1

Intermolecular Forces (IMF)

Part I: Types of IMF Effect of IMF on Physical Properties

(based on Chap. 12 Sec 1-3 of Jespersen 6th Ed)

Dr. C. Yau

Fall 2013

2

Review of the Postulates of the KMT

KMT = Kinetic Molecular TheoryKMT provides an explanation on the behavior of

an ideal gas:1. A gas consists of an extremely large number of

very tiny particles that are in constant, random motion.

2. The gas particles themselves occupy a net volume so small in relation to the volume of their container that their volume can be considered negligible.

3. The particles collide with each other and with the walls of the container in perfectly elastic collision. They move in straight lines between collisions neither attracting nor repelling each other.

3

Parameters of a Gas

When determining the amount of gas, one must specify all of the following (the parameters of the gas):

P = Pressure

V = Volume

T = Temperature

4

Review of the Postulates of the KMT

The KMT explains:• Boyle’s Law (P and V of a gas is inversely

proportional when T is kept constant).• Gay-Lussac’s Law (P and T of a gas is directly

proportional when V is kept constant). Increase in T increases the kinetic energy (KE) of the particles resulting in a higher frequency of collisions with the walls of the container, and thus increasing the P.

• Charles’ Law (V is proportional to T when P is kept constant.)

5

View of the physical states at the particulate level

Solid: Particles closely packed, moving slowly.

Liquid: Particles further apart, moving faster.

Gas: Particles VERY far apart, moving VERY fast.

6

1.Solid heating up

2.Solid melting into liquid

3.Liquid heating up

4. Liquid vaporizing

5. Gas heating up

Why is the temp not increasing while heat is added?

77

Changes in Physical State at the Particulate Level

SOLID LIQUID GASAs heat is added to a solid, the energy is

absorbed as it…1) weakens the attractive forces between the

particles (allowing them to move away from each other), and

2) increases the kinetic energy of the particles (causing them to move faster).

As a gas, the particles no longer have any attractive forces between them.

These attractive forces are called“intermolecular forces” (often abbreviated IMF)

8

Intermolecular ForcesThis is only a summary of what is covered on the

blackboard. Refer to your lecture notes for details.

Types of Intermolecular Forces:

1) Dipole Forces (or dipole-dipole attraction)

2) London Forces (or London Dispersion Forces, or Dispersion Forces)

3) Hydrogen Bonding (or Hydrogen Bond)

Other related forces:

4) Ion-dipole interaction (ionic solute in polar solvent)

5) Ion-induced dipole interaction (ionic solute in nonpolar solvent)

9

Relative Strengths of Intermolecular Forces

Dipole Forces:• The greater the difference between the

electronegativities of the atoms, the larger the dipole, andthe stronger is the dipole force.

• However, one must be careful to compare only molecules of comparable size because a variation in size has a separate effect on the strength of the IMF.

10

Relative Strengths of Intermolecular Forces

London Forces:• Effect of Size: The larger the particle, the

more polarizable it is, the stronger is the London forces.

• Effect of # of Atoms: The more atoms in the molecule, the larger the surface for interaction, the stronger is the London forces.

• Effect of Shape: The more compact the molecule, the less surface for interaction, the weaker is the London forces.

11

PolarizabilityLearn the term “polarizable.”It refers to how easily the electron cloud

surrounding a particle can be distorted by an outside charge.

It is how “fluffy” the electron cloud is.A large particle tends to have “fluffier”

electron clouds as the electrons are further from the nucleus and therefore less tightly bound. It is said to be more polarizable.

It is more easily polarized, more easily form an induced dipole.

12

What has IMF to do with BP?

If compound A has stronger IMF than compound B, which would have the higher BP?

A or B ?

Ans. A has the higher BP.

13

Effect of # Atoms on IMF Strength

C6H14 has stronger London forces than C3H8.

BP = 68.7oC BP = - 42.1oC

What is the significance of a BP being lower than 25 oC?

14

How do you explain the trend in the BP?

p.530

15

Gases

Liquids

Solids

Methane, ethane,

propane, butane

Pentane, hexane, heptane, octane, (gasoline) nonane, decane etc…kerosene, heating oil, lubricating oilsWax, paraffin (face cream, crayons) asphalt

These are hydrocarbons found in crude oil.

