes and plumbanes is primarily due to decrease in M-H bond energies. The tetrahalides of the type MX4 are all known except PbBr4 and PbI4 ,which are not expected to be stable due to the strong oxidizing power of Pb ( 4 +) and the reducing power of Br- and I- .
- In general, stability of halides decreases on moving down the group and also as the halogen changes from fluorine to iodine. All the tetrahalides are covalent and sp3 hybridised (tetrahedral) compounds (except SnF4 and PbF4) and have polymeric partly ionic structures. No dihalides of carbon and silicon are known. The dihalides of Ge, Sn and Pb are known. The dihalides are much less volatile than tetrahalides and become increasingly powerful reducing agent from Ge to Pb.
- The three important crystalline forms of silica are quartz, tridymite and crystobalite. Amorphous forms of silica are agate, jaspur and onyx. Transparent quartz is used in making optical instruments and lenses. Sand (SiO2) is an important flux in metallurgy. Quartz glass used in the manufacture of optical instruments and chemical apparatus is called vitreosil .
- Fuel is a substance, which burns in air to give heat energy. Fuels may be solid (e.g coke, coal, wood), liquid (petrol, kerosene. alcohol etc) and gaseous (e.g. water gas. coal gas, producer gas etc). All fuels have greater affinity for oxygen and burn to give heat energy. Water gas CO+ H2, producer gas is CO+N2 and carburetted water gas is water gas mixed hydrocarbons. Semi water gas is the product formed by passing a mixture of steam and air over incandescent coke. It is a mixture of CO2 + N2 + H2 . Approximate composition of water gas is H2 50% , CO = 40% and rest is a mixture of CO2 , CH4 and N2. The calorific value of water gas is high (2700 k.cals/ m3). The calorific value of producer gas is low (900 k.cals / m3). The composition of producer gas is CO = 30 -35% . N2 = 50-55% and rest is a mixture of CH4 , CO2 and H2. The calorific value of coal gas is 4500-5900 k.cals/ m3 . It is a good gaseous fuel as it contains 95% of combustible gases. Petrol gas is a mixture of CH4 , C2H6 , C2H4 etc .
- Cement is finely powdered mixture of calcium aluminates and silicates . It is called Portlant cement because it resembles to a famous variety of building stone quarried near Portland in England. Cement was first prepared by Joseph Aspidin .
- Portland cement is a mixture of limestone is : CaO2 = 20-25% , Al2O3 = 5-10% , MgO = 2-3% , Fe2O3 = 1-2% and SO3 = 1-2% . For a good quality cement the ratios between silica and alumina should be 2.5 - 4.0. The ratios between the weight of CaO and other oxides (SiO2 + Al2O3 + Fe2 O3 ) should be close to 2. The MgO content should be less than 5% . The mixture of calcium silicate and calcium aluminate produced in the manufacture of cement clinker. Cement clinker is mixed with 2-3% of gypsum to regulate the time of setting. The setting of cement is due to hydration of calcium silicates and aluminates .
- Chemical composition of ordinary glass is Na2SiO3. CaSiO3. 4SiO2 . Raw materials used for the manufacture of glass are Na2CO3, CaCO3 and SiO2 ( soda ash, limestone and silica ). The process of gradual cooling of glass is known as annealing. Lenses, prisms and optical instruments are made with flint glass. It has high refractive index. UV rays can be checked by Crookes glass . Pyrex glass has a very low coefficient of thermal expansion . It is mainly a mixture of sodium borosilicate and aluminium borosilicate. Different coloured glasses are obtained by the addition of metallic oxides to glass.
|Colour ||Substance Used ||Colour ||Substance Used |
|Cu2O & SeO |
Fe O3 & CuO in excess
SnO and Ca3 (PO4)2
- Semiconductors are those solid substances which are insulators under normal conditions but conduct electricity by the passage of electrons with the increases in temperature or by the presence of impurities. Silicon and Germanium are semiconductors.
- Tin exists in three allotropic forms - grey, white and rhombic.White variety is the most stable variety of tin . The crumbling of tin to a powder is known as tin pest, tin disease or tin plague. Tin is weakly electropositive and relatively less active metal. Tin is not attacked by organic acids.Thus it is used in tinning of cooking utensils. Tin is the only element which has maximum number of isotopes (ten). The process of depositing a thin protective layer over sheets ,of iron is called tin plating.Important alloys of tin are,
- Bronze (75 - 90% Cu, 10 - 20% Sn), Solder (76% Sn, 33% Pb), Bell metal (25% Sn, 75% Cu), Gun metal (88%Cu, 10% Sn 2% Zn) and Babbit metal (90% Sn, 7% Sb, 3% Cu). SnO2 is amphoteric. SnCl2 is soluable in water , alcohol and ether.
