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New Way Chemistry for Hong Kong A-Level Book 11 New Way Chemistry for Hong Kong A-Level Book 12 New Way Chemistry for Hong Kong A-Level Book 13 New Way Chemistry for Hong Kong A-Level Book 14 H HShared electrons The shared electron pair spends most of the time between the two nuclei.e-e- Attraction between oppositely charged nuclei and shared electrons ( _ in nature)electrostaticOverlapping of atomic orbitals covalent bond formation8.1 Formation of Covalent Bonds (SB p.213) New Way Chemistry for Hong Kong A-Level Book 15 8.1 Formation of Covalent Bonds (SB p.213) New Way Chemistry for Hong Kong A-Level Book 16Thus electrons are shared between the two atoms. Compare electron-density-map for ionic compounds: There is substantial electron density at all points along the internuclear axis. 8.1 Formation of Covalent Bonds (SB p.214) New Way Chemistry for Hong Kong A-Level Book 17 Dot and cross diagram 8.1 Formation of Covalent Bonds (SB p.214) Hydrogen molecule New Way Chemistry for Hong Kong A-Level Book 18 8.1 Formation of Covalent Bonds (SB p.215) Chlorine molecule Oxygen molecule New Way Chemistry for Hong Kong A-Level Book 19 8.1 Formation of Covalent Bonds (SB p.215) Nitrogen molecule New Way Chemistry for Hong Kong A-Level Book 110 8.1 Formation of Covalent Bonds (SB p.216) New Way Chemistry for Hong Kong A-Level Book 111 8.1 Formation of Covalent Bonds (SB p.216) New Way Chemistry for Hong Kong A-Level Book 112 8.1 Formation of Covalent Bonds (SB p.216 217)In forming chemical bonds, atoms tend to achieve the stable noble gas electronic configuration with 8 electrons in the valence shell (except helium which has 2 electrons in the valence shell) by gaining, losing or sharing of electrons. New Way Chemistry for Hong Kong A-Level Book 113 Why doesnt B form ionic compounds with F? B: small atomic size high I.E.s required to become a cation.electrons from Fnot fullfilling octect (electron deficient)8.1 Formation of Covalent Bonds (SB p.217) New Way Chemistry for Hong Kong A-Level Book 114 Why Phosphorus can expand its octet to form PCl5?There is low-lying vacant d-orbital in P. electrons from Cl8.1 Formation of Covalent Bonds (SB p.207) New Way Chemistry for Hong Kong A-Level Book 115 New Way Chemistry for Hong Kong A-Level Book 116 8.2 Dative Covalent Bonds (SB p.218)A dative covalent bond is formed by the overlapping of an empty orbital of an atom with an orbital occupied by a lone pair of electrons of another atom.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. New Way Chemistry for Hong Kong A-Level Book 117 8.2 Dative Covalent Bonds (SB p.218 219) New Way Chemistry for Hong Kong A-Level Book 118 8.2 Dative Covalent Bonds (SB p.219) New Way Chemistry for Hong Kong A-Level Book 119 AlCl3 Al: relative small atomic size; high I.E.s required to become a cation of +3 charge.8.2 Dative Covalent Bonds (SB p.219 220) New Way Chemistry for Hong Kong A-Level Book 120 Why doesnt Al form ionic compounds with Cl? (a dimer of AlCl3)8.2 Dative Covalent Bonds (SB p.219 220) New Way Chemistry for Hong Kong A-Level Book 121 New Way Chemistry for Hong Kong A-Level Book 122 8.3 Bond Enthalpies (SB p.220)Bond enthalpy is the energy associated with a chemical bond. When a chemical bond is broken or formed, a certain amount of energy is absorbed from or released to the surroundings. New Way Chemistry for Hong Kong A-Level Book 123 8.3 Bond Enthalpies (SB p.220) Example:Combustion of methane New Way Chemistry for Hong Kong A-Level Book 124 8.3 Bond Enthalpies (SB p.221) Standard enthalpy changes of combustion of the homologous series of alkanes and alkanols New Way Chemistry for Hong Kong A-Level Book 125 e.g. H-H(g) 2H(g) H = +431 kJ mol-1 8.3 Bond Enthalpies (SB p.221)Bond dissociation enthalpy is the enthalpy change when one mole of a particular bond in a particular environment is broken under standard conditions. New Way Chemistry for Hong Kong A-Level Book 126 Why do successive B.D.E. of C-H differ?