Changes of State

1. 10.4 Changes of State

2. Definitions • Phase – any part of a system with uniform composition and properties. • Condensation – gas changes to a liquid • Molecules of liquid can evaporate and then condense and re-enter the liquid phase

3. Equilibrium • Condensation and evaporation can happen at the same rate… • Equilibrium – a dynamic condition in which 2 opposing changes occur at equal rates in a closed system • No net change in amt. of substance in either phase.

4. Equilibrium and Vapor Pressure • P is directly proportional to [ ] • Equilibrium vapor pressure – pressure exerted by vapor in equilibrium with its corresponding liquid at a given temp • Different for each liquid • The higher the bond strength, the __________the EVP • Think about the KMT

5. Definitions • Volatile Liquids – liquids that evaporate readily • __________ forces of attraction. • Boiling – conversion of a liquid to a vapor within the liquid as well as at the surface • Boiling point – the temp at which the equilibrium vapor pressure of the liquid = atm pressure

6. Boiling • The ________ the atm pressure, the lower the b.p. • Temp is constant while boiling • Normal atmospheric pressure = 1 atm, 760 torr, 101.325kPa • E needs to be + constantly to continue boiling

7. Molar Enthalpy of Vaporization • MEV – the amt of E as heat that is needed to vaporize 1 mol of liquid at the liquids b.p. at constant pressure. • ΔHv • Measure of attraction of particles

8. Freezing and Melting • Freezing – physical change of liquid to solid • Loss of E • Freezing point – the temp at which solid and liquid are at equilibrium at 1 atm • At f. p. particles have same amt of ave K. E.

9. Melting • At m.p. the solid and liquid have the same amt of k.e. also. • For pure crystalline solids, f.p. and m.p. are at the same temp. • Temp is constant until all one phase • Example: water and ice

10. Molar Enthalpy of Fusion • MEF – the amount of energy as heat required to melt one mole of solid at the solid’s m.p. • ΔHf • Dependent on attractive forces b/t molecules

11. Sublimation and Deposition • Sublimation – the change of state from a solid directly to a gas = CO2 • Deposition – the change of state from a gas directly to a solid = Frost

12. Phase Diagrams • Phase Diagram – a graph of pressure versus temperature that shows the conditions under which the phases of a substance exist • Triple point – indicates the temp and press conditions at which the solid, liquid, and vapor coexist at equilibrium

13. Phase Diagrams • Critical point – indicates the critical t and critical p • Critical temperature tc– the t above which the substance cannot exist in the liquid state • Critical Pressure Pc – the lowest p at which the substance can exist as a liquid

14. Phase Diagrams • This is the phase diagram for water. • Notice the triple and critical points

15. Heating Curves • The Heating curve for water. • Notice the 2 flat lines

16. Heating Curves • Some constant for heating curve calcualtions • Δhvap = 40.67 kJ/mol • Δhfus = 6.01 kJ/mol • Cice = 2.09 J/g* K • Cwater = 4.18 J/g* K • Csteam = 1.84 J/g*K

17. Calculations • Calculate the enthalpy change upon converting 1.00 mol of ice at -25°C to water vapor (steam) at 125°C under a constant pressure of 1 atmosphere.

18. Calculations • What is the enthalpy change during the process in which 100 grams of H2O at 50.0°C is cooled to ice at – 30.0°C? • How much energy in kJ is needed to cool 385 grams of steam at 900°C to ice at -200°C?

19. Calculations • How much energy (in kJ) is needed to convert 50.0 grams of water at 75°C to steam at 120°C? • Calculate the enthalpy change upon converting 5.00 mol of ice at -75°C to water vapor (steam) at 225°C under a constant pressure of 1 atmosphere.