Ch. 10 – Changes of State (p. 324 – 330)

Phases  The phase of a substance is determined by three things. The temperature. The pressure. The strength of intermolecular forces.

A. Phase Changes

 Evaporation molecules at the surface gain enough energy to overcome IMF (intermolecular forces)  Volatility measure of evaporation rate depends on temp & IMF

A. Phase Changes Kinetic Energy # of Particles p. 477 Boltzmann Distribution tempvolatilityIMFvolatility

A. Phase Changes  Equilibrium trapped molecules reach a balance between evaporation & condensation

Condensation  Change from gas to liquid.  Achieves a dynamic equilibrium with vaporization in a closed system.  What is a closed system?  A closed system means matter can’t go in or out.  Put a cork in it.  What the heck is a “dynamic equilibrium?”

/ When first sealed the molecules gradually escape the surface of the liquid / As the molecules build up above the liquid some condense back to a liquid. Dynamic Equilibrium

/ As time goes by the rate of vaporization remains constant / but the rate of condensation increases because there are more molecules to condense. Dynamic Equilibrium

 Equilibrium is reached when Rate of Vaporization = Rate of Condensation  Molecules are constantly changing phase “Dynamic”  The total amount of liquid and vapor remains constant “Equilibrium ” Dynamic equilibrium

Vapor Pressure  The pressure above the liquid at equilibrium.  Liquids with high vapor pressures evaporate easily.  They are called volatile.

Vapor pressure  Decreases with increasing intermolecular forces. Bigger molecules (bigger LDF) More polar molecules (dipole-dipole)  Increases with increasing temperature.  Easily measured in a barometer.

Vapor Pressure  Vaporization - change from liquid to gas at boiling point.  Evaporation - change from liquid to gas below boiling point  Heat (or Enthalpy) of Vaporization (  H vap )- the energy required to vaporize 1 mol at 1 atm.

 Vaporization is an endothermic process - it requires heat.  Energy is required to overcome intermolecular forces.  Responsible for cool beaches.  Why we sweat.

Temperature Effect Kinetic energy # of molecules T1T1 Energy needed to overcome intermolecular forces

Kinetic energy # of molecules T1T1 Energy needed to overcome intermolecular forces T1T1 T2T2  At higher temperature more molecules have enough energy - higher vapor pressure. Energy needed to overcome intermolecular forces

A. Phase Changes  Vapor Pressure pressure of vapor above a liquid at equilibrium IMFv.p.tempv.p. depends on temp & IMF directly related to volatility p.478 temp v.p.

Boiling Point  Reached when the vapor pressure equals the external pressure.  Normal boiling point is the boiling point at 1 atm pressure.  Superheating - Heating above the boiling point.  Supercooling - Cooling below the freezing point.

A. Phase Changes  Boiling Point temp at which v.p. of liquid equals external pressure IMFb.p.P atm b.p. depends on P atm & IMF Normal B.P. - b.p. at 1 atm

 Which has a higher m.p.? polar or nonpolar? covalent or ionic? A. Phase Changes  Melting Point equal to freezing point polar ionic IMFm.p.

Melting Point  Melting point is determined by the vapor pressure of the solid and the liquid.  At the melting point the vapor pressure of the solid = vapor pressure of the liquid

Solid Water Liquid Water Water Vapor Vapor

Solid Water Liquid Water Water Vapor Vapor  If the vapor pressure of the solid is higher than that of the liquid the solid will release molecules to achieve equilibrium.

Solid Water Liquid Water Water Vapor Vapor  While the molecules of condense to a liquid.

 This can only happen if the temperature is above the freezing point since solid is turning to liquid. Solid Water Liquid Water Water Vapor Vapor

 If the vapor pressure of the liquid is higher than that of the solid, the liquid will release molecules to achieve equilibrium. Solid Water Liquid Water Water Vapor Vapor

Solid Water Liquid Water Water Vapor Vapor  While the molecules condense to a solid.

 The temperature must be below the freezing point since the liquid is turning to a solid. Solid Water Liquid Water Water Vapor Vapor

 If the vapor pressure of the solid and liquid are equal, the solid and liquid are vaporizing and condensing at the same rate. The Melting point. Solid Water Liquid Water Water Vapor Vapor

Changes of state  The graph of temperature versus heat applied is called a heating curve.  The temperature a solid turns to a liquid is the melting point.  The energy required to accomplish this change is called the Heat (or Enthalpy) of Fusion  H fus

A. Phase Changes  Sublimation solid  gas v.p. of solid equals external pressure  EX: dry ice, mothballs, solid air fresheners  Deposition gas  solid EX: frost, snow

Heating Curve for Water Ice Water and Ice Water Water and Steam Steam

B. Heating Curves Melting - PE  Solid - KE  Liquid - KE  Boiling - PE  Gas - KE 

n B. Heating Curves  Temperature Change change in KE (molecular motion)

B. Heating Curves  Phase Change change in PE (molecular arrangement) temp remains constant  Heat of Fusion (  H fus ) energy required to melt 1 gram of a substance at its m.p.

B. Heating Curves  Heat of Vaporization (  H vap ) energy required to boil 1 gram of a substance at its b.p. usually larger than  H fus …why?  EX: sweating, steam burns, the drinking bird

Phase Diagrams.  A plot of temperature versus pressure for a closed system, with lines to indicate where there is a phase change.

C. Phase Diagrams  Show the phases of a substance at different temps and pressures.

Temperature Solid Liquid Gas 1 Atm A A B B C C D D D Pressure D

Solid Liquid Gas Triple Point Critical Point Temperature Pressure

Solid Liquid Gas  This is the phase diagram for water.  The density of liquid water is higher than solid water. Temperature Pressure

Solid Liquid Gas 1 Atm  This is the phase diagram for CO 2  The solid is more dense than the liquid  The solid sublimes at 1 atm. Temperature Pressure