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1 Intermediate Type of Bonding 9.1Incomplete Electron Transfer in Ionic Compounds 9.2Electronegativity of Elements 9.3Polarity of Covalent Bonds 9.

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Presentation on theme: "1 Intermediate Type of Bonding 9.1Incomplete Electron Transfer in Ionic Compounds 9.2Electronegativity of Elements 9.3Polarity of Covalent Bonds 9."— Presentation transcript:

1 1 Intermediate Type of Bonding 9.1Incomplete Electron Transfer in Ionic Compounds 9.2Electronegativity of Elements 9.3Polarity of Covalent Bonds 9

2 2 Pure ionic and covalent bonds are only extremes of a continuum. Most chemical bonds are intermediate between the two extremes. Pure covalent Intermediate Pure ionic

3 3 Equal sharing of electrons Symmetrical distribution of electron cloud Non-polar molecule Complete transfer of electrons Spherical electron clouds Electron cloud of D  is not polarized by C +

4 4 Pure covalent Intermediate Pure ionic Incomplete transfer of electrons Or Unequal sharing of electrons Polar molecule with partial –ve charge on B and partial +ve charge on A

5 5 Polarization 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.

6 6 Polarization 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.

7 7 Pure ionic bond does not exist Li + FF LiF(g) Electron clouds are not perfectly spherical Slight distortion or sharing of electron cloud

8 8 Polarization of ionic bond - Incomplete Transfer of Electron

9 9 Determination of Lattice Enthalpy 1. Experimental method : - from Born-Haber cycle

10 10 -349 -791.4

11 11 Determination of Lattice Enthalpy 1. Experimental method : - from Born-Haber cycle 2.Theoretical calculation : - based on an ionic model

12 12 1. Ions are spherical and have no distortion of electron cloud, I.e. 100% ionic. Ionic model : Assumptions 2. Oppositely charged ions are in direct contact with each other. r + + r 

13 13 3.The crystal has certain assumed lattice structure. 5.Repulsive forces between oppositely charged ions at short distances are ignored. 4.The interaction between oppositely charged ions are electrostatic in nature.

14 14 Comparison of theoretical and experimental values of lattice enthalpy Discrepancy : - Reveals the nature of the bond in the compound

15 15 0.14-631.8-630.9KI 0.87-672.3-666.5KBr 0.84-697.8-692.0KCl 0.38-688.3-685.7NaI 0.74-733.0-730.5NaBr 0.04-766.4-766.1NaCl % deviationExperimentalTheoretical Lattice enthalpy (kJ mol -1 ) Compound Good 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

16 16 5.5-3615.0-3427.0Zns 12-867.0-774.0AgI 8.5-877.0-808.0AgBr 6.8-890.0-833.0AgCl % deviationExperimentalTheoretical Lattice enthalpy (kJ mol -1 ) Compound Silver halides and zinc sulphide show large discrepancies between the two values.  Silver halides and zinc sulphide are NOT purely ionic compounds

17 17 5.5-3615.0-3427.0Zns 12-867.0-774.0AgI 8.5-877.0-808.0AgBr 6.8-890.0-833.0AgCl % deviationExperimentalTheoretical Lattice enthalpy (kJ mol -1 ) Compound The experimental values are always more negative than the theoretical values  Polarization of a chemical bond always results in a stronger bond.

18 18 The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms. E.g.Ag + Cl   Ag–Cl Large % deviation of lattice enthalpy  greater b and more covalent character Purely ionic Purely covalent

19 19 The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms. E.g.Ag + Cl   Ag–Cl Small % deviation of lattice enthalpy  smaller b and less covalent character Purely ionic Purely covalent

20 20 Factors that Favour Polarization of Ionic Bond – Fajans’ Rules For cations Polarizing power : - The ability of a cation to polarize the electron cloud of an anion. Polarizing power  as the of the cation 

21 21 Q.50(a) Charge : Al 3+ > Mg 2+ > Na + Size : Al 3+ < Mg 2+ < Na + Al 3+ > Mg 2+ > Na + Polarizing power :Al 3+ > Mg 2+ > Na +

22 22 Q.50(b) Charge : Li + = Na + = K + Size : Li + < Na + < K + Li + > Na + > K + Polarizing power :Li + > Na + > K +

23 23 For anions Polarizability : - 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 

24 24 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 cations I  > Br  > Cl  > F  S 2  > O 2  Polarizability  as the size of the anion 

