chapter 7 intermolecular forces 2

7/30/2019 Chapter 7 Intermolecular Forces 2

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Forces Between Molecules


2/23

Bonding model for covalent

molecular substances

Bonding for covalent molecular substances

falls into two categories

1. The strong forces of attraction which holds

atoms together within molecules

2. The weak forces of attraction between

molecules


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Forces between molecules

(intermolecular forces)

So far we have spoken about the forces orbonds between atoms e.g. covalent, ionic and

metallic

Now we will learn about the forces betweenmolecules or compounds are called

intermolecular forces

Inter means between or among Internet, interstate, international

What would Interstellar travel be?


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Intramolecular forces

What would intramolecular forces be?

Forces within molecules e.g covalent,

metallic or ionic

intra means within Intrastate, intranet, intracellular

Most of the intermolecular forces we look

at occur between covalently bondedmolecules or covalent molecular

substances


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Overview

All matter is held together by force.

The force between atoms within amolecule is a chemical orintramolecular

force. The force between molecules are a

physical orintermolecular force.

These physical forces are what weovercome when a chemical changes itsstate (e.g. gas liquid).


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What causes intermolecular forces?

Molecules are made up of charged particles:

positive nuclei and negative electrons.

When one molecule approaches another there

is a multitude of forces between the particles in

the two molecules. Each electron in one molecule is attracted to

the nuclei in the other molecule but also

repelled by the electronsin the other molecule.

The same applies for nuclei


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Types of Intermolecular forces

The three main types of intermolecular

forces are:

1. Dipole-dipole attraction occur only btw polar

molecules

2. H bonding only with Hydrogen and Oxygen,

Fluorine and Nitrogen)

3. Dispersion forces

1 Di l Di l tt ti Di l


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1. Dipole Dipole attraction or Dipole

forces Dipole forces occur between two polar

molecules

The dipole force is the electrostatic attraction

between the partial positive on one hydrogen

on one molecule and the partial negativecharge on the sulfur of another molecule.

One type of intermolecular force is dipole dipole

+ +


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Draw the molecule and determine its shape

using VSPER

Determine polarity within molecule

Determine how the molecules would

interact


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2. Hydrogen bonding

Hydrogen bonds are a special case of dipoleforces

Hydrogen bonding occurs betweenmolecules in which hydrogen is bonded to

nitrogen, oxygen or fluorine It occurs because N, F and O are very

electronegative

This makes the molecule formed verypolarised

The partial positive charge on hydrogen and

the lone pair of electrons on N,F and O


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Hydrogen bonds are stronger than dipole

dipole attractions

We know this because of the difference in

melting points of molecules with dipole

dipole attractions and hydrogen bonding


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3. Dispersion forces Dispersion forces, or London forces, are the

weakest of the intermolecular forces.

Dispersion forces occur between polar and

non-polar molecules. They are generally

weak and only considered in the absence ofstronger intermolecular forces.

The greater the number of electrons within

a molecule, the more significant thedispersion forces btw the molecule.

They occur between non-polar molecules.


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Essentially they are the same as dipole forces- an

electrostatic attraction between a partial positive

charge and a partial negative charge.

However, the partial charges are more rare innonpolar molecules. It is perhaps better to think of

these interactions as those between a "temporary

dipole" and an "induced dipole".


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16/23

Stages for Dispersion Forces

1. There are two molecules

2. One molecule has a temporary dipole

3. The other molecule has an induced dipole


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http://www.youtube.com/watch?v=IgcGuE

wHHKY&feature=related


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Strength of intermolecular forces

1. Hydrogen bonds are the strongest

intermolecular force

2. Dipole Dipole Interactions

3. Disperson Forces are the weakest

intermolecular force

dispersion forces < dipole-dipole interactions

< hydrogen bonds

H d i l l f


19/23

How do intermolecular forces

affect melting point?

When heat is applied to a solid, the moleculesbegin to vibrate more and more. The energy

possessed by the molecules increases to the

point that some intermolecular attractions are

overcome.


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How to measure the strength

of a chemical bond? One measure of the strength of a chemical

bond is the energy required to break one

of those bonds.

http://www.youtube.com/watch?v=IgcGuE

wHHKY


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Overview Intermolecular forces (dispersion forces,

dipole-dipole interactions and hydrogenbonds) are much weaker than

intramolecular forces (covalent bonds,

ionic bonds or metallic bonds) dispersion forces are the weakest

intermolecular force (one hundredth-one

thousandth the strength of a covalentbond), hydrogen bonds are the strongest

intermolecular force (about one-tenth the

strength of a covalent bond).


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Quiz

1. Which attractions are stronger:intermolecular or intramolecular?

2. Which is stronger a covalent bond or a

dipole-dipole attraction?3. Suggest some ways that the dipoles in

London forces are different from the dipolesin dipole-dipole attractions.


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1. Intramolecular are stronger.2. A covalent bond is approximately100x stronger.3. London forces

Are present in all compounds

Are transient in nature (dipole-dipole are morepermanent).

London forces are weaker

chapter 7 intermolecular forces 2

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