Learning Check

Determine the VSEPRT geometry,

bond angle, and hybridization of each

indicated atom in the following

molecule.

1

Types of Covalent Bonds

1. A sigma (σ) bond is formed by end-to-end overlap of orbitals.

◦ All single bonds are σ bonds.

2. A pi (p) bond is formed by sideways overlap of orbitals.

◦ A p bond is weaker than a σ bond because sideways overlap is less effective than end-to-end overlap.

◦ A double bond consists of one σ bond and one p bond.

2

Double Bond (ie ethylene)

3

unhybridized 2p orbitals

Triple Bond (ie acetylene)

4

Each C is sp hybridized and has

two unhybridized p orbitals.

[11.43] Isoniazid is an antibacterial agent that is very useful against may common strains of tuberculosis. A valid Lewis structure is:

How many bonds are in the molecule? What is the hybridization of each C and

N atom? 5

Chapter 12 Intermolecular Forces: Liquids,

Solids, and Phase Changes

Outline

1. An Overview of Physical States and Phase

Changes

2. Quantitative Aspects of Phase Changes

3. Types of Intermolecular Forces

4. Properties of the Liquid State

5. The Uniqueness of Water

Sections 12.3, 12.4 only

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Molecules are held together by intramolecular

forces (bonds within a molecule). Also, there are

attractive intermolecular forces in solids, liquids,

and gases.

Influence chemical

properties

Influence physical

properties

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Intramolecular forces are bonding forces that exist within

a molecule or ionic compound holding it together (i.e.

internal chemical bonds).

Covalent molecules have

intramolecular bonds that

result from sharing one or

more electrons pairs

between non-metals.

Ionic substances have

intramolecular bonds

that result from

electron transfer.

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Intermolecular forces are attractive forces between

molecules that explain many physical properties of

compounds on the planet.

Wetting or Not Wetting

Surface Tension

Viscosity

Freezing and

Boiling Point

capillary action

10

Intermolecular forces play a critical role in life. For example,

they hold together the double helix of DNA.

11

When washing clothes, an oily portion of soap interacts

with grease and dirt, solubilizes it and removes it from

clothing (or hands).

CH3(CH2)16COO- Na+

12

Hydrophobic paint

• Video: https://www.youtube.com/watch?v=uoN5EteWCH8

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Time

Equilibrium

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Vapor pressure is the pressure exerted by the vapor at

liquid-gas equilibrium.

Molecules vaporizing and

condensing at such a rate

that no net change in vapor

pressure occurs.

Molecules in liquid

begin to vaporize

Factors affecting Vapor Pressure

higher T higher P

weaker forces higher P

temperature As temperature increases, the fraction of molecules

with enough energy to enter the vapor phase increases,

and the vapor pressure increases.

intermolecular forces The weaker the intermolecular forces, the more easily

particles enter the vapor phase, and the higher the

vapor pressure.

15

The temperature at which the vapor pressure equals

the external pressure over the liquid is the boiling point.

The normal boiling point is the temperature at which a

liquid boils when the external pressure is 1 atm.

Evaporation Boiling

16

The following diagram shows a close-up view of part of the

vapor pressure curves for two liquids: the green upper

curve for a pure solvent, and the red lower curve for a

solution. Which of the two is more volatile?

The more volatile solvent

will have a higher vapor

pressure (more gas

molecules in the gas

phase above its liquid).

The green curve has a

higher vapor pressure at

all temperatures. It has

weaker IMF’s.

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Intermolecular Forces

1. Ion-Dipole: occurs between an ion and a polar

molecule

2. Dipole-Dipole: occurs between neutral polar

molecules

3. Induced Dipoles: occurs when an ion or a dipole

induces a spontaneous dipole in a neutral

polarizable molecule

4. London Dispersion Forces: occurs in all

molecules.

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Ion-Dipole

• Attractive forces between an ion in solution and a

neighboring polar molecule

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an anion in solution

a cation in solution

(+) end of dipole

attracted to

anions

(-) end of dipole

attracted to

cations

Dipole-Dipole

• Attractive forces between polar molecules (same or

not the same)

20

Boiling point differences can be explained by dipole-dipole interactions.

21

Dipole-Dipole (H-bonding) • Hydrogen bonding is the attraction between the H

atom of one molecule and a lone pair of the N, O,

or F atom of another molecule

22

Learning Check

• Which of the following substances exhibits H

bonding? For any that do, draw the H bonds

between two of its molecules.

23

Induced Dipole

• Attraction between an ion or dipole charge and a

polarizable non-polar molecule

24

Non-polar

molecule

London Dispersion Forces

• Attraction that occur when temporary dipoles are

formed due to random electron motions in all

polarizable molecules.

25

Smaller particles are less polarizable than larger ones

because their electrons are held more tightly.

Molecular Shape and Boiling Point

26

There are more points

at which dispersion

forces act.

There are fewer points

at which dispersion

forces act.

Determining IMF in a Sample

27

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Practice Exercises

• For each pair of substances, identify the key

bonding and/or intermolecular force(s), and predict

which one of the pair has the higher boiling point:

▫ (a) CH3NH2 or CH3F

▫ (b) CH3OH or CH3CH2OH

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Announcement

• March 19, Thursday

▫ LT#3: 120 points: 40 items MC x 2.5 pts each + 2 PS

items x 10 pts each

▫ 6-7:30 PM

▫ Schmitt Hall C-114

▫ Coverage: Chapters 7-11

• March 20, Friday

▫ Quiz # 10, 11, 12

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