25 25 5.5-3615.0-3427.0ZnS 12-867.0-774.0AgI 8.5-877.0-808.0AgBr 6.8-890.0-833.0AgCl % deviationExperimentalTheoretical Lattice enthalpy (kJ mol -1 ) Compound Polarizability :I  > Br  > Cl  % deviation : AgI > AgBr > AgCl Covalent character : AgI > AgBr > AgCl

26 26 5.5-3615.0-3427.0ZnS 12-867.0-774.0AgI 8.5-877.0-808.0AgBr 6.8-890.0-833.0AgCl % deviationExperimentalTheoretical Lattice enthalpy (kJ mol -1 ) Compound Great % deviation of ZnS due to high polarizability of the large S 2  ion

27 27 Polarizability  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

28 28 12-867.0-774.0AgI 8.5-867.0-808.0AgBr 6.8-890.0-833.0AgCl 0.38-688.3-685.7NaI 0.74-733.0-730.5NaBr 0.04-766.4-766.1NaCl % deviationExperimentalTheoretical Lattice enthalpy (kJ mol -1 ) Compound Ionic radius : Ag + > Na + Why are AgX more covalent than NaX ?

29 29 Polarizing power : Ag + > Na + Ag + = [Kr] 5s 1 4d 9 Na + = Ne The valence 4d electrons are less penetrating  They shield less effectively the electron cloud of the anion from the nuclear attraction of the cation  The electron cloud of the anion experiences a stronger nuclear attraction Ag + has a higher ENC than Na +

30 30 Polarizing power : Ag + > Na + Ag + = [Kr] 5s 1 4d 9 Na + = Ne Noble gas configuration of the cation produces better shielding effect and less polarizing power

31 31 Q.51(a) Solubility in water : NaX >> AgX AgX has more covalent character due to higher extent of bond polarization. Thus, it is less soluble in water

32 32 Q.51(b) Solubility in water : AgF > AgCl > AgBr > AgI Polarizability : F  < Cl  < Br  < I  Extent of polarization : F  < Cl  < Br  < I  Ionic character : AgF > AgCl > AgBr > AgI

33 33 Q.51(c) Solubility in water : - Gp I carbonates >> other carbonates 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 Carbonate ions are large and carry two negative charges. Thus, they can be easily polarized by cations to exhibit more covalent character.

34 34 Q.51(d) Solubility in water : LiX << other Gp I halide Li + is very small and thus is highly polarizing. LiX has more covalent character Example 9-1 Example 9-1 Check Point 9-1 Check Point 9-1

35 35 Fajans’ rules – A summary IonicCovalent Low charge on ionsHigh charge on ions Large cationSmall cation Small anionLarge anion Noble gas configuration Valence shell electron configuration with incomplete d/f subshell

36 36 Apart from those compounds mentioned on p.63, list THREE ionic compounds with high covalent character. AlCl 3, MgI 2, CuCO 3

37 37 Polarization of Covalent Bond : – Unequal Sharing of electrons Evidence : - 1.Deflection of a jet of a polar liquid(e.g. H 2 O) in a non-uniform electrostatic field 2.Breakdown of additivity rule of covalent radii 3.Breakdown of additivity rule of bond enthalpies

38 38 Liquid shows deflection Contains polar molecules Liquid shows no deflection Contains non-polar molecules

39 39 a charged rod deflection of water Deflection of a polar liquid (water) under the influence of a charged rod.

40 40 a polar molecule a positively charged rod Orientation of polar molecules towards a positively charged rod. Demonstration

41 41 Tetrachloromethane Cyclohexane Benzene Carbon disulphide Trichloromethane, CHCl 3 Ethanol,CH 3 CH 2 OH Propanone Water, H 2 O Solvents showing no deflection Solvents showing a marked deflection

42 42 A stream of water is attracted (deflected) to a charged rod, regardless of the sign of the charges on the rod. Explain.              ++ ++ 

43 43 Additivity rule of covalent radii Assumption : Electrons are equally shared between A and B Pure covalent bond

44 44 9.91%5.59%12.12%-1.54% deviation 0.12750.15100.14800.1910 Estimated bond length/nm 0.11600.14300.13200.1940 Experimental value/nm C  O in CO 2 C  O in CH 3 OH C  F in CF 4 C  Br in CBr 4 Bond Failure of additivity rule indicates formation of covalent bond with ionic character due to polarization of shared electron cloud to the more electronegative atom.