Table 12.2

16

Effect of Shape on IMF StrengthC5H12 C5H12

Which has the higher IMF? Why?

Fig. 12.5 p.531

17

H-bonding is of particular significance to us

• It explains why H2O exists as a liquid rather than a gas.

• If we consider H2O only as a polar molecule, with only dipole forces, we would have expected it to be a gas, like H2S.

• Life would not exist as we know it if H2O were not a liquid!!!

18

Significance of H-bonding• Enzymes are biological catalysts that keep

us alive. They work only if they maintain their special shapes.

• These shapes are often held in place by H-bonding.

• Example: Proteins are made of polypeptides, which are chains of amino acids.

• Hair protein is made of polypeptides exist in

a shape known as -helix.

19

-Helix of Hair Protein

O of one amino acid is H-bonded to H of the fifth amino acid.

Result is the alpha-helix.

20

-Helix of Hair Protein

H-bonding of O to H of the 5th amino acid below it.

21

-Pleated Sheets of Silk

For silk, H-bonding holds strands of polypeptides in pleated sheets.

22

What holds the 2 strands in DNA is purely

H-bonding!

Significance of H-bonding

Fig. 12.9 p.534

23

Of the 3 IMF, if we compare molecules of comparable size (so that London forces are about the same for all)…

London forces < dipole forces < H-bonding

Relative Strengths of Intermolecular Forces

24

How do they compare with covalent bonds?

Dipole forces are about 1 - 4% of the strength of covalent bonds.

H—Cl H—Cl H—Cl

Where are the dipole forces in the structures shown above? Where are the covalent bonds?

H-bonding is 5 - 10% of the strength of covalent bonds.

London forces vary in strength depending on size. A large molecule could have quite significant London forces just due to size, usually weaker than covalent bonds.

Relative Strengths of Intermolecular Forces

25

Determination of Which IMF is in Effect

Dipole forces: exhibited by polar molecules

London forces: exhibited by all substances (polar and nonpolar molecules, ions, atoms)

H-bonding: exhibited by compounds containing H-F, H-O or H-N bonds

…..Ionic bond: exhibited by ionic compoundsCovalent bond: exhibited by molecular

compounds (made of all nonmetals) and polyatomic ions.

26

Overall Picture: Relative Strengths of All Substances of comparable size

27

Which of the following is not likely to have hydrogen bonding? Hint- sketch the molecules!

A. CH3CO2H

B. CH3OH

C. CH3OCH3

D. All of these exhibit hydrogen bonding

Ans. C is not likely to have H-bonding.

28

Learning CheckIdentify the kinds of intermolecular forces present in the following compounds and then rank them in order of increasing boiling point: H2S CH3OH CBr4 Ne

C Br

Br

Br

Br

HS

H C O

H

HH

HNe

hydrogenbonding

& L.forces

London forces

London forces

dipoleforces

& L.forces MM=331.63 MM=20.18

Ne < CBr4 < H2S < CH3OHBp 189.5 oCBp 65 oCBp - 60 oC Bp -246oC

Ne < H2S < CH3OH < CBr4

29

Arrange the following in terms of increasing strength of intermolecular forces:

CO2, CH4, HF

A. CO2 < CH4 < HFB. HF < CO2 < CH4

C. CH4 < HF < CO2

D. CH4 < CO2 < HFE. None of these

Ans. D. CH4 < CO2 < HF

How can we rate CH4 with CO2?How can we verify this is true? BP (CH4) = -161.5oC

BP (CO2) = -78oC

Compare molar masses.

30

Effect of IMF on Physical Properties

• Compressibility: Gases are compressible, but not solids or liquids. Why?

• Surface tension:

Gases have no

surface tension.

Water has a high

surface tension.

WHY?

31

What exactly is surface tension?It is the resistance of a substance for a

denser object from penetrating through the top layer of a liquid.

For example, a water insect can walk on water even though it is denser than water.

If you are careful, you can “float” paper clips on top of water.

32

Surface TensionThere is a layer of molecules at the surface

that has a higher cohesive force forming a membrane of sorts...and this is due to the strong IMF of water molecules: hydrogen bonding.