- Important ores of lead are galena (PbS) and cerlussite (PbCO3). Water containing dissolved air dissolves lead, forming water soluble lead hydroxide. This solvent effect of water is known as plumbo solvency. Litharge or massicot is PbO, red lead is Pb3O4 or PbO2. 2PbO (double oxide). Lead is soluble in acetic acid. Pb(CH3COO)2 is called sugar of lead. Tetra ethyl lead is an anti knock compound. Basic lead carbonate is known as white lead.
- Nitrogen is very important element in man’s environment . It is one of the five major elements required by living systems, in parts because of its presence in all amino acids, the building blocks from which proteins are cinstructed . Nitrogen is one of three major elements present in commercial fertilizeres ( the other two being P and K . ) Nearly all chemical explosives contain nitrogen .
- Phosphorus is also a major element essential to living systems . The phosphate linkages hold together the skeletons of enormous molecules of deoxy ribose nucleic acid (DNA) that contain the genetic code in animal cells. A series of adenosine phosphate compounds store and later release the energy obtained when high energy compounds such as glucose are oxidised in the body . Phosphorus is the tenth element in order of abundance in the earth’s crust and occurs to the extent of 1120 ppm. All the known minerals of phosphorus are orthophosphate.
- Arsenic , antimony and bismuth are much less abundant in the earth’s crust and much less important than nitrogen and phosphorus . The first two elements in the group , nitrogen and phosphorus are typical non-metals , arsenic and antimony are metalloids , while bismuth is a metal.
- Each of these elements has 5 electrons in the outermost shell (ns2p3 ), so they are short of three electrons to attain an octaelectronic noble gas configuration . All these elements are capable of completing their outermost shell. For example , in compounds with hydrogen these elements are strictly trivalent. The electronegativity of these elements , however , decreases with increase in their atomic number . Atoms of these elements can also give up electrons. In oxy compounds and in compounds with halogens ( except for Iodine ) and sulphur , these elements exhibit valence of +3 and +5 .
- All hydroxides formed by group VA elements in pentavalent state are acidic in nature. The strength of these acids decreases down the group. HNO3>H3PO4 >H3AsO4>H3SbO4>H2BiO4. Thus HNO3 is a strong acid , H3PO4 is moderately weak acid , H3AsO4 is a weak acid, while the remaining two acids are extremely weak.
- Nitrogen and phosphorus exist as anion in all their compounds , while As, Sb and Bi have a tendency to form cation , For example , these three elements are present in aqueous solution as cations, As3+ , Sb3+ and Bi3+ , with oxidation number of +3 , in a very insignificant amount , since their salts undergo hydrolysis.
- Though the elements have been placed in the same group VA, they differ from one another in many properties . The N and P have noble gas core beneath the valence shell of 5 elements , while As, Sb and Bi possess 18 electrons in their penultimate shell . due to this difference in electronic configuration , they have different properties.
- All members of the nitrogen family exhibit multiple oxidation states between -3 and +5 as is expected from the electronic configuration of their atoms . Common oxidation states of VA group elements are -3 , +3 and +5 . Nitrogen itself shows all the integral oxidation states of -2 , -1 , -1/3 , +1 , +12 , +4 etc.more electrons , either by gaining three more electrons , either by gaininig three electrons or by sharing electron pair with a less electronegative atom. In + 3 oxidation state one pair of s-electrons is inert and the three unpaired p-eletrons are used for chemical bonding to more electronegative element . The +5 oxidation state involves the use of all the 5 valence electrons to form covalent bonds. The higher positive oxidation states are represented by halogen and oxygen derivatives of the elements .
- Compounds such as NH3 , PH3, AsH3, SbH3 etc show -3 oxidation states . NH2 - NH2 exhibits -2 and NH2OH-1 oxidation state. N2O and NO show + 1 and +2 oxidation states respectively . Species such as NO2,P4O6,As4O6 , Sb4O6 and Bi2O3 show +3 oxidation state . NO2 , P4O8 , Sb2O4 etc. show +4 oxidation state and species such as N2O5 , NO3- , P4O10 , As2O5 , Sb2O5 and Bi2O5 show +5 oxidation states .
- The ionisation energies are high as would be expected from the electronic configuration of the elements ( stability of half filled p-orbitals ) and those for the removal of s-electrons encrease between Sb and Bi, just as they increase between In and Tl and between Sn and Pb , Bi like Pb, shows the 6s inert pair effect .
- The standard electrode potentials for As, Sb and Bi are quite close together. The decreasing ionisation energies of the ions being offset by the increase in ionic radii and decrease in heat of hydration of the cation formed .
- The gain of three electrons to form anion of the type N3- is favourable for N and that too only by metals which have low ionisation energies and form nitrides with high lattice energies . For example , Li3N , Be3N3 etc.
- The formation of p3- from P is very difficult , because addition of three electrons to a neutral atom requires a huge amount of energy . The elements As, Sb and Bi show no tendency to form anion by gaining electrons . This is probably due to the fact that increasing size of the atom makes it difficult for the nucleus to hold extra electrons. The loss of all the 5 electrons is more difficult to achieve because of the large ionisation energies involved . The M5+ ions, therefore, do not exist. Even the formation of M3+ ion needs considerable amount of energy because it requires destruction of the stable half filled p-subshell . The heavier elements however,form compounds containing cationic species , in which charge corresponds to M3+ , for example , BiF3. In this type of compounds , the pair of s-electrons become inert because of their ability to penetrate towards the nuclear charge . The tendency of the pair of electrons to remain inert increases as we go sown the group.