(Average) bond enthalpy; E(C-H)4 335)( 425)( 480)( 422)( = +415.5 kJ mol-1CH4(g) CH3(g) + H(g) H = +422 kJ mol-1CH3(g) CH2(g) + H(g) H = +480 kJ mol-1CH2(g) CH(g) + H(g) H = +425 kJ mol-1CH(g) C(g) + H(g) H = +335 kJ mol-18.3 Bond Enthalpies (SB p.221) New Way Chemistry for Hong Kong A-Level Book 127 8.3 Bond Enthalpies (SB p.222)Average bond enthalpy is the average of the bond dissociation enthalpies required to break a particular chemical bond. New Way Chemistry for Hong Kong A-Level Book 128 New Way Chemistry for Hong Kong A-Level Book 129 8.4 Estimation of Average Bond Enthalpies using Data from Enegetics (SB p.223)Atomization of a compound means the breaking down of one mole of the gaseous compound into its constituent atoms in the gaseous state. New Way Chemistry for Hong Kong A-Level Book 130 8.4 Estimation of Average Bond Enthalpies using Data from Enegetics (SB p.223) Example:Atomization of methaneC(g) + 4H(g) H = +1 662 kJ mol-1 E(C-H) = +415.5 kJ mol-1The 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= x (+1 662) kJ mol-1 = +415.5 kJ mol-1 New Way Chemistry for Hong Kong A-Level Book 131 8.4 Estimation of Average Bond Enthalpies using Data from Enegetics (SB p.223) Two ways to determine the enthalpy change of atomization of methane:1. From successive bond dissociation enthalpies2. From enthalpy cycle and Hesss law New Way Chemistry for Hong Kong A-Level Book 132 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.8.4 Estimation of Average Bond Enthalpies using Data from Enegetics (SB p.224 225) New Way Chemistry for Hong Kong A-Level Book 133 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 8.4 Estimation of Average Bond Enthalpies using Data from Enegetics (SB p.224 225) New Way Chemistry for Hong Kong A-Level Book 134 New Way Chemistry for Hong Kong A-Level Book 135 Sum of bond enthalpies of productsEnthalpy change of reaction = Sum of bond enthalpies of reactants - 8.5 Use of Average Bond Enthalpies to Estimate the Enthalpy Changes of Reactions (SB p.225) New Way Chemistry for Hong Kong A-Level Book 136 8.5 Use of Average Bond Enthalpies to Estimate the Enthalpy Changes of Reactions (SB p.226) New Way Chemistry for Hong Kong A-Level Book 137 8.5 Use of Average Bond Enthalpies to Estimate the Enthalpy Changes of Reactions (SB p.226) New Way Chemistry for Hong Kong A-Level Book 138 New Way Chemistry for Hong Kong A-Level Book 139 8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.227) Gives a direct measure of the strength of a covalent bond It is the energy required to break the bond Not in proportion to the bond order(The number of bonding electrons divided by two) New Way Chemistry for Hong Kong A-Level Book 140 The distance between the two bonded nuclei Inversely related to bond strength Not constant Depends on the local environment of that particular bond Determined experimentally by electron diffraction, X-ray diffraction or spectroscopic techniques8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.228) New Way Chemistry for Hong Kong A-Level Book 141 Any conclusion for the relationship between bond length & bond enthalpy?Usually a longer bond length corresponds to a lower value of bond enthalpy (weaker bond).Bond Bond length (nm) Bond enthalpy(kJ mol-1)H-HCl-ClBr-BrI-IH-FH-ClH-BrH-I 0.0740.1990.2280.2660.0920.1270.1410.161 4362421931515654313642998.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.228) New Way Chemistry for Hong Kong A-Level Book 142 Bond Bond Length /nm Bond Enthalpy / kJ mol-1F-F 0.142 158Cl-Cl 0.199 242Br-Br 0.228 193I-I 0.266 1518.