JEE Chemistry The P Block Elements Notes Set 20

General Trends in Groups 13 and 14

Group 13 (B, Al, Ga, In, Tl) has the valence configuration \( ns^2np^1 \) and group 14 (C, Si, Ge, Sn, Pb) has \( ns^2np^2 \). The first member of each group differs from the rest because it is small, highly electronegative, has a high ionisation enthalpy and has no d orbitals. Boron is a hard, high-melting non-metal (metalloid) and carbon is a non-metal; the heavier members are metals, with Si and Ge as metalloids.

Covalent radii increase down the group, but gallium is smaller than aluminium (Ga 135 pm, Al 143 pm) because ten 3d electrons shield the nucleus poorly. For the same reason the ionisation enthalpy of Ga is slightly higher than that of Al and that of Tl is higher than that of In. Electronegativity drops from B to Al and then rises slightly. In group 14, EN is C 2.5, Si 1.8, Ge 1.8, Sn 1.8 and Pb 1.9.

Oxidation states. The group oxidation state is +3 (group 13) and +4 (group 14). Because the ns pair becomes reluctant to take part in bonding in the heavier atoms (the inert pair effect), the state two units lower becomes more stable going down. Stability of +3 falls Al > Ga > In > Tl while +1 rises Ga < In < Tl. In group 14, +4 falls Ge > Sn > Pb while +2 rises Ge < Sn < Pb. So Tl(III), Pb(IV) and Sn(IV) behave as oxidising agents (for example \( \ce{PbO2} \)), and Sn(II) as a reducing agent (for example \( \ce{SnCl2} \)).

Catenation and the Anomalous Behaviour of Carbon

Catenation, the joining of atoms of one element into chains and rings, depends on the strength of the element-element bond. Bond enthalpies are C-C 348, Si-Si 297, Ge-Ge 260 and Sn-Sn 240 kJ/mol, so catenation falls in the order C >> Si > Ge approximately equal to Sn; lead does not show it. Carbon can hold only four pairs around it (maximum covalence 4) while Si and heavier atoms use d orbitals (for example \( \ce{[SiF6]^{2-}} \)). Carbon forms strong \( p\pi\text{-}p\pi \) multiple bonds (C=C, C=O, C triple bond N); silicon cannot, which explains why silicon has no graphite-like form and why \( \ce{CO2} \) is a gas but \( \ce{SiO2} \) is a network solid. Likewise boron cannot go beyond coordination number 4, so \( \ce{[BF6]^{3-}} \) does not exist while \( \ce{[AlF6]^{3-}} \) does.

Occurrence and Extraction

Boron occurs as borax \( \ce{Na2[B4O5(OH)4].8H2O} \), kernite, colemanite \( \ce{Ca2B6O11.5H2O} \) and boric acid. Aluminium is the most abundant metal and third most abundant element; its ores are bauxite \( \ce{Al2O3.xH2O} \), cryolite \( \ce{Na3AlF6} \) and aluminosilicates such as feldspar and mica. Silicon occurs as silica and silicates. Carbon is found as coal, graphite, diamond and carbonates. Tin occurs as cassiterite \( \ce{SnO2} \) and lead as galena PbS.

Boron. Impure amorphous boron: borax is treated with acid to give boric acid, which is heated to \( \ce{B2O3} \) and reduced with Mg or Na. \( \ce{Na2B4O7 + 2HCl + 5H2O -> 4H3BO3 + 2NaCl} \); \( \ce{2H3BO3 -> B2O3 + 3H2O} \); \( \ce{B2O3 + 3Mg -> 2B + 3MgO} \). Pure crystalline boron: \( \ce{2BX3 + 3H2 -> 2B + 6HX} \) on a red-hot tungsten or tantalum filament, the Van Arkel method for \( \ce{BI3} \), thermal decomposition of diborane, or \( \ce{KBF4 + 3K -> 4KF + B} \). Very high purity is hard to get because boron melts at about 2180 degrees C, forms borides, nitrides and oxides easily and corrodes its container when molten.

Aluminium is made by the Hall-Heroult process: purified alumina dissolved in molten cryolite with fluorspar (which lowers the melting point and increases conductivity) is electrolysed between a steel cathode and graphite anodes. \( \ce{2Al2O3 + 3C -> 4Al + 3CO2} \). The anodes are slowly burnt away.