- The most common mode of bonding by the elements in VA group is the use of their three unpaired electrons to form 3 covalent bonds and to form molecules such as NH3 , PH3 , PCl3 etc. Here the elements may be in - 3 or +3 in NCl3 or NF3, but -3 in NH3.
- For As and Bi, the +5 state is often less stable than the +3 state. Thus arsenates and bismuthates are oxidising agents and are reduced to arsenite , As (III) or Bi (III) . Preference of As for the +3 state may be attributed to the poor shielding of the nucleus afforded by the inner 3d-electrons,as a result of which it is more difficult the promotion of one of the 4s -electrons . For Sb, the poor shielding of the 4d-electrons is offset by the greater distance of the valence electrons from the nucleus . Atoms of high atomic number generally form weak covalent bonds and the preference of bismuth as Bi (III) is probably due to the fact that it more stable in ionic compounds than entirely in covalent form.
- Nitrogen does not posses d-orbitals in valence shell and so its covalency is limited to four. No pentavalent compound of nitrogen is therefore known . N can , however , achieve a formal oxidation state of +5 as in NO3- ion. The inability of N to unpair and promote its 2s-electron results in that N ( +5) is less stable than N ( +3 ) . Thus HNO3 is a strong oxidising agent . However , P ( + 5) is more stable than P ( + 3) and hence phosphoric acid , H3PO4 shows no oxidising properties.
- Catenation in nitrogen is limited to two or three atoms . Hydrazine , NH2 - NH2 , the simplest catenated N-N compound is unstable with respect to disproportionation to elements . The higher homologues become increaseingly unstable . The P-P compounds are comparatively more stable . In As , Sb and Bi , the property of catenation is still less and decrease in the order P > N > As >> Sb >>> Bi .
- Maximum co-ordination number for N is 4 and thus the only regular stereo chemical arrangements that it can exhibit are tetrahedral , pyramidal, planar, angular and linear as in NH3+ , NH3, NO3- , ClNO and NO3+ respectively .
- Phosphorus , like N, can complete as octet of 4 tetrahedrally arranged electron pairs either by forming three covalent bonds giving pyramidal structure as in PCl3 or by forming 4 covalent bonds in ions such as PH4+ , which is similar to NH4+. Distortios because of lone electron pair repulsion similar to those in N compounds are also operative in P compounds . The H-P-H bond angle in PH3 is 93° and H-N-H bond angle in NH3 is 107° . Because P=O bond repels the P-X bonds more than the single bonds repel each other , the X -P-X angle is always less than the tetrachedral angle (109° 28’ ) in PX3 compounds ( X = F , Cl , Br etc) .
- The tetrahedral geometry in N compounds can be achieved by (a) Forming 4 covalent bonds and losing an electron as NH4+ . (b) Forming three covalent bonds and donating a lone pair of electrons to form a fourth bond as in H3N. BF3 . In both cases there is a tetrahedral distribution of 4 bonds about the nitrogen atom . Pyramidal geometry is most common for N and the bond angle X-N-X in majority of compounds is close to 108° . The nitrogen atom is pyramidal in hydrazine and its derivatives. The X-N-N bond is 110° - 112° . Rotation about the N-N bond is prevented by the lone pair on each nitrogen atom . The lone pair repulsion are also responsible for the long N-N bonds . The bond is shortened when the lone pairs are involved in bonding , as in N2H5+ and N2H62+ .
- In all nitrogen compounds where the nitrogen octet is comleted by the formation of three electron pair bonds, the N atom has a pair of non-bonding (lone pair ) electrons . As a result of non-bonding pair all NR3 type of compounds act as Lewis bases and form complexes with Lewis acids. Substituted ammonias of the type of NR3 are also pyramidal .
- Trigonal planar compounds of nitrogen contain three sigma and one pi bonds . The p bond can be localised in one direction or delocalised . Examples of trigonal planar species of N are NO3- , O2N - X ( X = F , Cl , OH , C6H5 , CH3 , OCH3 etc).
- An important difference in stereochemistry of N and other elements of the group is the possibility of utilizing the low lying d-orbitals by these elements . Thus they can expand their valence shell and can have co-ordination numbers more than 4. For example , PH3 , AsCl3 are atrigonal pyramidal in shape , PCl43+ is tetrahedral , PCl3 is trigonal , PO43- is tetrahedral , PF5 , AsF5 , SbCl5 are trigonal bipyramidal and PC6- , SbCl6- are octahedral in shape .
- Gaseous nitrogen is generally inert because of great strength of the Nº N bond or due to its high bond energy . Very symmetrical electron distribution in the molecule and the absence of the bond polarity are also responsible for its inert nature. When these are modified as in isoelectronic NO+ , the reactivity is considerably increased .
