States of Matter Lesson 4.2 CHEMISTRY 2 HONORS Jeff Venables Northwestern High School
Phase Changes
Energy Changes Accompanying Phase Changes Sublimation: Hsub Vaporization: Hvap Melting or Fusion: Hfus Deposition: Hdep Condensation: Hcon Freezing: Hfre
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).
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.
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. cool gas condense cool liquid freeze cool solid is exothermic.
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.
From origin to “a” = From a to b = From b to c = From c to d = From d up =
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
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.
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.
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).
Property Stronger forces mean… Viscosity Surface tension Melting point (freezing) Boiling point (condensation) ΔHfus ΔHvap Vapor Pressure
Property Stronger forces mean… Viscosity higher Surface tension Melting point (freezing) Boiling point (condensation) ΔHfus ΔHvap Vapor Pressure lower
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.
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.
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.
The Phase Diagrams of H2O and CO2
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 50C as the pressure is decreased from 1.0 atm to 0.0010 atm?
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? 0.0098C 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? 374C and 218 atm What change occurs at 50C as the pressure is decreased from 1.0 atm to 0.0010 atm? vaporization
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?
Carbon Dioxide: At what temperature and pressure does the triple point occur? -56.4C and 5.11 atm What is the normal sublimation point? -78.5C What is the critical point? 31.1C and 73 atm What change occurs at 30. atm as you move from -60˚C to 0˚C? melting