Lecture 07 Intermolecular Forces

Topics: Intermolecular force, Chemical bonding, Chemical polarity Pages: 10 (1193 words) Published: December 18, 2014
Lecture 7

Intermolecular Force
Semester 1: 2014-2015
Instructor
Teach Assistants
Office
Office hours

1

Huynh Kim Lam

: Dr. Huỳnh Kim Lâm
: Ms. Võ Diệu Ánh Dương
Ms. Hồ Như Ngọc
: Rm. A1.705
: 9:00-11:00 AM, Thursday (LKH)
Chemistry for Engineers (CH011IU) - Lecture 7 - Semester 1: 2014-2015

huynhkimlam.iu@gmail.com / lamhuynh.info

Molecular forces

Intramolecular forces (bonding forces)
– exist within each molecule
– influence the chemical properties of the substance
– are relatively strong (larger charges that are closer together)

Intermolecular forces (nonbonding forces)
– exist between the molecules
– influence the physical properties of the substance
– are relatively weak (smaller charges that are farther apart) 2

Strength

Intramolecular Forces - Review

Copyright @ 2007 by The McGraw-Hill Companies, Inc.

3

Intermolecular forces

 exist between the molecules
 influence the physical properties of the substance
• boiling and melting points, vapor pressures, and
viscosities
 are relatively weak (smaller charges that are farther apart)

4

Review on physical states

5

Phase changes
Phase changes are also determined by the interplay
between kinetic energy and intermolecular forces.
Temperature ↑
→ average kinetic energy ↑
→ faster moving particles can overcome attractions
more easily
Condensation: gas → liquid
Vaporization: liquid → gas
Freezing: liquid → solid
Melting, or fusion: solid → liquid
Sublimation: solid → gas
Deposition: gas → solid
6

Heating-cooling curve
A cooling curve for the conversion of gaseous water to ice

Copyright @ 2007 by The McGraw-Hill Companies, Inc.

7

The Equilibrium Nature of Phase Changes (reading)

Liquid-Gas Equilibria
Solid-Liquid Equilibria
Solid-Gas Equilibria
Phase Diagrams: Effect of Pressure and Temperature
on Physical State

8

Intermolecular Forces

Four main types:
 Dipole-dipole
 Ion-dipole
 Hydrogen bonding
 London Dispersion force
Not every molecule has all of four types of forces.
Then, for a specific molecule, which types of forces does it exhibit?

9

Dipole-dipole Forces
Electrostatic interaction of between
positive and negative ends of two
polar molecules (the positive pole of
one molecule attracts the negative
pole of another)
These forces give polar molecules a higher
boiling point than nonpolar molecules of
similar molar mass

Gas

Liquid

Strength of intermolecular forces increase
with increasing polarity; thus more energy
Solid
is needed to separate them
Copyright @ 2007 by The McGraw-Hill Companies, Inc.
10

Hydrogen Bonding forces
A special type of dipole-dipole forces
Occur between two polar covalent particles one of which is
 A Hydrogen atom for the (+) dipole
 A small, highly electronegative (EN) atom with lone electron pairs such as O (EN = 3.5), F (4.0), or N (3.0) for the (-) dipole  It makes these atoms so electronegative that their covalently bonded H is highly positive.

 It allows the lone pair on the other N, O, or F to come close to the H .

Structure: (N/O/F)  H   (N/O/F) 
Examples:
11

Problem
Which of the following substances exhibits H bonding? For
those that do, draw two molecules of the substance with the H bond(s) between them.
a) C2H6 b) CH3OH c) CH3C(=O)NH2

12

Polarity and Boiling Point



The polarity of the molecules determines the forces of attraction between the molecules in the liquid state.



Polar molecules are attracted by the opposite charge effect (the positive end of one molecule is attracted to the negative end of another molecule.



Molecules have different degrees of polarity as determined by the functional group present.

The greater the forces of attraction the higher the boiling point or
the greater the polarity the higher the boiling point.
13

The Significance of Hydrogen Bonding
Hydrogen bonding has a profound impact...
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