Intermolecular Forces

Question 1

What are intermolecular forces?

Answer 1

Weak interactions between molecules of different molecules.

Question 2

What are the three main types of intermolecular forces?

Answer 2

Induced dipole-dipole interactions (London forces)
Permanent dipole-dipole interactions
Hydrogen bonding

Question 3

What type of property are intermolecular forces largely responsible for?

Answer 3

Physical properties

Question 4

Give an example of a physical property?

Answer 4

Melting and boiling points.

Question 5

What type of properties do covalent bonds determine?

Answer 5

The identity and chemical reactions of molecules.

Question 6

List the intermolecular forces in order of strength.

Answer 6

London forces
Permanent dipole-dipole interactions
Hydrogen bonds

Question 7

What is stronger, intermolecular forces of single covalent bonds?

Answer 7

Single covalent bonds

Question 8

What type of molecules do London forces exist between?

Answer 8

They exist between all molecules, regardless of polarity.

Question 9

What are London forces?

Answer 9

Weak intermolecular forces that exist between all molecules. They act between induced dipoles indifferent molecules.

Question 10

Explain the origin of induced dipoles.

Answer 10

Movement of electrons produces a changing dipole in a molecule.
At any instant, an instantaneous dipole exist, but its position is constantly shifting.
The instantaneous dipole induces a dipole on a neighbouring molecule.
The induced dipole induces further dipoles on neighbouring molecules, which then attract one another.

Question 11

Are induced dipoles temporary or permanent?

Answer 11

Temporary

Question 12

What do induced dipoles result from?

Answer 12

The interactions of electrons.

Question 13

What happens to the strength of London forces as E number of electrons in the molecule increases?

Answer 13

The more electrons the:
Larger the induced dipole
The greater the induced dipole-dipole interactions
The stronger threat traction forces between the molecules

Question 14

Why does the boiling point increase down the noble gases?

Answer 14

The larger number of electrons the larger the induced dipoles.
More energy is then needed to overcome the intermolecular forces, increasing the boiling point.

Question 15

What do van der Waals forces describe?

Answer 15

This term is used to describe both induced and permanent dipole-dipole interactions.

Question 16

What do permanent dipole-dipole interactions act between?

Answer 16

Permanent dipole-dipole interactions act between the permanent dipoles in different polar molecules.

Question 17

What increases the boiling and melting points of molecules?

Answer 17

The permanent dipole-dipole interactions.

Question 18

What is a simple molecular substance?

Answer 18

This is made up of small molecules containing a definite number of atoms with a definite molecular formula.

Question 19

What do simple molecules form in their solid state?

Answer 19

A regular structure called a simple molecular lattice.

Question 20

What are the bonds like in a simple molecular lattice?

Answer 20

The molecules are held in place by weak intermolecular forces.
The atoms within each molecule are bonded together strongly by covalent bonds.

Question 21

Why do simple molecular substances have low melting and boiling points?

Answer 21

The weak intermolecular forces can be broken by the energy present at low temperatures.

Question 22

What state are simple molecular substances in at low temperatures?

Answer 22

They can be solid, liquid or gases.

Question 23

How can you solidify simple molecular structures?

Answer 23

By lowering the temperatures.

Question 24

What happens when a simple molecular lattice is broken apart during melting?

Answer 24

Only the weak intermolecular forces break.

The covalent bonds are strong and do not break.

Question 25

What two categories do simple molecular substances fit into?

Answer 25

Polar and non-polar.

Question 26

What happens when a non-polar simple molecular compound is added to a non-polar solvent?

Answer 26

Intermolecular forces form between the molecules and the solvent.
These interactions weaken the intermolecular forces in the molecular lattice.
The intermolecular forces break and the compound dissolves.
Non-polar simple molecular substances tend to be soluble in non-polar solvents.

Question 27

What happens when a non-polar simple molecular substance is added to a polar solvent?

Answer 27

There is little interaction between the molecules in the lattice and the solvent molecules as the intermolecular bonding within the polar solvent is too strong to be broken.
Simple molecular substances tend to be insoluble in polar solvents.

Question 28

Why might polar covalent substances dissolve in polar solvents?

Answer 28

The polar solvent molecules and the polar solute molecules a can attract each other.

Question 29

What does the solubility of polar simple molecular substances depend on?

Answer 29

The strength of the dipole.

Question 30

Why are simple molecular substances not conductors of electricity?

Answer 30

There are no delocalised electrons so there are no charged molecules that can move, therefore the electrical circuit can not be completed.