CHM214 Inorganic Chemistry I Lecture 6 PDF

CHM214 Inorganic Chemistry I Prof. Dr. Gehad G. Mohamed Nanoscience Department BAS, EJUST Lecture 6  GROUP 14 (IVA) ELEMENTS Carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb) constitute the group 14 of th...

CHM214 Inorganic Chemistry I Prof. Dr. Gehad G. Mohamed Nanoscience Department BAS, EJUST Lecture 6  GROUP 14 (IVA) ELEMENTS Carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb) constitute the group 14 of the periodic table. This group is known as carbon family 11/24/2024 2  Element 6C 14Si 32Ge 50Sn 82Pb Carbon Silicon Germanium Tin Lead simple electron 2,8,18,32,18 2,4 2,8,4 2,8,18,4 2,8,18,18,4 config. ,3 [Ne]3s23p [Ar]3d104s24 [Kr]4d105s25 [Xe]4f145d10 electron config. [He]2s22p2 2 p2 p2 6s26p2 melting pt./oC 3547 sub 1410 937 232 328 boiling pt./oC 4827 sub 2355 2830 2270 1740 2.3(graphite) density/ gcm-3 2.3 5.3 5.8 11.4 3.5(diamond) Atomic Wt. 12.01 28.13 32.6 118.7 207.2 Oxidization - 4 CH4, +2 +4 +2, +4 +2, +4 +2, +4 State CO, +4 CO2 Covalent 0. 77 1. 17 1. 22 1. 410 1. 75 Radius/ (A) 1st IE/ kJmol-1 1086 786 762 709 716 11/24/2024Electronegativity 2. 50 1.74 2.01 1. 72 1. 8 3  Chemistry of Group 14 elements in relation to Electronic Structure: 1-The elements of group 14 have four electrons in the outermost orbitals. The general configuration may be written as ns2np2. They show a (+IV) oxidation state in their compounds; the oxidation state coinciding with the group number, the four – covalence state then arises due to sp3 hybridization of the atomic orbitals thus forming the tetrahedral coordination in the compounds. As an example, the electronic structure of C in its ground state is 1s2 2s2 2p2 so that to accommodate the normal four-covalence the atom must be promoted to a valence state 2s 2px 2py 2pz. Generally, we find, as a result, that the majority of the compounds of Gp 14 elements are four covalent 2-There is a decrease in stability of the (+IV) Oxidation State and an increase in the stability of the (+II) state on descending the group (the 11/24/2024inner pair effect). 4  1s 2s 2p Electronic Structure of carbon-ground state Two unpaired electrons thus confirm two covalent bonds Carbon atom-excited state Four unpaired electrons thus form four covalent bonds and sp3 hybridization results in a tetrahedral structure Thus Ge(+II) exists as a strong reducing agent and Ge(IV) is stable, Sn(+II) exists as simple ions but is strongly reducing and Sn(+IV) is covalent. Pb(+II) is ionic, stable and more common than Pb(+IV) which is oxidizing. The lower valencies are more ionic because the radius of M(II) is greater that M(IV) and according to Fajians’ results, the smaller the ion, the greater the tendency to form covalency. 11/24/2024 5  In a similar way, the small difference in size between Sn and Pb is because of the filling of the 4f shell. 4-The ionization energies decrease from C to Si, but then change in an irregular way because of the effects of filling the d and f shells. The extremely large amount of energy required to form M(IV) ions suggests that simple ionic compounds will be very rare. The only elements that will give a large enough electronegativity difference to give ionic character are F and O, and the compound SnF4, SnO2, PbF4 and PbO2 are significantly ionic. 3-The covalent radii increase down the group, but the difference in size between Si and Ge is less than might be otherwise expected because the filling of the 3d orbital increases the nuclear charge and provides only poorly 11/24/2024shielding d electrons. 6  5-The change from non metal to metal with increasing atomic number is well illustrated in group 14, where carbon and silicon are non metal, germanium has some metallic properties, and tin and lead are metals. The increase in metallic character shows itself in the structures and appearance of the elements, in physical properties such as malleability and electrical conductivity, and in chemical properties such as the increased tendency to form M(II) ions. And the acidic or basic properties of the oxides and hydroxides. 