Liquids and Solids
H/Chemistry/1213
Liquids and Solids
Forces of Attraction
Forces within Compounds
Intermolecular forces (Forces between
covalently-bonded compounds)
Between molecules
 Dispersion (London) forces
 Dipole-dipole forces
 Hydrogen bonds
 Relatively weak compared to intramolecular
forces
Chemical Bonds:
 Ionic
 Metallic
 Covalent (intramolecular force)
Intermolecular forces (van der Waals forces)
1.
Dispersion or “London” forces (nonpolar molecules)
(named after Fritz London, 1900-1954)

weakest intermolecular force, present between all molecules

results from an induced “temporary dipole” which induces a dipole in a nearby
molecule. The constant motion of electrons  uneven distribution of electrons at any
particular moment:

acts on all molecules all the time

only intermolecular force acting among noble gas atoms and nonpolar molecules

larger number of electrons  larger temporary dipole  stronger attractions between
molecules  higher m.p. and b.p.
e.g. halogens
F2, Cl2 gases at room T
Br2
liquid at room T (larger than F2 and Cl2)
I2
solid at room T (largest)
-1-
Liquids and Solids
H/Chemistry/1213
2.
3.
Dipole force (polar molecules)

attractions between polar molecules, stronger than London dispersion forces

(-) end of one polar molecule attracts the (+) end of another polar molecule

more polar  stronger dipole force

closer together  stronger dipole force
Hydrogen bonding

always involves H attached to an O, F or N (small, high electronegativity)

strongest intermolecular force
How strong? 5% of the strength of a covalent bond

 higher b.p. and higher viscosity

accounts for high b.p. of H2O
-2-
Liquids and Solids
H/Chemistry/1213
Liquids and Solids
How are liquids and solids similar to and different from gases? (in terms of the KMT)
Property
Gases
Liquids
Solids
Spacing
Movement
Average KE
Attraction between
Particles
Disorder/order
Volume
Shape
Fluidity
Density
Compressibility
Diffusing ability
-3-
Liquids and Solids
H/Chemistry/1213
Liquids
Viscosity = a measure of the resistance to flow
In liquids, viscosity is determined by the

