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					             Chapter 8
        Covalent Bonding
    8.1 Formation of Covalent Bonds
    8.2 Dative Covalent Bonds
    8.3 Bond Enthalpies
    8.4 Estimation of Average Bond
       Enthalpies using Data from Energetics

1           New Way Chemistry for Hong Kong A-Level Book 1
    8.5 Use of Average Bond Enthalpies to
        Estimate Enthalpy Changes of Reactions
    8.6 Relationship between Bond Enthalpies
        and Bond Lengths
    8.7 Shapes of Covalent Molecules and
        Polyatomic Ions
    8.8 Multiple Bonds
    8.9 Covalent Crystals

2             New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.203)

  Electron Sharing in Covalent Bonds
          H H                                   Attraction between
                                                oppositely charged nuclei
                                                and shared electrons
 Shared                                           electrostatic
                                                ( _____________ in
 electrons
                                                nature)
                               e-
                               e-                       The shared electron pair
                                                        spends most of the time
                                                        between the two nuclei.


      Overlapping of atomic orbitals  covalent bond formation


  3                 New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.203)
 A hydrogen molecule is achieved by partial
 overlapping of 1s orbitals




  4                 New Way Chemistry for Hong Kong A-Level Book 1
 8.1 Formation of Covalent Bonds (SB p.204)
  Electron density map for covalent compounds
  There is
  substantial
  electron density
  at all points along                                            Compare electron-
  the internuclear                                               density-map for
  axis.                                                          ionic compounds:
Thus electrons
are shared
between the
two atoms.




    5                   New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.204)

  Electron density map for ionic compounds




  6                 New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.205)

   Covalent Bonds in Elements
   Some Examples




                                               Dot and cross diagram




   7                 New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.205)

Covalent Bonds in Compounds
Some Examples                                      Octet rule
                    All the above examples obey ____________.




       Carbon
                     1s        2s                 2p
  8                 New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.205)

Covalent Bonds in Compounds
 Some Examples                                        Octet rule
                        All the above examples obey ____________.




                                        octet


      Carbon                                                         electrons from H
                   1s        2s                 2p
  9                 New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.206)
  Octet Rule and its limitations

                                                                     electrons from F




                        not fullfilling                       B: small atomic size
                        octect (electron
                        deficient)                            high I.E.‟s required
 BF3                                                          to become a cation.




 Why doesn‟t B form ionic compounds with F?
  10                New Way Chemistry for Hong Kong A-Level Book 1
8.1 Formation of Covalent Bonds (SB p.207)
   Octet Rule and its limitations




 PCl5                                                            electrons from Cl


                                                         There is low-lying
                                                         vacant d-orbital in P.




 Why Phosphorus can expand its octet to form
 PCl5?
 11             New Way Chemistry for Hong Kong A-Level Book 1
8.2 Dative Covalent Bonds (SB p.208)

 NH3BF3 Molecule




  12                New Way Chemistry for Hong Kong A-Level Book 1
8.2 Dative Covalent Bonds (SB p.208)
   Dative Covalent Bonds
  The dative covalent bond (also known as the
  coordinate bond) is a type of covalent bond in which
  the shared pair of electrons is supplied by only one of
  the bonded atoms.

   Remarks
   (1) The atom that supplies the shared pair of electrons is
       known as the donor while the other atom involved in the
      dative covalent bond is known as the acceptor.

   (2) Once formed, a dative covalent bond cannot be
       distinguished from a „normal‟ covalent bond.
  13                New Way Chemistry for Hong Kong A-Level Book 1
8.2 Dative Covalent Bonds (SB p.209)

  Ammonium Ion (NH4+)




  14                New Way Chemistry for Hong Kong A-Level Book 1
8.2 Dative Covalent Bonds (SB p.209)

 Aluminium Chloride Dimer (Al2Cl6)
                                                          Al: relative small
                                                          atomic size; high
                                                          I.E.‟s required to
                                                          become a cation of
             AlCl3                                        +3 charge.

