ch13-2web

Chem 5 Chapter 13 Liquids, Solids, and Intermolecular Forces Part 2 December 4, 2002

Phase Equilibria And Phase Transitions

Key Concepts: Vapor pressure, Clausius-Clapeyron Eq. Phase diagram Boiling point, normal boiling point Melting and freezing point Sublimation Triple point Critical point

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

Initially only vaporization occurs

Condensation begins

Rate of vaporization is equal to rate of condensation

Dynamic Equilibrium

A mercury barometer

Vapor pressure Vapor pressure depresses the independent of mercury level the volume of liquid

Vapor pressure independent of the volume of vapor present

Vapor pressure increases with an increase in temperature

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

Vapor Pressure vs. Temperature

Normal Boiling Point

P is not proportional to T!

Does PV = nRT apply ? 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 Clausius-Clapeyron Eq.

( )

∆H vap 1 1 P2 ln = − P1 R T1 T2

∆Hvap, enthalpy of vaporization

Why Logarithm Dependence?

P∝ e

− E/RT

Probability of having kinetic energy higher than E A Molecular Picture

P(T) ∝ e

− ∆ H vap /RT

∆Hvap  1    + B lnP = R T

EE Distributions of kinetic energies of surface molecules of a liquid at two temperatures. Only the molecules having kinetic energy higher than E can escape 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 pressure at which solids, liquids and gases exist, either as single phases or as two phases in equilibrium with one another.

lnP = -

lnP = -

∆Hsub +C RT

∆Hvap RT

+B

Phase Diagram of H2O

Heating Curve for H2O Enthalpy of vaporization

Enthalpy of fusion

Temperature ( C )

Heat capacities from the slopes

Demo: Reaching Triple Point of H2O A closed system

Temperature ( C )

Ice Skating

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

Temperature ( C )

Sublimation

∆Hsub = ∆Hfus+∆Hvap

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 stress between 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 and creates 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

Tc,Pc

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.

Demo: Critical Point of CO2

Vapor Liquid

Vapor Liquid

Capillaries sealed with CO2

Meniscus disappears above Tc.

Chemistry Goes Green! “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.” C&E News, August 2002

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? O CH3

CH3 N

N

O

N CH3

N

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

How is instant coffee made?

High-quality instant coffee is made by freezedrying 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.