5. Intermolecular Forces

Question 1

What determines a molecules physical properties?

Answer 1

The molecules intermolecular forces.

Question 2

What are intermolecular forces? (2)

Answer 2

• the forces between molecules.

- the forces between noble gas atoms.

Question 3

True or false, intermolecular forces are weaker than covalent, ionic and metallic bonding (intramolecular bonds).

Answer 3

True.

Question 4

Collectively name the forces involved in intermolecular bonding.

Answer 4

The Van Der Waal’s forces.

Question 5

What can covalent bonding be sub-divided into? (2)

Answer 5

• Polar bonds.

- Non-polar bonds.

Question 6

Name the force between atoms of a non-polar bond.

Answer 6

London Forces (induced dipole-dipole forces).

Question 7

Name the forces that can exist between atoms of a polar bond. (2)

Answer 7

• Dipole-dipole forces.

- Hydrogen bonding (forces).

Question 8

What occurs within molecules with dipole-dipole type forces?

Answer 8

They have permanent dipoles with oppositely charged ends.

Question 9

What makes dipole-dipole type forces stronger than non-polar molecules (with the same number of electrons)?

Answer 9

• The attractive force is greater as London forces act along with the dipole-dipole type forces between all molecules.
• Whereas non-polar molecules, with the same number of electron, only experience London forces between their molecules.

Question 10

Why do London forces occur?

Answer 10

As a result of non-permanent or fluctuating dipoles.

Question 11

Why does this electron fluctuation result in the weakest type of inter-molecular bonding?

Answer 11

As the fluctuation of electron distribution within the atom/molecule results in the atom/molecule being polarised but the magnitude and the direction of the dipole is constantly changing.

Question 12

True or false, there is no overall dipole moment in a non-polar molecule.

Answer 12

True.

Question 13

How is the magnitude of the dipole in London forces thus determined?

Answer 13

By the number of electrons in the molecule.

Question 14

Why is the strength of the London forces dependent on the number of electrons in a molecule/atom?

Answer 14

As the greater the number of electrons in an atom/molecule the greater the magnitude of the fluctuating dipoles.

Question 15

True or false, temperature and pressure do not effect the London forces between molecules.

Answer 15

False, the greater the temperature/pressure the greater the magnitude of the London forces as molecules have more kinetic energy and on average become closer together.

Question 16

What is a special type of dipole-dipole force?

Answer 16

Hydrogen bonding (forces).

Question 17

Why is it called ‘hydrogen’ bonding?

Answer 17

As it involves a partially positive hydrogen atom in one of the molecules.

Question 18

When is a hydrogen atom sufficiently partially positive?

Answer 18

When it is bonded to a small, highly electronegative atom such as Fluorine (F), Oxygen (O) and Nitrogen (N).

Question 19

Why is hydrogen bonding stronger than ‘normal’ dipole-dipole type bonding? (For molecules containing the same number of electrons).

Answer 19

As the force between the partially positive hydrogen atom, similar to a ‘bare proton’, is attracted to the centre of the F,O and/or N of adjacent molecules.

Question 20

What are the effects of the abundance of electrons have on the boiling point of substances?

Answer 20

The more electrons there are in a molecule/atom, the higher the boiling point becomes due to the stronger inter-molecular forces. (Moving down a group the atoms/molecules have more electrons.)

Question 21

True or false, therefore due to the increase in electrons the attractive forces due to fluctuating dipoles increase.

Answer 21

True.

Question 22

Why are the behaviour of ammonia, water and hydrogen fluoride anomalous?

Answer 22

As they exhibit hydrogen bonding. Therefore their boiling point will be higher than molecules/atoms with only London forces as they have both London and hydrogen bonding forces.

Question 23

Define viscosity.

Answer 23

The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of “thickness”: for example, syrup has a higher viscosity than water.

Question 24

How do we go about comparing the intermolecular forces between different molecules? (3)

Answer 24

-Identify the Intermolecular forces.
-Compare the IM forces (Which is stronger).
-How much energy is required to break the bonds.
Note: if it has a similar molecular mass, generally has the same number of electrons.

Question 25

How do dipole-dipole interaction originate?

Answer 25

• ___ is polar (___ molecules are permanent dipole)
• causing an attractive force
• between the slightly negative side of one molecule and the slightly positive side of another molecule

Question 26

How do London forces originate? (4)

Answer 26

• ___ is non-polar √
• random e- movement causes fleeting/temp/fluctuating dipole √
• temp dipole on one molecule induces dipole on adjacent molecule √
• oppositely charged dipoles on different molecules attract √ producing the IMF.