Slide 36-1

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 36-2

Chem 1101

A/Prof Sébastien PerrierRoom: 351

Phone: 9351-3366

Email: s.perrier@chem.usyd.edu.au

Prof Scott KableRoom: 311

Phone: 9351-2756

Email: s.kable@chem.usyd.edu.au

A/Prof Adam BridgemanRoom: 222

Phone: 9351-2731

Email: a.bridgeman@chem.usyd.edu.au

Slide 36-3

Highlights of last lecture

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.

Batteries and Corrosion…

Slide 36-4

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 36-5

IM Forces

Key Calculations:

• none

Key chemical concepts:

• Different types of forces between molecules

• H-bonding

• primary, secondary and tertiary structure of polymers

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 attractionand 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 36-7

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 attractionand 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

Covalent radius

VDW radius

Slide 36-8

Intramolecular ForcesAll 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 Model Attraction Energy Example

Ionic

Covalent

Metallic

Cation – anion

Nuclei sharing e- pair

Delocalised electrons

400-4000 kJ/mol

150-1100 kJ/mol

75-1000 kJ/mol

NaCl

H-H

Fe

Slide 36-9

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 eachother.

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 36-10

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 36-11

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 36-12

2) Dipole moment

Slide 36-13

2) Dipole moment

Blackman Figure 6.28

Slide 36-14

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 36-15

A dipole may be induced in an atom or non-polar molecule by the presence of an electric field. This process is called polarisation.

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

Induced dipole ∝ electric field strength∝ polarisability of the non-polar species

3) Polarizability

-

-

-

-

+

+

+

+

Slide 36-16

Type of IM Forces

Slide 36-17

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 36-18

INTERmolecular forces

Hydration shells around ions in water

Blackman Figure 10.8

Slide 36-20

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 36-21

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 thepresence of non-polar species (due to the insolubility of ionic solids in non-polar solvents).

Slide 36-22

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 36-23

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

- may be considered as an instantaneous dipole - induced dipole force

Points of note:

- occurs between all atoms and molecules

Slide 36-24

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 moleculese.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 36-25

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). Theirexistence 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% 237.8Slide 36-26

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 36-27

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 NH3Slide 36-28

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 36-29

Drawing H-bonds

� We indicate a H-bond using a dotted line. The H-bond is strongest when the bond angle is 180º.

Slide 36-30

NH3 (g) + H2O ⇔ NH3 (aq)

NH3 (aq) + H2O ⇔ NH4+ (aq) + OH- (aq)

The Ammonia Fountain

Slide 36-31

Exam-type question

Example: Which of the following substances show H-bonding? Draw the structure for those that do.

a) C2H6 b) CH3OH c) CH3(C=O)NH2

NO YES YES

H3C

O H

H

O

CH3C

O

N H

H

C

O

NH

H

CH3H3C

Slide 36-32

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 H-atoms and accepting two H-atoms to the lone pairs), forming a tetrahedral network.

1 2

3

4

Slide 36-33

Intramolecular H-bonds

� All the H-bonds we’ve seen thus far have been betweentwo molecules (intermolecular H-bonds). H-bonds can also occur within the same molecule (intramolecular H-bond).

o-hydroxybenzoic acid= salicylic acid

= metabolite of aspirin

p-hydroxybenzoic acid(no intramolecular

H-bonding)Slide 36-34

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