Phase Changes
Chapter 11
Vaporization
• An endothermic
process in which the
intermolecular
attractions of a liquid
are broken releasing
molecules as a gas.
• ΔH vap, The heat of
vaporization is the
heat required to
vaporize one mole
of a liquid at 1 atm.
Vapor Pressure
• In a closed container,
some liquid is initially
changed to a gas.
• Eventually the rate of
evaporation equals
the rate of
condensation.
• Vapor Pressure is at
equilibrium.
Vapor Pressure
• Vapor pressure varies depending on the
volatility of a liquid.
• Volatility is the ease of a liquid to form a
gas.
• Liquids with low IMF’s have high volatility.
Large molecules tend not to be volatile.
• Water is a small molecule, but is not highly
volatile. Why?
Vapor Pressure increases with
temperature
• Increasing the
average kinetic
energy of the
molecules, means
they require less
additional energy to
overcome their
intermolecular forces
to become a gas.
Calculating Enthalpy of
vaporization
• ln(Pvap) = - [ΔHvap/R] x (1/T) +C
• R is universal gas constant
• C is the constant characteristic of a given
liquid.
• ln is the natural log.
• y=mx+b
• Plotting the natural log versus 1/T in kelvin
gives a straight line.
Calculating the ΔH vap
Changes of State
•
•
•
•
•
•
Vaporization
Condensation
Fusion
Solidification
Sublimation
Deposition
Heating curve
• Energy can be converted into a heat
change of a substance.
• Energy is also required to change the
state of a substance.
• This change in energy is plotted in a
Heating Curve.
Phase Diagrams
• These are a
convenient way to
represent the phases
of any substance as a
function of
temperature and
pressure.
Phase Diagrams
• The Triple Point is the temperature and pressure that a
substance can exist as all three states of matter (solid,
liquid and gas) as all three have the same vapor
pressure.
• The Critical Temperature is defined as the temperature
above which the vapor cannot exist as a liquid no matter
what pressure is applied.
• The Critical Pressure is the pressure required to
condense a vapor at the critical temperature.
• Together the critical temperature and critical pressure
make the Critical Point.
• The melting point at 1 atm is the Normal Melting Point.
Phase Diagram for Carbon
Application for Phase Diagrams
• Note: on the phase
diagram for water that
the melting point of
ice decreases as the
external pressure
increases. This is
opposite to
observations for most
other liquids.
• Why is this possible?
Phase Diagrams
• For ice skating, the
narrow blade exerts a
large external energy
on the ice melting it
beneath the blade,
and refreezing as the
blade leaves the
surface. This allows
the smooth gliding
action.
Phase diagrams
• As pressure
decreases so does
the boiling point of
water. This is why
Doug in Caldwell, NJ
requires more heat to
boil water than
Kristen in Boulder,
CO.