Presentation on theme: "Intermediate Type of Bonding"— Presentation transcript:
1Intermediate Type of Bonding 9Intermediate Type of Bonding9.1 Incomplete Electron Transfer in Ionic Compounds9.2 Electronegativity of Elements9.3 Polarity of Covalent Bonds
2Pure ionic and covalent bonds are only extremes of a continuum. Most chemical bonds are intermediate between the two extremes.Pure covalent Intermediate Pure ionic
3Pure covalent Intermediate Pure ionic Equal sharing of electronsSymmetrical distribution of electron cloudNon-polar moleculeComplete transfer of electronsSpherical electron cloudsElectron cloud of D is not polarized by C+
4Pure covalent Intermediate Pure ionic Incomplete transfer of electronsOrUnequal sharing of electronsPolar molecule with partial –ve charge on B and partial +ve charge on A
5Polarization of a covalent bond means the displacement of shared electron cloud towards the more electronegative atom (Cl).Polarization of a covalent bond results in a covalent bond with ionic character.
6Polarization of an ionic bond means the distortion of the electron cloud of an anion towards a cation by the influence of the electric field of the cation.Polarization of an ionic bond results in an ionic bond with covalent character.
7Pure ionic bond does not exist Li+FLiF(g)Electron clouds are not perfectly sphericalSlight distortion or sharing of electron cloud
8Polarization of ionic bond - Incomplete Transfer of Electron
11Determination of Lattice Enthalpy 1. Experimental method : -from Born-Haber cycle2. Theoretical calculation : -based on an ionic model
12r+ + r Ionic model : Assumptions 1. Ions are spherical and have no distortion of electron cloud, I.e. 100% ionic.2. Oppositely charged ions are in direct contact with each other.r+ + r
133. The crystal has certain assumed lattice structure. 4. The interaction between oppositely charged ions are electrostatic in nature.5. Repulsive forces between oppositely charged ions at short distances are ignored.
14Comparison of theoretical and experimental values of lattice enthalpy Discrepancy : -Reveals the nature of the bond in the compound
15Lattice enthalpy (kJ mol-1) 0.14-631.8-630.9KI0.87-672.3-666.5KBr0.84-697.8-692.0KCl0.38-688.3-685.7NaI0.74-733.0-730.5NaBr0.04-766.4-766.1NaCl% deviationExperimentalTheoreticalLattice enthalpy (kJ mol-1)CompoundGood agreement between the two values for alkali halides The simple ionic model used for calculating the theoretical value holds true All alkali halides are typical ionic compounds
16Lattice enthalpy (kJ mol-1) 5.5Zns12-867.0-774.0AgI8.5-877.0-808.0AgBr6.8-890.0-833.0AgCl% deviationExperimentalTheoreticalLattice enthalpy (kJ mol-1)CompoundSilver halides and zinc sulphide show large discrepancies between the two values. Silver halides and zinc sulphide are NOT purely ionic compounds
17Lattice enthalpy (kJ mol-1) 5.5Zns12-867.0-774.0AgI8.5-877.0-808.0AgBr6.8-890.0-833.0AgCl% deviationExperimentalTheoreticalLattice enthalpy (kJ mol-1)CompoundThe experimental values are always more negative than the theoretical values Polarization of a chemical bond always results in a stronger bond.
18Large % deviation of lattice enthalpy The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms.E.g. Ag+ Cl Ag–ClPurely ionicPurely covalentLarge % deviation of lattice enthalpy greater b and more covalent character
19Small % deviation of lattice enthalpy The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms.E.g. Ag+ Cl Ag–ClPurely ionicPurely covalentSmall % deviation of lattice enthalpy smaller b and less covalent character
20Factors that Favour Polarization of Ionic Bond – Fajans’ Rules For cationsPolarizing power :- The ability of a cation to polarize the electron cloud of an anion.Polarizing power as the of the cation
23For anionsPolarizability : -A measure of how easily the electron cloud of an anion can be distorted or polarized by a cation.Polarizability as the size of the anion Polarizability as the charge of the anion
24I > Br > Cl > F S2 > O2 Polarizability as the size of the anion Larger size of anion outer electrons are further away from the nucleus electrons are less firmly held by the nucleus and are more easily polarized by cationsI > Br > Cl > FS2 > O2
26Lattice enthalpy (kJ mol-1) 5.5ZnS12-867.0-774.0AgI8.5-877.0-808.0AgBr6.8-890.0-833.0AgCl% deviationExperimentalTheoreticalLattice enthalpy (kJ mol-1)CompoundGreat % deviation of ZnS due to high polarizability of the large S2 ion
27Polarizability as the charge of the anion Higher charge in the anion results in greater repulsion between electrons electrons are less firmly held by the nucleus and are more easily polarized by cations
28Lattice enthalpy (kJ mol-1) 12-867.0-774.0AgI8.5-808.0AgBr6.8-890.0-833.0AgCl0.38-688.3-685.7NaI0.74-733.0-730.5NaBr0.04-766.4-766.1NaCl% deviationExperimentalTheoreticalLattice enthalpy (kJ mol-1)CompoundIonic radius : Ag+ > Na+Why are AgX more covalent than NaX ?