- White p is considerably more reactive than red phosphorus . Most reactive element of the group is phosphorus and its reactivity is due to very small bond ( 60°) or small bond strain .
- Phosphorus forms binary compounds with all elements except Sb, Bi and the inert gases .
- Arsenic and antimony are unaffected in dry air at room temperature. At high temperature , oxides of the form M2O3 are formed . Aqueous solutions of oxidising acids are able to dissove the elements but non-oxdising acids do not react . None of the elements reacts with alkalies.
- Compared to the amphoteric oxides of As and Sb and the acidic oxides of N and P, the bismuth oxide , Bi2O3 is basic. Direct reactions of Bi with O2 , S and X2 at elevated temperatures yields Bi2O3 , Bi2S3 and BiX3 respectively.
- VA group elements are called pnicogens and their compounds as pnictides . Most common valence of VA group elements is +5 . In bismuth stable oxidation state is +3 because of inert pair effect . All the binary compounds of VA elements are covalent , except BiF2 and SbF3. Arsenic undrgoes sublimation at high temperature and low pressure . Phosphorus is more reactive than nitrogen . In fact , the most reactive element of VA group is phosphorus because of its low bond dissociation energy . Except Bismuth all elements exhibit allotropy.
- Despite its ready availability in the atmosphere , nitrogen is relatively less abundant in the rocks and soil of the earth. It does not generally occur in nature in the form of inorganic compounds . The only major minerals containing nitrogen are KNO3 ( saltpetre) and NaNO3 (chile saltpetre ) . There is a continuous interchange of nitrogen between the atmosphere and the biosphere and is called Nitrogen cycle. P u r e N i t r o g e n can be obtained by thermal decomposition of sodium azide . 2NaN3 2Na + 3N2. Most important elements for life process is nitrogen contain about 16% N in them .
- N2 can be obtained by heating a mixture of NH4Cl and NaNO3 or NH4NO2 or decomposition of ammonium dichromate, ( NH4 )2 Cr2O7. N2 is less soluble in water than O2.
- Solid nitrogen exists in two allotropic crystalline forms : a -nitrogen ( cubic ) and b - nitrogen ( hexagonal ).
- Nitrogen exists as discrete diatomic molecule , N2 in the gaseous , liquid and solid state .
- Inspite of its high electronegativity ( 3.0) , nitrogen is relatively inert chemically . It reacts with simple substances ( except for Li) either at high temperatures or in the presence of catalysts . The chemical inertness is due to high strength of N º N bond and high bond dissociation energy . Because of the stability of N2 molecule , a number of nitrogen compounds are endothermic . N2 being a gas , the entropy of formation of such compounds is negative ( Exothermic ) .
- Most nitrogen compounds are thermally unstable and decompose relatively easily on heating . This explains why N2 is present in the earth mainly in the free form .
- Phosphorus does not occur in free state in nature because of its high reactivity . Important minerals of phosphorus are phosphorus rock , Ca3(PO4)2. Fluorapatite,CaF2.Ca3(PO4)2 and chlorapatite,Ca Cl2. Ca3(PO4)2. is the chief constituent of teeth and bones . Phosphorus can be obtained by the reduction of phosphate mineral such as bone as h with silica and coke in an electric furnace at 1500°C . White P is the most reactive form of P. Red P is the most stable form of P. White P contains P4 molecules and red P contains polymerised P units . In white P, the 4 atoms are pointed towards the 4 corners of a regular tetrahedron. White P is reactive because of small bond angle (60°) or bond strain . White P is the commonest form of P. It is most volatile , reactive solid but thermodynamically the least stable . The thermodynamically most stable form of P is black P. The P-P-P bond angle in white P is 60. The P-P bond distance is 221 p.m. The low bond angle indicates considerable strain in the molecules in white P. Red P is more dense than white P, has a higher m.p. and is much less reactive . White P is more volatile than red P because P4 molecules are held by weak vander Waals forces . White P is always kept under water because of its high reactivity and because its ignition temperature is very low, being 30° C. White P is soluble in non polar solvents such as C6H6 , CS2 etc. Red P is soluble in conc. HNO3 or alcoholic KOH. White P becomes yellow because of slow oxidation to P2O5. Red P is used in match industry . White P is poisonous and causes phossy jaw.So it is banned in match industry . Strike any where matches contain P4S3 .The head of safety match stick contains KNO3 or KNO2 or red lead with grounded glass and antimony sulphide. Sides of match box contain red P and sand powder or powdered glas. Black P has a graphite like appearance and a flaky layer lattice . The elements of VA group form trioxides and pentoxides . Pentoxides are more acidic than trioxides (N2O5 > N2O3). The acidic nature increases as the oxidation number of the element increases. For example , NO is neutral , N2O3 is weakly acidic and N2O5 is strongly acidic . N2O5 is the most acidic oxide in V group . It is formed by N. The most acidic oxide in the Iind period is also formed by N. Dimeric pentoxides are P4O10 , As4O10 and Sn4O10,while dimeric trioxides are P4O6 , As4O6 and Sb4O6. In P4O6 each P atom is linked by three oxygen atoms and in P4O10, each P atom is linked by 4-oxygen atoms .P4O10 is a powerful drying and dehydrating agent . Bi2O3 is basic , but As4O6 and Sb4O6 are amphoteric oxides . Oxides of an element in its highest oxidation state tend to be most strongly acidic and the least stable . Thus As4O6 is amphoteric and stable state decreases with increase in atomic number . Thus Bi2O5 has not been prepared in pure state. P4O6 is acidic, As4O6 and Sb4O6 are amphoteric and Bi2O3 is basic .