Carbon and silicon. Synthetic graphite comes from heating coke with sand (\( \ce{3C + SiO2 -> SiC + 2CO} \), then SiC at 2500 degrees C gives graphite and silicon vapour); synthetic diamond comes from graphite at about 1600 degrees C and 50000-60000 atm. Silicon is made by reducing silica with coke in an electric furnace (silica in excess, otherwise SiC forms). Purer silicon follows from \( \ce{Si + 2Cl2 -> SiCl4} \) then \( \ce{SiCl4 + 2Mg -> Si + 2MgCl2} \), or \( \ce{Na2[SiF6] + 4Na -> Si + 6NaF} \), and ultra-pure silicon for semiconductors is obtained by zone refining.

Allotropes of Carbon

Diamond is sp3 hybridised with a 3-D network (C-C 154 pm, density 3.51, \( \Delta_fH = +1.9 \) kJ/mol) and is the hardest natural substance, an electrical insulator and an excellent heat conductor. Graphite is sp2 hybridised, with hexagonal layers 340 pm apart held by weak van der Waals forces (C-C 141.5 pm, density 2.22). It conducts along the layers, is soft and a solid lubricant, and is the thermodynamically most stable form. Diamond does not turn into graphite spontaneously because the activation energy is very high. Fullerene \( \ce{C60} \) is a cage of 20 six-membered and 12 five-membered rings with 30 double bonds (two bond lengths, 138 and 146 pm, with hexagons fused to hexagons or pentagons and pentagons fused only to hexagons). Amorphous forms include coke, charcoal and carbon black (from incomplete combustion of hydrocarbons).

Chemical Reactions of the Elements

With oxygen. The group 13 elements give \( \ce{E2O3} \) and group 14 give \( \ce{EO2} \). Acid-base character: \( \ce{B2O3} \) acidic; \( \ce{Al2O3} \) and \( \ce{Ga2O3} \) amphoteric; \( \ce{In2O3} \) and \( \ce{Tl2O3} \) basic. \( \ce{CO2} \), \( \ce{SiO2} \), \( \ce{GeO2} \) acidic; \( \ce{SnO2} \), \( \ce{PbO2} \), SnO and PbO amphoteric; CO is neutral. With limited oxygen carbon gives CO and silicon gives \( \ce{SiO2} \). Boron burning in air gives \( \ce{B2O3} \) along with BN. Both B and Al combine with nitrogen at high temperature (BN is very hard and AlN hydrolyses to \( \ce{Al(OH)3} \) and \( \ce{NH3} \)).

With halogens. \( \ce{2E + 3X2 -> 2EX3} \) for group 13 (BX3 and AlCl3 are covalent; AlF3 and GaF3 are ionic; thallium gives TlI3 which is really \( \ce{Tl^+ I3^-} \)). Group 14 gives \( \ce{EX4} \); the stability of EX4 falls and that of EX2 rises down the group, so GeX4 > GeX2 but PbX2 > PbX4, and \( \ce{PbI4} \) does not exist because Pb(IV) would oxidise iodide.

With water and acids. Boron is unaffected by water. Aluminium is protected by its oxide film, which is why it resists corrosion despite being reactive, and it is passive in conc. nitric acid. Red-hot carbon gives water gas (\( \ce{C + H2O -> CO + H2} \)). Pb is protected in dilute \( \ce{H2SO4} \) and conc. HCl by insoluble \( \ce{PbSO4} \) and \( \ce{PbCl2} \) coatings; in soft water lead dissolves as \( \ce{Pb(OH)2} \) (plumbosolvency). Boron reacts only with oxidising acids (hot conc. \( \ce{HNO3} \) gives \( \ce{H3BO3} \)). Graphite is oxidised by hot conc. \( \ce{HNO3} \) to mellitic acid. Silicon dissolves in a HF/HNO3 mixture as \( \ce{H2SiF6} \).

With alkalis. Boron, aluminium, gallium, silicon, tin and lead dissolve in hot alkali and release hydrogen: \( \ce{2Al + 2NaOH + 6H2O -> 2Na[Al(OH)4] + 3H2} \); \( \ce{Si + 2NaOH + H2O -> Na2SiO3 + 2H2} \). Aluminium vessels should therefore not be cleaned with washing soda. With metals, boron gives borides (\( \ce{Mg3B2} \)) and silicon gives silicides (\( \ce{Mg2Si} \)), which give boranes and silanes with acid. Boron and aluminium are strong reducing agents at high temperature (the thermite reaction \( \ce{Cr2O3 + 2Al -> Al2O3 + 2Cr} \)).

Oxides, Boric Acid and Borax

\( \ce{B2O3} \) is the acidic anhydride of boric acid. It reacts slowly with water, gives borates with bases, and forms coloured metaborates with transition-metal salts, which is the basis of the borax bead test: \( \ce{Na2B4O7.10H2O -> 2NaBO2 + B2O3} \) on heating and \( \ce{CoO + B2O3 -> Co(BO2)2} \) (blue), copper gives blue and chromium green in the oxidising flame.

