Intermolecular Forces

• T O B E A B L E T O D E S C R I B E I N T E R M O L E C U L A R F O R C E S B A S E D O N P E R M A N E N T D I P O L E S , A S I N H Y D R O G E N C H L O R I D E A N D I N D U C E D D I P O L E S ( V A N D E R W A A L S ’ F O R C E S ) , A S I N T H E N O B L E G A S E S .

• T O B E A B L E T O D E S C R I B E H Y D R O G E N B O N D I N G A N D T H E A N O M A L O U S P R O P E R T I E S O F W A T E R R E S U L T I N G F R O M H Y D R O G E N B O N D I N G .

Learning Objectives

B E F O R E W E C A N U N D E R S T A N D T H E A T T R A C T I V E F O R C E S T H A T E X I S T B E T W E E N M O L E C U L E S , I T M I G H T B E H E L P F U L T O R E V I E W T H E K I N E T I C T H E O R Y O F M A T T E R .

The kinetic theory of matter

The solid state

Substances in the solid state are packed together in a regular ordered way.

This order breaks down when a substance is melted (requiring energy).

The liquid state

In the liquid state there may be some degree of order due to the remaining intermolecular forces between the particles, but most of the intermolecular forces have been overcome on melting, so the molecules are free to move past each other.

The gaseous state

However in the gaseous state, all molecules are free to move in any direction independently. This is because all the forces between the particles in the solid or liquid state have been overcome on vaporisation.

As the particles move randomly in any direction, they exert a force which causes pressure (vapour pressure) on the walls of the container.

Intermolecular forces

Substances with small molecules are dependent upon their weak intermolecular forces.

It is this attraction between the molecules that needs to be overcome with energy when boiling or melting to turn the substance into a liquid or gas.

Van Der Waals’ forces

Even atoms of noble gases must exert an attraction on each other.

The Enthalpy change of vaporisation is the energy required to convert the liquid to a gas.

The further down the group of noble gases you go, the more electrons and protons there are present. Therefore the further down the group of noble gases you go, the higher the enthalpy change of vaporisation is.

Enthalpy change of vaporisation of the noble gases plotted against the

number of electrons present.

We can see the trends in the graph.

As the number of electrons increase so does the enthalpy change of vaporisation (kJ mol-1).

This is a dynamic and positive relationship.

But why?

Both the noble gases and the alkanes have attractive forces between atoms and molecules, which depend on the number of electrons and protons present.

In other words the enthalpy change of vaporisation increases with the number of atoms in a molecule because of the increasing number of electrons and protons.

This applies to both the forces between atoms and the forces between molecules.

W H Y D O T H E S E F O R C E S A R I S E ?

Induced dipoles

Induced dipoles

Electrons in an atom or molecule move at very high speeds within orbitals.

Therefore at any point in time it is possible for more electrons to be on one side of the atom or molecule than the other.

When this happens an instantaneous electric dipole occurs.

The imbalance of electrons (more on one side) provides the slightly negative end of the dipole.

The positive atomic nucleus (or lack of electrons on that side) provides the positive end of the dipole.

What happens next?

This instantaneous dipole produces an induced dipole in a neighbouring atom or molecule which is hence attracted.

This kind of attraction is called a instantaneous dipole- induced dipole force or a Van Der Waals’ force.

This is the weakest type of attractive force between atoms or molecules.

Induced dipole attractions

This shows how a an induced dipole arises.

As you can see the separate atoms (or molecules) are attracted to one another because they have an induced opposite charge.

M O R E D I P O L E S ?

W H A T D O T H E S E I N V O L V E ?

Permanent dipole-dipole forces

A section of poly(ester) chain

Permanent dipole-dipole forces are involved in the bonding of poly(ester) string.

Many fabrics are made using of poly(ester) fibres because of its strength.

This strength is due to the strong permanenet dipole-dipole forces between ester groups of adjacent molecules.

Permanent dipole-dipole forces

Water molecules (H2O) are attracted to a charged nylon rod because water molecules have a permanent dipole.

When a permanent dipole –dipole attraction is between oxygen, fluorine or nitrogen and hydrogen it is called a hydrogen bond.

In water, this dipole arises because of the non-linear shape of the molecule and the greater electron density around the oxygen atom.

What do you think will happen if the nylon rod has an opposite charge (e.g. positive instead of negative)?

Answer

The jet of water will always be attracted to the nylon rod… it will not be repelled.

This is because if the positively charged rod can attract the oxygen, a negatively charged rod will also attract the water as the molecules will just move and the hydrogen side will be attracted.