- In a period , on going to right , the nature of the oxides of elements changes from strong basic to strong acidic.
- Nitrogen forms 5 oxides with oxygen . These are neutral or non- salt forming oxides ( N2O and NO) and acidic oxides ( N2O3 , NO2 and N2O5 ) .The acids and salts corresponding to the acidic oxides are HNO2, and nitrite for N2O3 and nitric acid (HNO3) and nitrates for N2O5 respectively . NO2 forms a mixture of HNO2 and HNO3 . Salts of nitric acid are called nitrates and these decompose on heating to yield oxygen .
- The compounds of N and O are expected to be quite different from the oxides of other elements in the VA group because the ability to form multiple bonds (pp -pp ) is much less for the elements such as As, Sb and Bi .
- In nitrogen oxides ( N2O , NO, N2O3, NO2( N2O4) , N2O5 ) , the nitrogen exhibits +ve oxidation states of +1 to +5 . All the oxides have positive free energies of formation because of high dissociation energy of nitrogen and oxygen molecules . All are gases except N2O5 , which is a white solid and sublimes at 305K. The reduction potential for each of the oxidation states of nitrogen is positive . Thus each positive oxidation state can be reduced and will serve as oxidiseng agen .
- With oxygen , phosphorus forms two oxides , P2O3 and P2O5 which are acidic in nature . The corresponding acid of P2O5 is orthophosphoric acid , H3PO4. Salts of this acid are called orthophosphates. H3PO4 does not exhibit oxidising properties . It is tribasic moderately weak acid and forms neutral salts like orthophosphates and acid salts such as hydrogen orthophosphates and dihydrogen orthophophates.
- Strong oxidising agents such as HNO3 and conc. H2SO4 oxidise phosphorus to H3PO4.
P + 5HNO3 H3PO4 + 5NO2 + H2O
- Paramagnetic oxides of nitrogen are NO and NO2 . Coloured gaseous oxide of nitrogen is NO2 . Most acidic oxide of nitrogen is N2O5 . Thermally most stable oxide of nitrogen is NO. The oxide of nitrogen that can not decolourise acidic KMnO4 is N2O5.
- N2O can obtained by heating NH4NO3. N2O supports combustion at 500°C because of dissociation . It is also called laughing gas and has a linear shape .It is used in making whipped cream . It is also used as anesthetic. NO ( nitric oxide ) is colourless gas but gives brown fumes on exposure to air due to the formation of NO2.NO dissolves in FeSO4 solution and gives brown solution of FeSO4 solution and gives brown solution of FeSO4.NO( Brown ring test for NO3). N2O and NO can be seperated by passing through FeSO4 solution . NO also supports combustion .NO2 is a reddish brown gas, which dissolves in water giving a mixture of HNO2 and HNO3. SO it may be regarded as mixed anhydride of HNO2 and HNO3.NO2 is a powerful oxidising agent . It supports combustion above 650° C. NO2 is paramagnetic but N2O4 is diamagnetic due to pairing of unpaired electrons . N2O3 is pale blue in liquid state. It is anhydride of HNO2 . N2O5 is anhydride od HNO3 . In solid state it exists as NO2+ , N2O5 is the strongest oxidising, agent among all the oxides of nitrogen . All oxides of nitrogen are monomeric (NO, N2O , N2O3 , NO2( N2O4) , and N2O5 , while oxides of other elements ate dimeric (P4O6, P4O10 etc.) .
- The per nitric acid (H2N2O2 ) is hydrate of N2O. It is unstable and explodes violently when dry . The acid and its salts (hyponitrites ) are reducing agents . It is dibasic acid .
- The nitrous acid is also unstable and known only in solution . It is monobasic acid and its salts are called nitrites. These salts are stable . The acid and its salts act as oxidising as well as reducing agents .
- Nitric acid (HNO3) , also known as aqua fortis, is manufactured by Birkland and Eyde process (arc prcess ) or by Ostwald’s process. In arc process raw material is dry air , while in Ostwald’s process it is ammonia . In Ostwald’s process oxidation of NH3 is carried out in presence of platinum gauze . Pure HNO3 is colouless liquid but turns yellow due to dissolved oxides of nitrogen . 96 - 98% HNO3 is called fuming nitric acid . HNO3 reacts with all metals , except Au, Pt , Rh, Ir and Ta .Cold conc. HNO3 makes al ,Fe , Cr stainless steel passive . The ability of nitric acid , in presence of H2SO4, to nitrate many organic compounds is due to the formation of NO2+ ion. Nitric acid is a powerful oxidising agent . Thus non-metals such as S, P, I2 etc oxidised to their respective oxyacids when heated with conc. HNO3.
