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New Way Chemistry for Hong Kong A-Level Book 11
Chapter 11Chapter 11Intermolecular ForcesIntermolecular Forces
11.111.1 Polarity of MoleculesPolarity of Molecules
11.211.2 Van der Waals’ ForcesVan der Waals’ Forces
11.311.3 Van der Waals’ RadiiVan der Waals’ Radii
11.411.4 Molecular Crystals Molecular Crystals
11.511.5 Hydrogen Bonding Hydrogen Bonding
New Way Chemistry for Hong Kong A-Level Book 12
Polarity of Molecules
Intermoleculear forcesIntermoleculear forces
Van der Waal’s forces
Van der Waal’s forces
hydrogen bonding
hydrogen bonding
(very weak when compare with covalent
bond between atoms in molecule)
electrostatic attraction between dipoles, i.e. the attraction between the +ve end of one molecule and the -ve end of another molecule
11.1 Polarity of Molecules (SB p.257)
New Way Chemistry for Hong Kong A-Level Book 13
3 types of dipoles3 types of dipoles
Permanent dipole
Permanent dipole
Instantaneous dipole
Instantaneous dipole
Induced dipole
Induced dipole
11.1 Polarity of Molecules (SB p.257)
Polarity of Molecules
New Way Chemistry for Hong Kong A-Level Book 14
Permanent Dipole
A permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms.
A permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms.
11.1 Polarity of Molecules (SB p.257)
New Way Chemistry for Hong Kong A-Level Book 15
Instantaneous Dipole
An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.
An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.
11.1 Polarity of Molecules (SB p.258)
New Way Chemistry for Hong Kong A-Level Book 16
Induced Dipole
An induced dipole is a temporary dipole that is created due to the influence of neighbouring dipole (which may be a permanent or an instantaneous dipole).
An induced dipole is a temporary dipole that is created due to the influence of neighbouring dipole (which may be a permanent or an instantaneous dipole).
11.1 Polarity of Molecules (SB p.258)
New Way Chemistry for Hong Kong A-Level Book 17
Van der Waals’ Forces
Van der Waals’ forces
Van der Waals’ forces
Dipole-Dipole
Interaction
Dipole-Dipole
Interaction
Dipole-Induced Dipole
Interaction
Dipole-Induced Dipole
Interaction
Instantaneous Dipole-
Induced DipoleInteraction
Instantaneous Dipole-
Induced DipoleInteraction
11.2 Van der Waal’s Forces (SB p.258)
New Way Chemistry for Hong Kong A-Level Book 18
Dipole-Dipole Interactions
Polar molecules have permanent dipole moments. They tend to orient themselves in such a way that the attractive forces between molecules are maximized while repulsive forces are minimized.
Polar molecules have permanent dipole moments. They tend to orient themselves in such a way that the attractive forces between molecules are maximized while repulsive forces are minimized.
11.2 Van der Waal’s Forces (SB p.258)
New Way Chemistry for Hong Kong A-Level Book 19
Dipole-Induced Dipole Interactions
When a non-poar molecule approaches a polar molecule (with a permanent dipole), a dipole will be induced in the non-polar molecule. The dipole induced will be in opposite orintation to that of the polar molecule.
When a non-poar molecule approaches a polar molecule (with a permanent dipole), a dipole will be induced in the non-polar molecule. The dipole induced will be in opposite orintation to that of the polar molecule.
11.2 Van der Waal’s Forces (SB p.259)
New Way Chemistry for Hong Kong A-Level Book 110
Instantaneous Dipole-Induced Dipole Interactions
The instantaneous dipole will induce a dipole moment in the neighbouring atom by attracting opposite charges. If the +ve end of the dipole is pointing towards a neighbouring atom, the induced dipole will then have its -ve end pointing towards the +ve pole of that dipole.
The instantaneous dipole will induce a dipole moment in the neighbouring atom by attracting opposite charges. If the +ve end of the dipole is pointing towards a neighbouring atom, the induced dipole will then have its -ve end pointing towards the +ve pole of that dipole.
11.2 Van der Waal’s Forces (SB p.259)
New Way Chemistry for Hong Kong A-Level Book 111
11.2 Van der Waal’s Forces (SB p.260)
Strength of Van der Waals’ Forces
Type of interaction Magnitude (kJ mol-
1)
Dipole-dipole 5-25
Dipole-induced dipole 2-10
Instantaneous dipole-induced dipole
0.05-50
New Way Chemistry for Hong Kong A-Level Book 112
The greater the no. of e-s in a molecule
The greater the no. of e-s in a molecule
The more weakly they are held by the nucleus
The more weakly they are held by the nucleus
The easier the instantaneous dipole can be set up (greater van der Waals’ forces)
The easier the instantaneous dipole can be set up (greater van der Waals’ forces)
11.2 Van der Waal’s Forces (SB p.260)
Molecule Boiling point (oC)
Helium
Neon
Argon
-269
-246
-186
Fluorine
Chlorine
Bromine
-188
-34.7
58.8
Methane
Ethane
Propane
-162
-88.6
-42.2
New Way Chemistry for Hong Kong A-Level Book 113
The van der Waals’ forces also increase with the surface area of the molecule.
