Phase Changes
Phase Changes
exothermic
sublimination
vaporizing
melting
solid
liquid
freezing
endothermic
gas
condensing
Phase changes and their enthalpy changes.
Heating Curve
Heating Curve of Water
Quantitative Aspects of Phase Changes
Within a phase, a change in heat is accompanied by a
change in temperature which is associated with a change
in average Ek as the most probable speed of the
molecules changes.
q = (specific heat )(amount)(ΔT)
During a phase change, a change in heat occurs at a
constant temperature, which is associated with a change
in Ep, as the average distance between molecules
changes.
q = (amount)(enthalpy of phase change or ΔH)
Heats of Fusion and
Vaporization
Problem
• How much energy is transferred when
125 g of water are converted from 25oC
to 70oC? to 120oC?
ΔHvap = 2.26 kJ/g
ΔHfus = 0.33 kJ/g
sp.htice = 2.09 J/g oC
sp.htwater = 4.18 J/g oC
sp.htsteam = 2.10 J/g oC
Phase Diagrams
Fusion Curve
critical
point
Pressure
melting
1 atm
freezing
Liquid
Solid
normal
boiling pt.
normal
melting pt.
Sublimation
Curve
sublimation
deposition
triple
point
vaporization
condensation
Gas
Temperature
Vapor Pressure
Curve
Phase diagrams for CO2 and H2O.
CO2
H 2O