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Brown's Chemistry Advanced A Level Revision Notes - Theoretical–Physical
Advanced Level
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 8
8.2.1 Intermolecular Forces -
Intermolecular Bonding – Van der Waals forces
and the boiling point comparison of 8 organic molecules of
similar molecular mass
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INDEX for Part 8.
Phase equilibria–vapour
pressure, boiling point and intermolecular forces
Index of ALL my chemical equilibrium
context revision notes Index
ALL my advanced A
level theoretical
chemistry revision study notes
The different types of intermolecular force
(intermolecular bond) are described, explained and discussed with
examples, collectively known as Van der Waals Forces.
i.e. Instantaneous dipole – induced dipole interaction (London
forces, dispersive/dispersion forces), permanent dipole – permanent
dipole interactions (Keesom forces, orientation forces), Permanent
dipole – induced dipole interactions (Debye forces, induction forces)
8.2 Survey of
8 selected organic molecules
– their boiling points and intermolecular forces
8.2.1 Introduction to intermolecular forces
– Van der
Waals forces
also referred to as 'intermolecular bonding'
forces
(do NOT confuse with
chemical bonding between atoms or ions i.e. so-called ionic, covalent or
metallic bonds)
A definition of Van der Waals forces
These can be
defined as weak, short-range electrostatic attractive forces
between uncharged molecules, arising from the interaction of
permanent or transient electric dipole moments and the different
types and their origin are described below.
-
From the start understand that:
-
Intermolecular forces are all about
partially positive (δ+)
sites and partially negative
(δ–)
sites on molecules causing the attraction between neighbouring
molecules - though their origin can differ.
-
The fact that molecules
congregate together to form liquids and solids suggests that there
must be attractive forces between the molecules independently from
the intramolecular bonds which hold the atoms together in the
molecule.
-
The origin of each
source of intermolecular force is summarised below and discussed further
for particular molecules.
-
In the context of this
page, the word dipole
means an asymmetric distribution of electron electrical charge to
give partially positive (δ+) and partially negative
(δ–) regions in the
same molecule.
-
In a simple sense its a molecule with
a partially positive end and a partial negative charge at the other
end.
-
Electric dipoles (δ+
and δ–) may be
permanent or transient (temporary) and the molecules
discussed here are electrically neutral overall.
-
There
are always attractive forces operating between ANY particles whatever
their particle constitution in gases, liquids or solids composed of
atoms, ions or molecules.
-
They are referred to as intermolecular attractive forces
or intermolecular bonding.
-
Collectively they are often
referred to as Van der Waals forces.
-
DO NOT confuse
intermolecular bonds with the very much stronger intramolecular
bonds e.g. between atoms in a molecule like the O-H bond holding
atoms together in water, or the C-C and C-H bonds holding atoms
together in hydrocarbon molecules.
-
The total intermolecular
force is quoted as a summation of the various possible dipoles
interaction and the principal attractive forces are shown in bold for selected
molecules.
-
Wherever possible,
albeit just for a few cases, I've quoted % contributions from the three
types of intermolecular attractive force that I've been able to
obtain from internet searches or textbooks and if I couldn't match
the molecule then I may quote percentages for a similar molecule.
-
One source used by writers of research papers is A. L. McClellan,
Tables of Experimental Dipole Moments.
-
In pre–university
advanced chemistry exams I suggest you
use the terms in bold to
describe the intermolecular force component
-
Summary of the types of
intermolecular bonding forces (Van Waals forces)
-
(ii)
δ–:O–Hδ+
e.g. in water H2O (above), alcohols ROH (above), carboxylic acids
RCOOH (R = alkyl or aryl)
-
(iii)
δ+H–Fδ–
in hydrogen fluoride HF
-
and via these highly polar bonds you get molecule to
molecule attraction via so called hydrogen bonding.
-
Note the spatially
important non-bonding pairs of electrons (:)
on the most electronegative atom.
-
These are the strongest permanent dipole permanent
dipole intermolecular forces
-
e.g. using
llll to indicate a hydrogen bond
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δ–O–Hδ+llllδ–:O–Hδ+llllδ–:O–Hδ+llllδ–:O–Hδ+llll in water (liquid or solid ice)
-
llll
δ–N–Hδ+llllδ–:N–Hδ+llllδ–:N–Hδ+ in amines
or liquid ammonia,
-
in the case of
carboxylic acids the dominant interaction is the hydrogen bonding
via
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In hydrogen fluoride, in
all physical states you get a 'chain connection'
llll
δ–F–Hδ+llllδ–F–Hδ+llllδ–F–Hδ+
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You can also get hydrogen
bonding between these different molecules e.g.
-
δ–N–Hδ+llllδ–:O–Hδ+ or δ–O–Hδ+llllδ–:N–Hδ+ in aqueous ammonia solution (NH3(aq),
-
C-δ–O–Hδ+llllδ–:O–Hδ+ in aqueous carboxylic
acid solutions RCOOH(aq),
-
δ–F–Hδ+llllδ–:O–Hδ+
or δ–O–Hδ+llllδ–:F–Hδ+
in hydrofluoric acid solution (HF(aq)).
