Forces Between Molecules

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

Forces between molecules (intermolecular forces)

• So far we have spoken about the forces or bonds between atoms e.g. covalent, ionic and metallic

• Now we will learn about the forces between molecules or compounds are called intermolecular forces

• Inter means between or among

• Internet, interstate, international

• What would Interstellar travel be?

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 bonded molecules or covalent molecular substances

Overview

• All matter is held together by force. • The force between atoms within a

molecule is a chemical or intramolecular force.

• The force between molecules are a physical or intermolecular force.

• These physical forces are what we overcome when a chemical changes its state (e.g. gas liquid).

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 electrons

in the other molecule.

• The same applies for nuclei

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. 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 negative charge on the sulfur of another molecule.

• One type of intermolecular force is dipole dipole

H Cl

+ –+ –

+ –+ –

• Draw the molecule and determine its shape using VSPER

• Determine polarity within molecule

• Determine how the molecules would interact

2. Hydrogen bonding

• Hydrogen bonds are a special case of dipole forces

• Hydrogen bonding occurs between molecules 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 very polarised

• The partial positive charge on hydrogen and the lone pair of electrons on N,F and O

• 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

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 of stronger intermolecular forces.

• The greater the number of electrons within a molecule, the more significant the dispersion forces btw the molecule.

• They occur between non-polar molecules.

• 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 in nonpolar molecules. It is perhaps better to think of these interactions as those between a "temporary dipole" and an "induced dipole".

Stages for Dispersion Forces

1. There are two molecules

2. One molecule has a temporary dipole

3. The other molecule has an induced dipole

• http://www.youtube.com/watch?v=IgcGuEwHHKY&feature=related

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

How do intermolecular forces affect melting point?

• When heat is applied to a solid, the molecules begin to vibrate more and more. The energy possessed by the molecules increases to the point that some intermolecular attractions are overcome.

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=IgcGuEwHHKY

Overview• Intermolecular forces (dispersion forces, dipole-

dipole interactions and hydrogen bonds) 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 covalent bond), hydrogen bonds are the strongest intermolecular force (about one-tenth the strength of a covalent bond).

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 dipoles in dipole-dipole attractions.

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 more

permanent).– London forces are weaker