45 45 9.91%5.59%12.12%-1.54% deviation 0.12750.15100.14800.1910 Estimated bond length/nm 0.11600.14300.13200.1940 Experimental value/nm C  O in CO 2 C  O in CH 3 OH C  F in CF 4 C  Br in CBr 4 Bond Polarization of a covalent bond always results in the formation a stronger bond with shorter bond length. ++ 

46 46 Breakdown of additivity rule of bond enthalpy E(H – H) = 436 kJ mol  1 E(F – F) = 158 kJ mol  1 E(H – F) = 565 kJ mol  1 >> A.M. or G.M. A.M. G.M. Equal sharing of electrons

47 47 E(H – F) = 565 kJ mol  1 >> A.M. or G.M. Greater difference  Higher extent of bond polarization  Greater difference in electronegativity values of bonding atoms Pauling Scale of Electronegativity (1932)

48 48 For the molecule A–X n A and n X are the electronegativity values of A and X respectively n F = 4.0

49 49 Q.52 n H = 2.2 n Cl = 3.3 More electronegative Given : E(H–H)  436 kJ mol  1, E(F–F)  158 kJ mol  1, E(H–F)  565 kJ mol  1, E(Cl–Cl)  242 kJ mol  1, E(H–Cl)  431 kJ mol  1 Calculate the electronegativity values of H and Cl.

50 50 Estimation of Ionic Character of Chemical Bonds Two methods : - 1.The difference in electronegativity between the bonding atoms  n A – n X  (Qualitative) 2.The electric dipole moment of diatomic molecule (Quantitative)

51 51  n A – n X  2.0ionic or nearly ionic bond  n A – n X  0.4covalent or nearly covalent bond 0.4   n A – n X  2.0 covalent bond with ionic character or ionic bond with covalent character e.g. C – H bond (2.5 – 2.1) = 0.4 e.g. Li – F bond (4.0 – 1.0) = 3.0 1.The difference in electronegativity between the bonding atoms  n A – n X  (Qualitative)

52 52 2.The electric dipole moment of diatomic molecule (Quantitative)  = q  d SI units : - Coulomb meter 1 Debye (D) = 3.336  10  30 Coulomb meter

53 53 Electric dipole moment is a vector pointing from the positive pole to the negative pole Centre of postive charge

54 54 Estimating the % ionic character of H–Cl bond by dipole moment Molecule Dipole moment (Coulomb meter) Bond length meter H–Cl3.689  10  30 1.284  10  10 Electronic charge, e  1.602  10  19 Coulomb

55 55 If H–Cl is 100% ionic, dipole moment  1.602  10  19 Coulomb  1.284  10  10 meter  2.057  10  29 Cm The measured dipole moment of H–Cl  3.689  10  30 Cm

56 56 Q.53 70.041.111.314.82.87 % ionic character 7.8841.8270.8880.4480.159 Dipole moment( D) 2.3470.9261.6321.6201.154 Bond length(Å) CsFHFClFHINOMolecule Electronic charge, e  1.602  10  19 Coulomb

57 57 Q.53 83.980.182.279.3 % ionic character 6.32710.2698.5939.001 Dipole moment( D) 1.5702.6712.1762.365 Bond length(Å) LiFKClKFNaClMolecule Electronic charge, e  1.602  10  19 Coulomb

58 58 Calculated from dipole moment  n A – n X  Good correlation between two methods

59 59 How do you expect the bond type to change for the chlorides of the third period elements, NaCl, MgCl 2, AlCl 3, SiCl 4, PCl 5, SCl 2 and Cl 2, going from left to right? Explain the change in the bond type. NaCl MgCl 2 AlCl 3 SiCl 4 PCl 5 SCl 2 Cl 2 Purely Ionic Purely covalent Ionic with covalent character Polar covalent

60 60  difference in electronegativity values  difference in electronegativity values NaCl MgCl 2 AlCl 3 SiCl 4 PCl 5 SCl 2 Cl 2 Purely Ionic Purely covalent Ionic with covalent character Polar covalent

61 61  extent of polarization of ionic bond  extent of polarization of covalent bond NaCl MgCl 2 AlCl 3 SiCl 4 PCl 5 SCl 2 Cl 2 Purely Ionic Purely covalent Ionic with covalent character Polar covalent

62 62 Polarity of Molecules depends on : - 1.Polarity of bonds   n A – n X  or dipole moment 2.Geometry of molecules Symmetrical molecules are usually non-polar due to symmetrical arrangements of dipole moments

63 63 Non-polarSymmetricalNon-polar AsymmetricalNon-polar SymmetricalPolar AsymmetricalPolar Polarity of molecule Geometry of molecule Bond polarity

64 64 The 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

65 65 The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.

