How can a gecko cling to any surface and yet still be able to move?

Intermolecular forces

Intermolecular forces arise between molecules. They can be thought of as holding individual molecules together. These weak forces must be overcome for substances to boil.

There are three types of intermolecular forces

1) Van der Waals forces2) Permanent dipole-dipole forces

3) Hydrogen bonds

If there were no attractions between molecules it would be impossible to turn a gas into a liquid by cooling.

But why are there attractions between uncharged non-polar molecules?

Van der Waals forces

These are weak forces that arise from the fluctuating movement of electrons. This movement causes temporary dipoles, which, in turn, induce attraction between molecules (or atoms). These attractions are called Van der Waals forces.

Temporary dipoles occur in one molecule owing to an unequal distribution of the electron cloud around it.

This temporary dipole induces a dipole in another molecule and the two attract each other.

London forces are also known as induced dipole-dipole forces or Van der Waals forces.

Van der Waals

These weak intermolecular forces explain how a gecko (and a spider) can climb a wall!

The strength of Van der Waals forces increases:

The larger the molecule (due to a larger electron cloud)

The larger the surface area of the molecule (due to a larger electron cloud exposed).

Van der Waals forces affect:

The boiling point

The melting point

Viscosity

The more Van der Waals forces exist between molecules, the more ‘sticky’ they become as they attract each other more strongly.

Energy is required to overcome these forces of attraction.

Boiling temperatures of noble gases

Have a go at explaining this graph

What about the Halogens?

At room temperature.....

Chlorine is a gas Bromine is a liquid Iodine is a solid

Alkanes

Draw 2 isomers of butane

Which do you think will have the highest boiling temperature?

Permanent dipole-dipole forces

These forces occur between molecules with a permanent dipole. This happens when the electronegativities of the elements are very different, resulting in a polar bond.

The δ+ atom of one molecule attracts the δ- atom of another molecule

This results in a weak electrostatic force of attraction.

Remember, no overall dipoles are seen in symmetrical molecules, due to the equal pull on the bonding electrons.

The Saturated Hydrocarbons

methane CH4 -164 ethane C2H6 -89

propane C3H8 -42 butane C4H10 -0.5pentane C5H12 36 hexane C6H14 69 heptane C7H16 98 octane C8H18 125

nonane C9H20 151 decane C10H22 174

Using what you know about inter-molecular

forces explain the following graph

Why is the boiling point of water 100ᵒC, when the boiling point of Hydrogen Sulphide is -60ᵒC and that of Hydrogen Selenide is -40ᵒC

Hydrogen bonds

These are very strong intermolecular interactions. They are a special case of a permanent

dipole-dipole interaction.

They are stronger than Van der Waals forces and permanent

dipole-dipole interactions (but still 10X weaker than covalent bonds)

Hydrogen bonds

Hydrogen bonds ONLY form when Hydrogen is covalently bonded to

one of three very small electronegative atoms.....

Oxygen, Nitrogen and Fluorine

A lone pair is also required.

Nitrogen, Oxygen and Fluorine are very electronegative and cause the bond with

hydrogen to be polar.

The bond is so polarised that the hydrogen forms a weak bond with the nitrogen, fluorine

or oxygen of another molecule.

In addition, N, O and F are all small enough to approach a hydrogen atom – their lone pair

on the N, O or F forms a bond with the hydrogen.

δ+

δ-

Because Oxygen is so electronegative, Hydrogen is effectively a proton – it has no outer shell.

This means it can interact strongly with a lone pair from another oxygen to produce.............

The three atoms associated with a

hydrogen bond are ALWAYS in a straight line.

The structure of ice

Molecules in ice are held together by hydrogen bonding. The molecules form a giant lattice in which each is bonded to two H atoms by covalent bonds, and two others by hydrogen bonds.

The structure of ice

In liquid water hydrogen bonds break and reform easily as the molecules move about.

When water freezes the hydrogen bonds hold the water molecules in fixed positions.

In order to fit into this structure the molecules are less tightly packed than in liquid water. This means that ice is less dense than water and floats on water.

**This is thought to have helped life to continue during the ice ages – ice would form an insulating layer on bodies of water**

Questions:

1) In which of these does hydrogen bonding NOT occur? H2O, NH3, HBr, HF?

2) Explain why hydrogen bonds do not form between methane molecules

3) Draw a dot and cross diagram of water

A) How many lone pairs does it have?B) How many hydrogen atoms does it have?C) Explain why water molecules form two hydrogen bonds

per molecule, but ammonia molecules only form one.

4) Why do alcohols have a lower volatility compared to alkanes of a similar size?

Quiz – spot the inter-molecular force

Neon and neon

Non-polar because it’s symmetrical