States of Matter Lesson 4.2

1. States of MatterLesson 4.2 CHEMISTRY 2 HONORS Jeff VenablesNorthwestern High School

2. Phase Changes

3. Energy Changes Accompanying Phase Changes • Sublimation: Hsub • Vaporization: Hvap • Melting or Fusion: Hfus • Deposition: Hdep • Condensation: Hcon • Freezing: Hfre

4. Energy Changes Accompanying Phase Changes • Sublimation: Hsub> 0 (endothermic). • Vaporization: Hvap> 0 (endothermic). • Melting or Fusion: Hfus> 0 (endothermic). • Deposition: Hdep< 0 (exothermic). • Condensation: Hcon< 0 (exothermic). • Freezing: Hfre< 0 (exothermic).

5. Energy Changes Accompanying Phase Changes • Generally heat of fusion (enthalpy of fusion) is less than heat of vaporization: • it takes more energy to completely separate molecules, than partially separate them.

7. Energy Changes Accompanying Phase Changes • All phase changes are possible under the right conditions. • The sequence • heat solid  melt  heat liquid  boil  heat gas • is endothermic. • The sequence • cool gas  condense  cool liquid  freeze  cool solid • is exothermic.

8. Heating and Cooling Curves • Plot of temperature change versus heat added is a heating curve. • Plot of temperature change versus heat removed is a cooling curve. • During a phase change, adding heat causes no temperature change. • These points are used to calculate Hfus and Hvap. • Supercooling: When a liquid is cooled below its melting point and it still remains a liquid. • Achieved by keeping the temperature low and increasing kinetic energy to break intermolecular forces.

9. From origin to “a” = From a to b = From b to c = From c to d = From d up =

10. From origin to a = solid From a to b = melting/freezing point From b to c = liquid From c to d = boiling/condensation point From d up = gas

12. Vapor Pressure • Explaining Vapor Pressure on the Molecular Level • Some of the molecules on the surface of a liquid have enough energy to escape the attraction of the bulk liquid. • These molecules move into the gas phase. • As the number of molecules in the gas phase increases, some of the gas phase molecules strike the surface and return to the liquid. • After some time the pressure of the gas will be constant at the vapor pressure.

14. Dynamic Equilibrium: the point when as many molecules escape the surface as strike the surface. • Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium. • Volatility, Vapor Pressure, and Temperature • If equilibrium is never established then the liquid evaporates. • Volatile substances evaporate rapidly. • The higher the temperature, the higher the average kinetic energy, the faster the liquid evaporates.

17. Vapor Pressure and Boiling Point • Liquids boil when the external pressure equals the vapor pressure. • Temperature of boiling point increases as pressure increases. • Two ways to get a liquid to boil: increase temperature or decrease pressure. • Pressure cookers operate at high pressure. At high pressure the boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked, reducing the cooking time required. • Normal boiling point is the boiling point at 760 mmHg (1 atm).

20. Phase Diagrams • Phase diagram: plot of pressure vs. Temperature summarizing all equilibria between phases. • Given a temperature and pressure, phase diagrams tell us which phase will exist. • Any temperature and pressure combination not on a curve represents a single phase.

21. Features of a phase diagram: • Triple point: temperature and pressure at which all three phases are in equilibrium. • Vapor-pressure curve: generally as pressure increases, temperature increases. • Critical point: critical temperature and pressure for the gas. • Melting point curve: as pressure increases, the solid phase is favored if the solid is more dense than the liquid. • Normal melting point: melting point at 1 atm.

22. Critical Temperature and Pressure • Gases liquefied by increasing pressure at some temperature. • Critical temperature: the maximum temperature for liquefaction of a gas using pressure. • Critical pressure: pressure required for liquefaction.

24. The Phase Diagrams of H2O and CO2

25. Water: • Why does the melting point curve slope to the left? • What are the temperature and pressure at the triple point? • What are the normal freezing and boiling points? • What are the critical temperature and pressure? • What change occurs at 50C as the pressure is decreased from 1.0 atm to 0.0010 atm?

26. Water: • Why does the melting point curve slope to the left? • ice is less dense than water • What are the temperature and pressure at the triple point? • 0.0098C and 4.58 mmHg • What are the normal freezing and boiling points? • Freezing = 0 C and Boiling = 100 C • What are the critical temperature and pressure? • 374C and 218 atm • What change occurs at 50C as the pressure is decreased from 1.0 atm to 0.0010 atm? • vaporization

27. Carbon Dioxide: • At what temperature and pressure does the triple point occur? • What is the normal sublimation point? • What is the critical point? • What change occurs at 30. atm as you move from -60˚C to 0˚C?

28. Carbon Dioxide: • At what temperature and pressure does the triple point occur? • -56.4C and 5.11 atm • What is the normal sublimation point? • -78.5C • What is the critical point? • 31.1C and 73 atm • What change occurs at 30. atm as you move from -60˚C to 0˚C? • melting