Unit 3 Chp 11 notes B and L.ppt

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UNIT 3

Intermolecular

Forces

and

Liquids

Chapter 11

Intermolecular Forces of Attraction

•

Ch 13 is about

liquids

and

solids

… where the

attraction between particles allows the

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States of substances depends largely on:

1. The balance between the KE of the particles and

2. The

intermolecular forces of

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Phases of Matter

• Chp 10 is about gases… particles that really don’t attract each other.

• Chp 11 and 12 are about liquids and solids…

where the attraction

between particles allows the formation of solids and liquids. (requires closer contact between particles)

4

Intermolecular Forces of Attraction

• Ch 13 is about liquids and solids… where the attraction between particles allows the

formation of solids and liquids.

Intermolecular Forces of Attraction

•

Ch 13 is about

liquids

and

solids

… where the

attraction between particles allows the

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Forces that hold Molecules together:

• Intra

molecular force (inside

molecules, bonds)

• Inter

molecular force refers to the

forces between the molecules, (IMFs).

Intermolecular Forces of Attraction

•

These attractions are called “

i

nter

m

olcular

f

orces of attractions” or IMF’s for s

h or t.

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Relative Magnitudes of Bonds

Relative Magnitudes of Bonds

and Forces

The types of bonding forces vary in their

strength as measured by average bond

energy.

Covalent bonds (400 kcal/mol)

Hydrogen bonding (12-16 kcal/mol )

Dipole-dipole interactions (2-0.5 kcal/mol)

London forces (less than 1 kcal/mol)

Strongest

Weakest

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3 IMFs to Know:

1. London Dispersion Forces (LDFs) non-polar molecules

2. Dipole-Dipole Forces polar molecules

3. Hydrogen Bonding

polar molecules H bonded to F, O, N

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1. London Dispersion

1. London Dispersion

Forces

The temporary separations of

charge that lead to the

London force attractions are

what attract one

what attract one nonpolarnonpolar molecule to its neighbors.

molecule to its neighbors.

Fritz London

1900-1954

London forces increase with

the size of the molecules or

masses. We refer to this as

“

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London Dispersion Forces

• Forces that are among noble gases and nonpolar molecules.

• Much, much weaker than other forces. Short lived.

• LDF increases with size of the molecule b/c more electrons.

• More electrons, more polarizable.

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London Dispersion Forces

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Can you interpret?

London Forces in

London Forces in

Hydrocarbons: More C,

more e-, more

polarizable, higher BP

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2. Dipole - Dipole Forces

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3. Hydrogen Bonding

• Bonding between hydrogen and more

electronegative atoms such as oxygen, fluorine and nitrogen (FON elements)

• Very strong dipole-dipole interaction (10x stronger) • Small molecules, highly EN, strong IMFs

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Be prepared to draw:

15 Bonds: always represented

with solid lines

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Hydrogen Bonding in DNA

T

A

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Hydrogen Bonding in DNA

C

G

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Boiling point as a measure of intermolecular

Boiling point as a measure of intermolecular

attractive forces

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More interactions

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Dipole-induced Dipole Interactions

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What type(s) of intermolecular forces exist between each of the following molecules?

HBr

HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.

CH₄

CH₄ is nonpolar: dispersion forces.

SO₂

SO₂ is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO₂ molecules.

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AP Sample Question

In the diagram to the right, which of the

labeled arrows identifies hydrogen bonding in

water?

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AP Sample Question

In which of the following liquids do the

intermolecular forces include dipole-dipole forces?

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AP Sample Question

The London (dispersion) forces are weakest for which of the following gases under the

same conditions of temperature and pressure?

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AP Sample Question

Which of the following is the strongest type of

interaction that occurs between the atoms within the circled areas of the two molecules represented above?

A.Polar covalent bond

B.Nonpolar covalent bond C.Hydrogen bond

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Some Properties of a Liquid

1. Surface Tension

1. Surface Tension: The : The resistance to an increase

resistance to an increase

in its surface area (polar

molecules, liquid metals).

Molecules at the top are

only pulled inside.

Molecules in the middle

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2. Capillary Action

2. Capillary Action: : Spontaneous rising of a

Spontaneous rising of a

liquid in a narrow tube.

