Chem 5Chapter 13

Liquids, Solids, and Intermolecular Forces

Part 2

December 4, 2002

Phase EquilibriaAnd Phase Transitions

Key Concepts: Vapor pressure, Clausius-Clapeyron Eq.Phase diagramBoiling point, normal boiling pointMelting and freezing pointSublimationTriple pointCritical point

Vapor PressureThe vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor states are in dynamic equilibrium.

Dynamic Equilibrium

Rate of vaporization is equal to rate of condensation

Initially only vaporization occurs

Condensation begins

Vapor pressureincreases with an increase in temperature

Vapor pressureindependent of the volume of vapor present

Vapor pressure Vapor pressuredepresses themercury level

A mercury barometer independent of

the volume of liquid

Pvap is only dependent on T (independent of V and n) as long as both phases exist!

Vapor Pressure vs. Temperature

Normal Boiling Point

Does PV = nRT apply ?P is not proportional to T!

Yes, PV = nRT does hold for the vapor phase!

For a fixed vapor phase volume, V, n is not const when T is increased,because the liquid phase is a reservoir.

Demo: Boiling at reduced pressure

lnP = - A (1/T) + B

( )21

vap

1

2

T1

T1

R∆H

PP

ln −=Clausius-Clapeyron Eq.

∆Hvap, enthalpy of vaporization

Why Logarithm Dependence?

E

E/RTeP−

∝Probability of having kinetic energy higher than E

BT1

R

H-lnP

vap+

∆=

/RTvapHeP(T)

∆−∝

E

A Molecular Picture

Distributions of kinetic energies of surface molecules of a liquid at twotemperatures. Only the molecules having kinetic energy higher than E canescape from the intermolecular interaction in liquid. The higher the temp., the larger the fraction of escaping molecules.

Phase Diagram

A graphic representation of the conditions of temp. and pressureat which solids, liquids and gases exist, either as single phases or as two phases in equilibrium with one another.

CRTH

-lnP sub +∆

=

BRT

H-lnP

vap+

∆=

Phase Diagram of H2O Heating Curve for H2O

Enthalpy of fusion

Enthalpy of vaporization

Heat capacities from the slopesTemperature ( C )

Demo: Reaching Triple Point of H2O

A closed system

Temperature ( C )

Ice Skating

According to one theory:the pressure of the skateblades melts the ice, and the skate skims along a thin lubricating film of water.

Temperature ( C )

∆Hsub = ∆Hfus+∆HvapSublimation

Temperature ( C )

A Story of Stanford Library

Some years ago, a Stanford University library suffered flooding that soaked thousands of books. Hoping to preserve the books, Stanford took an unusual approach: they froze them and allowed the ice to sublimate.

How did it work? When paper gets wet, it becomes less rigid. The process of evaporation, which occurs quickly, creates stressbetween the dry and wet parts of a page, causing wrinkles.

By freezing the wet books, the original structure of the paper was maintained. Sublimation is a slower process than evaporation andcreates essentially no stress on the paper. The library collection was saved!

Sublimation of Iodine

Sublimation of CO2 (s)- Dry Ice

Demo: Liquification of CO2 in a Coke Bottle

Critical temp., Tc: the highest temp. at which a distinct liquid phase can form.Critical pressure: the pressure required to bring about liquefaction at Tc.

Tc,Pc

Demo: Critical Point of CO2

Vapor

Liquid

Vapor

Liquid

Capillaries sealed with CO2 Meniscus disappears above Tc.

Chemistry Goes Green!

C&E News, August 2002

“Over recent decades, the chemical industry has been increasingly regulated and leaned that it is often best to cooperate with environmental legislators. Organic solvents are particularly problematic, because many are toxic and are not broken down easily in the environment. Green chemistry focuses on making whole chemical processes less wasteful and damaging to the environment.”

Green chemistry is defined as “The design, development and implementation of chemical processes and products to reduce or eliminate substances hazardous to human health and the environment”

Science 297, 807 (2002)

Supercritical Fluid Extraction Technology- supercritical fluids as solvents

Solubility of Naphthalene (C10H8) in Supercritical CO2

How is decaffeinated coffee made?

Decaffeinated coffee is made through caffeine extraction from coffee beans by supercritical fluid CO2. Afterwards the beans are roasted.

N

N

CH3

CH3

O

N

N

CH3

O

How is instant coffee made?

High-quality instant coffee is made by freeze-drying with dry ice. Fresh-made coffee is frozen and undergoes sublimation under vacuum. The dried coffee flakes are then packed and ready to serve by adding hot water. Compared to instant coffee made by drying at high temperature, this method preserves the flavor of coffee better and has a longer shelf life.

Super cooled liquid How is artificial rain made?

Clouds are composed of small water droplets; in order to condense them into raindrops, nucleation sites are needed. In principle, small ice crystals formed at 0°C can be used as the nucleation site. However, because of the supercooling, rain does not form even at –10°C. In order to accelerate the formation of the crystal nucleus, dry ice is dispersed in clouds. Sublimation of dry ice cools down the water droplets and creates nucleation sites that result in rain formation over a small region.