ch08---covalent-bonding.ppt

New Way Chemistry for Hong Kong A-Level Book 11Chapter 8Covalent Bonding8.1 Formation of Covalent Bonds8.2 Dative Covalent Bonds8.3 Bond Enthalpies8.4 Estimation of Average Bond Enthalpies using Data from EnergeticsNew Way Chemistry for Hong Kong A-Level Book 128.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of Reactions8.6 Relationship between Bond Enthalpies and Bond Lengths8.7 Shapes of Covalent Molecules and Polyatomic Ions8.8 Multiple Bonds8.9 Covalent CrystalsNew Way Chemistry for Hong Kong A-Level Book 13 Electron Sharing in Covalent BondsH HShared electronsThe shared electron pair spends most of the time between the two nuclei.e-e-Attraction between oppositely charged nuclei and shared electrons(_ in nature)electrostaticOverlapping of atomic orbitals covalent bond formation8.1 Formation of Covalent Bonds(SB p.203)New Way Chemistry for Hong Kong A-Level Book 148.1 Formation of Covalent Bonds(SB p.203)A hydrogen molecule is achieved by partial overlapping of 1s orbitalsNew Way Chemistry for Hong Kong A-Level Book 15Thus electrons are shared between the two atoms.8.1 Formation of Covalent Bonds(SB p.204)Electron density map for covalent compoundsThere is substantial electron density at all points along the internuclear axis.New Way Chemistry for Hong Kong A-Level Book 168.1 Formation of Covalent Bonds(SB p.204)Electron density map for ionic compoundsNew Way Chemistry for Hong Kong A-Level Book 17 Covalent Bonds in ElementsSome ExamplesDot and cross diagram8.1 Formation of Covalent Bonds(SB p.205)New Way Chemistry for Hong Kong A-Level Book 18Covalent Bonds in CompoundsSome ExamplesCarbon1s2s2pAll the above examples obey _.Octet rule8.1 Formation of Covalent Bonds(SB p.205)New Way Chemistry for Hong Kong A-Level Book 19Some ExamplesAll the above examples obey _.Carbon1s2s2pelectrons from HOctet ruleoctet8.1 Formation of Covalent Bonds(SB p.205)Covalent Bonds in CompoundsNew Way Chemistry for Hong Kong A-Level Book 110Octet Rule and its limitationsBF3Why doesnt B form ionic compounds with F?B:small atomic sizehigh I.E.s required to become a cation.electrons from F8.1 Formation of Covalent Bonds(SB p.206)not fullfilling octect(electron deficient)New Way Chemistry for Hong Kong A-Level Book 111Octet Rule and its limitationsPCl5Why Phosphorus can expand its octet to form PCl5?There is low-lying vacant d-orbital in P.electrons from Cl8.1 Formation of Covalent Bonds(SB p.207)New Way Chemistry for Hong Kong A-Level Book 112NH3BF3 Molecule8.2 Dative Covalent Bonds(SB p.208)New Way Chemistry for Hong Kong A-Level Book 113Dative Covalent BondsThe dative covalent bond(also known as the coordinate bond)is a type of covalent bond in which the shared pair of electrons is supplied by only one of the bonded atoms.Remarks(1)The atom that supplies the shared pair of electrons is known as the donor while the other atom involved in the dative covalent bond is known as the acceptor.(2)Once formed,a dative covalent bond cannot be distinguished from a normal covalent bond.8.2 Dative Covalent Bonds(SB p.208)New Way Chemistry for Hong Kong A-Level Book 114Ammonium Ion(NH4+)8.2 Dative Covalent Bonds(SB p.209)New Way Chemistry for Hong Kong A-Level Book 115Aluminium Chloride Dimer(Al2Cl6)AlCl3Why doesnt Al form ionic compounds with Cl?Al:relative small atomic size;high I.E.s required to become a cation of+3 charge.(a dimer of AlCl3)8.2 Dative Covalent Bonds(SB p.209)New Way Chemistry for Hong Kong A-Level Book 116Bond EnthalpiesWhy do successive B.D.E.of C-H differ?