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Chapter 10Liquids, Solids, and Phase

Change

CHM 112 M. Prushan

Why Do Solids and Liquids Exist?

• Intermolecular Forces

gases – Little to no attraction between

molecules

CHM 112 M. Prushan

Why Do Solids and Liquids Exist?

• Intermolecular Forces

liquids – Fairly strong forces between a

few molecules

CHM 112 M. Prushan

Why Do Solids and Liquids Exist?

• Intermolecular Forces

solids – Strong forces prevent large amount of motion between molecules/atoms

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What makes a molecule polar?• Polar molecules are said to possess a

permanent dipole moment.

• Dipole moment measured in units of

Debye (Q x r ) [charge x distance]

1 D = 3.336 x 10-30 coulomb metersProf. Peter DebyeNoble Prize 1936

CHM 112 M. Prushan

Dr. Prushan, enough physics, what does it mean?

• OK… Let’s go back to Lewis Structures and VSEPR

• Examples:

Draw Lewis Structures and Predict the Geometry of : NH3, H2O, CH4, CH3Cl,CHCl3 CO2, CCl4

(Yes, there was a reason you needed this)

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Examples

NH

HH O

HH C OO

NH3 H2O CO2

Trigonal Pyramidal Bent Linear

Tetrahedral

CCl4

H

CH

HH

Cl

CCl

ClCl

CH3ClCH4

Cl

CH

HH

H

CCl

ClCl

CHCl3

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NH

HH O

HH C OO

Polar or Not Polar That is the Question?

NH3 H2O CO2

Trigonal Pyramidal Bent Linear

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NH

HH O

HH C OO

Polar or Not Polar That is the Question?

= 1.47 D = 1.85 D = 0 D

+

-

+

-

- - +

NH3 H2O CO2

Trigonal Pyramidal Bent Linear

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Polar or Not Polar That is the Question?

CCl4

H

CH

HH

Cl

CCl

ClCl

CH3ClCH4

Cl

CH

HH

H

CCl

ClCl

CHCl3

+

-

-

+

= 0 D = 0 D = 1.87 D = 1.90 D

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Types of Intermolecular Forces

• 4 main types of intermolecular forces

(1) London (Dispersion) Forces

(2) Dipole-Dipole Forces

(3) Ion-Dipole Forces

(4) Hydrogen Bonds

Increasing

Strength

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London (Dispersion) Forces

Weakest of the IM forces. All molecules have Dispersion forces

Result of interactions between instantaneous dipole moments.

Prof. Fritz London

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Dipole-Dipole Forces

• Result of permanent dipole moments in polar molecules

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Ion-Dipole Forces

The force which dissolves ionic compounds

A result of interactions between ions and solvent molecules

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Hydrogen Bonds

• Strongest of the intermolecular forces

• NOT a chemical bond

• MUST contain hydrogen covalently bonded to F, Cl, O or N.

• Essential for Life!

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If the boiling point of CO2 is , and CS2 is

Molecules with larger molecular weights have higher boiling points.

So why is H2O a liquid, but H2S is a gas?

Hydrogen Bonds

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BONDS… Hydrogen Bonds

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Why are Snow Flakes Hexagons?

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Hydrogen Bonds and Life

• Life Depends of H-bonds

• The “Stuff” of Life relies on them to stay together

Proteins, Enzymes Nucleic Acids (DNA, RNA)

Amino Acids Sugar-phosphate Backbond andBase Pairs

CHM 112 M. Prushan

Amino acids bond together to make polypeptides

The polypeptides fold to make the secondary structure of proteins

CH

C

N

HC

C

N

CH

N

CHN

HC

N

HCN

CH

N

HCN

O

O H

HO

HO

HO

HO

HO

HO

H

RRRR

R

R

R

How and Why does the alpha structure form??

Linus Pauling Discovered How, in 1948

While in bed with a cold…. Lets let him describe how…

CHM 112 M. Prushan

Now let’s try that…

Come on down and get a polypeptide

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Hydrogen Bonds are also important in DNA

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N

NNH

N

NH2

NH

NNH

N

O

NH2

N

NH

NH2

O

NH

NH

O

O

Adenine

Guanine

Cytosine

Thymine

Which goes with which ???

A – T

G – C

Right??? BUT WHY????

Come on down and get some base pairs

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The Guanine-Cytosine (GC) Base Pair

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The Guanine-Cytosine (GC) Base Pair

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Effect of Intermolecular forces on melting and

boiling points of molecular covalent substances

Increase only due to increase in MW,Since all are non-polar

1632

77132

Molecular wts.20

37

81

128

Why does HF have the highest BP?

Molecular wts.

BONDS… Hydrogen Bonds