Why doesn‟t Al form ionic compounds with Cl? (a dimer of AlCl3)




  15                 New Way Chemistry for Hong Kong A-Level Book 1
8.3 Bond Enthalpies (SB p.210)

  Bond Enthalpies
  Bond Dissociation Enthalpies
  B.D.E of a certain bond is the amount of energy required to
  break one mole of that bond under standard conditions.
  e.g. H-H(g)  2H(g)            H (H-H) = +431 kJ mol-1
   CH4(g)          CH3(g) + H(g)           H = +422 kJ mol-1
   CH3(g)          CH2(g) + H(g)                       H = +480 kJ mol-1
   CH2(g)          CH(g) + H(g)                       H = +425 kJ mol-1
   CH(g)            C(g) + H(g)                       H = +335 kJ mol-1

 Why do successive B.D.E. of C-H differ?
                                           ( 422)  ( 480)  ( 425)  ( 335)
(Average) bond enthalpy; E(C-            
                                                             4
H)
                                          = +415.5 kJ mol-1
  16                New Way Chemistry for Hong Kong A-Level Book 1
8.3 Bond Enthalpies (SB p.211)

       Bond Enthalpies
          Bond        Average bond enthalpy (kJ mol-1)
         H-H                                  +436
         C-C                                  +348
         C═ C                                 +612
         C≡ C                                 +837
         C-H                                  +412
         O-H                                  +463
       Why is this value of E(C-H) still different from the
       previously calculated one (+415.5 kJ mol-1)?

  17                  New Way Chemistry for Hong Kong A-Level Book 1
8.3 Bond Enthalpies (SB p.211)

  Bond Enthalpies
       Remarks
   • Bond enthalpies refer to the energy required to
     break bonds (+ve)
   • For the formation of a certain bond, the enthalpy
     change is represented by
     “- bond enthalpy” (-ve)
   • The bond enthalpy is a measure of bond strength.
     How?



  18                New Way Chemistry for Hong Kong A-Level Book 1
8.4 Estimation of Average Bond Enthalpies using
    Data from Energetics (SB p.212)
From the Enthalpy Change of Atomization of
a Compound
The enthalpy change of atomization of methane (CH4) is +1662 kJ
mol-1. Find a value for the bond enthalpy of C-H based on the above
data.


                        C(g) + 4H(g) ΔH = +1 662 kJ mol-1


 The atomization of methane involves the breaking of a four C-H
 bonds. Assume that all four C-H bonds are equal in strength.
 The average bond enthalpy of C-H bonds
 = ¼ x (+1 662) kJ mol-1 = +415.5 kJ mol-1
         E(C-H) = +415.5 kJ mol-1
 19                 New Way Chemistry for Hong Kong A-Level Book 1
8.4 Estimation of Average Bond Enthalpies using
    Data from Energetics (SB p.212)

  The standard enthalpy change of atomization of a compound is the
  enthalpy change when one mole of gaseous compound is broken
  down into its constituent atoms in the gaseous state under standard
  conditions, e.g.
    CH4(g)  C(g) + 4H(g)
   1 mole

  The standard enthalpy change of atomization of a element is the
  enthalpy change when one mole of gaseous atoms is formed into
  its constituent atoms in the gaseous state under standard conditions,
  e.g.
       ½Cl2(g)  Cl(g)
                1 mole



  20                New Way Chemistry for Hong Kong A-Level Book 1
8.4 Estimation of Average Bond Enthalpies using
    Data from Energetics (SB p.214)
From the Enthalpy Changes of Atomization of
Two Compounds
 The enthalpy change of atomization of butane (C4H10) and pentane
 (C5H12) are +5165 kJ mol-1 and +6337 kJ mol-1 respectively. Find a
 values for the bond enthalpies of C-H and C-C based on the above
 data. For butane,
       3 E(C-C) + 10 E(C-H) = +5 165 kJ mol-1 …….(1)
       For pentane,
       4 E(C-C) + 12 E(C-H) = +6 337 kJ mol-1 ……..(2)
       Solving simultaneous equations (1) and (2), we
       obtain the following bond enthalpy values.
       E (C-H) = +412.25 kJ mol-1
       E (C-C) = +347.5 kJ mol-1
21                 New Way Chemistry for Hong Kong A-Level Book 1
8.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of
    Reactions (SB p.214)

Reaction of ethene and hydrogen




                                  Sum of bond                         Sum of bond
  Enthalpy change
                          =       enthalpies of            -          enthalpies of
       of reaction                reactants                           products

  22                 New Way Chemistry for Hong Kong A-Level Book 1
8.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of
    Reactions (SB p.215)
  Enthalpy profile for the reaction of ethene
  and hydrogen




  23                New Way Chemistry for Hong Kong A-Level Book 1
8.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of
Reactions (SB p.214)

  Reaction of ethene and hydrogen
                                                       ?




