New Way Chemistry for Hong Kong A-Level Book 11 Intermediate Type of Bonding 9.1Incomplete Electron Transfer in Ionic Compounds 9.2Electronegativity of Elements 9.3Polarity of Covalent Bonds 9

New Way Chemistry for Hong Kong A-Level Book Incomplete Electron Transfer in Ionic Compounds

New Way Chemistry for Hong Kong A-Level Book 13 Born-Haber cycle for the formation of sodium chloride  H lattice  can be found from the Born-Haber Cycle Exptal value of  H lattice  Theoretical values of lattice enthalpy 9.1 Incomplete electron transfer in ionic compounds (SB p.247)

New Way Chemistry for Hong Kong A-Level Book 14 Another way of getting the value of  H lattice In the ionic model, assume: 1. Ions are perfect spheres with uniform charge distribution. 2. The cations and anions are in contact with each other and with electrostatic interactions in between.  9.1 Incomplete electron transfer in ionic compounds (SB p.247)

New Way Chemistry for Hong Kong A-Level Book 15 Lattice enthalpy is calculated in terms of electrostatic interactions within the lattice The potential energy (P) required to bring two ions with charges q 1 and q 2 from an infinite distance to a distance r is: R _ q2q2 q1q Incomplete electron transfer in ionic compounds (SB p.248) Another way of getting the value of  H lattice 

New Way Chemistry for Hong Kong A-Level Book Incomplete electron transfer in ionic compounds (SB p.248) Comparison of theoretical and experimental values of lattice enthalpy Theoretical value of lattice enthalpy Compound is purely ionic Lattice enthalpy obtained from the Born- Haber cycle True value Based on the experimentally derived enthalpy changes

New Way Chemistry for Hong Kong A-Level Book Incomplete electron transfer in ionic compounds (SB p.248) Comparison of theoretical and experimental values of lattice enthalpy Reveal the nature of the bond in the compound

New Way Chemistry for Hong Kong A-Level Book Incomplete electron transfer in ionic compounds (SB p.248) Comparison of theoretical and experimental values of lattice enthalpy CompoundLattice enthalpy (kJ mol -1 ) TheoreticalExperimentalDifference NaCl NaBr NaI KCl KBr KI AgCl AgBr AgI ZnS

New Way Chemistry for Hong Kong A-Level Book 19 Increasing polarization of a negative ion by a positive ion Polarization / Distortion of electron cloud = Covalent character in ionic bond Polarizing power of cation polarizability of anion 9.1 Incomplete electron transfer in ionic compounds (SB p.249)

New Way Chemistry for Hong Kong A-Level Book 110 Polarizing power of a cation increases as its charge density (charge/volume ratio) increases.  polarizing power: Al 3+ > Mg 2+ > Na + and Li + > Na + > K + Polarizing power of cation 9.1 Incomplete electron transfer in ionic compounds (SB p.249) Polarization of ion

New Way Chemistry for Hong Kong A-Level Book 111 Polarizability of an anion increases as its size increases.  polarizability: I - > Br - > Cl - > F - and S 2- > O 2- polarizability of anion 9.1 Incomplete electron transfer in ionic compounds (SB p.247) Polarization of ion Example 9-1 Example 9-1 Check Point 9-1 Check Point 9-1

New Way Chemistry for Hong Kong A-Level Book Electronegativity of Elements

New Way Chemistry for Hong Kong A-Level Book 113 Electronegativity 9.2 Electronegativity of elements (SB p.250) Electronegativity is defined as the relative tendency of an atom to attract a bond pair(s) of electrons towards itself in a covalent bond.

New Way Chemistry for Hong Kong A-Level Book 114 XYXY X : Y Electronegativity of some representative elements 9.2 Electronegativity of elements (SB p.251)

New Way Chemistry for Hong Kong A-Level Book Polarity of Covalent Bonds

New Way Chemistry for Hong Kong A-Level Book 116 Bond polarization a pure covalent bond a polar covalent bond 9.3 Polarity of covalent bonds (SB p.251)

New Way Chemistry for Hong Kong A-Level Book 117 Relationship between the type of bond and difference in electronegativity between two bonded atoms BondDifference in electronegativity between the two bonded atoms Type of bond Cl  Cl Cl  C Cl  H F  Li = = = = 3.0 Covalent Polar covalent Ionic 9.3 Polarity of covalent bonds (SB p.252)

New Way Chemistry for Hong Kong A-Level Book Polarity of covalent bonds (SB p.252) Relationship between the type of bond and difference in electronegativity between two bonded atoms Check Point 9-3A Check Point 9-3A

New Way Chemistry for Hong Kong A-Level Book 119 Polarity of molecules  (Debye, D) = q  d How polar is the molecule? represented by dipole moment (  ) 9.3 Polarity of covalent bonds (SB p.253)

New Way Chemistry for Hong Kong A-Level Book 120 Effect of an electric field on polar molecules 9.3 Polarity of covalent bonds (SB p.254)

New Way Chemistry for Hong Kong A-Level Book Polarity of covalent bonds (SB p.254) Effect of an electric field on non- polar molecules

New Way Chemistry for Hong Kong A-Level Book 122 Non-polar molecules 9.3 Polarity of covalent bonds (SB p.255) Tetrahedral Trigonal planar Linear Cancelling out of dipole moments MoleculeShape

New Way Chemistry for Hong Kong A-Level Book Polarity of covalent bonds (SB p.255) Non-polar molecules Octahedral Trigonal bipyramidal Cancelling out of dipole moments MoleculeShape

New Way Chemistry for Hong Kong A-Level Book Polarity of covalent bonds (SB p.255) Polar molecules Tetrahedral Trigonal pyramidal V-shaped ( or bent) Net resultant dipole moment Dipole moment of individual polar bonds MoleculeShape

New Way Chemistry for Hong Kong A-Level Book 125 Use of dipole moments 9.3 Polarity of covalent bonds (SB p.256) Provide important structural information about molecules

New Way Chemistry for Hong Kong A-Level Book 126 Effect of a non-uniform electric field on polar and non-polar solvents Tetrachloromethan e, CCl 4 Cyclohexane Benzene Trichloromethane, CHCl 3 Ethanol,CH 3 CH 2 OH Propanone, Water, H 2 O Solvents showing no deflection Solvents showing a marked deflection 9.3 Polarity of covalent bonds (SB p.256)

New Way Chemistry for Hong Kong A-Level Book Polarity of covalent bonds (SB p.256) Check Point 9-3B Check Point 9-3B Example 9-3 Example 9-3

New Way Chemistry for Hong Kong A-Level Book 128 The END

New Way Chemistry for Hong Kong A-Level Book 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) Y+Br-(s)

New Way Chemistry for Hong Kong A-Level Book 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.

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book 132 Apart from those compounds mentioned in Table 9-1, list three other compounds with a large difference between the theoretical and experimentally derived lattice enthalpies. They are the compounds with a cation of high polarizing power and an anion of high polarizability. Examples are AlCl 3, MgI 2, Al 2 S 3, CuO and Ag 2 S. Back Answer 9.1 Incomplete electron transfer in ionic compounds (SB p.250)

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book 134 How do you expect the bond type to change form 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. Moving along the chlorides of the third period elements, the bond type changes from pure ionic bond, ionic bond with covalent character, polar covalent bond to pure covalent bond. This is because the difference in electronegativity between the atoms of the third period elements and the chlorine atom decreases across the period. Back Answer 9.3 Polarity of covalent bonds (SB p.252)

New Way Chemistry for Hong Kong A-Level Book 135 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 H2OH2O0.65 HF1.07

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book 138 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.

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book 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