Polar liquids spontaneously rise in a narrow tube due to….

Cohesive Forces: the

inter-molecular forces of the liquid itself.

Adhesive forces: force between

the liquid and it’s container.

Glass is polar therefore it

attracts water molecules but not mercury.

Some Properties of a Liquid

adhesive forces are stronger than the cohesive forces meniscus

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Some Properties of a Liquid

Some Properties of a Liquid

3. Viscosity:

Resistance to flow

High viscosity_{High viscosity} is an is an

indication of

indication of strong strong intermolecular

intermolecular forces forces

Larger forces, more viscous.

_{Large molecules can get tangled}

up.

Cyclohexane has a lower viscosity

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4. Vapor pressure

• Defined as the pressure above the liquid at

equilibrium.

• Liquids with high vapor

pressures evaporate easily. They are called volatile.

• Vapor Pressure decreases with increasing

intermolecular forces.

– Bigger molecules (bigger

LDF)

– More polar molecules

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Vapor

pressure Con’d

• Smaller Molecules with weak IMFs tend to be most volatile. (High Vapor Pressures)

• Vapor Pressure also ↑ with ↑

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Effects of Temperature on Vapor

Pressure

• Energy is needed to overcome

intermolecular forces.

• As temperature (average KE) is

increased, more of these forces are

broken creating more vapor.

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BP of any

substance is when vapor pressure is at or greater than atmospheric

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AP Sample Question

At 298 K and 1 atm, bromine is a liquid with a high vapor pressure, whereas chlorine is a gas. This

provides evidence that, under these conditions, the

A.forces among Br₂ molecules are greater than those among Cl₂ molecules

B.forces among Br₂ molecules are weaker than the Br —Br bond

C.forces among Cl₂ molecules are stronger than the Cl —Cl bond

D.Br— Br bond is stronger than the Cl—Cl bond E.Br— Br bond is weaker than the Cl—Cl bond

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AP Sample Question

Based on the data in the table above, which of the following liquid substances has the

weakest intermolecular forces? A.C₆H₆(l)

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AP Sample Question

Ne, HF, C₂H₆, CH₄

Which of the substances listed above has the highest boiling point, and why?

A.Ne, because its atoms have the largest radius

B.HF, because its molecules form hydrogen bonds C.C₂H₆ , because each molecule can form multiple hydrogen bonds

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AP Sample Question

Based on the structures shown above, which of the following statements identifies the compound with the higher boiling point and provides the best explanation for the higher boiling point?

A.Compound 1, because it has stronger dipole-dipole forces than compound 2

B.Compound 1, because it forms hydrogen bonds, whereas compound 2 does not

C.Compound 2, because it is less polarizable and has weaker London dispersion forces than compound 1

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The molecular formula and molar mass of two straight-chain hydrocarbons are listed in the table above. Based on the

information in the table, which compound has the higher boiling point, and why is that compound’s boiling point higher?

A.C₄H₁₀ ,because it has more hydrogen atoms, resulting in more hydrogen bonding

B.C₄H₁₀ , because it has more electrons, resulting in greater polarizability and stronger dispersion forces

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Useful processes to note:

• Phase changes for

different states of matter. • S  L fusion

• L  g vaporization • Can be shown on a

Heating Curve

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Changes of state

• The graph of temperature versus

heat applied is called a

heating

curve

.

• The temperature at which a solid

turns to a liquid is the

melting point

.

• The energy required to accomplish

this change is called the

Heat of

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Changes of state

•

The temperature at which a liquid

turns to a gas is the

boiling point

.

•

The energy required to accomplish

this change is called the

Heat of

Vaporization

Δ

H

_vap

(or Enthalpy of

Vaporization)

•

*IMFs need to be completely

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Water phase

Water phase

changes

Temperature remains __________

during a phase change. constant

Energy

*Able to predict whether or not a certain substance is water by temperatures on graph MP

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The following questions refer to the

graph below, which shows the heating

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The enthalpy of vaporization of water is 40.7 kJ/mol. Which of the following best explains why

the enthalpy of vaporization of methane is less than that of water?

A.Methane does not exhibit hydrogen bonding, but water does.

B.Methane has weaker dispersion forces. C.Methane has a smaller molar mass.