Intermolecular interactions: London forces

Question 1

interactions between molecules are described as (2 items)

Answer 1

• ‘non-bonded interactions’

- ‘intermolecular’

Question 2

London forces are an important

Answer 2

non-bonded interaction also known as dispersion forces

Question 3

London forces are weaker than

Answer 3

covalent bonds and polar covalent bonds

Question 4

on average, diatomic molecules such as nitrogen are non-polar because

Answer 4

the electron density is symmetrically distributed throughout the molecule

Question 5

the electron density in molecules can

Answer 5

change over time, generating a dipole as the molecule becomes unsymmetrical in electron density

Question 6

when the electron density in a molecule changes, London forces arise because

Answer 6

• one molecule gains an unsymmetrical distribution of electron density, with partial charges forming at each end of the molecule
• this partial charge is called an instantaneous dipole
• the partial charge of a molecule will affect the electron densities of the molecules on either side of it as their electrons will respond to the partial charges of the instantaneous dipole by either repelling or being attracted to a partial positive charge
• the resulting dipole is called an induced dipole
• the two dipoles are arranged so that they interact favourably, and it is this force that is responsible for the London force of attraction between two molecules

Question 7

the induced dipole will ……………..be aligned……………….

Answer 7

ALWAYS be aligned in such a way that the interaction with the instantaneous dipole is FAVOURABLE

Question 8

the fluctuations that lead to the generation of an instantaneous dipole and the subsequent induced dipole are

Answer 8

very rapid processes compared to the rates which the molecules are moving owing to the kinetic and rotational energy they have

Question 9

the relative speed of the dipole generating processes means that as molecules move around, they will continue to

Answer 9

attract each other regardless of the orientation of the two molecules

Question 10

the attractive force of London forces increases with

Answer 10

the increasing number of electrons in the molecule

Question 11

we can easily see how the increasing number of electrons in a molecule increases the London forces with the study of

Answer 11

• the boiling temperatures of the noble gases (monatomic molecules)
• as the London force is the only force of attraction between them
• the boiling temperatures of each member down the group increases

Question 12

using noble gas boiling temperature as a study of London forces is suitable because

Answer 12

London forces are the only force of attraction between the monatomic molecules

Question 13

the more electrons there are in a molecule, the greater

Answer 13

the fluctuation in electron density and the larger the instantaneous and induced dipoles created

Question 14

London forces also depend on

Answer 14

the shape and size of the molecule

Question 15

the greater the points of contact between the molecules, the greater

Answer 15

the overall London force

Question 16

London forces are always

Answer 16

present between molecules