Energy and Phase Changes
Energy and Phase Changes
• Three states or phases of matter
Solid
Liquid
Gas
• Energy is involved in every phase change.
– Some require energy to occur (endothermic)
– Some release energy (exothermic)
– Based on the breaking or formation of bonds
between the particles (atoms, molecules, fun)
Intermolecular Forces
• Differing attractive forces between
molecules cause some materials to be
solids, some to be liquids, and some to be
gases at the same temperature.
Gases
• Kinetic Molecular Theory
• Constant random motion
 High energy movement spreads
molecules throughout container.
• Large space between gas molecules
• Elastic collisions
• Little or no intermolecular attraction
or repulsion.
Liquids
• Kinetic molecular theory still involved, but
intermolecular forces are strong enough to
greatly limit movement.
• Forces of attraction keep molecules closely
packed in a fixed volume, but not in a fixed
position. Molecules can flow past each other.
• Liquids are much denser than gases because
of the stronger intermolecular forces holding
the particles together.
Solids
• Solids: particles’ attractive intermolecular
forces are greater than their kinetic energy
of motion.
• Particles in a solid vibrate in a fixed position.
• Most solids are more dense than their liquid.
• Note: Ice is not more dense than liquid water.
 It FLOATS!
Solids (cont.)
• Crystalline solids are solids with atoms,
ions, or molecules arranged in an orderly,
geometric shape.
Heating Curve for Water
http://media.wwnorton.com/college/chemistry/chemtou
rs/interface.asp?chapter=chapter_05&folder=heating_c
urves
Phase Changes
• Phase changes that require energy to occur:
 Melting
 Vaporization
 Sublimation
• Phase changes that release energy:
 Freezing
 Condensation
 Deposition
Phase Changes Involve Heat Transfer
• Heat is the transfer of energy from an object
at a higher temperature to an object at a
lower temperature.
Phase Changes That Require Energy (cont.)
• When ice is heated, the ice eventually
absorbs enough energy to break the
hydrogen bonds that hold the water
molecules together.
• When the bonds break, the particles move
apart and ice melts into water.
• The melting point of a crystalline solid is the
temperature at which the forces holding the
crystalline structure together are broken and
it becomes a liquid.
Phase Changes That Require Energy (cont.)
• Particles with enough energy escape from
the liquid and enter the gas phase.
Phase Changes That Require Energy (cont.)
• Vaporization is the process by which a
liquid changes to a gas or vapor.
• Evaporation is vaporization only at the
surface of a liquid.
Phase Changes That Require Energy (cont.)
• In a closed container, the pressure exerted
by a vapor over a liquid is called vapor
pressure.
Phase Changes That Require Energy (cont.)
• The boiling point is the temperature at
which the vapor pressure of a liquid equals
the atmospheric pressure.
Phase Changes That Require Energy (cont.)
• Sublimation is the process by which a solid
changes into a gas without becoming a
liquid.
• Dry ice = solid CO2 changing directly to gas.
Phase Changes That Release Energy
• As heat flows from water to the
surroundings, the particles lose energy.
• The freezing point is the temperature at
which a liquid is converted into a crystalline
solid.
• The bonds that are formed create a more
stable situation and excess energy is
released.
Phase Changes That Release Energy (cont.)
• As energy flows away (cooling) from water
vapor, the velocity decreases.
• The process by which a gas or vapor
becomes a liquid is called condensation.
• Deposition is the process by which a gas or
vapor changes directly to a solid, and is the
reverse of sublimation.
Heat Energy of Phase Changes
• Heat of fusion (Hf) = heat absorbed when a solid melts
– Energy needed to break bonds (endothermic)
• Heat of solidification or crystallization (Hs) = heat
released when a liquid freezes (= - Hf )
– Energy released from the formation of bonds (exothermic)
• Heat of vaporization (Hv) = heat absorbed when a liquid
changes into a gas
– Energy needed to break bonds
• Heat of condensation (Hc) = heat released when a gas
condenses to become a liquid (= - Hv )
– Energy released from the formation of bonds
Heating Curves - Where is the energy going?
•
Heat of vaporization:
•
Heat of fusion
Heat Energy of Phase Changes
q = c x m x ∆T
Leads to:
q = m x Hf (or Hv, Hs, Hc)
For water (memorize):
Hf = 80 cal/g or 6.02 kJ/mol (melting/freezing)
Hv = 540 cal/g or 40.6 kJ/mol (boiling/condensing)
C liquid water = 1.00 cal/g*deg C or 4.184J/ g*deg C
C ice
= 0.50 cal/g*deg C or 2.1 J/ g*deg C
C water vapor = 0.50 cal/g*deg C or 2.1 J/ g*deg C
Phase Diagrams
• A phase diagram is a graph of pressure versus
temperature that shows in which phase a
substance will exist at various T and P.
Phase Diagrams (cont.)
• The triple point is the temperature and pressure
at which all three phases of a substance can
coexist.
• Above the
critical point
temperature the
substance can
no longer be
found in a liquid
state at any P.
Phase Diagrams (cont.)
• The phase diagram for different
substances are different from water.