Transcript Unit 5 Phase Changes power point 2014
Phase Changes
• Matter can change from one form to another. As this occurs, energy also changes.
*As one proceeds from ice to water to water vapor, there is an
increase
in kinetic energy.
* The changes of phase are
not chemical
, they are
physical
changes.
Heating and Cooling Curves
Heating Curve
If there is a change in the temperature there is a change in
kinetic energy
because there is a change in the
average motion of the particles.
During a phase change there is no change in temperature, therefore no change in
kinetic
energy. Instead, energy goes to breaking bonds so it is a
potential
energy change.
Along Energy Changes AB BC
KE, no
PE no
KE,
PE
CD
KE, no
PE
DE
no
KE,
PE
EF
KE, no
PE
A cooling curve would be the opposite.
Gases • Gases (g): Transparent, compressible, expand without limit, have no shape/volume. **Take the shape and volume of their container.
Gases exert pressure: STP: defined as
standard temperature and pressure
*Found on Table A
101kPa or 1atm
*Pressure can also be
760
or
760
mm Hg torr
Liquids • Liquids:
no definite
shape/
but definite
volume with very
low
compressibility.
• Compressibility is the ability to occupy less space.
Boiling Point • The
temperature
at which the
vapor pressure
of a liquid reaches
atmospheric pressure
; therefore allowing particles to escape as a gas. *When vapor pressure of a liquid = atmospheric pressure
As atmospheric pressure
increases
one must
raise
the vapor pressure of the liquid by
increasing
its temperature.
Normal Boiling Point is measured at standard pressure For water it is:
100
C or 373K
• • Vapor Pressure See Table H The pressure exerted by the vapor evaporating off the surface of a liquid.
• Each liquid has its own vapor pressure.
• As the temperature of the liquid
increases
, the vapor pressure of that liquid
increases
.
Evaporation • The change of phase from
liquid
to
gas
.
Heat of Vaporization: The amount of heat energy required to
vaporize
a given mass of a liquid to
gas
at a constant
temperature
. This is an
endothermic
process, energy is being
absorbed
.
Each substance has its own Heat of Vaporization.
For water at its normal boiling temperature of 100 C and standard pressure the heat of vaporization is
2260
joules per gram. (Table B)
Condensation • The change of phase from
gas
to
liquid
. This is an
exothermic
process.
• The Heat of Condensation is the direct opposite of the heat of vaporization. The quantity of heat energy is the same as for the heat of vaporization, but instead of being
absorbed
the heat energy is being
released
.
• Solids
Definite
shape/
definite
volume.
***Regular Geometric Pattern***
Melting (fusion): An
endothermic
process in which a
solid
becomes a
liquid
. For water at Standard Pressure the temperature at which melting occurs is
0
C / 273K
.
Freezing (Solidification) • Freezing is the direct opposite of melting, but instead of being an
endothermic
process where energy is
absorbed
, it is an
exothermic
process where energy is
released
. Water freezes at
0
C / 273K
Heat of Fusion • The amount of heat energy required to
change
a given mass of
solid
to
liquid
at a
constant temperature
.
Each substance has its own Heat of Fusion.
For water at standard pressure, this quantity of heat is
334 J/gK (Table B)
Heat of Solidification (Crystallization) • The direct opposite of the Heat of Fusion. Since solidification is an
exothermic
process, the heat energy is
released absorbed
.
instead of
Sublimation • The change of phase from
solid
to
gas
, completely skipping the
liquid
phase.
• This generally occurs only in solids with
high
vapor pressures and
weak
intermolecular forces of attractions.
• • Examples:
Dry ice (CO2), paradichlorobenzene (moth balls), I 2