Intermediate Type of Bonding

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

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

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

8. Polarization of ionic bond - Incomplete Transfer of Electron

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

10. -349 -791.4

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

12. r+ + 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.

13. 3. 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.

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

15. Compound Lattice enthalpy (kJ mol-1) Theoretical Experimental % deviation NaCl -766.1 -766.4 0.04 NaBr -730.5 -733.0 0.74 NaI -685.7 -688.3 0.38 KCl -692.0 -697.8 0.84 KBr -666.5 -672.3 0.87 KI -630.9 -631.8 0.14 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. Compound Lattice enthalpy (kJ mol-1) Theoretical Experimental % deviation AgCl -833.0 -890.0 6.8 AgBr -808.0 -877.0 8.5 AgI -774.0 -867.0 12 Zns -3427.0 -3615.0 5.5 Silver halides and zinc sulphide show large discrepancies between the two values.  Silver halides and zinc sulphide are NOTpurely ionic compounds

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

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 Purely ionic Purely covalent Large % deviation of lattice enthalpy  greater b and more covalent character

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 Purely ionic Purely covalent Small % deviation of lattice enthalpy  smaller b and less covalent character

20. as the of the cation  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 

21. Al3+ > Mg2+ > Na+ Q.50(a) Charge : Al3+ > Mg2+ > Na+ Size : Al3+ < Mg2+ < Na+ Polarizing power : Al3+ > Mg2+ > Na+

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

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. 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 cations I > Br > Cl > F S2 > O2

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

26. Compound Lattice enthalpy (kJ mol-1) Theoretical Experimental % deviation AgCl -833.0 -890.0 6.8 AgBr -808.0 -877.0 8.5 AgI -774.0 -867.0 12 ZnS -3427.0 -3615.0 5.5 Great % deviation of ZnS due to high polarizability of the large S2 ion

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

29. Ag+ = [Kr] 5s14d9 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+ Polarizing power : Ag+ > Na+

30. Ag+ = [Kr] 5s1 4d9 Na+ = Ne Noble gas configuration of the cation produces better shielding effect and less polarizing power Polarizing power : Ag+ > Na+

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. 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. Q.51(c) Solubility in water : - Gp I carbonates >> other carbonates Carbonate 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

34. Example 9-1 Check Point 9-1 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

35. Fajans’ rules – A summary

36. Apart from those compounds mentioned on p.63, list THREE ionic compounds with high covalent character. AlCl3 , MgI2 , CuCO3

37. Polarization of Covalent Bond : – Unequal Sharing of electrons Evidence : - • Deflection of a jet of a polar liquid(e.g. H2O) in a non-uniform electrostatic field • Breakdown of additivity rule of covalent radii • Breakdown of additivity rule of bond enthalpies

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

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

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

41. Solvents showing a marked deflection Solvents showing no deflection Trichloromethane, CHCl3 Ethanol,CH3CH2OH Propanone Water, H2O Tetrachloromethane Cyclohexane Benzene Carbon disulphide

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

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

44. 0.1910 0.1480 0.1510 0.1275 -1.54% 12.12% 5.59% 9.91% Bond CBr in CBr4 CF in CF4 CO in CH3OH CO in CO2 Experimental value/nm 0.1940 0.1320 0.1430 0.1160 Estimated bond length/nm % deviation 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. 0.1910 0.1480 0.1510 0.1275 -1.54% 12.12% 5.59% 9.91% +  Bond CBr in CBr4 CF in CF4 CO in CH3OH CO in CO2 Experimental value/nm 0.1940 0.1320 0.1430 0.1160 Estimated bond length/nm % deviation Polarization of a covalent bond always results in the formation a stronger bond with shorter bond length.

46. Equal sharing of electrons A.M. G.M. 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.

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. For the molecule A–X nA and nX are the electronegativity values of A and X respectively nF = 4.0

49. More electronegative Q.52 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. nH = 2.2 nCl = 3.3

50. Estimation 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)