States of MatterStates of MatterLesson 4.2Lesson 4.2

CHEMISTRY 2 HONORS

Jeff VenablesNorthwestern High School

Phase ChangesPhase 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.• The sequence

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

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.

Vapor PressureVapor 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 higher

Melting point (freezing) higher

Boiling point (condensation) higher

ΔHfus higher

ΔHvap higher

Vapor Pressure lower

• 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.

Phase DiagramsPhase Diagrams

• 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