chapter 11

Question 1

what are intermolecular forces? how do we change physical state? look at chart

Answer 1

• Weak electrostatic forces that occur between molecules
• Originate from interactions between charges, partial
charges, and temporary charges between atoms/ions
• Intermolecular forces are responsible for making a liquid
and liquid, a gas a gas, and a solid a solid
• You must overcome intermolecular forces to change
the physical state

Question 2

what do electrons in an atom form?

Answer 2

Electrons in an atom form an ‘electron cloud’ with
varying sizes
• Electrons are in constant motion around the nucleus, but
we cannot know where the electrons are:
üHeisenberg Uncertainty Principle
• The relative position of electrons around the nucleus can
be distorted by neighbouring charged particles

Question 3

what does polarizability refer to?

Answer 3

• Polarizability refers to how easy an ‘electron cloud’ can

be distorted

Question 4

how do ions induce dipoles?

Answer 4

• Ions can approach non-polar molecules and distort the
electron cloud
• It is not a permanent effect; the cloud will redistribute
back to normal once the ion is removed
• Larger electron clouds are more polarizable

Question 5

what are dispersion forces?

Answer 5

weakest and first to exist in substances
• Can be called London dispersion forces
• Exist in all molecules

Question 6

what causes LD forces? what do they cause? occur in?

Answer 6

Due to fluctuations in the electron distribution within
an atom or molecule in the absence of an ion
• Can create and instantaneous dipole or temporary
dipole
• Regions in the molecule will have a partial (+) or partial
(-) charge
üSeparate from an actual dipole moment and can
occur in molecules with purely covalent bonds

Question 7

what can one dipole create?

Answer 7

One instantaneous dipole can create a chain reaction
with other molecules
• Continues spreading through all molecules
• Although weak, these forces contribute to the energy of
attraction between all molecule

Question 8

what do dipole dipole forces result from?

Answer 8

• Remember: a permanent dipole exists
on a polar molecules
üDipole moment results in a partial (+)
and partial (-) charge on the 
• Dipole-dipole forces result
from the attraction between
the (+) end of one molecule
with the (-) end of another

Question 9

what is hydrogen bonding? why?

Answer 9

“Super” dipole-dipole force
• Hydrogen is a small atom that can approach
neighbouring heteroatoms very closely
• Attraction felt between H and heteroatom of another
molecule is hydrogen bonding

Question 10

what are ion dipole forces?

Answer 10

the positively charged end of a polar molecule is attracted to the negative ions and vise versa

Question 11

ion induced

Answer 11

find in notes later

Question 12

how is boiling point related to intermolecular forces?

Answer 12

Remember that the boiling point of a substance is
related to the temperature at which the vapour pressure
is equal to the external pressure
• Liquids and solids with a lower vapour pressure have
stronger intermolecular forces (molecules cannot escape
into the gas phase as easily)
• This means more heat needs to be put into a substance
in order increase the vapour pressure, ∴ higher boiling
point

Question 13

what is related to the polarizability of noble gases?

Answer 13

The magnitude of the dispersion
force is related to the polarizability of
an atom
üPolarizability: how easily the
electron cloud around a molecule
can be distorted
üDepends on the number of
electrons
üRoughly correlated with molar
mass
ØIncreasing mass leads to larger
molecules or more atoms...either
produces a greater number of
electrons
more dispersion forces/size= higher boiling point

Question 14

what are dispersion

forces correlated to?

Answer 14

• Dispersion forces are correlated with molar mass- there
is also a larger surface area from which transient dipoles
can be induced
• A homologous series can be ranked based on the molar
masses

Question 15

what do larger electron clouds result in?