- When nitric acid reacts with metals , the products of reaction ( other than metal nitrates ) are largely determined by (a) Position of the metal in the electrochemical series (b) Concentration of the acid (c) Temperature . The reaction products may contain one or more of the substances HNO2 , NO2 , NO, N2O , N2 , NH2OH or NH3 . H2 is produced only when dilute HNO3 reacts with Mg , Mn or Zn. Metals above hydrogen in the electrochemical series react to yield products such as H2 , N2 , NH3 , NH2 OH or N2O when treated with nitric acid .
- Nitrous acid plays an important role in reaction between nitric acid and noble metals such as Cu , Ag and Hg . Copper is not attacked by HNO3 in presence of urea or H2O2 , because these compounds destroy any HNO2 as soon as it is formed .
- With Cu, dilute HNO3 gives NO, but conc. HNO3 gives NO2 . With Zn, cold dilute HNO3 gives N2O , but hot conc. HNO3 gives NO2 . With Sn , Mg or Zn , cold dilute HNO3 gives NH4NO3 but conc. HNO3 gives hydrated oxide. With Ag , Co , Ni and Pb , cold dilute HNO3 gives NO, but conc. HNO3 gives NO2 .
- The metals such as Mn, Mg react with cold dilute HNO3 to form metal nitrate and H2 . The metals such as Fe , Zn etc. react with very dilute HNO3 to form metal nitrate and NH4NO3. Metals such as Pb , Cu , Ag and Hg react with dilute HNO3 to form metal nitrate and NO. Fe and Zn react with dilute HNO3 to form metal nitrate and NO2. Sn reacts with dilute HNO3 to form Sn ( NO3)2 + NH4 NO3 . Al does not react with dilute or conc. HNO3. Au , Pt etc do not react with even conc . HNO3 . Fe becomes passive with conc. HNO3 . Zn, Fe , Pb , Cu , Ag and Hg react with conc. HNO3 to form metal nitrate and NO2.Sn reacts with conc. HNO3 to form H2SnO3 + NO2 .In general reaction of dilute HNO3 on metals above hydrogen in the electrochemical series can be represented as ,
3Zn +8HNO33Zn(NO3)2 + 2NO+ 4H2O
- The reaction of dilute HNO3 on metals below hydrogen can be represented as ,
3 Cu + 8HNO3 3 Cu(NO3)2 + 2NO + 4H2O
- Cold dilute HNO3 reacts with Mg and Mn to liberate H2 .
- Very dilute and cold HNO3 reacts with Zn , Fe and Sn in the following manner .
- Dilute and cold HNO3 reacts with Zn as under ,
- Concentrated HNO3 reacts with metals above hydrogen in the following manner ,
- Concentrated HNO3 reacts with metals below hydrogen as follows .
- Vapours of concentrated HNO3 react with Zn and Cu in the following manner.
- HNO3 reacts with non metals in the following manner,
- Oxidising properties of HNO3 are shown by reactions of the type.
- Nitric acid acts as nitrating agent in presence of conc. H2SO4 .Nitric acid is slowly decomposed by light giving NO2
- Action of heat on HNO3 can be represented as ,
- Aqua regia is a mixture of 3 parts conc. HCl and 1 part conc. HNO3 .
- Action of heat on some nitric acid salts ( nitrates ) can be represented as follows :
- Conc. HNO3 produces yellow stains on the skin because of formation of xanthoproteins .
- The oxyacids of phosphorus are more than those of any other element .
- The structure of the acids are such that each P atom is tetrahedrally surrounded by 4 neighbouring atoms . That is , it is 4 co- ordinate.
- Each oxyacid contains atleast one P = O unit . All P atoms in the oxyacids have atleast one P-OH linkage . The basicity of the acid is equal to the number of such H atoms bonded to P through oxygen .
- Some oxyacids have one or more P-H links, Such directly bonded H-atoms are not acidic, because they are not ionisable . The H atom directly bonded to P confer strong reducing properties on the molecules . Catenation takes place through P-O-P links or directly through P-P bonds . Oxyacids containing P in the lower oxidation states of +1 and +3 tend to disproportionate, as a result , some P atoms attain the relatively stable + 5 state.
- Hypophosphorus acid ( H3PO3) is generally prepared as its barium salt by heating white P with aqueous Ba (OH)2 . The free acid is obtained by treating the barium salt, barium hypophosphite with calculated amount of dilute H2SO4 . Pure H3PO2 is white crystalline solid . It is a monobasic acid. H3PO2 and its salts , known as hypophosphites are strong reducing agents . They decompose on heating and liberate PH3.
