1. Changes of State (Phase Changes)

2. Cyclohexane vaporizing and solidifying at the same time
YOUTUBE

3. 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

4. Phase Changes cont. . 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)

5. Temperature and Phase Change
During a phase change, as energy is added or removed, there is NO temperature change.

6. Latent Heat
Since there is no temperature change, we cannot use Q = m(T)Cp 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

7. 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 = kJ/mol

8. 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 = kJ/mol

9. Changes of State
Latent Heat of Fusion is the heat required to melt one mole of a solid substance.
Going from solid to liquid
For water, Hfus = 6.01 kJ/mol (Endothermic)
Latent Heat of Solidification is the heat required to solidify one mole of a liquid substance.
Going from liquid to solid
For water, Hsolid = kJ/mol (Exothermic)

10. 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 = kJ/mol

11. Explanation of Heating Curve
When there is a temperature change (slopes) use Q=m(T)Cp 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.

12. Heating Curve q=cmT Hvap/ Hcond Q=m(T)Cp Latent Heat Q=m(T)Cp
Hfus/ Hsolid
Latent Heat
q=cmT
Q=m(T)Cp

13. Question Time
What state of matter is at each portion of the heating curve?
Liquid/Gas
Gas
Liquid
Solid/Liquid
Solid

14. Example 1
Calculate the heat required to melt 45.6 g of water at its melting point .
Latent heat of fusion (∆Hfus) = 6.01 kJ/mol
∆H = 15.2 kJ

15. Example 2
Calculate the heat evolved to condense 75.4 g of water at its boiling point
Latent heat of vaporization (∆Hvap) = 40.7 kJ/mol
∆Hcond = kJ/mol
∆H = kJ or
170. kJ evolved

16. Example 3
What mass of ammonia (NH3) must be vaporized to absorb 345 kJ of heat?
Latent heat of vaporization (∆Hvap) = 23.3 kJ/mol
m = 252 g NH3