Transcript Changes of State (Phase Changes)

Changes of State (Phase
Changes)
Phase Change
• A phase change is going from one state of
matter to another (Physical change)
– Gas to liquid
– Liquid to solid
• Changes of state either absorb or release
(evolve/liberate) energy
• Endothermic phase changes (Absorb energy)
– Melting/Fusion (solid to liquid)
– Evaporation/Vaporization (liquid to gas)
– Sublimation (solid to gas)
• Exothermic phase changes (Release energy)
– Condensation (gas to liquid)
– Freezing/solidification (liquid to solid)
– Deposition (gas to solid)
Temperature and Phase Change
• During a phase change, as energy is added or
removed, there is no temperature change.
Latent Heat
• Since there is no temperature change, we cannot use
Q = mC(T) to calculate enthalpy change.
• Instead we use latent (hidden) heat, which is the
quantity of heat released or absorbed during a phase
change
• Units are usually Joule/gram or kilojoule/mole
• Q=mL
Changes of State
• Latent Heat of Vaporization is the heat required to vaporize
one mole of a liquid
– Going from liquid to gas
– For water, Hvap = 40.7 kJ/mol
• Latent Heat of Condensation is the heat required to
condense one mole of a gas
– Going from gas to liquid
– For water, Hcond = -40.7 kJ/mol
Thermochemical Equations for
Vaporization and Condensation
• The reverse process of vaporization is
condensation
– The reverse process has an opposite sign
Hvap = - Hcond
H2O(l)  H2O(g) Hvap = 40.7 kJ/mol
H2O(g)  H2O(l) Hcond = -40.7 kJ/mol
Thermochemical Equations for
Fusion and Solidification
• The reverse process of fusion is solidification.
– The reverse process has an opposite sign
Hfus = - Hsolid
– H2O(s)  H2O(l) Hfus = 6.01 kJ/mol
– H2O(l)  H2O(s) Hsolid = -6.01 kJ/mol
Changes of State
• Latent Heat of fusion is the heat required to melt one mole
of a solid
– Going from solid to liquid
– For water, Hfus = 6.1 kJ/mol
• Latent Heat of solidification is the heat required to solidify
one mole of a liquid
– Going from liquid to solid
– For water, Hsolid = -6.1 kJ/mol
Heating Curve
Explanation of Heating Curve
• When there is a temperature
change use Q=mC(T) to
calculate the amount of
energy used.
• When the heating curve is flat
(no temperature change),
there is a phase change.
Use the latent heat to
calculate the amount of
energy used. Q=mL
Hvap/ Hcond
Latent Heat
Hfus/ Hsolid
Latent Heat
Q=m(T)Cp
Q=m(T)Cp
Q=m(T)Cp
Example 1
Calculate the heat required to melt 45.6g of water at its
melting point.
Latent heat of fusion (∆Hfus)=6.01 kJ/mol
Example 2
Calculate the heat evolved to condense 75.4g of water
at its boiling point.
Latent heat of vaporization (∆Hvap)= 40.7 kJ/mol
Example 3
What mass of ammonia (NH3) will vaporize when 345
kJ of heat is absorbed?
Latent heat of vaporization (∆Hvap)=23.3 kJ/mol