- Phosphorus acid (H3PO3) can be obtained by the hydrolysis of PCl3 in cold water. It is moderately strong dibasic acid and forms normal salts as Na2HPO3 and acidic salts such as NaH2PO3 . Its salts are known as phosphites . H3PO3 decomposes on heating to yield PH3 and H3PO4 . The acid and its salts are good reducing agents. Phosphorus acid is an extermely hygroscopic white crystalline solid.
- Phosphoric acid or orthophosphoric acid ( H3PO4 ) can be prepared by heating Ca3 (PO4)2 with H2SO4 (b) Dissolving P2O5 in hot water (c) Hydrolysis of PCl5 (d) Refluxing red P with excess conc. HNO3 on a water both . It is hydrate of P2O5 (P4O10). Dehydration on H3PO4 gives pyrophosphoric acid , H4P2O7. Commercial phosphoric acid has about 85% and H3PO4 is called syrupy phosphoric acid . It is syrupy because of extensive hydrogen bonding . Phosphoric acid is tribasic acid and forms three series of salts . The acidic NaH2PO4 , neutral or weakly basic NaHPO4 and basic Na3PO4.
- Dilute aqueous solutions of the acid are strongly acidic but only one of the hydrogen atoms is readily ionisable. Solutions of H2PO4 are weakly acidic, those of HPO2-4 weakly alkaline and those containing PO3-4 ions are strongly alkline due to hydrolysis . On heating to 220° C, H3PO4 gives pyrophophoric acid and at 316°C it gives meta phosphoric acid .
- Hypophosphoric ( H4P2O4 ) can be prepared by oxidising phosphorus acid with iodine. The acid shows no reducing properties and decomposes on heating above its m.p. (346K). It undergoes rearrangement and disproportionation even at room temperature.
- Pyrophosphoric acid (H4P4O7) can be obtained by heating H3PO4 to 220° C. It is tetrabasic acid, which on heating to 316°C gives metaphosphoric acid , HPO3 .
- The metaphosphoric acid (HPO3) can be obtained by heating ammonium phosphate . It can also be obtained by heating microcosmic salt , NaNH4 .It is known as glacial phosphoric acid . It is a monobasic acid .
- As4O6 dissolves in water to give a weakly acidic solution of arsenious acid . The acid forms salts with alkalies most of which are meta-arsenates, for example , NaAsO3 . Arsenic acid ( H3AsO4 ) is weaker than H3PO4 and is obtained by the oxidation of As4O6 with conc. HNO3 . H3AsO4 is a strong oxidising agent . This reflects the greater stability of As (III) compared to As(V) . Sb4O6 is much less soluble in water than As4O6, but its saturated solution is acidic .Very little is known about H3SbO4 and their salts.No oxyacids of bismuth are known , Bi2O3 is not acidic and is insoluble in alkalies .
- All the possible simple trrihalides of elements of VA group are known . The number of pernahalides is limited . Nitrogen does not form penta halides , because its maximum covalency is 4 . Bismuth only forms BiF5, because of marked instability of Bi (V) . AsBr5 , SbBr5 , AsI5 and SbI5 do not exist , because of reducing character of Br- and I- and oxidising properties of As (V) and Sb (V).
- All the possible trihalides of N, P , as Sb and Bi are known . They are all covalent , except BiF3, which shows ionic character . The stability of MX3 increases as one moves down a group .
- NF3 is a colourless gas , NCl3 is an oily liquid , which may explode violently on heating , Nbr3 and NI3 are unstable. The instability of the nitrogen halides is due to enormous strength of N º N bond . NF3 is stable probably due to ionic resonance of the molecule .
- Trihalides of P, As, Sb and Bi (except BiF3 ) can be prepared by direct combination of elements . The group VA element should be in excess otherwise pantahalides are formed . The tihaledes are all hydrolysed with water . The products formed are different. On hydrolysis NCl3 gives NH3 and HOCl, but PCl3 gives H3PO3 . The hydrolysis of As Cl3 is complete but reversible . Both SbCl3 and BiCl3 are hydrolysed to the insoluble basic chlorides .
- Phosphours trihalides act as electron pair donors. All the known pentahalides can be prepared by halogenation of the trihalides. PF5 is chemically reactive gas. PX5 are readily hydrolysed. PF5 appears to be stable but Pcl5 dissociates reversible to Pcl3 and Cl2. Pbr5 undergoes dissociation in the liquid state . In the vapour state PF5, Pcl5, PCl3F2 molecules have a trigonal bipyramidal structure. Pcl5 is molecular in the has phase and ionic in the solid phase, [PCl4]+[PCl6]-. Pbr5 is also ionic in solid state but exists as [PBr4]+[Br]-. Four pentahalides of As Sb and Bi are known. These are AsF5, SbF5, BiF5 and SbCl5. AsCl5 is relatively unstable is comparision to Pcl5, probably due to incomplete shielding of the nucleus, as a result of which energy of 4s2 electrons is decreased making it more difficult to promote one of the 4s2 electrons for the formation of AsCl5.
