New Way Chemistry for Hong Kong A- Level Book 1 1 Chapter 8 Chapter 8 Covalent Bonding Covalent Bonding 8.1 8.1 Formation of Covalent Bon Formation of Covalent Bon ds ds 8.2 8.2 Dative Covalent Bonds Dative Covalent Bonds 8.3 8.3 Bond Enthalpies Bond Enthalpies 8.4 8.4 Estimation of Average Bon Estimation of Average Bon d d Enthalpies using Data f Enthalpies using Data f rom Energetics rom Energetics
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
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.
New Way Chemistry for Hong Kong A-Level Book 113
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.
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.
8.2 Dative Covalent Bonds (SB p.218)
New Way Chemistry for Hong Kong A-Level Book 114
Ammonium Ion (NH4+)
8.2 Dative Covalent Bonds (SB p.219)
New Way Chemistry for Hong Kong A-Level Book 115
Aluminium Chloride Dimer (Al2Cl6)
AlCl3
Why doesn’t Al form ionic compounds with Cl?Why doesn’t Al form ionic compounds with Cl?
Al: relative small atomic size; high I.E.’s required to become a cation of +3 charge.
Al: relative small atomic size; high I.E.’s required to become a cation of +3 charge.
(a dimer of AlCl3)
8.2 Dative Covalent Bonds (SB p.220)
New Way Chemistry for Hong Kong A-Level Book 117
Bond Enthalpies
Why do successive B.D.E. of C-H differ?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-1
8.3 Bond Enthalpies (SB p.221)
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
øø
øø
Bond Dissociation EnthalpiesB.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-1ø
New Way Chemistry for Hong Kong A-Level Book 118
Bond Enthalpies
Why is this value of E(C-H) still different from the previously calculated one (+415.5 kJ mol-1)?
Why is this value of E(C-H) still different from the previously calculated one (+415.5 kJ mol-1)?
8.3 Bond Enthalpies (SB p.222)
Bond Average bond enthalpy (kJ mol-1)
H—H
C—C
C═ C
C≡ C
C—H
O—H
+436
+348
+612
+837
+412
+463
New Way Chemistry for Hong Kong A-Level Book 119
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.
E(C-H) = +415.5 kJ mol-1E(C-H) = +415.5 kJ mol-1
8.4 Estimation of Average Bond Enthalpies using Data from Energetics (SB p.223)
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
C(g) + 4H(g) ΔH = +1 662 kJ mol-1
New Way Chemistry for Hong Kong A-Level Book 120
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 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 moleThe 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
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
8.4 Estimation of Average Bond Enthalpies using Data from Energetics (SB p.223)
New Way Chemistry for Hong Kong A-Level Book 121
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 Energetics (SB p.224-225)
From the Enthalpy Changes of Atomization of Two Compounds
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
New Way Chemistry for Hong Kong A-Level Book 122
8.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of Reactions (SB p.225)
Reaction of ethene and hydrogen
Sum of bond enthalpies of products
Enthalpy change
of reaction =
Sum of bond enthalpies of reactants
-
New Way Chemistry for Hong Kong A-Level Book 123
8.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of Reactions (SB p.226)
Enthalpy profile for the reaction of ethene and hydrogen
New Way Chemistry for Hong Kong A-Level Book 124
?
Sum of bond enthalpies of reactants= E(C=C) + 4E(C-H) + E(H-H)= (612) + 4(412) + (436)= +2696 kJmol-1
Sum of bond enthalpies of products= E(C-C) + 6E(C-H)= (348) + 6(412) = +2820 kJmol-1
Hr = 2696 - (+2820)
= -124 kJ mol-1
Only an estimated value for H
r .Why?Only an estimated value for H
r .Why?
8.5 Use of Average Bond Enthalpies to Estimate Enthalpy Changes of Reactions (SB p.225)
Reaction of ethene and hydrogen
New Way Chemistry for Hong Kong A-Level Book 125
Consider C-C, C=C and CC
bond order = 1
bond order = 2
bond order = 3
As the bond order increases, the bond strength also increases.As the bond order increases, the bond strength also increases.
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.227)
Bond Average bond enthalpy (kJ mol-1)
H—H
C—C
C═ C
C≡ C
C—H
O—H
+436
+348
+612
+837
+412
+463
Bond Enthalpies as an Indication of the Strength of Covalent Bonds
New Way Chemistry for Hong Kong A-Level Book 126
Bond length (for covalent bond)Bond length (for covalent bond)
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.228)
Bond Lengths
New Way Chemistry for Hong Kong A-Level Book 127
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.228)
Bond lengths of some covalent bondsBond Molecule Bond lengths (nm)O-H H2O
H2O2
CH3OHHCOOH
0.0960.0970.0960.096
C-C DiamondC2H6
C3H8
C2H5OH
0.1540.1540.1540.155
C-H CH4
C2H4
0.1100.109
By what technique can these bond lengths be measured?
Bond lengths of some covalent bonds
X-ray diffraction
New Way Chemistry for Hong Kong A-Level Book 128
Any conclusion for the relationship between bond length & bond enthalpy?
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).
Usually a longer bond length corresponds to a lower value of bond enthalpy (weaker bond).