Lone pairs and electric dipole of the water molecule

The diagram shows the lone pairs and electric dipole of the water molecule.

Note that the arrow head shows the negative end of the dipole.

Enthalpy changes of vaporisation of group 6 hydrides, including water, plotted against number of electrons present

Water is peculiar. It has a much higher enthalpy change of vaporisation than expected. Water has several other peculiar properties:

o The boiling point of water is much higher than predicted by the trend in boiling points for the hydrides of other group 6 elements.

o Water has a very high surface tension and a high viscosity.

o The density of ice is less than the density of water.

Most solids are denser than their liquids, as molecules usually pack closer in solids than in liquids.

W H A T I S T H E R O L E O F T H E L O N E P A I R ?

H O W D O E S T H I S R E L A T E T O M O L E C U L E S C O N T A I N I N G – O H A N D – N H G R O U P S ?

Hydrogen bonding

Hydrogen bonding

The peculiar nature of water is explained by the strongest type of intermolecular force- the hydrogen bond; indicated on diagrams by dotted lines.

Water is highly polar owing to the large difference in electronegativity between hydrogen and oxygen.

The resulting intermolecular attraction between oxygen atoms and hydrogen atoms on neighbouring water molecules is a very strong permanent dipole-dipole attraction called a hydrogen bond.

Each water molecule can form two hydrogen bonds to other water molecules. These form in the directions of lone pairs.

Hydrogen bonding

Water molecules collect in groups in the liquid state.

On boiling, the hydrogen bonds must be broken.

This raises the boiling point significantly as the hydrogen bonds are stronger than the other intermolecular forces.

The enthalpy change of vaporisation is also much higher than it would be if no hydrogen bonds were present (it takes more energy to convert the liquid to a gas).

Will it float or sink?

In ice, a three-dimensional hydrogen-bonded lattice is produced.

Each oxygen atom is surrounded by a tetrahedron of hydrogen atoms bonded to further oxygen atoms.

The meltinging point is raised significantly above that predicted by the trend for other group 6 hydrides by the extensive network of hydrogen bonds.

Model of ice

Oxygen atoms are red.

Hydrogen atoms are white.

Hydrogen bonds are lilac.

This hydrogen-bonded arrangement makes ice less dense than water.

Breaking the bonds

The presence of a hydrogen-bonded network of water molecules at the surface explains the high surface tension of water.

This network is sufficiently strong to enable a needle to be floated on the surface of water.

Within the bulk of water, small groups of molecules are attracted together via hydrogen bonds.

The hydrogen bonds are constantly breaking and reforming at room temperature.

As the temperature of water is raised towards the boiling point, the number of hydrogen bonds reduces.

On boiling, the remaining hydrogen bonds are broken.

Water vapour consists of widely separated water molecules.

Why are hydrogen bonds important?

Hydrogen bonds play a very important part in the structures and properties of biochemical polymers.

For example, protein chains often produce a helical structure, and the ability of DNA molecules to replicate themselves depends primarily on the hydrogen bond, which hold the two parts of the molecules together in a double helix.

Plenary

Here are a few quick questions to test what you have learned, write your answers in your blog:

1. What is the term for the energy required to convert a liquid to a gas?

2. Explain in 30 words how an induced dipole arises.

3. Explain why water will always be attracted to a charged nylon rod.

4. State and explain two anomalous properties of water which arise from hydrogen bonding.

Answers

1. Enthalpy change of vaporisation.

2. More electrons on one side of an atom/molecule than the other forming an instantaneous dipole. This causes an induced dipole in a neighbouring atom/molecule, which is hence attracted.

3. The jet of water will always be attracted to the nylon rod… it will not be repelled. This is because if the positively charged rod can attract the oxygen (delta negative side of dipole) a negatively charged rod will also attract the water as the molecules will just rearrange themselves and the hydrogen side (delta positive side of dipole) will be attracted.

Answers

4. Any two of the following: Water has a much higher enthalpy change of

vaporisation than expected of group 6 hydrides. The boiling point of water is much higher than

predicted by the trend in boiling points for the hydrides of other group 6 elements.

Water has a very high surface tension and a high viscosity.

The density of ice is less than the density of water. These are due to the strong hydrogen bonds between molecules.

W H A T I N S T A N T A N E O U S A N D I N D U C E D D I P O L E S A R E A N D H O W T H E Y A R I S E .

W H A T H Y D R O G E N B O N D S A R E A N D W H A T A N O M A L O U S P R O P E R T I E S T H E Y C A U S E I N

W A T E R .

You should now know…