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.228)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. New Way Chemistry for Hong Kong A-Level Book 143 As the size of fluorine atom is very small, the repulsion between the non-bonding pairs of electrons on the fluorine atoms weaken the F-F bond. FFNon-bonding e-/ lone pair of e-8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.228) New Way Chemistry for Hong Kong A-Level Book 144 8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.228) Half the internuclear distance between two atoms in a covalently bonded molecule Generally taken as half of the bond length of homoatomic covalent molecules (where identical atoms are bonded together) New Way Chemistry for Hong Kong A-Level Book 145 8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.228)The covalent radius of an atom is taken as half of the bond length of a homoatomic molecule New Way Chemistry for Hong Kong A-Level Book 146 8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.229)The covalent radii (in nm) of some elements New Way Chemistry for Hong Kong A-Level Book 147 Predicting bond length of A-B if rA & rB are knownBond length of a covalent bond A-B = Covalent radius of atom A + Covalent radius of atom B 8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.229) New Way Chemistry for Hong Kong A-Level Book 148 Bond Calculated bond length (nm) Experimentally determined bond length (nm)C-OC-FC-ClC-BrC-CH-ClC-HN-Cl 0.1500.1490.1760.1910.1540.1360.1140.173 0.1430.1380.1770.1930.1540.1280.1090.174By what technique can the bond lengths be determined experimentally? Similarelectronegativity8.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.229 230) New Way Chemistry for Hong Kong A-Level Book 149 Bond Calculated bond length (nm) Experimentally determined bond length (nm)C-OC-FC-ClC-BrC-CH-ClC-HN-Cl 0.1500.1490.1760.1910.1540.1360.1140.173 0.1430.1380.1770.1930.1540.1280.1090.1748.6 Bond Enthalpies, Bond Lengths and Covalent Radii (SB p.229 230)Quite differentelectronegativity New Way Chemistry for Hong Kong A-Level Book 150 New Way Chemistry for Hong Kong A-Level Book 151 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.231) Geometric arrangement of atoms within the molecules or ions The non-bonding electrons (i.e. the lone pair electrons) are not taken into account New Way Chemistry for Hong Kong A-Level Book 152 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.231)Electron pairs in the valence shell of the central atom of a molecule will stay as far apart as possible to minimize the electrostatic repulsion between electron pairs in the valence shell. The electron pairs are oriented with the maximum separation in space so as to minimize the coulombic repulsion of electron clouds. New Way Chemistry for Hong Kong A-Level Book 153 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.231) Examples:1. Beryllium Chloride (BeCl2) Molecule2. Boron Trifluoride (BF3) Molecule3. Methane (CH4) Molecule4. Ammonium Ion (NH 4+)5. Phosphorus Pentachloride (PCl5) Molecule6. Sulphur Hexafluoride (SF6) Molecule New Way Chemistry for Hong Kong A-Level Book 154 BeCl Cl Electronic DiagramShape in word Linear Bond angle= angle between 2 bondsShape in Diagram8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.231) New Way Chemistry for Hong Kong A-Level Book 155 Shape in wordTrigonal planarBF F F8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)Electronic Diagram Shape in Diagram New Way Chemistry for Hong Kong A-Level Book 156 TetrahedralCH H HH8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)Shape in wordElectronic Diagram Shape in Diagram New Way Chemistry for Hong Kong A-Level Book 157 TetrahedralNH H HH8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)Shape in wordElectronic Diagram Shape in Diagram New Way Chemistry for Hong Kong