intermolecular forces – more attractive   viscosity

shape of the particles – longer chains   viscosity

temperature – colder   viscosity
Surface tension - result of attractive forces of particles in the liquid = E required to  the SA of a
liquid by a given amount
 intermolecular forces   surface tension
Surfactants = compounds that lower the surface tension of water (e.g. detergent or soap)
Capillary action – the result of cohesion and adhesion
Cohesion = force of attraction between identical molecules
Adhesion = force of attraction between different types of molecules
e.g. Water in capillary tube – adhesion between water molecules and glass > cohesion
between water molecules
Solids
Density of solids is higher than density of liquids, and may be crystalline or amorphous
Crystalline solids
 made of crystals – particles are arranged in an orderly, geometric, repeating pattern
 have definite geometric shape
 crystal lattice = total 3-D array of points that describe the arrangements of particles,
smallest unit of which is the unit cell
 crystal has the same symmetry as its unit cell
 abrupt melting points – all bonds break at once
-4-
Liquids and Solids
H/Chemistry/1213
7 shapes, based on arrangement of atoms in unit cell, cell lengths and cell angles (see
textbook for details if you are particularly interested)
Categories of Crystalline Solids
1.
2.
3.
4.
5.
atomic (e.g. noble gases)
molecular (e.g. table sugar)
covalent network (e.g. diamond, quartz)
ionic
metallic
Binding forces in crystals
1. Atomic and 2. Covalent molecular crystals
Weak intermolecular forces
 low m.p., easily vaporized, relatively soft, good insulators
3. Covalent network (e.g. diamond, graphite)
3-D covalent bonds (giant covalent molecules)
 very hard, brittle, high m.p., nonconductors or semiconductors;
some are planar, e.g. graphite – in sheets
4.
Ionic (e.g. NaCl)
strong positive and negative ions, electrostatically attracted to one another
 hard, brittle, high m.p., good insulators
5. Metallic (metals)
positive ions surrounded by a cloud of electrons (electrons can move freely through
lattice)  high electrical conductivity, malleability, ductility
Amorphous solids
e.g. glasses and plastics – particles are arranged randomly  nearly any shape, depending on
molding
 no definite melting point, gradually soften to  thick, sticky liquids
 cool too fast for crystals to form
-5-
Liquids and Solids
H/Chemistry/1213
Phase Changes
Process
Phases involved
Endothermic or
Exothermic?
melting
solid  liquid
-6-
endothermic
Liquids and Solids
H/Chemistry/1213
Some Phase Change Terminology
Melting point (m.p.) = T at which the forces holding the crystal lattice of a crystalline solid together
are broken and it becomes a liquid
Note: Amorphous solids act somewhat like liquids even when solid
Vapor Pressure (VP)= P exerted by a vapor over a liquid
Boiling point (b.p.)= T at which the VP of a liquid = atmospheric P
Evaporation = vaporization only at the surface of liquid, below the b.p.
Freezing point = T at which a liquid is converted into a crystalline solid
Phase Diagrams
Graph showing the relationships between solid, liquid and gaseous phases over a range of
conditions, e.g. P vs. T
Triple point = T and P conditions at which the solid, liquid and vapor of a substance can coexist at
equilibrium
Critical T = T above which the substance cannot exist in the liquid state
Critical P = P at which a gas at its critical T is converted to a liquid
Refer to diagram on next page for the following questions
1. What variables are plotted on a phase diagram? __________________________
2. How many phases of water are represented in its phase diagram? _____
What are they? ___________________________
3. What phases of water coexist at each point along the red curve? _________________
Along the yellow curve? _________________________
4. Look at the phase diagram for carbon dioxide. Above which pressure and temperature is
carbon dioxide unable to exist as a liquid? _______________
5. At which pressure and temperature do the solid, liquid, and gaseous phases of carbon dioxide
coexist? ____________________________
-7-
Liquids and Solids
H/Chemistry/1213
6. Use the phase diagram for water to complete the following table.
Temperature (oC)
Pressure (atm)
200
1
-2
1
150
100
-2
0.001
30
0.8
1
100.00
Phase
liquid
vapor
-8-
Liquids and Solids
H/Chemistry/1213
Gas vs. Vapor
Gas = state of particles at room temperature
Vapor = a gas formed from a substance that normally exists as a solid or liquid at room T and P
Vaporization = conversion of a liquid to gas or vapor
Boiling point = T at which vapor pressure of liquid = atmospheric pressure

Boiling point at 1 atm = “normal boiling point”

H2O enters vapor state within liquid. Vapor is less dense, so rises to surface

Needs constant energy (heat) to keep it boiling - cooling process

Liquid never rises above its boiling point! (at constant P)

If atm P , less energy is required for particles to escape atm P

Mountains vs. pressure cooker
Evaporation
Evaporation occurs when particles have enough KE to overcome their I.M. forces
In a contained vessel (closed):
“dynamic equilibrium” occurs when rate of vaporization = rate of condensation
vapor pressure = partial pressure of a vapor above a liquid depends on:
1.
# gas particles: # particles   vapor pressure
2.
Temperature: as T  v.p.  because evaporation  (particles have more energy to
escape)
3.
Intermolecular forces: the stronger the I.M. forces   v.p. (fewer particles have enough
energy to break the I.M. bonds and escape)
-9-
Liquids and Solids
H/Chemistry/1213
In an uncontained vessel (open):
evaporation as a cooling process: the most energetic particles leave so average KE of remaining
particles is lower
Rate of evaporation increases with increase in:
1.
T (more particles have energy to escape the liquid)
2.
air currents
3.
surface area
Evaporation
vs.
Boiling
takes place at surface
occurs throughout liquid
below boiling T
at boiling T
Vapor Pressure
1. What is the VP of ethanol at 60oC? ________________________
2. What is the VP of water at the same T (60oC)? ____________________
3. Which compound boils at a lower T? How can you tell? ____________________
_________________________________________________________________
4. Which exhibits stronger IMFs? Ethanol or water? Explain your answer.
_________________________________________________________________
_________________________________________________________________
- 10 -