(Average)bond enthalpy;E(C-H)=+415.5 kJ mol-18.3 Bond Enthalpies(SB p.210)CH4(g)CH3(g)+H(g)H =+422 kJ mol-1CH3(g)CH2(g)+H(g)H =+480 kJ mol-1CH2(g)CH(g)+H(g)H =+425 kJ mol-1CH(g)C(g)+H(g)H =+335 kJ mol-1Bond Dissociation EnthalpiesB.D.E of a certain bond is the amount of energy required to break one mole of that bond under standard conditions.e.g.H-H(g)2H(g)H(H-H)=+431 kJ mol-1New Way Chemistry for Hong Kong A-Level Book 117Bond EnthalpiesWhy is this value of E(C-H)still different from the previously calculated one(+415.5 kJ mol-1)?8.3 Bond Enthalpies(SB p.211)BondAverage bond enthalpy(kJ mol-1)H-HC-CC CC CC-HO-H+436+348+612+837+412+463New Way Chemistry for Hong Kong A-Level Book 118Bond EnthalpiesRemarksBond enthalpies refer to the energy required to break bonds(+ve)For the formation of a certain bond,the enthalpy change is represented by“-bond enthalpy”(-ve)The bond enthalpy is a measure of bond strength.How?8.3 Bond Enthalpies(SB p.211)New Way Chemistry for Hong Kong A-Level Book 119From the Enthalpy Change of Atomization of a CompoundThe enthalpy change of atomization of methane(CH4)is+1662 kJ mol-1.Find a value for the bond enthalpy of C-H based on the above data.E(C-H)=+415.5 kJ mol-18.4 Estimation of Average Bond Enthalpies using Data from Energetics(SB p.212)The atomization of methane involves the breaking of a four C-H bonds.Assume that all four C-H bonds are equal in strength.The average bond enthalpy of C-H bonds=x(+1 662)kJ mol-1=+415.5 kJ mol-1 C(g)+4H(g)H=+1 662 kJ mol-1 New Way Chemistry for Hong Kong A-Level Book 120The standard enthalpy change of atomization of a compound is the enthalpy change when one mole of gaseous compound is broken down into its constituent atoms in the gaseous state under standard conditions,e.g.CH4(g)C(g)+4H(g)1 moleThe standard enthalpy change of atomization of a element is the enthalpy change when one mole of gaseous atoms is formed into its constituent atoms in the gaseous state under standard conditions,e.g.Cl2(g)Cl(g)1 mole8.4 Estimation of Average Bond Enthalpies using Data from Energetics(SB p.212)New Way Chemistry for Hong Kong A-Level Book 121The enthalpy change of atomization of butane(C4H10)and pentane(C5H12)are+5165 kJ mol-1 and+6337 kJ mol-1 respectively.Find a values for the bond enthalpies of C-H and C-C based on the above data.8.4 Estimation of Average Bond Enthalpies using Data from Energetics(SB p.214)From the Enthalpy Changes of Atomization of Two CompoundsFor butane,3 E(C-C)+10 E(C-H)=+5 165 kJ mol-1.(1)For pentane,4 E(C-C)+12 E(C-H)=+6 337 kJ mol-1.(2)Solving simultaneous equations(1)and(2),we obtain the following bond enthalpy values.E(C-H)=+412.25 kJ mol-1 E(C-C)=+347.5 kJ mol-1 New Way Chemistry for Hong Kong A-Level Book 1228.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of Reactions(SB p.214)Reaction of ethene and hydrogenSum of bond enthalpies of productsEnthalpy change of reaction=Sum of bond enthalpies of reactants-New Way Chemistry for Hong Kong A-Level Book 1238.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of Reactions(SB p.215)Enthalpy profile for the reaction of ethene and hydrogenNew Way Chemistry for Hong Kong A-Level Book 124?Sum of bond enthalpies of reactants=E(C=C)+4E(C-H)+E(H-H)=(612)+4(412)+(436)=+2696 kJmol-1Sum of bond enthalpies of products=E(C-C)+6E(C-H)=(348)+6(412)=+2820 kJmol-1Hr=2696-(+2820)=-124 kJ mol-1Only an estimated value for Hr.Why?8.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of Reactions(SB p.214)Reaction of ethene and hydrogenNew Way Chemistry for Hong Kong A-Level Book 125Consider C-C,C=C and CCbond order =1bond order =2bond order =3As the bond order increases,the bond strength also increases.8.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.217)BondAverage bond enthalpy(kJ mol-1)H-HC-CC CC CC-HO-H+436+348+612+837+412+463Bond Enthalpies as an Indication of the Strength of Covalent BondsNew Way Chemistry for Hong Kong A-Level Book 126Bond length(for covalent bond)8.