Changes of State (Phase Changes)

1. Changes of State (Phase Changes)

2. 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

3. Phase Changes cont. . • Endothermic phase changes (Absorb energy) • Melting (solid to liquid) • Evaporation/Vaporization (liquid to gas) • Exothermic phase changes (Release energy) • Condensation (gas to liquid) • Freezing (liquid to solid)

4. Temperature and Phase Change • During a phase change, as energy is added or removed, there is no temperature change.

5. Question Time • What is a phase change? • Which phase changes are exothermic? • Which phase changes are endothermic? • What happens to the temperature of a substance as it undergoes a phase 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 = -40.7 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 = -40.7 kJ/mol

9. Changes of State • Latent Heat of Fusionis 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 Solidificationis the heat required to solidify one mole of a liquid substance. • Going from liquid to solid • For water, Hsolid = -6.01 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 = -6.01 kJ/mol

11. Question Time • When do you use Q = m(T)Cp to calculate heat? • When do you use the latent heat to calculate energy? • What is the H called going from a liquid to a gas and gas to a liquid? • What is the H called going from a solid to a liquid and liquid to solid? • Which phase changes are have a positive enthalpy change? • Which phase changes are have a negative enthalpy change?

12. Heating Curve

13. Explanation of Heating Curve • When there is a temperature change 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.

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

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

16. Question Time How would you calculate the amount of energy used at each portion of the heating curve? Latent Heat Q=m(T)Cp Q=m(T)Cp Latent Heat Q=m(T)Cp

17. 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

18. 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

19. Example 3 What mass of ammonia (NH3) must be vaporizes that absorbs 345 kJ of heat? Latent heat of vaporization (∆Hvap) = 23.3 kJ/mol