29Ag+ = [Kr] 5s1 4d9 Na+ = Ne Polarizing power : Ag+ > Na+ The valence 4d electrons are less penetratingThey shield less effectively the electron cloud of the anion from the nuclear attraction of the cationThe electron cloud of the anion experiences a stronger nuclear attractionAg+ has a higher ENC than Na+Polarizing power : Ag+ > Na+
30Ag+ = [Kr] 5s1 4d9 Na+ = Ne Polarizing power : Ag+ > Na+ Noble gas configuration of the cation produces better shielding effect and less polarizing powerPolarizing power : Ag+ > Na+
31Q.51(a)Solubility in water : NaX >> AgXAgX has more covalent character due to higher extent of bond polarization.Thus, it is less soluble in water
32Q.51(b)Solubility in water : AgF > AgCl > AgBr > AgIPolarizability : F < Cl < Br < IExtent of polarization : F < Cl < Br < IIonic character : AgF > AgCl > AgBr > AgI
33Q.51(c)Solubility in water : -Gp I carbonates >> other carbonatesCarbonate ions are large and carry two negative charges. Thus, they can be easily polarized by cations to exhibit more covalent character.However, ions of group I metals have very small charge/size ratio and thus are much less polarizing than other metal ions.Gp I carbonates have less covalent character
34Example 9-1 Check Point 9-1 Q.51(d) Solubility in water : LiX << other Gp I halideLi+ is very small and thus is highly polarizing.LiX has more covalent characterExample 9-1Check Point 9-1
35Fajans’ rules – A summary Ionic Covalent Low charge on ions High charge on ionsLarge cationSmall cationSmall anionLarge anionNoble gas configurationValence shell electron configuration with incomplete d/f subshell
36Apart from those compounds mentioned on p Apart from those compounds mentioned on p.63, list THREE ionic compounds with high covalent character.AlCl3 , MgI2 , CuCO3
37Polarization of Covalent Bond : – Unequal Sharing of electrons Evidence : -Deflection of a jet of a polar liquid(e.g. H2O) in a non-uniform electrostatic fieldBreakdown of additivity rule of covalent radiiBreakdown of additivity rule of bond enthalpies
39a charged roddeflectionof waterDeflection of a polar liquid (water) under the influence of a charged rod.
40a positively charged rod a polar moleculea positively charged rodOrientation of polar molecules towards a positively charged rod.Demonstration
41Solvents showing no deflection Solvents showing a marked deflection Trichloromethane, CHCl3Ethanol,CH3CH2OHPropanoneWater, H2OTetrachloromethaneCyclohexaneBenzeneCarbon disulphide
42A stream of water is attracted (deflected) to a charged rod, regardless of the sign of the charges on the rod. Explain. +
43Additivity rule of covalent radii Assumption : Electrons are equally shared between A and BPure covalent bond
44Experimental value/nm 0.1940 0.1320 0.1430 0.1160 BondCBr in CBr4CF in CF4CO in CH3OHCO in CO2Experimental value/nm0.19400.13200.14300.1160Estimated bond length/nm0.12750.15100.14800.1910% deviation9.91%5.59%12.12%-1.54%Failure of additivity rule indicates formation ofcovalent bond with ionic character due to polarization of shared electron cloud to the more electronegative atom.
45Experimental value/nm 0.1940 0.1320 0.1430 0.1160 BondCBr in CBr4CF in CF4CO in CH3OHCO in CO2Experimental value/nm0.19400.13200.14300.1160Estimated bond length/nm0.12750.15100.14800.1910% deviation9.91%5.59%12.12%-1.54%Polarization of a covalent bond always results in the formation a stronger bond with shorter bond length.+
46Equal sharing of electrons Breakdown of additivity rule of bond enthalpyE(H – H) = 436 kJ mol1E(F – F) = 158 kJ mol1Equal sharing of electronsA.M.G.M.E(H – F) = 565 kJ mol1 >> A.M. or G.M.