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.217)Bond LengthsNew Way Chemistry for Hong Kong A-Level Book 1278.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.217)Bond lengths of some covalent bondsBondMoleculeBond lengths(nm)O-HH2OH2O2CH3OHHCOOH0.0960.0970.0960.096C-CDiamondC2H6C3H8C2H5OH0.1540.1540.1540.155C-HCH4C2H40.1100.109By what technique can these bond lengths be measured?Bond lengths of some covalent bondsX-ray diffractionNew Way Chemistry for Hong Kong A-Level Book 128Any conclusion for the relationship between bond length&bond enthalpy?Usually a longer bond length corresponds to a lower value of bond enthalpy(weaker bond).8.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.217)BondBond length(nm)Bond enthalpy(kJ mol-1)H-HCl-ClBr-BrI-IH-FH-ClH-BrH-I0.0740.1990.2280.2660.0920.1270.1410.161436242193151565431364299Bond enthalpies and bond lengthsNew Way Chemistry for Hong Kong A-Level Book 129Explain why the bond enthalpy of F-F is smaller than that of Cl-Cl even though the bond length of F-F is the shortest among the halogens.As the size of fluorine atom is very small,the repulsion between the non-bonding pairs of electrons on the fluorine atoms weaken the F-F bond.Bond Bond Length/nm Bond Enthalpy/kJ mol-1Cl-Cl 0.199 242Br-Br 0.228 193 I-I 0.266 151FFNon-bonding e-/lone pair of e-F-F 0.142 1588.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.217)Special Situation for F2New Way Chemistry for Hong Kong A-Level Book 130(often referred as Atomic radius)Where b is the bond length r is the covalent radiusThe space occupied by an atom in a covalently bonded molecule in the direction of the covalent bond(generally taken as half of the bond length of homoatomic molecules)8.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.218)Covalent RadiusNew Way Chemistry for Hong Kong A-Level Book 1318.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.218)The covalent radii(in nm)of some elementsNew Way Chemistry for Hong Kong A-Level Book 132Predicting bond length of A-B if rA&rB are known8.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.218)Bond length of a covalent bond A-B=Covalent radius of atom A+Covalent radius of atom BBond lengths of atom A and atom BNew Way Chemistry for Hong Kong A-Level Book 1338.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.219)BondCalculated bond length(nm)Experimentally determined bond length(nm)C-OC-FC-ClC-BrC-CH-ClC-HN-Cl0.1500.1490.1760.1910.1540.1360.1140.1730.1430.1380.1770.1930.1540.1280.1090.174Calculated and experimentally determined bond lengthBy what technique can the bond lengths be determined experimentally?SimilarelectronegativityNew Way Chemistry for Hong Kong A-Level Book 134BondCalculated bond length(nm)Experimentally determined bond length(nm)C-OC-FC-ClC-BrC-CH-ClC-HN-Cl0.1500.1490.1760.1910.1540.1360.1140.1730.1430.1380.1770.1930.1540.1280.1090.1748.6 Relationship between Bond Enthalpies and Bond Lengths(SB p.219)Calculated and experimentally determined bond lengthQuite differentelectronegativityNew Way Chemistry for Hong Kong A-Level Book 135Shape of a molecule=geometric arrangement of atoms within the moleculesValence Shell Electron Pair Repulsion Theory(VSEPR)The electron pairs in the outermost shell of the central atom in a molecule will stay as far apart as possible to minimize electronic repulsion.The electron pairs are oriented at a maximum separation in space so as to minimize the coulombic repulsion of electron clouds.8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.220)Shapes of Molecules and Polyatomic IonsNew Way Chemistry for Hong Kong A-Level Book 136Beryllium chloride molecule(BeCl2)BeClClElectronic