At the surface, water molecules do not have any attraction to the air above it. Each molecule only have the IMF to the neighboring molecules to each side and below it. This increases the IMF giving a “sturdier” membrane at the surface.

33

Surface TensionSurface tension is also described as the

resistance of a substance to increasing its surface area.

This is observed in water forming beads on a surface that is not attractive to the water:

This is the “wetting ability” of a liquid, which is also related to the strength of the IMF.

34

Wetting of a surface by a liquid:"Wetting" refers to the spreading of a liquid

across a surface.In the lab, if water forms beads of droplets

on the sides of a glass container (such as a buret or grad cylinder) it means the glassware is dirty! Why?

If the glassware is clean, the water form a thin film across the surface and no droplets are observed.

Effect of IMF on Wetting Ability

35

Wetting Ability/Surface TensionWetting ability is not totally dependent on just the IMF of a substance. It is also dependent on the IMF of the surface on which the substance is placed.

The top diagram shows an ink drop on an untreated film to which the ink has little attraction. The ink shows a high surface tension.

The bottom diagram shows a treated surface that is now

attractive to the ink. The surface tension is lowered & wetting ability has increased.

36

Surface Tension & MeniscusYou should have learned that

water has a meniscus curving downwards. That is because water molecules are attracted to the glass and “creeps” up the sides.

Mercury on the other hand has a strong London force and no attraction to the Si-O bonds of the glass.

37

Wetting Ability of WaterGlass contains a lot of Si-O bonds. Water form H-bonds to the O and therefore spreads out on the surface of the glass.

Dirty (greasy) glassware, however is coated with nonpolar grease. The water is not attracted to the grease and prefers to bonds to itself (H-bonding between water molecules). The result is they form water droplets on the surface (observed as beads).

38Water on clean Water on dirtyglass surface. glass surface.

Layer of grease, which is nonpolar

39

Wetting AbilitySurfactants

– substances that have polar and non-polar characteristics

– improve a liquid’s wetting properties – allow non-polar substances to dissolve in

polar solvents

Detergents contain surfactants, which causes water to be come “wetter” and allow detergents to spread out better on the surfaces we are trying to clean.

40

Effect of IMF on Viscosity

Viscosity is the ease of flow of a liquid.

Honey is very viscous.

What do you think the effect of IMF is on the viscosity of a liquid?

CH3 C CH3

O

O C C OH H

H H

H H acetone ethylene glycol

Which is more viscous? and why?

4141

Solubility• “Like dissolve like”- the more similar the

polarity of two substances, the greater their ability to interact with each other rather

• This explains why oil and water don’t mix: H2O molecules form very strong H-bonding to each other.

• Oil is made of mostly hydrocarbons (nonpolar) and are attracted to each other by London forces.

42

SolubilityWhy does table salt dissolve well in water

but not in oil?

What are the attractive forces in table salt?

When table salt is added to water, what new forces are formed?

What kind of IMF are in oil?

What happens when table salt is added to oil?

43

Role of IMF in How We Wash Dishes

This is the type of structure soap has:

The function of soap is to help dirt (grease) dissolve in the water. Think about how soap could get grease to dissolve in water.

44

Effect of IMF on Evaporation

What do you think an increase in IMF would do to the rate of evaporation?

A.Increase the rate of evaporation

B.Decrease the rate of evaporation

Ans. B. Decrease the rate of evaporation.

45

Understanding Evaporation

Evaporation depends on...

• Surface area

• Temperature

• Strength of IMF

46

Understanding E of Evaporation

T2 is a higher temperature than T1

This is called “activation energy” (Ea) or “threshold energy.” It is the energy a molecule requires to break off all IMF and leave (as a gas).

Shaded area shows fraction of molecules with E higher than Ea

How does this tell you the effect of T on evaporation?

47

Understanding E of Evaporation

At a higher temperature, more molecules have beyond the activation energy and they break off all IMF to form a gas.

This means that the liquid would evaporate faster at a higher temperature.

48

Understanding E of Evaporation

This kind of graph is important to understand. You will see it again in later chapters. Be able to draw the 2 curves and label them.

When T is increased, what happens to the curve?

Peak moves to higher KE & curve flattens out.