6.3 intermolecular forces

Question 1

INTERmolecular forces are

Answer 1

are forces, which hold molecules together (between one molecule
and its neighbours). They are relatively weak.

Question 2

INTRAmolecular forces.

Answer 2

Covalent bonding is strong and holds the atoms in a molecule together. These are an
example if INTRAmolecular forces.

Question 3

The molecules in simple covalent substances are not entirely isolated from one another. There
are forces of attraction between them. These are called intermolecular forces.

There are 3 types of intermolecular forces:

Answer 3

• london forces (induced dipole-dipole interactions)
• permanent dipole dipole interactions
• hydrogen bonds

Question 4

Induced dipole-dipole interactions (London forces) exsist between….

Answer 4

ALL molecules.

Question 5

when do

Induced dipole-dipole interactions (London forces) arise

Answer 5

These forces arise because the electrons in an atom are in continual motion.

It produces a changing dipole.

At any particular instant, the electron charge around the molecular may not be perfectly
symmetrical (giving one side of the molecule more negative charge)

It possesses an instantaneous dipole

The instantaneous
dipole can induce a
dipole in any
non-polar
neighbouring
molecules.

Question 6

The strength of induced dipole-dipole interactions (London forces) is affected by:

Answer 6

The number of electrons directly relates to the strength of the induced dipole-dipole
interactions.

The greater the number of electrons in a molecule the greater the fluctuation in the
asymmetry of the electron charge cloud, and the stronger the London forces will be.

The greater the London forces (induced dipole-dipole interactions) the stronger the
attractive forces between one molecule and its neighbour.

This means more energy is needed to break the forces.

Increasing the boiling point of an element/compound.

Question 7

Permanent dipole-dipole interactions:

Answer 7

These forces of attraction exist between polar molecules, which have a permanent dipole. A
molecule with permanent dipole contains two elements with different electronegativity and is
not symmetrical in shape.

The negative end of one molecule is attracted towards the positive end of another.

Question 8

what determins the strength of london forces

london forces

Answer 8

The shape of the molecule
* The more branches the lower the boiling point which means weaker London forces.

• The long shape of pentane allows close packing and maximum surface area for London
  forces. This gives optimum interactions. More branches means the molecules can’t pack as
  closely.
• It also means that the surface area for London forces is reduced, therefore the energy
  needed to overcome London forces is reduced (lower boiling point)

Question 9

coment solubility of simple molecular substances

Solubility:

Answer 9

Simple molecular substances can be non-polar or polar.

‘Like dissolves like’

This means..

non-polar substances dissolve in non-polar solvents

Polar substances dissolve in polar solvents

Non-polar simple molecular substances

When a non-polar molecule substance is added to a non-polar solvent, the intermolecular forces
weaken in the simple molecular structure.

They eventually break allowing intermolecular interactions to form between the solvent and the
molecules from the substance.

Polar simple molecular substances

The solubility depends on the strength of the dipole (making it hard to predict) but in general
they dissolve in polar solvents similarly to the way ionic compounds dissolve.

N.B. some molecules have polar hydrophobic and non-polar hydrophilic groups (e.g. organic
molecules – carboxylic acids/alcohols) this allows them to dissolve in polar or non-polar solvents
depending upon which group takes priority.