6-The elements in this group are relatively unreactive, but reactivity increases down the group. They are generally attacked by acids, alkalis and the halogens; graphite by F, Si by HF, Ge by 11/24/2024 H2SO4 and HNO3, and Sn and Pb by a number of acids. 7  Differences between Carbon, Silicon and the Remaining Elements: The first element in a group differs from the rest of the group because of its smaller size, higher electronegativity and the non-availability of d orbitals. Carbon differs from the other elements in: 1- Its limitations to a coordination number of four (because there are no d orbitals in the second shell) 2- Its unique ability to form multiple bonds, such as C = C, C ≡ C, C =O, C = S, and C ≡ N and its marked ability to form chains (catenation). The tendency to catenation is related to the strength of the atom-atom bond. The greater the strength of the atom-atom bond, greater is the extent of catenation. The elements of group 14 exhibit catenation. It is most extensive in the case of carbon because C-C bond is very strong. On going down the group, the atom-atom bond strength decreases and hence the extent of catenation also decreases. It varies in the order: 11/24/2024 8  C > > Si > Ge Sn > > Pb Bond Bond Energy Remarks kJ Mol-1 C-C 348 Forms many chains Si - Si 222 Forms a few chains Ge - Ge 167 Very little tendency to form chains Sn - Sn 155 11/24/2024 9  Carbon and silicon have only s and p electrons, but the other elements follow a completed transition series with ten d electrons. Thus some differences are expected, and carbon and silicon differ both from one to another and from the rest of the group, while germanium, tin and lead form a graded series. For example a-Carbon and silicon are non-metals; their compounds are mainly covalent in character and have acidic properties, while the remaining elements show metallic type behavior in their chemistry. b-Carbon and Silicon have higher melting points than the remaining elements. The extremely high M.P. for C and Si result from the very stable arrangement of the diamond type of lattice, since these strong covalent bonds must be broken on melting. The decrease in melting point on descending the group is because the M-M bonds become weaker as the atoms increase in size. The low melting points for Sn and Pb indicate that they do not use all four outer electrons for metallic bonding. 11/24/2024 10  c-Carbon and silicon are relatively unreactive, while germanium, tin and lead are readily attacked by acids and oxidize on heating. d-The tendency to catenation which is such a feature of carbon chemistry is less revealed by the other elements, the tendency decreasing in the order C>> Si > Ge  Sn  Pb. e-Carbon is in the second period and has a maximum of eight electrons in its outer shell. In four covalent compounds of carbon, the second shell contains the maximum of eight electrons. Because this structure resembles that of an inert gas, these compounds are stable and carbon does not form complexes. f-Carbon exists in two allotropic forms, diamond and graphite which differ in their physical and chemical properties because of differences in the arrangement and 13.2 oC bonding of the atoms Si,  Sn  Sn Ge and Sn also have a diamond type of structure, Grey tin White tin though Sn and Pb also existdiamond in metallic form. metallic 11/24/2024 structure 11  Chemical Reactivity. 1-Reaction with Oxygen: Oxides and Oxygen Compounds of Carbon: Five oxides of carbon are known. CO, CO2, C3O2, C5O2 and C12O9, though only the first two are important. Carbon monoxide CO Carbon monoxide is a poisonous gas, sparingly soluble in water and neutral oxide. It is formed when carbon is burned in a limited amount of air. It is prepared by dehydrating formic acid with conc. sulphuric acid HCOOH ⎯H⎯ ⎯ SO2 → CO + H2O 4 The gas burns in air and evolves a considerable amount of heat; hence carbon monoxide is an important fuel 2CO + O2 → 2 CO2 + 565 k. J mole-1 Water gas, an equimolecular mixture of CO and H2, producer gas, a mixture of CO and N2, and coal gas, a mixture of CO, H2, CH4 and 11/24/2024 CO2 are all important industrial fuels. 