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Factors that Favour Polarization of Ionic Bond – Fajans’ Rules

Polarization of an ionic bond means the distortion of the electron cloud of an anion towards a cation Polarization of an ionic bond results in an ionic bond with covalent character. as the of the cation . Factors that Favour Polarization of Ionic Bond – Fajans’ Rules.

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Factors that Favour Polarization of Ionic Bond – Fajans’ Rules

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  1. Polarization of an ionic bond means the distortion of the electron cloud of an anion towards a cation Polarization of an ionic bond results in an ionic bond with covalent character.

  2. 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 

  3. Al3+ > Mg2+ > Na+ Charge : Al3+ > Mg2+ > Na+ Polarizing power : Al3+ > Mg2+ > Na+

  4. Polarizability  as the size of the anion  Larger size of anion  outer electrons are further away from the nucleus  electrons are less held by the nucleus and more easily polarized by cations I > Br > Cl > F S2 > O2

  5. 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 Covalent character : AgI > AgBr > AgCl

  6. 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

  7. Polarizability  as the charge of the anion  Higher charge in the anion results greater repulsion between electrons  electrons are less held by the nucleus and more easily polarized by cations

  8. 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 ?

  9. Ag+ = [Kr] 5s14d9 Na+ = Ne • The valence 4d electrons are less penetrating • They shield less 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+

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

  11. 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

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

  13. 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

  14. 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

  15. Fajans’ rules – A summary

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

  17. 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

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

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

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

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

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

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

  24. 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.

  25. 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.

  26. 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.

  27. 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)

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

  29. 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

  30. 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)

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

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

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

  34. Estimating the % ionic character of H–Cl bond by dipole moment Electronic charge, e  1.6021019 Coulomb

  35. 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

  36. 2.87 14.8 11.3 41.1 70.0 Q.53 Electronic charge, e  1.6021019 Coulomb Molecule NO HI ClF HF CsF Bond length(Å) 1.154 1.620 1.632 0.926 2.347 Dipole moment(D) 0.159 0.448 0.888 1.827 7.884 % ionic character

  37. 79.3 82.2 80.1 83.9 Q.53 Electronic charge, e  1.6021019 Coulomb Molecule NaCl KF KCl LiF Bond length(Å) 2.365 2.176 2.671 1.570 Dipole moment(D) 9.001 8.593 10.269 6.327 % ionic character

  38. Calculated from dipole moment Good correlation between two methods nA – nX 

  39. Ionic with covalent character Polar covalent 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 Cl2 Purely Ionic Purely covalent

  40.  difference in electronegativity values  difference in electronegativity values Ionic with covalent character Polar covalent NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2 Purely Ionic Purely covalent

  41.  extent of polarization of ionic bond  extent of polarization of covalent bond Ionic with covalent character Polar covalent NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2 Purely Ionic Purely covalent

  42. Polarity of Molecules • depends on : - • Polarity of bonds • nA – nX  or dipole moment • Geometry of molecules • Symmetrical molecules are usually non-polar • due to symmetrical arrangements of dipole moments

  43. Bond polarity Geometry of molecule Polarity of molecule Polar Asymmetrical Polar Polar Symmetrical Non-polar Non-polar Asymmetrical Non-polar Non-polar Symmetrical Non-polar

  44. 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

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

  46. 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

  47. 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

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

  49. +   +  + or Q.54

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