Answer 15

Larger electron clouds result in greater dispersion forces
because the electrons are held less tightly by the nucleus
and can be distorted more easily
• But, the number of electrons is not the sole determinant in
dispersion forces
• Also have to consider the shape of the molecule
ümore branched and compact molecules have less
surface area that can interact with other molecules and as
a result the dispersion forces are weaker, ergo lower
boiling point

Question 16

polar vs non polar- dipole dipole forces

Answer 16

• Forces between permanent dipoles are greater than
between transient or induced dipoles
• This results in higher boiling point for polar molecules
with similar masses and shapes

Question 17

hydrogen bonds and boiling points

Answer 17

H-bonds also increase the expected boiling points of
compounds
• H-bonds disrupt
the trend we would
expect in boiling
point due to
increased
dispersion forces
(more electrons,
higher
polarizability)

Question 18

what is surface tension?

Answer 18

Molecules in the interior of a liquid have maximal
contact with other molecules and have the most
intermolecular forces
• Molecules on the surface only interact with molecules
beside or below them
A liquid wants to resist increasing surface area - surface
tension is the energy required to increase the surface
area by a unit amount

Question 19

why does liquid form drops? which molecules have higher surface tension?

Answer 19

Why does liquid form spherical droplets?
• Intermolecular forces are maximized when as many
molecules as possible can be on the interior rather than
exterior
• A liquid will assume a shape with the smallest surface
area: a sphere
• Molecules with stronger intermolecular forces will have
higher surface tension

Question 20

what is viscosity?

Answer 20

• Resistance of a liquid to flow

Question 21

describe low viscosity

Answer 21

üLow viscosity liquids flow easily, while high viscosity

liquids flow more slowly

Question 22

describe high viscosity

Answer 22

• As viscosity increases to extreme levels the liquid starts
to behave more as a solid
üGlass can be described as a liquid with very high
viscosity

Question 23

what does viscosity arise from? strong IF? chains?

Answer 23

Viscosity arises from intermolecular forces
üStrong IF means molecules will be more resistant to
flow around each other
• Molecule shape is also important - long chain molecules
tend to be more viscous
üChains get entangled with each other

Question 24

saturation

Answer 24

• Margarines can be made by taking fatty acids (oils) that have a lot of
C=C double bonds and converting them to C-C single bonds
• Fatty acids/oils with double bonds have lower melting points - the
chains are more restrained, less tangling - weaker IF therefore lower
melting point - liquid
• When you ‘saturate’ fats by introducing hydrogens to make C-C
single bonds, there is more rotational freedom of the atoms, which
means more tangling and more surface area to interact with - strong
IF therefore a higher melting point - solid

Question 25

how do we account of the evaporation of a glass of water at room temp?

Answer 25

How do we account for the evaporation of a glass of
water at room temperature?
• Within the bulk liquid
(or solid), molecules
have a distribution of
energies
• Some of these molecules will have enough energy
to break the intermolecular forces holding them
together and will become gaseous molecules

Question 26

what happens when we seal a container?

Answer 26

• Gas molecules
can’t escape
• Some gas
molecules do not
have enough
energy to remain
in the gaseous
state and
condense
• When the rate of evaporation is equal to the rate of
condensation, a dynamic equilibrium is established

Question 27

what is the vapor of pressure of a chemical?

Answer 27

The vapor pressure of a chemical is the pressure of the
gas when it is in dynamic equilibrium with its liquid
• Chemicals with high vapor
pressures are said to be volatile
- Weak intermolecular forces
between molecules results in
volatile compounds
- Strong intermolecular forces
leads to non-volatile
substances

Question 28

what us vapor pressure dependent on?

Answer 28

• The vapor pressure of a
substance is dependent on
temperature
üAs temperature increases,
a greater proportion of the
molecules will have enough
kinetic energy to break
intermolecular bonds
• Vapor pressure is also
dependent on the strength of
the intermolecular force

Question 29

what is miscibility?

Answer 29

Water and oil are not miscible
üMiscibility is the ability of two
liquids to mix and form a
homogenous solution
• In order to be miscible, the
intermolecular forces between
solvent and solute must be equal to
or greater than the solvent-solvent
interactions
ü“Like dissolves like”

Question 30

what is required to break bonds?

Answer 30

Energy is required to break intermolecular bonds. If the
energy gained by dissolving something isn’t greater than
or equal to this energy, a solution will not form