12  Carbon monoxide is a good reducing agent and can reduce many metal oxides to the metal Fe2O3 + 3CO ⎯Blast ⎯⎯ ⎯→ 2Fe + 3CO2 Furnace CuO + CO → Cu + CO2 CO is an important ligand which can donate a share in a lone pair of electrons, that is, it can form a coordinate bond with many transition metals and form carbonyl compounds. The metal-carbon bonding in carbonyls may be represented M  C  O Carbon monoxide is quite reactive, and combines readily with O, S and the halogens F, Cl and Br. CO + ½ O2 → CO2 CO + S → COS Carbonyl sulphide CO + Cl2 → COCl2 Carbonyl Chloride (phosgene) The carbonyl halides are readily hydrolyzed by water, and react with ammonia to form urea. COCl2 + H2O → 2HCl + CO2 11/24/2024 13  Cl H2N C O + 2 NH3 C O + 2 HCl Cl H2N CO reacts with sodium hydroxide solution (above 150 C) under pressure to give sodium formate, and with sodium methoxide forming sodium acetate CO + NaOH → HCOONa CO + CH3ONa → CH3 COONa Carbon Dioxide CO2 CO2 is obtained by the action of dilute acids on carbonates, by burning carbon in excess of air, and on an industrial scale by heating limestone CaCO3. Solid CO2 sublimes directly to the vapor state (without going through the liquid state) at – 78C under atmospheric pressure. It is widely used as a refrigerant, and is often called ‘dry ice’ or 11/24/2024‘cardice’ 14  The gas is detected by its action on lime water Ca (OH)2 or baryta water Ba(OH)2. CO2 is an acidic oxide, and is the anhydride of carbonic acid. CO2 + H2O → H2CO3 The acid is unstable and has never been isolated, but it gives rise to two series of salts, namely bicarbonates and carbonates NaHCO3 Sod. Bicarbonate (acid salt) NaOH + H2CO3 Na2 CO3 Sod. Carbonate (normal salt) The structure of CO2 is linear O = C = O  -O  C–O+  +O–C  O- The carbon atom thus forms four bonds. Biologically, CO2 is important in the process of photosynthesis, respiration and fermentation. 11/24/2024 15  Oxides of Silicon and Oxygen Containing Compounds: Two oxides of silicon, SiO and SiO2 have been reported. Silicon monoxide is thought to be obtained by high temperature reduction of SiO2 with Si, but its existence at room temperature is in doubt. SiO2 + Si → 2SiO Silicon dioxide SiO2 (Silica) is a high-melting solid, which exists in three forms: quartz, tridymite and cristobalite unlike CO2 gas, which is discrete molecule, SiO2 forms an infinite three- dimensional structure. This is because carbon can form double bonds while silicon cannot form double bonds. Forms of Silica Silica is widely distributed in nature as sand. It is found in nature in many different forms: i) Quartz or rock crystal: This is the purest form of silica. It is used in the preparation of expensive lenses and prisms. The colored variety of quartz is used as gems. 11/24/2024 16  ii) Sand: It is crushed form of quartz, which is produced in nature by weathering of rock. iii) Artificial silica: It can be obtained in laboratory by burning of silicon: or by hydrolysis of silicon tetrachloride: Silicates: Silicate is the general term given to solids with silicon-oxygen bonds. A large percentage of the earth's crust consists of silicate minerals. Some of the important silicate minerals are quartz, asbestos (calcium magnesium silicate, CaMgSi2O6), feldspar (potassium aluminium silicate, KAlSi3O8) and zeolites (sodium aluminium silicate, Na2Al2Si2O8.xH2O). Reaction of SiO2 with NaOH and other alkalis yields silicates. SiO2 + NaOH → (Na2 SiO3)n and Na4 SiO4 11/24/2024 17  The silicates are complex network solids having silicate ion (SiO4)4- as the