- The hybridisation of elements in trihalides is sp3 and the shape of trihalides is pyramidal. The hybridisation of elements is pentahalides is sp3d and the shape of pentahalides is trigonal bipyramidal.
- All the elements of VA group form volatile hybrides of the type NH3. Nitrogen and phosphorus also form hydrides, N2H4 and P2H4. In addition nitrogen also forms H N (hydrazoic acid) . The trihydrides are all colourless gases. The st6ability ease of formation and basicity all decrease repidly with increase is atomic number. The much greater stabilty of NH3 greater solubility greater basically high melting and boiling points, large heat of vapourisation, high dielectric constant high polarity and ease of liquefaction can all be attributed to the much greater electronegativity of N with its lone pair of electrons ready for donation or hydrogen bonding.
- All the trihydrides are pyramidal molecules and the bond angle decreases on moving down the group or as the electronegativity of the central atom decreases. The decrease in electronegativity down the group causes the bond pair to come closer to the H-atom. Thus repulsion between bond pairs in the vicinity of the central atom decreases and hence bond angle decreases.
- The thermal stability of MH3 hydrides decreases with increase in size of M. The hydrides are good reducing agents (except for NH3 in acid solution). The reducing power increases as we go down the group. Ammonia is rather inert to oxidation.
- Basic character of hydrides decreases in the order NH3> PH3 > AsH3 > SbH3 > BiH3 .
- Stability of hydrides decreases in the order NH3 > PH3> AsH3> SbH3 > BiH3 .
- Decomposition temperature decreases in the oreder NH3 > (1573K) > PH3 (673K) > AsH3
- (553K ) > SbH3 (423K) > BiH3 (Unstable ).
- Solubility in water decreases in the oreder NH3 > PH3 > AsH3 > SbH3 > BiH3 .
- Reducing nature increase from NH3 to BiH3 .
- Bond angle decreases from NH3 (107°) to BiH3 (90°)
- Ease of formation of hydrides decreases in the order N > P > As > Sb > Bi.
- Volatility of hydrides increases in the NH3< SbH3 < AsH3 < PH3.
- Acidity of triooxides decreases in the oreder N2O3 > P4O6 > As4O6 > Sb4O6 > Bi2O3 is strongly acidic and Bi2O3 is basic .
- Strength of oxyacids decreases in the order HNO3 > H3PO4 > H3asO4 .
- Ionic nature of trihalides increases in the order NF3 < PCl3 < AsCl3 < SbCl3 < SbCl3 < BiCl3
- Poisonous nature of hydrides increases in the order NH3 < PH3 < AsH3 < SbH3 < BiH3 .
- Most poisonous compound of nitrogen is KCN . Nitrogen compound used as smelling salt is ammonium carbonate . HNO2 acts as oxidising , reducing and complexing agent . Nitrogen is the most abundant gas in the atmosphere . Nitrogen is capable of forming 3 covalent and 1 co-ordinate covalent bond . The oxide of nitrogen produced during lightening in atmosphere is nitric oxide which reacts further with oxygen to form NO2. NO is paramagnetic and contains three electron bond . The ionic character of halides increases with increase with increase in atomic number in VA group . As the oxidation number of the central atom in oxyacids of VA group elements increases , the nature of the acid changes from reduding to oxidising . Antidote of arsenic compounds is Fe (OH)3. NH3 may be manufactured by Haber process or cyanamide process . A mixuture of CaCN2+ C is called nitrolim and used as fertilizer .Superphosphate of lime is Ca (H2PO4)2 .H2O +2CaSO4. 2H2O.NH3 can be absorbed by quick lime ( CaO) . Conc. H2SO4 , forms ammonium sulphate , CaCl2 forms CaCl2 . 8NH3 and P4O10 forms ammonium phosphate . Ammonium nitrate is the explosive fertilizer of nitrogen . Au, Pt, Pd etc . are soluble in aqua regia. NOCl formed dissolves the noble metals . Formula of nitre cake is NaHSO4 . NCl3 is explosive liquid . Black phosphorus is semiconductor and good conductor of electricity . Most abundant allotrope of P is white phosphorus . Violet P is the dense variety of phosphorus ( density = 2.35). Liquid ammonia is used as refriherant because of its high heat of vapourisation . Conc . HNO3 turns skin yellow due to conversion of proteins into xantho protein . Before opening liquid NH3 bottle,it is to be cooled because it has high vapour pressure. The disease developed by the constant touch with white P is called Phosy Jaw .N2 is neutral and paramagnetic .Chemical used in smoke screens is Ca3P2. ( Calcium phosphide ).PH3 forms vortex rings when omes in contact with air due to combustion . White P is used as rat poison . When 100 ml of air is inhaled , it contains about 80 ml of N2. Nitrous oxide is soluble in alcohol . Phosphorous atom differs by phosphide ion by 3 electrons . Strike anywhere matches contain red P on the tip of the stick .