66 66 The 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

67 67 The 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

68 68 The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.

69 69 ++ ++ ++  or Q.54

70 70 Non-polarPolar Q.55

71 71 Q.55 Symmetrical  Non-polar

72 72 Xe F F F F Dipole moments of the two lone pairs point in opposite directions Non-polar

73 73 Q.55 Non-polar

74 74 Non-zero vector sum  Polar molecule Q.55

75 75 Q.56(a) >>

76 76 >> Q.56(b)

77 77 Explain the following phenomena: (a)PCl 3 is polar but BCl 3 is non-polar. BCl 3 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.

78 78 Explain the following phenomena: (a)PCl 3 is polar but BCl 3 is non-polar. PCl 3 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.

79 79 Explain the following phenomena: (b) Both NBr 3 and NF 3 are polar but their molecules align differently in a non-uniform electrostatic field.

80 80 (b) As the order of electronegativity is F > N > Br, the resultant dipole moments of NBr 3 and NF 3 are pointing to different directions. The situations are shown below:

81 81 In a non-uniform electrostatic field, the nitrogen end of NBr 3 will point to the positive pole while the nitrogen end of NF 3 will point to the negative pole.

82 82 Non-polar molecules Tetrahedral Trigonal planar Linear Cancelling out of dipole moments MoleculeShape

83 83 Non-polar molecules Octahedral Trigonal bipyramidal Cancelling out of dipole moments MoleculeShape

84 84 Polar molecules Tetrahedral Trigonal pyramidal V-shaped ( or bent) Net resultant dipole moment Dipole moment of individual polar bonds MoleculeShape

85 85 Use of dipole moments Provide important structural information about molecules

86 86 9.1 Incomplete electron transfer in ionic compounds (SB p.250) The 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? Answer CompoundTheoretical lattice enthalpy (kJ mol -1 ) Experimental lattice enthalpy (kJ mol -1 ) X+Br-(s)-665-670 Y+Br-(s)-758-890

87 87 9.1 Incomplete electron transfer in ionic compounds (SB p.250) (a)Since 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.

88 88 9.1 Incomplete electron transfer in ionic compounds (SB p.250) (b)To 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

89 89 9.3 Polarity of covalent bonds (SB p.252) Pure ionic bond and pure covalent bond are two extreme bond types. Why? In 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. Answer Back

90 90 How do you expect the bond type to change for the chlorides of the third period elements, NaCl, MgCl 2, AlCl 3, SiCl 4, PCl 5, SCl 2 and Cl 2, going from left to right? Explain the change in the bond type. Back 9.3 Polarity of covalent bonds (SB p.252) NaCl MgCl 2 AlCl 3 SiCl 4 PCl 5 SCl 2 Cl 2 Purely Ionic Ionic with covalent character Polar covalentPurely covalen t

91 91 Explain the variation in dipole moment of the following molecules. Answer 9.3 Polarity of covalent bonds (SB p.257) MoleculeDipole moment (D) CH 4 0 NH 3 0.35 H2OH2O0.65 HF1.07

92 92 9.3 Polarity of covalent bonds (SB p.257) The dipole moment of a molecule is based on two factors: 1.Bond polarity This 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. HCNOF 2.12.53.03.54.0

93 93 9.3 Polarity of covalent bonds (SB p.257) 2.The geometry If the molecule have symmetrical arrangements of polar bonds, the dipole moments of the bonds will cancel out each other. CH 4 NH 3 No net dipole momentNet dipole moment resulted

94 94 Back 9.3 Polarity of covalent bonds (SB p.257) H 2 O HF Net dipole moment resulted (Note: Lone pair(s) is/are not shown in the above diagrams) Hence, zero dipole moment is only observed in CH 4. HF has the largest dipole moment since the difference in electronegativity between the hydrogen atom and the fluorine atom is the largest. H 2 O comes the second, followed by NH 3.

95 95 9.3 Polarity of covalent bonds (SB p.257) Give the shapes and structural formulae of the following molecules. State whether each molecule is polar or non-polar. (a) BCl 3 (b) NH 3 (c) CHCl 3 Answer

96 96 9.3 Polarity of covalent bonds (SB p.257) Back PolarTetrahedral(c) CHCl 3 PolarTrigonal pyramidal (b) NH 3 Non-polarTrigonal planar(a) BCl 3 Polar or non- polar Structural formula ShapeMolecule


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