Boric acid \( \ce{H3BO3} \) is made from borax with acid or from colemanite with \( \ce{SO2} \) and water. It has a planar layered structure with hydrogen bonds between \( \ce{B(OH)3} \) units. It is a weak monobasic Lewis acid: it does not donate a proton, but accepts \( \ce{OH^-} \) from water, \( \ce{B(OH)3 + 2H2O <=> [B(OH)4]^- + H3O^+} \) (pK about 9.25). Adding a cis-diol such as glycerol or mannitol (but not ethanol or a trans-diol) forms a stable chelate with \( \ce{[B(OH)4]^-} \), pulls the equilibrium forward and makes it titratable with NaOH using phenolphthalein. On heating, it gives metaboric acid \( \ce{HBO2} \) (about 100 degrees C) and then \( \ce{B2O3} \) (red heat).

Borax is \( \ce{Na2[B4O5(OH)4].8H2O} \); its anion contains two sp2 and two sp3 boron atoms. It is made by boiling colemanite with \( \ce{Na2CO3} \) (\( \ce{Ca2B6O11 + 2Na2CO3 -> 2CaCO3 + Na2B4O7 + 2NaBO2} \); passing \( \ce{CO2} \) turns the metaborate into borax). In water it hydrolyses to equal amounts of \( \ce{B(OH)3} \) and \( \ce{[B(OH)4]^-} \), so the solution is alkaline and acts as a buffer. Acids give boric acid. Borax is used in glazes, in the bead test and as a flux.

Hydrides and Halides of Boron

Boranes are electron-deficient compounds of formula \( \ce{B_nH_{n+4}} \) or \( \ce{B_nH_{n+6}} \). Diborane \( \ce{B2H6} \) is made by \( \ce{4BF3 + 3LiAlH4 -> 2B2H6 + 3LiF + 3AlF3} \) or \( \ce{2NaBH4 + I2 -> B2H6 + 2NaI + H2} \). Its structure has four terminal B-H bonds (2c-2e, H-B-H about 121 degrees) in a plane and two bridging hydrogens above and below it (3c-2e banana bonds, H-B-H about 97 degrees); boron is sp3.

Reactions: it burns spontaneously, \( \ce{B2H6 + 3O2 -> B2O3 + 3H2O} \); it is hydrolysed, \( \ce{B2H6 + 6H2O -> 2H3BO3 + 6H2} \); with alcohols it gives borate esters and hydrogen. Cleavage by Lewis bases: bulky bases such as \( \ce{N(CH3)3} \), CO and \( \ce{PF3} \) give symmetric cleavage (\( \ce{BH3.L} \)); ammonia and small amines give unsymmetric cleavage to \( \ce{[BH2(L)2]^+[BH4]^-} \). Diborane with excess ammonia at high temperature gives borazine \( \ce{B3N3H6} \) (\( \ce{3B2H6 + 6NH3 -> 2B3N3H6 + 12H2} \)), called inorganic benzene; it is polar and unlike benzene undergoes addition reactions. Heating further gives boron nitride, (BN)n, a slippery layered white solid resembling graphite.

Halides. The boron trihalides are planar, monomeric covalent molecules that are Lewis acids. BF3 and BCl3 are gases, BBr3 a liquid and BI3 a solid. Lewis acid strength increases \( \ce{BF3 < BCl3 < BBr3} \) because the 2p-2p back bonding is best in BF3. Back bonding also makes the B-F bond shorter than expected. On adduct formation boron becomes sp3, back bonding is lost and the B-F bond lengthens. BCl3, BBr3 and BI3 are fully hydrolysed to \( \ce{H3BO3} \); BF3 gives \( \ce{H3BO3} \) and \( \ce{HBF4} \). Aluminium chloride dimerises to \( \ce{Al2Cl6} \) (Al sp3, with Cl bridges) in the vapour and in non-polar solvents; in water it gives \( \ce{[Al(H2O)6]^{3+}} \), and heating the hydrate gives \( \ce{Al2O3} \) instead of anhydrous chloride. Anhydrous \( \ce{AlCl3} \) is made from alumina, carbon and chlorine. In solid AlCl3 the coordination number of Al is 6.