A-Level Book 158 Trigonal bipyramidal 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)Electronic Diagram Shape in DiagramP Shape in word ClClCl ClCl New Way Chemistry for Hong Kong A-Level Book 159 Octahedral 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)Electronic Diagram Shape in DiagramShape in wordS FF FFF F New Way Chemistry for Hong Kong A-Level Book 160 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.235) The valence shell electron pair repulsion theory statesElectrostatic repulsion decreases in the following order:Lone pair lone pair Lone pair bond pair Bond pair bond pair New Way Chemistry for Hong Kong A-Level Book 161 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.235) Examples:1. Ammonia (NH3) Molecule2. Water (H2O) Molecule3. Amide Ion (NH2) New Way Chemistry for Hong Kong A-Level Book 162 Trigonal pyramidal H HHNlp-lp repulsion lp-bp repulsion bp-bp repulsion8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.235)Electronic Diagram Shape in DiagramShape in word New Way Chemistry for Hong Kong A-Level Book 163 V-shapedlp-lp repulsion lp-bp repulsion bp-bp repulsion 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.236)Electronic Diagram Shape in DiagramShape in wordH HO New Way Chemistry for Hong Kong A-Level Book 164 V-shapedlp-lp repulsion lp-bp repulsion bp-bp repulsion 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.236)Electronic Diagram Shape in DiagramShape in wordH HN New Way Chemistry for Hong Kong A-Level Book 165 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.237 238) New Way Chemistry for Hong Kong A-Level Book 166 New Way Chemistry for Hong Kong A-Level Book 167 8.8 Multiple Bonds (SB p.238) A covalent bond with two shared electrons Some atoms share more than two electrons in a bonde.g. double bond, triple bond New Way Chemistry for Hong Kong A-Level Book 168 Bond Bond order Bond length (nm) Bond enthalpy(kJ mol-1)C CC = CC C 123 0.1540.1340.120 +348+612+837N NN = NN N 123 0.1460.1200.110 +163+409+944C OC = O 12 0.1430.122 +360+7438.8 Multiple Bonds (SB p.239) New Way Chemistry for Hong Kong A-Level Book 169 8.8 Multiple Bonds (SB p.239) Predict the shapes of molecules or polyatomic ions with multiple bonds Examples:1. Ethene (CH2 = CH2) Molecule2. Ethyne (CH CH) Molecule3. Carbon Dioxide (CO 2) Molecule4. Sulphur Dioxide (SO2) Molecule New Way Chemistry for Hong Kong A-Level Book 170 Trigonal planarC C HHHH8.8 Multiple Bonds (SB p.239)Electronic Diagram Shape in DiagramShape in word New Way Chemistry for Hong Kong A-Level Book 171 Linear 8.8 Multiple Bonds (SB p.239)Electronic Diagram Shape in DiagramC C HH Shape in word New Way Chemistry for Hong Kong A-Level Book 172 Linear 8.8 Multiple Bonds (SB p.240)Electronic Diagram Shape in DiagramShape in wordCO O New Way Chemistry for Hong Kong A-Level Book 173 V-shaped 8.8 Multiple Bonds (SB p.240)Electronic Diagram Shape in DiagramShape in wordSO O New Way Chemistry for Hong Kong A-Level Book 174 New Way Chemistry for Hong Kong A-Level Book 175 8.9 Covalent Crystals (SB p.240) May have simple molecular structures or giant covalent structures New Way Chemistry for Hong Kong A-Level Book 176 8.9 Covalent Crystals (SB p.240) Consist of discrete molecules held together by weak intermolecular forces Atoms in a molecule are held together by strong covalent bonds Examples: H 2 , O2 , H2O, CO2, I2 New Way Chemistry for Hong Kong A-Level Book 177 8.9 Covalent Crystals (SB p.240) Consist of millions of atoms bonded covalently together in a structural network No simple molecules present Examples: diamond, graphite and quartz (silicon(IV) oxide) New Way Chemistry for Hong Kong A-Level Book 178 8.9 Covalent Crystals (SB p.240) 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 New Way Chemistry for Hong Kong A-Level Book 179 A