47Pauling Scale of Electronegativity (1932) E(H – F) = 565 kJ mol1 >> A.M. or G.M.Greater difference Higher extent of bond polarizationGreater difference in electronegativity values of bonding atomsPauling Scale of Electronegativity (1932)
48For the molecule A–XnA and nX are the electronegativity values of A and X respectivelynF = 4.0
50Estimation of Ionic Character of Chemical Bonds Two methods : -1. The difference in electronegativity between the bonding atoms nA – nX (Qualitative)2. The electric dipole moment of diatomic molecule (Quantitative)
511. The difference in electronegativity between 1. The difference in electronegativity between the bonding atoms nA – nX (Qualitative)nA – nX 2.0 ionic or nearly ionic bonde.g. Li – F bond (4.0 – 1.0) = 3.0nA – nX 0.4 covalent or nearly covalent bonde.g. C – H bond (2.5 – 2.1) = 0.40.4 nA – nX 2.0covalent bond with ionic character orionic bond with covalent character
521 Debye (D) = 3.3361030 Coulomb meter 2. The electric dipole moment of diatomic molecule (Quantitative) = q dSI units : - Coulomb meter1 Debye (D) = 3.3361030 Coulomb meter
53Centre of postive charge Electric dipole moment is a vector pointing from the positive pole to the negative pole
54Dipole moment (Coulomb meter) Estimating the % ionic character of H–Cl bond by dipole momentMoleculeDipole moment (Coulomb meter)Bond length meterH–Cl3.68910301.2841010Electronic charge, e 1.6021019 Coulomb
55If H–Cl is 100% ionic,dipole moment1.6021019 Coulomb1.2841010 meter 2.0571029 CmThe measured dipole moment of H–Cl 3.6891030 Cm
56Q.53Electronic charge, e 1.6021019 CoulombMoleculeNOHIClFHFCsFBond length(Å)1.1541.6201.6320.9262.347Dipole moment(D)0.1590.4480.8881.8277.884% ionic character70.041.111.314.82.87
57Q.53Electronic charge, e 1.6021019 CoulombMoleculeNaClKFKClLiFBond length(Å)2.3652.1762.6711.570Dipole moment(D)9.0018.59310.2696.327% ionic character83.980.182.279.3
58Good correlation between two methods Calculated from dipole momentGood correlation between two methodsnA – nX
59Ionic with covalent character How do you expect the bond type to change for the chlorides of the third period elements, NaCl, MgCl2, AlCl3, SiCl4, PCl5, SCl2 and Cl2, going from left to right?Explain the change in the bond type.NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2Ionic with covalent characterPolar covalentPurely IonicPurely covalent
60Ionic with covalent character difference in electronegativity values difference in electronegativity valuesNaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2Ionic with covalent characterPolar covalentPurely IonicPurely covalent
61Ionic with covalent character extent of polarization of ionic bond extent of polarization of covalent bondNaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2Ionic with covalent characterPolar covalentPurely IonicPurely covalent
62Polarity of Moleculesdepends on : -Polarity of bonds nA – nX or dipole momentGeometry of moleculesSymmetrical molecules are usually non-polardue to symmetrical arrangements of dipole moments
63Bond polarityGeometry of moleculePolarity of moleculePolarAsymmetricalPolarPolarSymmetricalNon-polarNon-polarAsymmetricalNon-polarNon-polarSymmetricalNon-polar
64The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.Net dipole moment (the vector sum) is zero Non-polar
65The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.
66The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.Net dipole moment (the vector sum) is zero Non-polar
67The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.Net dipole moment (the vector sum) is zero Non-polar
68The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.
77Explain the following phenomena: PCl3 is polar but BCl3 is non-polar.BCl3 has three polar B−Cl bonds and is trigonal planar in shape. As the dipole moments of the three polar bonds cancel out each other, the molecule is non-polar.
78Explain the following phenomena: PCl3 is polar but BCl3 is non-polar.PCl3 has three polar P−Cl bonds and is trigonal pyramidal in shape. As there is a resultant dipole moment arising from the three polar bonds, the molecule is polar.
79Explain the following phenomena: (b) Both NBr3 and NF3 are polar but their molecules align differently in a non-uniform electrostatic field.
80(b) As the order of electronegativity is F > N > Br, the resultant dipole moments of NBr3 and NF3 are pointing to different directions. The situations are shown below:
81In a non-uniform electrostatic field, the nitrogen end of NBr3 will point to the positive pole while the nitrogen end of NF3 will point to the negative pole.