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A slightly different
and modestly intriguing case of hydrogen bonding!
-
Trichloromethane
(CHCl3) and propanone (CH3COCH3)
are both polar molecules, but do not hydrogen bond with
themselves.
-
BUT, if you mix the
two liquids, they readily dissolve in each other via
hydrogen bonding!
-
Cl3C-Hδ+llllδ-:O=C(CH3)2
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Note that the three
electronegative chlorine atoms have such an effect on
the carbon atom (δ+),
that the hydrogen atom also acquires a sufficient (δ+)
to hydrogen bond with the oxygen atom (via a lone pair
of electrons).
The survey and a preliminary
summary table
hopefully justified by the arguments outlined after the
table and in on a separate page in
section
8.2.2
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Ins =
instantaneous (temporary) dipole – induced dipole attraction (a sort
of baseline force since it applies to all molecules, in fact it
operates between ANY adjacent particles - atoms, ions or molecules).
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WP = weaker
permanent dipole – permanent dipole attraction (doesn't seem to have
much effect on the boiling point)
-
SP stronger
permanent dipole – permanent dipole attraction (NOT H
bonding, but has a definite effect on the boiling point)
-
HB = hydrogen
bonding attraction - the strongest permanent dipole – permanent dipole
attractive force, i.e. the strongest SP and has the largest effect
on the boiling point)
-
MHB multiple hydrogen bonding
attraction sites on the molecule
(i.e. where there are at least two 'functional' groups capable of two permanent
dipole – permanent dipole interactions including hydrogen bonding,
hence
producing an even bigger effect on raising the boiling point)
-
Note that permanent dipole – induced
dipole attractive forces are not mentioned much and generally only
contribute a small portion of the total intermolecular
force.
-
Also, where I can obtain
data, I've indicated the percentage contribution of the three
types of intermolecular attraction which contribute to the total
intermolecular force i.e. the % contributions to Van der Waals
force.
-
D = Debye dipole moment
units
8.4 Table 1a. Comparing 32–34
electron species – linear organic
molecules (4 C/O/N atoms)
1.to 8. are discussed in detail
on a separate page |
MOLECULE |
formula |
Mr |
electrons |
boiling point
K/oC |
ΔHvap
kJmol–1 |
Dipole moment
D |
Intermolecular
forces |
1. butane |
CH3CH2CH2CH3 |
58 |
34 |
272.5K/–0.5oC |
22 |
0.00 |
Ins |
2. methoxyethane |
CH3OCH2CH3 |
60 |
34 |
280K/7oC |
21 |
1.23 |
Ins, WP |
3. chloroethane |
CH3CH2Cl |
64.5 |
34 |
285.5K/12.5oC |
25 |
2.06 |
Ins, WP |
4. propylamine |
CH3CH2CH2NH2 |
59 |
34 |
321K/48oC |
30 |
1.17 |
Ins, HB |
5. propanone |
CH3COCH3 |
58 |
32 |
329K/56oC |
29 |
2.88 |
Ins, SP |
6. propan–1–ol |
CH3CH2CH2OH |
60 |
34 |
370K/97oC |
45 |
1.69 |
Ins, HB |
7. ethanoic acid |
CH3COOH |
60 |
32 |
391K/118oC |
58 |
1.74 |
Ins,
SP, MHB |
8. ethanamide |
CH3CONH2 |
59 |
32 |
494K/221oC |
46 |
3.60 |
Ins,
SP, MHB |
WHAT NEXT?
INDEX for Part 8.
Phase equilibria–vapour
pressure, boiling point and intermolecular forces
Index of ALL my chemical equilibrium
context revision notes Index
Part 8 sub–index:
8.1 Vapour pressure origin and examples * 8.2.1
Introduction to Intermolecular Forces * 8.2.2
Detailed
comparative discussion of boiling points of 8 organic molecules * 8.3
Boiling point plots for six
organic
homologous series * 8.4 Other case studies of
boiling points related to intermolecular forces * 8.5
Steam
distillation – theory and practice * 8.6 Evidence and theory
for hydrogen bonding in simple covalent hydrides *
8.7
Solubility of covalent compounds, miscible and
immiscible liquids
Advanced Equilibrium Chemistry Notes Part 1. Equilibrium,
Le Chatelier's Principle–rules * Part 2. Kc and Kp equilibrium expressions and
calculations * Part 3.
Equilibrium and industrial processes * Part 4.
Partition,
solubility product and ion–exchange * Part 5.
pH, weak–strong acid–base theory and
calculations * Part 6. Salt hydrolysis,
Acid–base titrations–indicators, pH curves and buffers *
Part 7.
Redox equilibria, half–cell electrode potentials,
electrolysis and electrochemical series
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