  Sum of bond enthalpies of reactants          Sum of bond enthalpies of products
  = E(C=C) + 4E(C-H) + E(H-H)                  = E(C-C) + 6E(C-H)
  = (612) + 4(412) + (436)                     = (348) + 6(412)
  = +2696 kJmol-1                              = +2820 kJmol-1
   Hr = 2696 - (+2820)                       Only an estimated value
        = -124 kJ mol-1                        for Hr .Why?



  24                   New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.217)

 Bond Enthalpies as an Indication of the Strength
 of Covalent Bonds
       Bond        Average bond
                 enthalpy (kJ mol-1)                    Consider
       H-H              +436                            C-C, C=C and CC
       C-C              +348                         bond        bond    bond
       C═ C             +612                         order       order   order
                                                     =1          =2      =3
       C≡ C             +837
       C-H              +412
       O-H              +463

 As the bond order increases, the bond strength also increases.

  25                New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.217)

  Bond Lengths
       Bond length (for covalent bond)




  26                  New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.217)

 Bond lengths of some covalent bonds
          Bond             Molecule                Bond lengths (nm)
           O-H               H2O                           0.096
                            H2O2                           0.097
                           CH3OH                           0.096
                           HCOOH                           0.096
           C-C             Diamond                         0.154
                            C2H6                           0.154
                            C3H8                           0.154
                           C2H5OH                          0.155
           C-H               CH4                           0.110
                            C2H4                           0.109

         By what technique can these bond lengths be measured?
       X-ray diffraction
  27                 New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.217)

Bond enthalpies and bond lengths
   Bond    Bond length (nm) Bond enthalpy                    Any conclusion for
                              (kJ mol-1)                     the relationship
                                                             between bond length
  H-H            0.074                      436              & bond enthalpy?
  Cl-Cl          0.199                      242
  Br-Br          0.228                      193
                                                                Usually a longer
   I-I           0.266                      151                 bond length
   H-F           0.092                      565                 corresponds to a
  H-Cl           0.127                      431                 lower value of
  H-Br           0.141                      364                 bond enthalpy
                                                                (weaker bond).
   H-I           0.161                      299

   28                New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.217)
 Special Situation for F2
   Bond          Bond Length /nm                     Bond Enthalpy / kJ mol-1

   F-F                 0.142                                         158
   Cl-Cl               0.199                                         242
   Br-Br               0.228                                         193
   I-I                 0.266                                         151
Explain why the bond enthalpy of F-F is smaller than that of
Cl-Cl even though the bond length of F-F is the shortest among
the halogens.              Non-bonding e-
                                 / lone pair of e-
As the size of fluorine atom is very small,
the repulsion between the non-bonding                           F          F
pairs of electrons on the fluorine atoms
weaken the F-F bond.

  29                New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.218)

   Covalent Radius
       (often referred as „Atomic radius‟)


                              The space occupied by an atom in a
                              covalently bonded molecule in the
                              direction of the covalent bond (generally
                              taken as half of the bond length of
                              homoatomic molecules)

                               Where b is the bond length
                                     r is the covalent radius



  30                   New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.218)

 The covalent radii (in nm) of some elements




  31                New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.218)

 Bond lengths of atom A and atom B




       Predicting bond length of A-B if rA & rB are known
       Bond length                    Covalent                          Covalent
       of a covalent         =        radius of             +           radius of
       bond A-B                       atom A                            atom B


  32                   New Way Chemistry for Hong Kong A-Level Book 1
 8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.219)
Calculated and experimentally determined bond length
    Similar                            By what technique can the bond
    electronegativity                  lengths be determined experimentally?

