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Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille,
Chemistry, 2007 (John Wiley)
ISBN: 9 78047081 0866
Slide 36-1
Chem 1101
A/Prof Sébastien Perrier
Room: 351
Phone: 9351-3366
Email: [email protected]
Prof Scott Kable
Room: 311
Phone: 9351-2756
Email: [email protected]
A/Prof Adam Bridgeman
Room: 222
Phone: 9351-2731
Slide 36-2
Email: [email protected]
Highlights of last lecture
Batteries and Corrosion…
CONCEPTS
Difference between primary and secondary batteries
How common batteries work
Chemical reactions for common batteries
Fuel cells and how they work
Corrosion and corrosion protection
CALCULATIONS
None specifically, except redox calculations involving
batteries.
Slide 36-3
Intermolecular Forces
Themes:
Forces that hold molecules
together
Biopolymers and their structure
References: Most general chemistry texts will
have a satisfactory chapter on
intermolecular forces, e.g.
Silberberg: Chap 12.3 (IM Forces) and pp.468-73 (Polymers)
Slide 36-4
IM Forces
Key chemical concepts:
•
Different types of forces between molecules
•
H-bonding
•
primary, secondary and tertiary structure of
polymers
Key Calculations:
•
none
Slide 36-5
Q: If matter is so attracted to itself, why
doesn’t it squash down to nothing?
Answer:
There is a balance in energy between the forces of attraction
and the strong force of repulsion that atoms experience when
they get very close together.
The separation distance is known either as the
-
Bond length (for intramolecular), or the
-
Van der Waals (VDW) distance (for
intermolecular) - these are larger than bond
lengths since the forces of attraction are
weaker
We can assign atomic radii (= dist/2):
- Covalent radius and Van der Waals (VDW)
radius
Slide 36-6
Q: If matter is so attracted to itself, why
doesn’t it squash down to nothing?
Answer:
There is a balance in energy between the forces of attraction
and the strong force of repulsion that atoms experience when
they get very close together.
The separation distance is known either as the
-
Bond length (for intramolecular), or the
-
Van der Waals (VDW) distance (for
Covalent radius
VDW radius
intermolecular) - these are larger than bond
lengths since the forces of attraction are
weaker
We can assign atomic radii (= dist/2):
- Covalent radius and Van der Waals (VDW)
radius
Slide 36-7
Intramolecular Forces
All attractive and repulsive interaction between atoms arise from
electrostatic forces. We have already examined the forces that hold
atoms together to form a molecule, which we separate into ionic,
covalent and metallic bonds, although there is a continuum between all
three of these descriptions.
Force
Attraction
Energy
Example
Ionic
Cation – anion
400-4000
kJ/mol
NaCl
Covalent
Nuclei sharing 150-1100
kJ/mol
e- pair
Metallic
Model
Delocalised
electrons
75-1000
kJ/mol
H-H
Fe
Slide 36-8
Intramolecular Forces
One thing they have in common, however, is that they are
all quite STRONG (typically 80-4000 kJ/mol). Making
and breaking these bonds requires a lot of energy, which
we call a chemical reaction.
INTRAmolecular forces are NOT all there is to chemical species.
Individual molecules ALSO feel attraction or repulsion from each
other.
A trivial example is water as a solid (ice), liquid (water) and gas (steam).
If there were no attraction between individual water molecules then
water would only exist as a gas.
Interactions between water molecules (INTERmolecular interactions)
are responsible for liquid and solid water.
Slide 36-9
Charge distribution
Intramolecular bonding is due to electrostatic forces, it
should come as no surprise that INTERmolecular forces
also arise through electrostatics. We recognise three
different ways that the electron distribution can affect
the electrostatic effects:
1-Ionic Charge
2-Dipole moment
3-Polarizability
1) Ionic charge
In this case the molecule itself has an excess of
negative (anion) or positive (cation) charge.
Slide 36-10
2) Dipole moment
Ionic and polar covalent bonds have an unequal sharing
of electrons between the two atoms. In these cases one
end of the bond is more negative and the other more
positive. If the molecule is a diatomic species then we
call the molecule “polar”
These “bond dipoles” can be added up in more
complicated molecules. Frequently the sum of the bond
dipoles gives rise to an overall dipole moment in a
polyatomic molecule. However, sometimes, due to
symmetry or accident, the bond dipoles can cancel to
give rise to a non-polar molecule (even though the
individual bonds are polar).
Slide 36-11
2) Dipole moment
Slide 36-12
2) Dipole moment
Blackman Figure 6.28
Slide 36-13
3) Polarizability
The electrons on a molecule are never stationary, nor
rigidly held. When a molecule is brought into the
vicinity of other charges, the electrons on the molecule
will move in response to this charge. The freedom of
the electrons to move around is called the
“polarizability” of the molecule. Small and first row
atoms hold their electrons tightly and are not very
polarizable. Larger atoms, with electrons in outer
orbitals are more polarizable.
If an atom or molecule is polarizable, then a small dipole
can be induced in the atom/molecule when brought up to
another molecule. We call this an “induced dipole”.
Slide 36-14
3) Polarizability
A dipole may be induced in an atom or non-polar molecule by
the presence of an electric field. This process is called
polarisation.
+
-
+
-
+
-
+
-
Induced dipole
∝ electric field strength
∝ polarisability of the non-polar species
Polarisability is a measure of the ease with which an electron
cloud of an atom or molecule is distorted.