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.228)
Bond Bond length (nm)
Bond enthalpy
(kJ mol-1)
H-H
Cl-Cl
Br-Br
I-I
H-F
H-Cl
H-Br
H-I
0.074
0.199
0.228
0.266
0.092
0.127
0.141
0.161
436
242
193
151
565
431
364
299
Bond enthalpies and bond lengths
New Way Chemistry for Hong Kong A-Level Book 129
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.
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.
Bond Bond Length /nm Bond Enthalpy / kJ mol-1
Cl-Cl 0.199 242Br-Br 0.228 193 I-I 0.266 151
FF
Non-bonding e-
/ lone pair of e-
F-F 0.142 158
8.6 Relationship between Bond Enthalpies and Bond Lengths
Special Situation for F2
New Way Chemistry for Hong Kong A-Level Book 130
(often referred as ‘Atomic radius’)
Where b is the bond length r is the covalent 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)
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)
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.228)
Covalent Radius
New Way Chemistry for Hong Kong A-Level Book 131
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.229)
The covalent radii (in nm) of some elements
New Way Chemistry for Hong Kong A-Level Book 132
Predicting bond length of A-B if rA & rB are knownPredicting bond length of A-B if rA & rB are known
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.229)
Bond length of a covalent bond A-B
=Covalent radius of atom A
+Covalent radius of atom B
Bond length of Covalent molecule A-B
New Way Chemistry for Hong Kong A-Level Book 133
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.230)
Bond Calculated bond length (nm)
Experimentally determined bond length (nm)
C-O
C-F
C-Cl
C-Br
C-C
H-Cl
C-H
N-Cl
0.150
0.149
0.176
0.191
0.154
0.136
0.114
0.173
0.143
0.138
0.177
0.193
0.154
0.128
0.109
0.174
Calculated and experimentally determined bond length
By what technique can the bond lengths be determined experimentally?
By what technique can the bond lengths be determined experimentally?
Similarelectronegativity
New Way Chemistry for Hong Kong A-Level Book 134
Bond Calculated bond length (nm) Experimentally determined bond length (nm)
C-O
C-F
C-Cl
C-Br
C-C
H-Cl
C-H
N-Cl
0.150
0.149
0.176
0.191
0.154
0.136
0.114
0.173
0.143
0.138
0.177
0.193
0.154
0.128
0.109
0.174
8.6 Relationship between Bond Enthalpies and Bond Lengths (SB p.230)
Calculated and experimentally determined bond length
Quite differentelectronegativity
New Way Chemistry for Hong Kong A-Level Book 135
Shape of a moleculeShape of a molecule
= geometric arrangement of atoms within the molecules
Valence Shell Electron Pair Repulsion Theory (VSEPR)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.
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.231)
Shapes of Molecules and Polyatomic Ions
TWO bond pairs in linear geometry, THREE bond pairs in trigonal planar geometry FOUR bond pairs in tetrahedral geometry FIVE bond pairs in trigonal bipyramidal geometry SIX bond pairs in octahedral geometry
New Way Chemistry for Hong Kong A-Level Book 136
Beryllium chloride molecule (BeCl2)
BeClCl
Electronic Diagram
Shape in word linear
Bond angle= angle between
2 bonds
Bond angle= angle between
2 bonds
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)
Shape in Diagram
New Way Chemistry for Hong Kong A-Level Book 137
Boron trifluoride molecule(BF3)
Electronic DiagramShape in Diagram
Shape in word
trigonal planar
B
F F
F
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)
New Way Chemistry for Hong Kong A-Level Book 138
Methane (CH4) Molecule
Electronic Diagram Shape in Diagram
Shape in word
tetrahedral
CH H
H
H
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.221)
New Way Chemistry for Hong Kong A-Level Book 139
Electronic DiagramShape in Diagram
Shape in word
tetrahedral
CH H
H
H
C
H
H HH
bond in front of paper
bond behind paper
bond in the plane of paper
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)
Methane (CH4) Molecule
New Way Chemistry for Hong Kong A-Level Book 140
Phosphorus Pentachloride (PCl5) Molecule
Electronic DiagramShape in Diagram
Shape in word
trigonal bipyramidal
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)
New Way Chemistry for Hong Kong A-Level Book 141
Electronic Diagram Shape in Diagram
Shape in word
trigonal bipyramidal
P
Cl
Cl
Cl
Cl
Cl
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)
Phosphorus Pentachloride (PCl5) Molecule
New Way Chemistry for Hong Kong A-Level Book 142
Sulphur Hexafluoride (SF6)Electronic Diagram Shape in Diagram
Shape in word
octahedral
S
F
F
F
F
F
F
8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.232)
• 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
8.9 Covalent Crystals (SB p.241)
New Way Chemistry for Hong Kong A-Level Book 155
Why graphite has a high m.p. than that of diamond?Why graphite has a high m.p. than that of diamond?
8.9 Covalent Crystals (SB p.242)
Comparison of the properties of diamond and graphite
Property Diamond Graphite
Density (g cm-3)
Hardness
Melting point (oC)
Colour
Electrical conductivity
3.51
10 (hardest)
3 827
Colourless transparent
None
2.27
< 1 (very soft)
3 652 (sublime)
Shiny black
High (along sheet)
New Way Chemistry for Hong Kong A-Level Book 156
Each Si atom is bonded tetrahedrally to 4 neighbouring O atomsEach O atom is bonded to 2 Si atoms, one at the centre of each of two adjacent tetrahedral