Ch08 - Covalent Bonding

New Way Chemistry for Hong Kong A-Level Book 11

Chapter 8Chapter 8Covalent BondingCovalent Bonding

8.18.1 Formation of Covalent Bonds Formation of Covalent Bonds

8.2 8.2 Dative Covalent Bonds Dative Covalent Bonds

8.3 8.3 Bond Enthalpies Bond Enthalpies

8.48.4 Estimation of Average Bond Estimation of Average Bond

Enthalpies using Data from EnergeticsEnthalpies using Data from Energetics


8.58.5 Use of Average Bond Enthalpies to Use of Average Bond Enthalpies to

Estimate Enthalpy Changes of ReactionsEstimate Enthalpy Changes of Reactions

8.6 8.6 Relationship between Bond Enthalpies Relationship between Bond Enthalpies

and Bond Lengthsand Bond Lengths

8.7 8.7 Shapes of Covalent Molecules and Shapes of Covalent Molecules and

Polyatomic IonsPolyatomic Ions

8.88.8 Multiple Bonds Multiple Bonds

8.98.9 Covalent Crystals Covalent Crystals


Electron Sharing in Covalent Bonds

H H

Shared 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)

electrostatic

Overlapping of atomic orbitals covalent bond formationOverlapping of atomic orbitals covalent bond formation

8.1 Formation of Covalent Bonds (SB p.213)



A hydrogen molecule is achieved by partial overlapping of 1s orbitals


Electrons are shared between the two atoms.

Compare electron-density-map for ionic compounds:


Electron density map for covalent compoundsThere is substantial electron density at all points along the inter-nuclear axis.



Electron density map for ionic compounds


Covalent Bonds in ElementsSome Examples

Dot and cross diagramDot and cross diagram



Covalent Bonds in CompoundsSome Examples

Carbon

1s 2s 2p

All the above examples obey ____________.All the above examples obey ____________.Octet rule



Some Examples All the above examples obey ____________.All the above examples obey ____________.

CarbonExcited state 1s 2s 2p

electrons from H

Octet rule

octet


Covalent Bonds in Compounds


Octet Rule and its limitations

BF3

Why doesn’t B form ionic compounds with F?Why doesn’t B form ionic compounds with F?

B: small atomic size

high I.E.’s required to become a cation.

B: small atomic size

high I.E.’s required to become a cation.

electrons from F

8.1 Formation of Covalent Bonds (SB p.217-8)

not fulfilling octet (electron deficient)


Octet Rule and its limitations

PCl5

Why Phosphorus can expand its octet to form PCl5?Why Phosphorus can expand its octet to form PCl5?

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

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

electrons from Cl

8.1 Formation of Covalent Bonds (SB p.217-8)


NH3BF3 Molecule8.2 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.


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)


Ammonium Ion (NH4+)



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)



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


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


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


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)


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


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

-



Enthalpy profile for the reaction of ethene and hydrogen


?

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?


Reaction of ethene and hydrogen


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


Bond length (for covalent bond)Bond length (for covalent bond)


Bond Lengths



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


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).


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


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


(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)


Covalent Radius



The covalent radii (in nm) of some elements


Predicting bond length of A-B if rA & rB are knownPredicting bond length of A-B if rA & rB are known


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



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


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

Quite differentelectronegativity


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


Beryllium chloride molecule (BeCl2)

BeClCl

Electronic Diagram

Shape in word linear

Bond angle= angle between

2 bonds

Bond angle= angle between

2 bonds


Shape in Diagram


Boron trifluoride molecule(BF3)

Electronic DiagramShape in Diagram

Shape in word

trigonal planar

B

F F

F



Methane (CH4) Molecule

Electronic Diagram Shape in Diagram

Shape in word

tetrahedral

CH H

H

H




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


Methane (CH4) Molecule


Phosphorus Pentachloride (PCl5) Molecule


Shape in word

trigonal bipyramidal




Shape in word

trigonal bipyramidal

P

Cl

Cl

Cl

Cl

Cl


Phosphorus Pentachloride (PCl5) Molecule


Sulphur Hexafluoride (SF6)Electronic Diagram Shape in Diagram

Shape in word

octahedral

S

F

F

F

F

F

F



Ammonia (NH3) Molecule


Shape in word

Trigonal pyramidal

H H

H

N

lp-lp repulsion > lp-bp repulsion > bp-bp repulsion





Shape in word

Trigonal pyramidal

H H

H

N

bond in front of paper

bond behind paper

bond in the plane of paper

N

H HH

lone pair of e-


Ammonia (NH3) Molecule


Water (H2O) Molecule


Shape in word

V-shaped / bent

H HO






Shape in word

V-shaped / bent

H H

lone pair of e-

O

O

H H

Water (H2O) Molecule8.7 Shapes of Covalent Molecules and Polyatomic Ions (SB p.236)


8.8 Multiple Bonds (SB p.239)

Comparison of bond lengths and bond enthalpies between single and multiple bonds

Bond Bond order Bond length

(nm)

Bond enthalpy

(kJ mol-1)

C—C

C═ C

C≡ C

1

2

3

0.154

0.134

0.120

+348

+612

+837

N—N

N═ N

N≡ N

1

2

3

0.146

0.120

0.110

+163

+409

+944

C—O

C ═O

1

2

0.143

0.122

+360

+743


Ethene (CH2=CH2)


Shape in word

(planar)

C C

H

HH

H


Effect of Multiple Bonding on Shapes of Molecules


Ethyne (CHCH)

Shape in Diagram Shape in word

linear



Carbon dioxide (CO2)


linear




Angular / (bent)

Sulphur dioxide (SO2)



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

8.9 Covalent Crystals (SB p.241)

Giant Covalent Structures


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



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



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


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)


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

Quartz /Silicon(IV) oxide8.9 Covalent Crystals (SB p.242)

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Ch08 - Covalent Bonding