Vapor Pressure and Changes of State

Heat of vaporization

• Enthalpy of vaporization• energy required to vaporize 1 mole of a liquid at a pressure of 1 atmHvap

Vapor pressure-in closed container

• Vapor molecules reform to a liquid

• condensation

• eventually rate of condensation = rate of evaporation

equilibrium

• When no further net change occurs in the amt of liquid or vapor b/c the two opposite processes exactly balance each other

No net change?

• System is highly dynamic on the molecular level!

• Means molecules are constantly escaping and entering the liquid

Vapor pressure

• Determined by intermolecular forces

• large IM forces = low vp

• the molecules need a lot of energy to escape

High vapor pressure

• Evaporate readily from an open dish

• volitile

Temperature?

• Vapor pressure for a given liquid increases significantly with temperature

• why?

graphs

• vp verses temperature

• nonlinear increase

• straight line by plotting ln(Pvap) versus 1/T (in K)

Straight line

• ln(Pvap) = -Hvap/R (1/T) + C

Hvap = enthalpy of vap

• R = universal gas const

• C = const for each liquid

Impt relationship

• Can find Hvap by measuring Pvap at several temps and evaluating slope

Two temps

• Can combine the eqn b/c C does not dept on temp in order to solve for Pvap at another temp

Equation• Ln(Pvap

T1) - ln(PvapT2) = Hvap/R (1/T2 - 1/T1)

• OR

• Ln(PT1/PT2) = Hvap/R (1/T2 - 1/T1)

Solve

• The vp of water at 25oC is 23.8 torr and the Hvap at 25oC is 43.9 kJ/mol. What is the vp at 50. oC?

Changes of state

• What happens when a solid is heated?

• Heat solid --> melt to liquid --> liquid will boil to gas state

Heating curve

• Plot of time vs temp for a process where energy is added at a constant rate

Energy into ice

• Random vibrations of water molecules increase

• break from lattice and change to liquid

Enthalpy of fusion

• energy added to break (or disrupt) the ice structure by breaking H-bonds

• enthalpy change that occurs to melt a solid at the melting point (kJ/mol)

0oC

• Temp is constant until all solid changes to liquid

• then temp will increase again

100oC

• Temp is constant until all the liquid changes to a gas

• physical changes

Melting point

• As the temp of the solid is increased, a point is eventually reached where the liquid and solid have identical vapor pressures

Normal melting pt

• The temp at which the solid and liquid states have the same vp under the conditions where the total pressure is 1 atm

Normal boiling pt

• Temperature at which the vp of the liquid is exactly 1 atm

• boiling occurs when the vp of the liq is equal to the pressure of its environment

Phase Diagrams

• Represent the phases of a substance as a function of temperature and pressure

Phase Diagrams

• Shows which state can exist as given temp and pressure

• conditions of CLOSED system

Experiment 1

• Pressure is 1 atm

• initial- temp -20oC

• no vapor in cylinder (b/c at 0oC the vp is less than 1atm)

• Ice melts to liquid (still no vapor)

• at 100oC, vp is 1 atm and water boils

• changes until all steam

Experiment 2• Pressure is 2.0 torr

• ice is only component (-20oC, 2 torr)

• at -10oC, ice --> vapor

• sublimation (vp of ice = external pressure)

Experiment 3

• Pressure is 4.588 torr

• -20oC (ice only component)

• cylinder heated- no new phase until .0098oC

• TRIPLE point- solid and liquid have identical vp at 4.588 torr

• only at these conditions (.01oC) can all three states of water coexist

Experiment 4

• Pressure is 225 atm

• start with liquid water (300oC, 225 atm) b/c of high pressure

• Liquid changes to vapor as temp increases, but goes through intermediate “fluid” region which is neither true vapor or liquid

Critical temp

• temp above which vapor cannot be liquified no matter what pressure is applied

Critical Pressure

• Pressure required to produce liquification at the critical temp

Critical point

• Defined by critical pressure and temp (374oC, 218 atm)

• beyond this point is intermediate “fluid” region

Phase diagram for water

• Solid/liquid line has negative slope

• mp of water decreases as external pressure increases

Phase diagram for water

• Opposite of most substance b/c density of ice less than water at mp

Phase diagram for CO2

• Solid/liquid line has positive slope

• solid CO2 is more dense than liquid CO2

Phase diagram for CO2

• Triple point at 5.1 atm and -56.6oC

• Critical point at 72.8 atm and 31oC

• at 1 atm CO2 sublimes