Chapter 11
Intermolecular Forces (IMF)- the forces that exist
between molecules
Gases
-IMF not strong enough to hold particles together
-assumes shape and volume of container
-expands to fill container
-compressible
-flow readily
-diffusion within a gas occurs readily
Liquids
-IMF strong enough to hold particles close
together
-assumes shape of container
-does not expand to fill container
-incompressible
-flows readily
-diffusion within a liquid occurs slowly
-known as a condensed phase b/c particles are
fairly close together
Solids
-IMF very strong- virtually locks particles in
place
-has definite shape and volume
-incompressible
-does not flow
-diffusion within a solid occurs slowly
-known as a condensed phase
-states of matter can be changed by heating or
cooling
-changes the average kinetic energy
EX-dec the temp of a gas will dec KE and form a
liquid and then lock particles in place to form a
solid
-inc pressure of a gas can bring molecules closer
together to form a liquid and then a solid
IMF
-usually much weaker than intramolecular forces
(ionic, metallic or covalent bonds)
-less energy is required to change a state of
matter than to break a bond
-boiling points increase with stronger IMF
-melting points increase with stronger IMF
-page 428 Table 11.2
-all IMF are electrostatic- attractions between +
and -weaker than bond b/c distances between
molecules are often larger than the distances
between atoms held together by bonds
Ion-Dipole Forces
-exists between an ion and a polar molecule
-cations are attracted to - end of a dipole
-anions are attracted to + end
-magnitude of attraction increases as either the
ionic charge or the magnitude of the dipole
increases
-important for solutions of ionic substances in
polar liquids
-strongest IMF
*only found in ionic compounds dissolved in
solution
Ex: salt in water
Dispersion Forces (London Dispersion)
-present in all molecules and atoms
-weakest IMF
-caused by motion of e- and instantaneous dipole
moment
-can cause an instantaneous dipole moment on
an adjacent atom, causing the atoms to be
attracted
-page 429 Fig 11.4
-fluctuations in the electron distribution in atoms
and molecules result in a temporary dipole
– region with excess electron density has
partial (─) charge
– region with depleted electron density has
partial (+) charge
-strength depends on ease with which the charge
distribution can be distorted
-called polarizability
-the greater the polarizability the more easily an
e- cloud can be distorted to give an instantaneous
dipole
-more polarizable molecules have larger
dispersion forces
-polarizability inc. as the # of e- in an atom or
molecule inc.
-strength inc. with inc. molecular weight
-shape also influences- longer straight chains are
stronger b/c molecules can come into contact
along entire molecule
-more compact = less contact
Dipole-Dipole Forces
-exist in all polar molecules
-polar molecules have permanent dipoles that
interact with the permanent dipoles of
neighboring molecules
*+ end of polar molecule is attracted to – end of
its neighbor
-effective only when moleules are very close
together
-all molecules (polar and non-polar) have
dispersion forces
-only polar also have dipole-dipole forces
-this extra force raises melting and boiling points
compared to nonpolar molecules with similar
molar masses
-for molecules of about equal mass and size, the
strength of IMF inc. with inc. polarity
Ex:
CH2CN vs. CH2CH2CH3
Boiling pt.= 355K and 231K
Mass= 40amu and 43amu
*close masses so dispersion forces are similar
*higher bp in CH2CN due to dipole-dipole
forces
-page 431 Figure 11.8
Which of the following have dipole-dipole
forces?
1) CO2
-has polar bonds, but is nonpolar b/c it is linear
 no dipole-dipole forces
2) CH2Cℓ2
-has polar bonds and net dipole
 has dipole-dipole forces
3) CH4
-has nonpolar bonds
 no dipole-dipole forces
Hydrogen Bonding
-occurs when polar molecules have H atoms bonded
directly to very electronegative atoms (F, O, and
N)
-H atom is attracted to nonbonding e- pair on
another similar molecule
-causes strong interaction between H and other
atoms (O, F and N) in other molecules
-second strongest IMF
-substances with H bonding have higher melting and
boiling points
-water exhibits very strong H-bonding and this
explains its behavior
-these two compounds have the same formula,
but different structures which results in one
having strong H-bonding and one not
In which of these substances is H bonding likely
to play an important role in determining physical
properties?
methane CH4
methyl fluoride CH3F
hydrazine H2NNH2
hydrogen sulfide H2S
*consider Lewis structures
**hydrazine**
Which one of the following compounds has a
higher boiling point and why?
-all have similar molar masses- similar
dispersion forces
-all are polar- all have dipole-dipole forces
**hydrogen peroxide b/c it has H-bonding,
harder to break apart
List the substances BaCℓ2, H2, CO, HF, and Ne
in order of inc boiling point.
* BaCℓ2 is ionic
*all others have dispersion forces
*MW= 2, 28, 19.9, 20.2
*CO and HF have dipole-dipole b/c polar
*HF has H-bonding
H2 < Ne < CO < HF < BaCℓ2
Identify the IMF present in the following
substances and select the substance with the
highest boiling point.