- increases with number of electrons
- increases with size
Slide 36-15
Type of IM Forces
Slide 36-16
INTERmolecular forces
1) Ion-Dipole:
-
dipoles align (partially or fully) in the electric field of an ion
-
typical energy 40-600 kJ/mol
(per bond)
Slide 36-17
INTERmolecular forces
Hydration shells
around ions in water
Blackman Figure 10.8
Slide 36-18
INTERmolecular forces
2) Dipole-dipole
- dipoles align (partially or fully) in the electric field of
the neighboring dipole.
- typical energy 5-25 kJ/mol
Solid
Liquid
Slide 36-20
INTERmolecular forces
3) Ion-induced dipole
- a dipole is induced in a polarizable molecule by the presence of an ion.
- typical energy: 3-15 kJ/mol
Point of note:
- A comparatively rare force, since ions do not often exist in the
presence of non-polar species (due to the insolubility of ionic
solids in non-polar solvents).
Slide 36-21
INTERmolecular forces
4) Dipole – induced dipole
- a dipole is induced in a polarizable molecule by the presence of
another dipole.
- typical energy: 2-10 kJ/mol
Points of note:
- A comparatively rare force, since dipoles do not often exist in the
presence of non-polar species (due to the insolubility/immiscibility
of polar molecules in non-polar solvents and vice versa).
- Some highly polarisable molecules can have significant solubility in
polar solvents (e.g. Xe is 25 times more soluble in H2O than is He)
Slide 36-22
INTERmolecular forces
5) Induced dipole – induced dipole (or Dispersion or
London forces)
- Two polarizable molecules induce transient dipole moments in each
other.
- typical energy: 0.05 – upwards kJ/mol
Points of note:
- may be considered as an instantaneous
dipole - induced dipole force
- occurs between all atoms and molecules
Slide 36-23
INTERmolecular forces
5) Induced dipole – induced dipole (or Dispersion or
London forces)
Generally regarded as being weak, but can in fact
be very strong for large molecules
e.g. benzene and bromine are liquid at RT
I2 and S8 are solid at RT
The strength of the
interaction depends on
two features:
• polarisability
• surface contact area
Slide 36-24
Summary of IM Forces
Dispersion forces exist between ALL molecules. The force increase
in strength with molecular mass.
Forces associated with permanent dipoles are found only in
substances with overall dipole moments (polar molecules). Their
existence adds to the dispersion forces.
When comparing substances of widely different masses, dispersion
forces are usually more significant than dipolar forces.
When comparing substances of similar molecular mass, dipole forces
can produce significant differences in molecular properties (e.g.
boiling point).
Molecule
MW
(amu)
Dipole moment,
(Debye)
dispersion
dipolar
BP (K)
F2
38
0
100%
0%
85
HCl
36.5
1.08
81%
19%
188.1
HBr
80.9
0.82
94.5%
5.5%
206.4
HI
127.9
0.44
99.5%
0.5%
237.8
Slide 36-25
Type of IM Forces
INTERmolecular forces arises from the combination of any pair of the
three interactions we have just examined.*
* except ion-ion, which is ionic bonding
Slide 36-26
Hydrogen bonds
A H-bond arises from an unusually strong dipole-dipole force. When
H is bonded to a very electronegative element (F, O, N) the bond is
polar covalent. H is unusual because with only one electron, it leaves
a partially exposed nucleus (H has no other core electrons to shield
the nucleus).
The bond can be thought of as forming between the hydrogen atom
and the lone pairs of the F, N, or O.
HF
H2O
NH3
Slide 36-27
Only F, O, and N?
The H-bond occurs between H-atom and a lone pair. Can
other elements with lone pairs form H-bonds? e.g. Cl?
F, O and N are all 2nd row elements, which conveys
certain properties:
They are the most electronegative elements
They are small (only 2s, 2p in outer shell)
They have lone pairs
Consequently, although, e.g. Cl lone pairs DO contribute
to the dipole-dipole force, this force is not unusual in
size (see plot later in lecture). So we consider H-bonds
ONLY to form between bonds involving FH, OH and
NH
Slide 36-28
Drawing H-bonds
We indicate a H-bond using a
dotted line. The H-bond is
strongest when the bond angle
is 180º.
Slide 36-29
The Ammonia Fountain
NH3 (g) + H2O ⇔ NH3 (aq)
NH3 (aq) + H2O ⇔ NH4+ (aq) + OH- (aq)
Slide 36-30
Exam-type question
Example: Which of the following substances show H-bonding? Draw
the structure for those that do.
a) C2H6
b) CH3OH
NO
YES
c) CH3(C=O)NH2
YES
H
H
O H
H3C
O
O
CH3
H3C
H N
C
C
N
H
CH3
O
H
Slide 36-31
The special case of water
Water is perhaps the most unusual liquid. Each water molecule
is H-bonded to FOUR other water molecules (donating 2 Hatoms and accepting two H-atoms to the lone pairs), forming
a tetrahedral network.
1
2
4
3
Slide 36-32
Intramolecular H-bonds
All the H-bonds we’ve seen thus far have been between
two molecules (intermolecular H-bonds). H-bonds can
also occur within the same molecule (intramolecular Hbond).
o-hydroxybenzoic acid
= salicylic acid
= metabolite of aspirin
p-hydroxybenzoic acid
(no intramolecular
H-bonding)
Slide 36-33
Summary
CONCEPTS
How to determine whether a molecule is polar
Three type of charge distribution: ion, dipole,
polarizability
Different type of IM Forces
Which forces are present in different molecules
Which forces are more important
Effect of IM Forces on boiling point
CALCULATIONS
None
Slide 36-34