The van der Waals’ forces also increase with the surface area of the molecule.
11.2 Van der Waal’s Forces (SB p.261)
Surface Area of Molecule
New Way Chemistry for Hong Kong A-Level Book 114
Van der Waals’ Radii11.3 Van der Waal’s Radii (SB p.262)
New Way Chemistry for Hong Kong A-Level Book 115
Radii of iodine
The covalent radius is one half of the distance between two atoms in the same molecule.
The covalent radius is one half of the distance between two atoms in the same molecule.
The van der Waals’ radius is one half of the distance between two atoms in adjacent molecule.
The van der Waals’ radius is one half of the distance between two atoms in adjacent molecule.
11.3 Van der Waal’s Radii (SB p.262)
New Way Chemistry for Hong Kong A-Level Book 116
11.3 Van der Waal’s Radii (SB p.263)
Radii of some elements
New Way Chemistry for Hong Kong A-Level Book 117
sum of covalent radii
sum of covalent radii
sum of van der Waals’ radii
sum of van der Waals’ radii
11.3 Van der Waal’s Radii (SB p.263)
Structure of graphite
New Way Chemistry for Hong Kong A-Level Book 118
Molecular Crystals
A molecular crystal is a structure which consists of individual molecules packed together in a regular arrangement by weak intermolecular forces.
A molecular crystal is a structure which consists of individual molecules packed together in a regular arrangement by weak intermolecular forces.
11.4 Molecular Crystals (SB p.264)
New Way Chemistry for Hong Kong A-Level Book 119
very +vevery +veF being very
electronegativeF being very
electronegative
11.5 Hydrogen Bonding (SB p.264)
HF molecule
F atom being small enough to approach very close to the H atom in the neighbouring molecule
F atom being small enough to approach very close to the H atom in the neighbouring molecule
New Way Chemistry for Hong Kong A-Level Book 120
11.5 Hydrogen Bonding (SB p.265)
The relative strength of van der Waals’ forces, hydreogen bond and covalent bond
Phenomenon Energy involved
(kJ mol-1)
Forces overcome
Sublimation of solid helium
0.11 Van der Waals’ forces
Sublimation of
ice
46.90 Hydrogen bonds
Dissociation of hydrogen molecules
436.00 Cocalent bonds
New Way Chemistry for Hong Kong A-Level Book 121
11.5 Hydrogen Bonding (SB p.265)
Formation of hydrogen bonds in hydrogen fluoride
New Way Chemistry for Hong Kong A-Level Book 122
11.5 Hydrogen Bonding (SB p.265)
Formation of hydrogen bonds in water
New Way Chemistry for Hong Kong A-Level Book 123
Formation of hydrogen bonds in ammonia11.5 Hydrogen Bonding (SB p.265)
New Way Chemistry for Hong Kong A-Level Book 124
11.5 Hydrogen Bonding (SB p.265)
Formation of hydrogen bonds in methanol
New Way Chemistry for Hong Kong A-Level Book 125
Experimental Determination of the Strength of Hydrogen Bond
trichloromethane
Cl
Cl
Cl
Cl
very +vevery +ve
?Any H-bond formed?
11.5 Hydrogen Bonding (SB p.266)
New Way Chemistry for Hong Kong A-Level Book 126
Experimental Determination of the Strength of Hydrogen Bond
ethyl ethanoate
11.5 Hydrogen Bonding (SB p.266)
?Any H-bond formed?
New Way Chemistry for Hong Kong A-Level Book 127
Experimental Determination of the Strength of Hydrogen Bonds
?Any H-bond formed?
(YES!)
H bond formed between trichloromethane & ethyl ethanoate
How strong is it?
Hint: When you mix the 2 liquids together, what will happen?Hint: When you mix the 2 liquids together, what will happen?
11.5 Hydrogen Bonding (SB p.266)
New Way Chemistry for Hong Kong A-Level Book 128
Intramolecular Hydrogen bonding
Butenedioic acid
C
C
O
OH
H
C
H
C
O
OH
CC
H
C
O
OH
C
O
OH
H
cis- butenedioic acid trans- butenedioic acidm.p. = 1300C m.p. = 2900C
cis-trans isomers(geometric isomers)
cis-trans isomers(geometric isomers)
C C
O
OH
H
C
H
C
O
OH
11.5 Hydrogen Bonding (SB p.267)
New Way Chemistry for Hong Kong A-Level Book 129
Intramolecular Hydrogen bonding
m.p. = 1300C
cis- butenedioic acid trans- butenedioic acid
m.p. = 2900C
Owing to the formation of intramolecular H bonds, cis-butenedioic acid forms less extensive intermolecular H bonds with neighbouring molecules.
Owing to the formation of intramolecular H bonds, cis-butenedioic acid forms less extensive intermolecular H bonds with neighbouring molecules.