diamond crystal 8.9 Covalent Crystals (SB p.241) New Way Chemistry for Hong Kong A-Level Book 180 The structure of diamond 8.9 Covalent Crystals (SB p.241) New Way Chemistry for Hong Kong A-Level Book 181 8.9 Covalent Crystals (SB p.241) Each C atom is covalently bonded to 3 other C atoms in the same layer. A network of coplanar hexagons is formed Weak van der Waals forces hold the layers togetherDelocalized e- free to move within layers Properties: soft and slippery (used as pencil lead), conductor New Way Chemistry for Hong Kong A-Level Book 182 8.9 Covalent Crystals (SB p.241)Graphite New Way Chemistry for Hong Kong A-Level Book 183 8.9 Covalent Crystals (SB p.241) The structure of graphite New Way Chemistry for Hong Kong A-Level Book 184Why graphite has a high m.p. than that of diamond? Property Diamond GraphiteDensity (g cm-3)HardnessMelting point (C)ColourElectrical conductivity 3.5110 (hardest)3 827ColourlessNone 2.27 lone pair and bond pair bond pair and bond pairThus, the bond angle of H 2O is less than that of NH3. Answer New Way Chemistry for Hong Kong A-Level Book 1120 (c) Match the following bond enthalpies to the bonds in the above three molecules:+562 kJ moll, +388 kJ moll, +463 kJ moll8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.237 238)Answer(c) The bond enthalpies are:H N H O H F+388 kJ moll +463 kJ moll +562 kJ mollThe bond enthalpies increase as shown owing to the decrease in bond length and increase in polarity of bonds. Back New Way Chemistry for Hong Kong A-Level Book 1121 What are the shapes of a H2S molecule and a H3O+ ion?Explain their shapes in terms of the valence shell electron pair repulsion theory. Answer8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.238)H2S molecule is V-shaped. In H2S molecule, there are two bond pairs and two lone pairs of electrons in the outermost shell of the central sulphur atom. All three types of electrostatic repulsion (lone pair lone pair, lone pair bond pair, bond pair bond pair) are present. The two lone pairs will stay the furthest apart and the separation between the lone pair and a bond will be greater that that between the two bond pairs. Therefore, the H S H bond angle in the H 2S molecule is about 104.5 instead of 109.5 in tetrahedron. New Way Chemistry for Hong Kong A-Level Book 1122 8.6 Shapes of Covalent Molecules and Polyatomic Ions (SB p.238)H3O+ ion has a trigonal pyramidal shape. In H3O+ ion, the central oxygen atom forms two covalent bonds with two hydrogen atoms respectively. Also, one dative covalent bond is formed between the oxygen atom and the remaining hydrogen ion. We can regard the central oxygen atom has three bond pairs and one lone pair of electrons. According to the valence shell electron pair repulsion theory, the lone pair will stay further away from the three bond pairs. The three bond pairs are in turn compressed closer together. Thus, the H O H bond angles in the H 3O+ ion are about 107 instead of 109.5 in tetrahedron. Back New Way Chemistry for Hong Kong A-Level Book 1123 (a) Does sulphur obey the octet rule in forming a SO2 molecule? Explain your answer. Answer(a) In the formation of SO2 molecule, sulphur does not obey the octet rule because sulphur has 10 electrons in its outermost shell.8.8 Multiple Bonds (SB p.240) New Way Chemistry for Hong Kong A-Level Book 1124 (b) Draw a “dot and cross” diagram of the hydrogen cyanide molecule (HCN). Describe and explain the shape of the molecule. Answer8.8 Multiple Bonds (SB p.240)(b) HCN molecules has a linear shape as the central carbon atom does not have any lone pair electrons. In order to minimize electrostatic repulsion, the two electron clouds of the central carbon atom are separated at a maximum with bond angles of 180.Back
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