Aluminium Compounds and Alums

\( \ce{Al(OH)3} \) is amphoteric: it dissolves in acid and in excess alkali as \( \ce{[Al(OH)4]^-} \) (written \( \ce{[Al(H2O)2(OH)4]^-} \) in water). Ammonia precipitates \( \ce{Al(OH)3} \) without redissolving it. Alums have the formula \( \ce{M(I)2SO4.M(III)2(SO4)3.24H2O} \): potash alum (K, Al), chrome alum (K, Cr), ferric alum (K, Fe) and ammonium alum. Solutions are acidic because of hydrolysis of the hydrated trivalent ion. Alum is used as a mordant in dyeing, to purify water and to coagulate colloidal impurities.

Carbon Compounds and Oxides

CO is formed from formic acid with conc. \( \ce{H2SO4} \), and from coke and limited oxygen as producer gas (CO and \( \ce{N2} \)); water gas is CO and \( \ce{H2} \), coal gas is CO, \( \ce{H2} \), \( \ce{CH4} \) and \( \ce{CO2} \), and natural gas is mostly methane. Malonic acid with \( \ce{P4O10} \) gives carbon suboxide \( \ce{C3O2} \). Carbides: \( \ce{CaC2} \) gives ethyne, \( \ce{Al4C3} \) gives methane and \( \ce{Mg2C3} \) gives propyne. Carbon halides such as \( \ce{CCl4} \) are inert to water because carbon has no vacant d orbitals, while \( \ce{SiCl4} \) is hydrolysed at once to silicic acid. \( \ce{CCl4 + HF} \) (Swarts) gives freons and \( \ce{(C2F4)n} \) is Teflon. The tetrahalides of silicon are also hydrolysed, though \( \ce{SiF4} \) only partly (\( \ce{3SiF4 + 4H2O -> H4SiO4 + 2H2SiF6} \)).

Tin and Lead Compounds

Tin has two allotropes: white tin changes to brittle grey tin below 13.2 degrees C (tin pest). SnCl2 is a reducing agent and \( \ce{PbO2} \) is an oxidising agent. Red lead \( \ce{Pb3O4} \) is \( \ce{2PbO.PbO2} \), white lead is \( \ce{2PbCO3.Pb(OH)2} \) (heating it gives \( \ce{Pb3O4} \), CO, \( \ce{CO2} \) and water). Hydrolysis of \( \ce{SnCl4} \) is suppressed by adding HCl, which gives \( \ce{H2SnCl6} \).

Silicones, Silicates and Zeolites

Silicones are polymers with repeating \( \ce{R2SiO} \) units. Alkyl chlorosilanes are made from Si and RCl with Cu catalyst (570 K), then hydrolysed. \( \ce{R2SiCl2} \) gives the diol, which condenses to linear chains or rings; \( \ce{R3SiCl} \) has one Cl and terminates a chain and so controls its length; \( \ce{RSiCl3} \) gives a cross-linked polymer. Silicones are water-repellent (organic groups point outwards), chemically inert, heat resistant, good insulators and biocompatible, and are used as oils, greases, sealants and resins.

Silicates are built from \( \ce{SiO4^{4-}} \) tetrahedra. By the number of oxygen atoms shared per tetrahedron: orthosilicates 0 (\( \ce{SiO4^{4-}} \), phenacite, zircon); pyrosilicates 1 (\( \ce{Si2O7^{6-}} \), thortveitite); cyclic 2 (\( \ce{(SiO3)_n^{2n-}} \), beryl, benitoite, with \( \ce{Si3O9^{6-}} \) and \( \ce{Si6O18^{12-}} \)); single chains 2 (\( \ce{(SiO3)_n^{2n-}} \)); double chains and sheets 3 (\( \ce{(Si2O5)_n^{2n-}} \), talc, mica, kaolin); three-dimensional 4 (quartz, feldspar, zeolites). A chain of three tetrahedra is \( \ce{Si3O10^{8-}} \). Charge on any silicate anion = 4 x (number of Si) - 2 x (number of O).

Zeolites are aluminosilicates in which some Si are replaced by Al, leaving a negative framework balanced by \( \ce{Na^+} \) ions (permutit, \( \ce{Na2Al2Si2O8.xH2O} \), is used for softening). The \( \ce{Na^+} \) ions are exchanged for \( \ce{Ca^{2+}} \) and \( \ce{Mg^{2+}} \) of hard water (both temporary and permanent hardness are removed) and the exhausted zeolite is regenerated with brine.

Points to Remember

Inert pair effect drives the stability trends of +1/+3 and +2/+4. Boron is the only element that cannot show coordination number above 4. Boric acid is a Lewis acid, not a proton donor. Anhydrous aluminium chloride cannot be made by heating the hydrate. Of the tetrahalides of group 14 only the carbon ones resist hydrolysis. Cross-linking in silicones needs three Si-Cl bonds; chain stopping needs one.