         Bond       Calculated bond length (nm) Experimentally determined
                                                    bond length (nm)

         C-O                     0.150                                   0.143
         C-F                     0.149                                   0.138
         C-Cl                    0.176                                   0.177
         C-Br                    0.191                                   0.193
         C-C                     0.154                                   0.154
         H-Cl                    0.136                                   0.128
         C-H                     0.114                                   0.109
         N-Cl                    0.173                                   0.174

   33                   New Way Chemistry for Hong Kong A-Level Book 1
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.219)

Calculated and experimentally determined bond length
   Quite different
   electronegativity


       Bond        Calculated bond length (nm)          Experimentally determined
                                                            bond length (nm)
       C-O                     0.150                                    0.143
       C-F                     0.149                                    0.138
       C-Cl                    0.176                                    0.177
       C-Br                    0.191                                    0.193
       C-C                     0.154                                    0.154
       H-Cl                    0.136                                    0.128
       C-H                     0.114                                    0.109
       N-Cl                    0.173                                    0.174

  34                   New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.220)

Shapes of Molecules and Polyatomic Ions
   Shape of a molecule
       = geometric arrangement of atoms within the molecules


   Valence Shell Electron Pair Repulsion Theory (VSEPR)

   The electron pairs in the outermost shell of the central
   atom in a molecule will stay as far apart as possible to
   minimize electronic repulsion.
   The electron pairs are oriented at a maximum separation in
   space so as to minimize the coulombic repulsion of
   electron clouds.
  35                  New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.220)

Beryllium chloride molecule (BeCl2)


       Electronic Diagram                               Shape in Diagram




                  Be             Cl
        Cl


                                                             Bond angle
                                                           = angle between
                                                             2 bonds
        Shape in word              linear
  36                   New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.220)

  Boron trifluoride molecule(BF3)
       Electronic Diagram
                                                     Shape in Diagram

                 F




                B

        F                    F
                                                          Shape in word

                                                       trigonal planar

  37                 New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.221)

   Methane (CH4) Molecule
       Electronic Diagram                              Shape in Diagram


                  H




        H        C               H



                                                          Shape in word
                  H
                                                           tetrahedral

  38                  New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.221)

  Methane (CH4) Molecule
                                                          Shape in Diagram
  Electronic Diagram
                                                                      H

                 H
                                                                      C
                                                                                H
                                                              H             H
       H        C               H                            bond in the plane of paper
                                                             bond in front of paper
                                                             bond behind paper

                 H
                                                 Shape in word
                                                  tetrahedral
  39                 New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.221)

 Phosphorus Pentachloride (PCl5) Molecule
                                                       Shape in Diagram
       Electronic Diagram




                                              Shape in word
                                          trigonal bipyramidal
  40                 New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.221)

 Phosphorus Pentachloride (PCl5) Molecule
       Electronic Diagram                              Shape in Diagram

                                                                     Cl
                                                                          Cl
                                                     Cl              P
                                                                          Cl
                                                                     Cl

                                                         Shape in word

                                                     trigonal bipyramidal

  41                New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.221)

 Sulphur Hexafluoride
 (SF6)
    Electronic Diagram                                  Shape in Diagram


                 F

        F                      F

                 S
                               F
        F
                     F                                    Shape in word
                                                              octahedral

  42                 New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.223)

 Ammonia (NH3) Molecule
                                                      Shape in Diagram
       Electronic Diagram



       H        N               H



                 H


                                                         Shape in word
 lp-lp repulsion > lp-bp repulsion
 > bp-bp repulsion                                  Trigonal pyramidal

  43                 New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.223)

 Ammonia (NH3) Molecule
                                                          Shape in Diagram
   Electronic Diagram
                                                                         lone pair
                                                                           of e-
                                                                  N
                N
                                                                             H
       H                        H
                                                         H               H

                                                             bond in the plane of paper
                 H
                                                             bond in front of paper
                                                             bond behind paper

                               Shape in word
                         Trigonal pyramidal
  44                 New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.223)


   Water (H2O) Molecule
           Electronic Diagram                           Shape in Diagram



       H          O              H




lp-lp repulsion > lp-bp repulsion
> bp-bp repulsion
                Shape in word
                  V-shaped / bent
  45                  New Way Chemistry for Hong Kong A-Level Book 1
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.224)