DiagramShape in wordlinear Bond angle=angle between 2 bonds8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.220)Shape in DiagramNew Way Chemistry for Hong Kong A-Level Book 137Boron trifluoride molecule(BF3)Electronic DiagramShape in DiagramShape in wordtrigonal planarBFFF8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.220)New Way Chemistry for Hong Kong A-Level Book 138Methane(CH4)MoleculeElectronic DiagramShape in DiagramShape in wordtetrahedralCHHHH8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.221)New Way Chemistry for Hong Kong A-Level Book 139Electronic DiagramShape in DiagramShape in wordtetrahedralCHHHH8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.221)Methane(CH4)MoleculeNew Way Chemistry for Hong Kong A-Level Book 140Phosphorus Pentachloride(PCl5)MoleculeElectronic DiagramShape in DiagramShape in wordtrigonal bipyramidal8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.221)New Way Chemistry for Hong Kong A-Level Book 141Electronic DiagramShape in DiagramShape in wordtrigonal bipyramidal8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.221)Phosphorus Pentachloride(PCl5)MoleculeNew Way Chemistry for Hong Kong A-Level Book 142Sulphur Hexafluoride(SF6)Electronic DiagramShape in DiagramShape in wordoctahedralSFFFFFF8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.221)New Way Chemistry for Hong Kong A-Level Book 143Ammonia(NH3)MoleculeElectronic DiagramShape in DiagramShape in wordTrigonal pyramidalHHHNlp-lp repulsion lp-bp repulsion bp-bp repulsion8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.223)New Way Chemistry for Hong Kong A-Level Book 144Electronic DiagramShape in DiagramShape in wordTrigonal pyramidalHHHNlone pair of e-8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.223)Ammonia(NH3)MoleculeNew Way Chemistry for Hong Kong A-Level Book 145Water(H2O)MoleculeElectronic DiagramShape in DiagramShape in wordV-shaped/bentHHOlp-lp repulsion lp-bp repulsion bp-bp repulsion8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.223)New Way Chemistry for Hong Kong A-Level Book 146Electronic DiagramShape in DiagramShape in wordV-shaped/bentHHlone pair of e-OWater(H2O)Molecule8.7 Shapes of Covalent Molecules and Polyatomic Ions(SB p.224)New Way Chemistry for Hong Kong A-Level Book 1478.8 Multiple Bonds(SB p.227)Comparison of bond lengths and bond enthalpies between single and multiple bondsBondBond orderBond length(nm)Bond enthalpy(kJ mol-1)C-CC CC C1230.1540.1340.120+348+612+837N-NN NN N1230.1460.1200.110+163+409+944C-OC O120.1430.122+360+743New Way Chemistry for Hong Kong A-Level Book 148Ethene(CH2=CH2)Electronic DiagramShape in DiagramShape in word(planar)CCHHHH8.8 Multiple Bonds(SB p.227)Effect of Multiple Bonding on Shapes of MoleculesNew Way Chemistry for Hong Kong A-Level Book 149Ethyne(CHCH)Shape in DiagramShape in wordlinear8.8 Multiple Bonds(SB p.227)New Way Chemistry for Hong Kong A-Level Book 150Carbon dioxide(CO2)Shape in DiagramShape in wordlinear8.8 Multiple Bonds(SB p.227)New Way Chemistry for Hong Kong A-Level Book 151Shape in DiagramShape in wordAngular/(bent)Sulphur dioxide(SO2)8.8 Multiple Bonds(SB p.227)New Way Chemistry for Hong Kong A-Level Book 152Structures consist of millions of atoms covalently bonded together in a structural network and there is no simple molecule present.Examplesdiamond,graphite and quartz/silicon(IV)oxide8.9 Covalent Crystals(SB p.228)Giant Covalent StructuresNew Way Chemistry for Hong Kong A-Level Book 153DiamondEach C atom is covalently bonded to 4 other C atoms to form a three-dimensional networkThe C-C bonding pattern accounts for the high m.p.,stability and extreme hardness Applications:scratch proof cookware,watch crystals,ball bearings and