basic structural unit. The silicate ion has a tetrahedral structure. As a result of sharing of one or more oxygen atoms between such tetrahedrons, a complex structure arises. The basic classification of silicates into chain silicates, ring silicates, cyclic silicates, sheet silicates, three dimensional silicates depends on the way in which the (SiO4)4- tetrahedral units are linked together. O O O O O Si O O O O O O Si Si Si Si Si O O -O O O O Ring anion Si3O96- Infinite chain anion (pryoxene) (SiO32-)n Silicates are the most important compounds of silicon because the glass, 11/24/2024ceramic and cement industries are based on their chemistry. 18  Silicones Silicones are polymeric compounds containing Si-O-Si linkages. These have the general formula (R2SiO)n. These may be linear cyclic or cross-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 reaction of alkyl halides with silicon in the presence of metallic copper, which acts as a catalyst. The polymers are obtained by the hydrolysis of the above chloro derivatives as: 11/24/2024 19  The hydrolysis of trichloro 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. 11/24/2024 20  Chemically silicones are inert, water repellant, heat resistant and good insulators. Uses of silicones The important uses of silicones are: 1) in making electrical insulations. 2) in making heat resistant containers. 11/24/2024 21  Glass Ordinary glass is a mixture of sodium and calcium silicates and is made by heating a mixture of sand (essentially SiO2) with sodium carbonate and calcium oxide in a furnace at around 1700K and cooling rapidly. This type of glass is called soda lime glass or soft which has an approximate composition Na2SiO3.CaSiO3.4SiO2. Addition of small amounts of transition metal compounds to the glass mixture imparts color to glasses. For e.g., Cr(III) and Mn(IV) compounds impart green and violet colors respectively. Lead-potash g1ass has a high refractive index and is used for making cut glass objects and lenses for optical purposes. Addition of boric oxide or borax to replace part of SiO2 results in the formation of heat-resistant borosilicate glass with a low coefficient of thermal expansion. This type-of glass (trade-name- Pyrex/corning/Borosil) can withstand sudden changes in temperature 11/24/2024 22 and is used for making laboratory glassware.  Glass is not a true solid and has no definite melting point. It softens when heated to a certain temperature. It is a vitreous amorphous material. Though built up from SiO4 tetrahedral units, their arrangement in glass is not regular as in crystalline silicates or in quartz. Reaction with Halogens: All the tetrahalides are known except PbI4. They are all tetrahedral, and almost all are very volatile and covalent. The exceptions are SnF4 and PbF4, which are ionic and high melting. CF4 is unreactive and very stable, and fluorocarbons are useful lubricants, solvents and insulators. ⎯⎯Halides The Carbon ⎯ ⎯→anhyodrous Are not hydrolysed under normal conditions and CCl4 is very stable in solution. CCl4 is manufactured mainly from carbon disulphide CS2. CS2 + 2Cl2 ⎯FeCl ⎯⎯ ⎯→ CCl4 + 2S Catalyst 3 It is used as a solvent, in fire extinguishers, and to produce freons. CCl4 + 2HF CCl2 F2 + 2HCl 11/24/2024 conditions, SbCl5 23  Freons are mixed chlorofluoro hydrocarbons such as CFCl3, CF2Cl2 and CF3Cl. They are unreactive and non-toxic and are widely used as refrigeration fluids and as propellant in aerosols. Carbon also forms a number of catenated halides, perhaps the best known being Teflon or polytetrofluoroethylene. This is formed when tetrafluoroethylene is subjected to pressure, and the polymers formed have chain lengths of several hundred carbon atoms. C2F4 ⎯⎯ ⎯→ -CF2. CF2. CF2. CF2 – pressure Teflon is extremely useful because it is resistant to chemical attack and is a good electrical insulator. It is also used as a coating for non-stick cooking. 11/24/2024 24