chapter 11 Flashcards
what are intermolecular forces? how do we change physical state? look at chart
• 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
what do electrons in an atom form?
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
what does polarizability refer to?
• Polarizability refers to how easy an ‘electron cloud’ can
be distorted
how do ions induce dipoles?
• 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
what are dispersion forces?
weakest and first to exist in substances
• Can be called London dispersion forces
• Exist in all molecules
what causes LD forces? what do they cause? occur in?
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
what can one dipole create?
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
what do dipole dipole forces result from?
• 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
what is hydrogen bonding? why?
“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
what are ion dipole forces?
the positively charged end of a polar molecule is attracted to the negative ions and vise versa
ion induced
find in notes later
how is boiling point related to intermolecular forces?
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
what is related to the polarizability of noble gases?
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
what are dispersion
forces correlated to?
• 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
what do larger electron clouds result in?
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
polar vs non polar- dipole dipole forces
• 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
hydrogen bonds and boiling points
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)
what is surface tension?
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
why does liquid form drops? which molecules have higher surface tension?
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
what is viscosity?
• Resistance of a liquid to flow
describe low viscosity
üLow viscosity liquids flow easily, while high viscosity
liquids flow more slowly
describe high viscosity
• 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
what does viscosity arise from? strong IF? chains?
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
saturation
• 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
how do we account of the evaporation of a glass of water at room temp?
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
what happens when we seal a container?
• 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
what is the vapor of pressure of a chemical?
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
what us vapor pressure dependent on?
• 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
what is miscibility?
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”
what is required to break bonds?
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
