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 atm  H vap

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(P vap ) versus 1/T (in K)

Straight line ln(P vap ) = -  H vap /R (1/T) + C  H vap = enthalpy of vap R = universal gas const C = const for each liquid

Impt relationship Can find  H vap by measuring P vap 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 P vap at another temp

Equation Ln(P vap T1 ) - ln(P vap T2 ) =  H vap /R (1/T 2 - 1/T 1 ) OR Ln(P T1 /P T2 ) =  H vap /R (1/T 2 - 1/T 1 )

Solve The vp of water at 25 o C is 23.8 torr and the  H vap at 25 o C is 43.9 kJ/mol. What is the vp at 50. o C?

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)

0oC0oC Temp is constant until all solid changes to liquid then temp will increase again

100 o C 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 -20 o C no vapor in cylinder (b/c at 0 o C the vp is less than 1atm)

Ice melts to liquid (still no vapor) at 100 o C, vp is 1 atm and water boils changes until all steam

Experiment 2 Pressure is 2.0 torr ice is only component (-20 o C, 2 torr) at -10 o C, ice --> vapor sublimation (vp of ice = external pressure)

Experiment 3 Pressure is torr -20 o C (ice only component) cylinder heated- no new phase until.0098 o C

TRIPLE point- solid and liquid have identical vp at torr only at these conditions (.01 o C) can all three states of water coexist

Experiment 4 Pressure is 225 atm start with liquid water (300 o C, 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 (374 o C, 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 CO 2 Solid/liquid line has positive slope solid CO 2 is more dense than liquid CO 2

Phase diagram for CO 2 Triple point at 5.1 atm and o C Critical point at 72.8 atm and 31 o C at 1 atm CO 2 sublimes