 Water (H2O) Molecule
       Electronic Diagram                            Shape in Diagram


                                                                      lone pair
                 O                                                      of e-
        H                       H
                                                               O

                                                      H               H
                         Shape in word

                          V-shaped / bent


  46                 New Way Chemistry for Hong Kong A-Level Book 1
8.8 Multiple Bonds (SB p.227)

 Comparison of bond lengths and bond enthalpies
 between single and multiple bonds
       Bond       Bond order            Bond length                   Bond enthalpy
                                           (nm)                         (kJ mol-1)
       C-C            1                     0.154                         +348
       C═ C           2                     0.134                         +612
       C≡ C           3                     0.120                         +837
       N-N            1                     0.146                         +163
       N═ N           2                     0.120                         +409
       N≡ N           3                     0.110                         +944
       C-O            1                     0.143                         +360
       C ═O           2                     0.122                         +743


  47                 New Way Chemistry for Hong Kong A-Level Book 1
8.8 Multiple Bonds (SB p.227)

 Effect of Multiple Bonding on Shapes of Molecules
   Ethene (CH2=CH2)
                                                      Shape in Diagram
       Electronic Diagram

        H                   H

            C          C

       H
                            H
                                                          Shape in word

                                                         (planar)

  48                 New Way Chemistry for Hong Kong A-Level Book 1
8.8 Multiple Bonds (SB p.227)

 Ethyne (CHCH)

      Shape in Diagram                                 Shape in word

                                                           linear




49                New Way Chemistry for Hong Kong A-Level Book 1
8.8 Multiple Bonds (SB p.227)


       Carbon dioxide (CO2)

       Shape in Diagram                                  Shape in word


                                                                linear




  50                 New Way Chemistry for Hong Kong A-Level Book 1
8.8 Multiple Bonds (SB p.227)


     Sulphur dioxide (SO2)

      Shape in Diagram                              Shape in word



                                                    Angular / (bent)




51                New Way Chemistry for Hong Kong A-Level Book 1
8.9 Covalent Crystals (SB p.228)

 Giant Covalent Structures

   Structures consist of millions of atoms covalently
   bonded together in a structural network and there is
   no simple molecule present.


   Examples
   diamond, graphite and quartz/silicon(IV) oxide



  52                 New Way Chemistry for Hong Kong A-Level Book 1
8.9 Covalent Crystals (SB p.228)
        Diamond




       • Each C atom is covalently bonded to 4 other C
         atoms to form a three-dimensional network
       • The C-C bonding pattern accounts for the high
         m.p., stability and extreme hardness
       • Applications: scratch proof cookware, watch
         crystals, ball bearings and razor blade
  53                 New Way Chemistry for Hong Kong A-Level Book 1
8.9 Covalent Crystals (SB p.229)
   Graphite




 • Each C atom is covalently bonded to 3 other C atoms in the
   same layer. A network of coplanar hexagons is formed (C-C
   bond length: 0.142 nm)
 • Weak van der Waals‟ forces hold the layers together (distance
   between adjacent layers: 0.335 nm)
 • Delocalized e- free to move within layers
 • Properties: soft and slippery (used as pencil „lead‟), conductor
  54                 New Way Chemistry for Hong Kong A-Level Book 1
8.9 Covalent Crystals (SB p.230)

Comparison of the properties of diamond and graphite

  Property                    Diamond                             Graphite
  Density (g cm-3)            3.51                                2.27
  Hardness                    10 (hardest)                        < 1 (very soft)
  Melting point (oC)          3 827                               3 652 (sublime)
  Colour                      Colourless transparent              Shiny black
  Electrical conductivity     None                                High (along sheet)

       Why graphite has a high m.p. than that of
       diamond?


  55                 New Way Chemistry for Hong Kong A-Level Book 1
8.9 Covalent Crystals (SB p.230)
  Quartz /Silicon(IV) oxide




       Each Si atom is bonded
       tetrahedrally to 4 neighbouring
       O atoms
       Each O atom is bonded to 2 Si
       atoms, one at the centre of each
       of two adjacent tetrahedral
  56                   New Way Chemistry for Hong Kong A-Level Book 1
             The END




57   New Way Chemistry for Hong Kong A-Level Book 1