11.5 Hydrogen Bonding (SB p.267)
New Way Chemistry for Hong Kong A-Level Book 130
Intramolecular Hydrogen bonding
NO2
OH
2-nitrophenol
NO2
OH
4-nitrophenol
Can you match the two compounds with the following m.p.’s?
m.p. = 2160Cm.p. = 2160C m.p. = 2590Cm.p. = 2590C
11.5 Hydrogen Bonding (SB p.267)
New Way Chemistry for Hong Kong A-Level Book 131
Intramolecular Hydrogen bonding
m.p. = 2160Cm.p. = 2160C m.p. = 2590Cm.p. = 2590C
11.5 Hydrogen Bonding (SB p.267)
New Way Chemistry for Hong Kong A-Level Book 132
Anomalous Properties of the Second Period Hydrides
Molecular mass Molecular mass
Van der Waals’ forces (b.p. )
Van der Waals’ forces (b.p. )
There must be some type of intermolecular force (which is much stronger than van der Wa
als’ forces) in NH3, H2O & HF
There must be some type of intermolecular force (which is much stronger than van der Wa
als’ forces) in NH3, H2O & HF
(Hydrogen bonding)
11.5 Hydrogen Bonding (SB p.267)
New Way Chemistry for Hong Kong A-Level Book 133
Essential requirements for the formation of a hydrogen bond
1. A hydrogen atom must be directly bonded to a highly electronegative atom (F, O, N).1. A hydrogen atom must be directly bonded to a highly electronegative atom (F, O, N).
2. An unshared pair of electrons (lone pair electrons) on the electronegative atom.2. An unshared pair of electrons (lone pair electrons) on the electronegative atom.
11.5 Hydrogen Bonding (SB p.267)
New Way Chemistry for Hong Kong A-Level Book 134
Boiling Points and Solubilities of Alcohol11.5 Hydrogen Bonding (SB p.268)
New Way Chemistry for Hong Kong A-Level Book 135
Organic compounds are usually insoluble in water,e.g. ethane (C2H6) or chloroethane (C2H5Cl)Organic compounds are usually insoluble in water,e.g. ethane (C2H6) or chloroethane (C2H5Cl)
But alochols of low molecular mass are soluble in water, e.g methanol (CH3OH) and ethanol (C2H5OH).
But alochols of low molecular mass are soluble in water, e.g methanol (CH3OH) and ethanol (C2H5OH).
11.5 Hydrogen Bonding (SB p.268)
Boiling Points and Solubilities of Alcohol
New Way Chemistry for Hong Kong A-Level Book 136
Dimerization of Carboxylic Acids
In vapour phase or in organic solvents, carboxylic acids (alkanoic acids) exist as dimers.In vapour phase or in organic solvents, carboxylic acids (alkanoic acids) exist as dimers.
A dimer of ethanoic acid
11.5 Hydrogen Bonding (SB p.268)
New Way Chemistry for Hong Kong A-Level Book 137
Hydrogen Bonding in Water and Ice
H bonding in water
In water, the molecules are in constant motion. H bonds are formed and broken continually. The arrangement of molecules are thus in random.
In water, the molecules are in constant motion. H bonds are formed and broken continually. The arrangement of molecules are thus in random.
11.5 Hydrogen Bonding (SB p.269)
New Way Chemistry for Hong Kong A-Level Book 138
H-bonding in ice
In ice, the molecular motion is of a minimum and the molecules are oriented in such a way that the max. no. of H bonds are formed. This creates an open structure. (density of ice < density of water)
In ice, the molecular motion is of a minimum and the molecules are oriented in such a way that the max. no. of H bonds are formed. This creates an open structure. (density of ice < density of water)
11.5 Hydrogen Bonding (SB p.269)
New Way Chemistry for Hong Kong A-Level Book 139
Hydrogen Bonding in Proteins
The primary structure of a protein consists of a sequence of amino acids.
The primary structure of a protein consists of a sequence of amino acids.
11.5 Hydrogen Bonding (SB p.270)
New Way Chemistry for Hong Kong A-Level Book 140
11.5 Hydrogen Bonding (SB p.270)
Hydrogen Bonding in Proteins
H bonds formed bewteen NH and CO groups of protein chains. This creates the secondary coiled (helix) structure of the protein chain.
H bonds formed bewteen NH and CO groups of protein chains. This creates the secondary coiled (helix) structure of the protein chain.
New Way Chemistry for Hong Kong A-Level Book 141
Hydrogen Bonding in DNA
A model of the DNA helix
DNA (deoxyribonnuclei acid) is present in the nuclei of living cells and carries genetic information. It consists of two macromolecular strands spiraling round each other in the form of a double helix.
11.5 Hydrogen Bonding (SB p.271)
New Way Chemistry for Hong Kong A-Level Book 142
H bonding in the double helix of DNA
11.5 Hydrogen Bonding (SB p.271)
New Way Chemistry for Hong Kong A-Level Book 143
The END