IM Forces
Section 10.1
States of Matter
Forces Between Particles in Solids
and Liquids
• Ionic compounds
– Attractive forces between oppositely charged
ions hold ionic compounds together.
– Ionic bonds are the strongest interparticle
force.
– Smaller the ion and the larger the charge on
the ion the stronger the attractive forces
among the ions
Ionic
Bonding
Forces Between Particles in Solids
and Liquids
• Forces between molecular compounds
– Intermolecular (IM) forces between molecules
attract molecules to each other in the liquid
and solid state.
• IM forces are very weak as compared to ionic or
covalent bonds
IM Forces
Three types of IM Forces
1. Dipole-dipole force
2. Hydrogen “bonding”
3. London dispersion forces
See pages 440-442
Interparticle Forces and
Physical Properties
• The stronger the attractive forces between
particles in a liquid or solid, the
– Higher the:
•
•
•
•
Melting point
Boiling point
Surface tension
Viscosity
– Lower the:
• Vapor pressure
IM Forces
• Dipole-dipole forces
– Attractive forces between oppositely charged
dipoles.
– Dipole-dipole forces are found between polar
compounds.
• The more polar the compound the stronger the
dipole-dipole force.
IM Forces
• Hydrogen “bonds”
– Attractive force between a d+ H bonded to an
O, N, or F and a d- O, N, or F generally on
another molecule.
• Really a relatively strong dipole-dipole force
– Hydrogen bonding is the strongest of the IM
forces.
– H bonding is very important in water and in
many biological molecules.
• Hydrogen “bond” is a weak attractive
force between a d + hydrogen and a
d- O, N, or F in a second polar bond
London Dispersion Forces
• London Dispersion force
– Very weak and short-lasting attractive forces
between temporary dipoles
• See figure 10.5
– Weakest of the IM forces
London Dispersion Forces
• London Dispersion forces
– Found between all molecules in liquid/solid
state.
• Of greatest significance in nonpolar molecules as
it’s the only IM force between nonpolar molecules
– The larger the molecule the stronger the
dipersion forces.
Dispersion Forces
Occur between every compound and arise from the net attractive forces
amount molecules which is produced from induced charge imbalances
The magnitude of the Dispersion Forces
is dependent upon how easily it
is to distort the electron cloud.
The larger the molecule the greater
it’s Dispersion Forces are.
Dispersion Forces and
Molecular Shape
• Elongated molecules have higher
dispersion forces than compact molecules
• Ringed structures have higher dispersion
forces than straight chain molecules.
– Consider:
• Hexane
• Cyclohexane
• 2,2 – dimethyl butane
Interparticle Forces
• Weakest to Strongest:
Intermolecular forces – all relatively weak
London dispersion forces
Dipole-dipole force
Hydrogen Bonding
Ionic bond - BY FAR THE Strongest:
- not an IM Force
Properties of Liquids
• Freezing and boiling point
• Surface tension
• Capillary action
• Viscosity
Which are directly related to the strength
of the IM forces present between
molecules?
Change of State
• Normal Freezing/Melting point
– temperature at which the liquid and solid state
co-exist at 1 atm pressure
• Normal boiling point
– temperature at which the liquid and gaseous
state co-exist at 1 atm pressure
• Predict the relative BP of:
– Methane, acetone, methanol, ethanol, NaCl
Surface Tension
• Surface tension
– Resistance of a liquid to increase its surface
area
– Measure of the energy needed to break the
IM forces at the surface
Capillary Action
• Capillary action
– Spontaneous rising of a liquid in a narrow
tube
• Related terms:
– Cohesive forces – attractive forces among like
molecules
– Adhesive forces – attractive forces among
dislike molecules
See Figure 10.7, page 444
Concave meniscus
Adhesion > Cohesion
Convex meniscus
Cohesion > adhesion
Viscosity
• Viscosity – resistance of a liquid to flow
– Highly viscous liquids are thick (syrupy)
– Consider relative viscosity of:
• Propanol -
• Glycerol
Solid State
• Amorphous solids – random arrangement
of particles
• Crystalline solids – highly ordered
arrangement of particles
– Arrangement referred to as a lattice structure
• Unit cell – smallest repeating unit in the lattice
– Use x-ray crystallography to determine the structure of
crystalline solids
Common Unit Cells – All Cubic
Closest Packing and Unit Cells
• Often see a lattice structure based on the
closest packing possible for the particles.
• Two common closest packing
arrangements (pages 451/452)
– Abab close packing – creates a hexagonal
prism unit cell (hcp)
– Abca close packing – creates a face-centered
cubic unit cell
Types of Crystalline Solids
• Crystalline solids are defined by what’s in
the lattice points
• The properties of crystalline solids
depends upon the strength of the
attractions between the particles
– Stronger the attractive forces, the higher the
mp, bp…..
Crystalline Solids
Type of Solid
In Lattice Points
Attractive forces
between particles
Properties
Ionic
Ions
-- NaCl
-- CaO
Ionic bonds
High mp, poor
conductors of heat
and electricity in
solid state, hard,
brittle
Molecular
Molecules
-- H2O
-- CO2
-- H2
IM Forces
-- dispersion forces
-- dipole-dipole
-- H bonds
Relatively low mp
and bp (depends
on force present),
poor conductors of
heat and electricity,
soft
Crystalline Solids – Atomic Solids
Type of ATOMIC In Lattice Points Attractive forces
Solid
between particles
Properties
Group 8a –
frozen noble
gases
Noble gas atoms
-- He
-- Ar
Dispersion forces
VERY low mp,
insulators
Metallic
Metal atoms
-- Cu
-- Fe
Non-directional
covalent bonds
--sea of delocalized
valence electrons
Ductile, malleable,
good conductors of
heat and electricity
Network – giant
molecules, not
individual atoms
or molecules
Nonmetal atoms
-- C, diamond
-- SiO2
-- C, graphite
(atypical network
solid)
Covalent bonds
Typical – hard,
high mp, poor
conductors heat
and electricity
(insulators)
Atypical – see
graphite
Metallic Solids
• Metal atoms in lattice points
– Variety of unit cells possible
• Electron sea model
– delocalized valence electrons form strong
nondirectional bonds
• Valence e are free to move – conduct electricty
and heat
• Atoms remain bonded to each other as they bend/
move – malleable and ductile
Diamond – Network Solid
• A diamond is a gigantic molecule, each C
atom is bonded to 4 other C atoms
• Each C is sp3 hybridized
SiO2 – Network Solid
Why isn’t SiO2classified as a molecular solid?
Graphite – Atypical Network Solid
• Layers of ringed carbon structures
– Each C is bonded to 3 other C
– Each C is sp2 hybridized
Change of State Terms
• Freezing
– Normal freezing point
• Melting
• Vaporization
– Normal boiling point
• Condensation
• Sublimation
• Deposition
Phase Diagram
Phase Diagram Terms
• Triple Point
– temperature and pressure conditions when (any)
3 states co-exist
• Critical Point
– Critical Temp – temp
– above which a gas cannot
be liquefied
– Critical P – P required to
create a liquid at critical T
Phase Diagram of Water
11.9
11.9
Phase Diagram for Sulfur