1) CH3CH3
2) CH3OH
3) CH3CH2OH
1) dispersion, MW= 30amu
2) dispersion, H-bonding, MW= 32amu
3) dispersion, H-bonding, MW= 46amu
*#3 has highest boiling point
Results of IMF
1) viscosity- the resistance of a liquid to flow
-the higher the viscosity, the more slowly it
flows
-increases with greater IMF (or molar mass) b/c
molecules cannot flow as easily
-also depends on molecular shape- higher in
longer molecules b/c they can become
entangled
-temp plays a part- the higher the temp, the lower
the viscosity- higher KE can overcome the
IMF
2) surface tension- inward force or pull that tends
to minimize the surface area of a liquid
-energy required to inc the surface area of a liquid
by a unit amount
-molecules with increased IMF have increased
surface tension
-water has an extremely high surface tension b/c of
the strong H-bonding
ex- forming spherical droplets, things being able to
float on water
-it is difficult to break the H-bonds
-will decrease by adding a surfactant (soap)
3) capillary action- ability of liquid to flow
against gravity up a narrow tube
cohesive forces- bind similar molecules to one
another
adhesive forces- bind a substance to a surface
-if adhesive are greater than cohesive then the
liquid will be drawn up
-if cohesive are greater than adhesive than liquid
does not rise
-shape of meniscus determined by cohesion and
adhesion
Phase Changes
heat of fusion (∆Hfus)
-amount of heat required to melt one mole of a
solid
- endothermic= +
heat of solidification (∆Hsolid)
-amount of heat required to solidify one mole of
a liquid
-exothermic= -same magnitude as ∆Hfus, but opposite sign b/c
energy is given off
heat of vaporization (∆Hvap)
-amount of heat required to vaporize one mole of
a liquid
- endothermic
heat of condensation (∆Hcond)
-amount of heat required to condense one mole
of a gas to a liquid
-exothermic
-same magnitude as ∆Hvap, but opposite sign b/c
energy is given off
heat of sublimation (∆Hsub)
-goes from solid to gas without passing through
liquid phase
∆Hsub= ∆Hfus + ∆Hvap
-endothermic
ex- frozen foods
heat of deposition (∆Hdep)
-gas to solid skipping liquid phase
-exothermic
-same magnitude as ∆Hsub, but opposite sign b/c
energy is given off
Heating Curves
-graph of temp vs. amount of heat added
-can calculate enthalpy change of a system for
each segment of the heating curve
-in AB, CD, and EF a single phase is heated from
one phase to another use ∆H = mC∆T
-for BC use ∆Hfus
-for DE use ∆Hvap
*energy used up to increase distance between
particles
-a gas normally liquefies at some point when
pressure is applied
-if temp inc, pressure must inc even more
-if temp reaches a certain point no amount of
pressure can cause a liquid to form
-at that point, as pressure inc and the gas
becomes more steadily compressed
critical temp- the highest temp at which a
distinct liquid phase can form
critical pressure- pressure required to bring
about liquefaction at this critical temp
-above the critical temp, the KE of the molecules
is greater than the attractive forces that lead to
the liquid state
-the greater the IMF, the higher the critical temp
supercritical fluid- occurs when critical temp
and pressure are exceeded and the liquid and gas
phases are indistinguishable from each other
vapor pressure- pressure above the surface of
the liquid, causes evaporation
-will be lower with stronger intermolecular
forces
-if a container is sealed, the liquid will still
vaporize and condense, just not into the
atmosphere
-when rate of condensation equals rate of
vaporization the liquid has reached dynamic
equilibrium
-vapor pressure of a liquid is the pressure exerted
by its vapor when liquid and vapor are in
dynamic equilibrium
-equilibrium never occurs when vaporization
happens in an open container
volatile- liquids that evaporate easily
-have high vapor pressure
ex- nail polish remover
nonvolatile- liquids that do not vaporize easily
-have low vapor pressure
ex- motor oil
-vapor pressure inc with inc temp- particles
move more and can escape to gas phase
-a liquid boils when vapor pressure equals the
external pressure acting on the liquid surface
normal boiling point- boiling point of a liquid at
1 atm
*the higher the pressure the higher the boiling pt
*food takes longer to cook at higher elevations
WHY?
-pressure is lower, lowering boiling point of
water below 100°C, taking longer to cook
phase diagram- summarizes conditions under
which equilibria exists between the different
states of matter
-can be used to predict which phase of a
substance is present at any given temp and
pressure
-page 445 Fig 11.27
1) red curve is vapor pressure curve of liquid
-equilibrium between liquid and gas phase
-@ 1 atm is the normal boiling point
-ends at the critical point (C)- critical temp
and pressure
-supercritical fluid beyond critical point
2) green curve is sublimation curve
-separates the solid from the gas phase
3) blue curve is melting curve
-separates solid phase from liquid phase
-usually slopes to the right as pressure inc
b/c most solids are denser than their liquids
-@ 1 atm is the normal melting point
triple point
-point T, where the three curves intersect
-all three phases exist in equilibrium