82Cancelling out of dipole moments Non-polar moleculesTetrahedralTrigonal planarLinearCancelling out of dipole momentsMoleculeShape
83Cancelling out of dipole moments Non-polar moleculesOctahedralTrigonal bipyramidalCancelling out of dipole momentsMoleculeShape
84Polar molecules Tetrahedral Trigonal pyramidal V-shaped ( or bent) Net resultant dipole momentDipole moment of individual polar bondsMoleculeShape
85Use of dipole momentsProvide important structural information about molecules
869.1 Incomplete electron transfer in ionic compounds (SB p.250) Example 9-1The following gives the theoretical and experimental values of the lattice enthalpies of two metal bromides. X+Br- and Y+Br-.(a) There is a high degree of agreement between the theoretical and experimental values in the case of X+Br-(s) but a large discrepancy in the case of Y+Br-(s). What can you tell about the bond type of the two compounds?CompoundTheoretical lattice enthalpy (kJ mol-1)Experimental lattice enthalpy (kJ mol-1)X+Br-(s)-665-670Y+Br-(s)-758-890Answer
879.1 Incomplete electron transfer in ionic compounds (SB p.250) Example 9-1Since the theoretical value of the lattice enthalpy is calculated based on a simple ionic model, the good agreement for X+Br-(s) suggests that the compound is nearly purely ionic. The ions are nearly spherical with nearly uniform distribution of charges. The bond type in the compound is thus nearly purely ionic.For Y+Br-(s), the large discrepancy suggests that the simple ionic model does not hold due to the distortion of the electron cloud of the anion. Thus the bond type in this compound has a certain degree of covalent character.
889.1 Incomplete electron transfer in ionic compounds (SB p.250) Example 9-1To which group in the Periodic Table does metal X belong? Explain your answer.Answer(b) As X+ ion must have a low polarizing power, its charge to size ratio should be small. X is a Group I metal.Back
899.3 Polarity of covalent bonds (SB p.252) Let's Think 1Pure ionic bond and pure covalent bond are two extreme bond types. Why?AnswerIn pure ionic bonding, the bonded atoms are so different that one or more electrons are transferred to form oppositely charged ions. Two identical atoms share electrons equally in pure covalent bonding. This type of bonding results from the mutual attraction of the two nuclei for the shared electrons. Between these extremes are intermediate cases in which the atoms are not so different that electrons are incompletely transferred and unequal sharing results, forming polar covalent bond.Back
90Ionic with covalent character 9.3 Polarity of covalent bonds (SB p.252)BackCheck Point 9-3AHow do you expect the bond type to change for the chlorides of the third period elements, NaCl, MgCl2, AlCl3, SiCl4, PCl5, SCl2 and Cl2, going from left to right?Explain the change in the bond type.NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2Purely IonicIonic with covalent characterPolar covalentPurely covalent
919.3 Polarity of covalent bonds (SB p.257) Example 9-3Explain the variation in dipole moment of the following molecules.MoleculeDipole moment (D)CH4NH30.35H2O0.65HF1.07Answer
92Example 9-3 9.3 Polarity of covalent bonds (SB p.257) The dipole moment of a molecule is based on two factors:Bond polarityThis depends on the electronegativity of the atoms involved in a bond. A bond is said to be polar if there is a difference in electronegativity between two bonded atoms. The larger the difference, the more polar is the bond.H C N O F
93Example 9-3 9.3 Polarity of covalent bonds (SB p.257) The geometry If the molecule have symmetrical arrangements of polar bonds, the dipole moments of the bonds will cancel out each other.CH4 NH3No net dipole moment Net dipole moment resulted
94Example 9-3 Back 9.3 Polarity of covalent bonds (SB p.257) H2O HF Net dipole moment resulted Net dipole moment resulted(Note: Lone pair(s) is/are not shown in the above diagrams)Hence, zero dipole moment is only observed in CH4. HF has the largest dipole moment since the difference in electronegativity between the hydrogen atom and the fluorine atom is the largest. H2O comes the second, followed by NH3.
959.3 Polarity of covalent bonds (SB p.257) Check Point 9-3BGive the shapes and structural formulae of the following molecules. State whether each molecule is polar or non-polar.(a) BCl3(b) NH3(c) CHCl3Answer
96Check Point 9-3B Back 9.3 Polarity of covalent bonds (SB p.257) Polar Tetrahedral(c) CHCl3Trigonal pyramidal(b) NH3Non-polarTrigonal planar(a) BCl3Polar or non-polarStructural formulaShapeMolecule