Chapter 11 Lecture 3: Phase ChangesA. Molecular Solids

1. Molecules are at the lattice points of the crystalline solid

2. Ice = H2O at each lattice point

a. It takes 6 kJ/mol of energy to melt ice (470 kJ/mol to break O—H bond)

b. Weak, intermolecular H-bonds hold ice together

3. CO2 = no dipole moment; London forces hold solid together (gas at 25 oC)

4. P4, S8 = strong covalent bonds within the unit, but weak intermolecular forces between molecules = larger molecules, more London force, solids at higher T

B. Ionic Solids1. Ions at lattice points of the crystalline solid

2. Strong (Coulombic) inter-ionic forces: stable, high melting point solids

3. Packing

a. Larger anions are usually in one of the closest packing forms

b. Smaller cations fit into the holes among the anions to maximize the attraction and minimized repulsions

c. Types of holes for the smaller ion

i. Trigonal holes = between 3 spheres in the same layer

ii. Tetrahedral holes = between 3 spheres in one layer and 1 in another

iii. Octahedral holes = between 3 spheres in one layer and 3 in another

d. Hole size: trigonal < tetrahedral < octahedral

Trigonal too small to be occupied in most ionic solids

e. There are more holes than cations needed, usually

i. ZnS Example: twice the number of tetrahedral holes as cations needed

ii. Ccp structure: 4 S2- in the unit cell, 8 tetrahedral holes, 4 Zn2+ needed

iii. NaCl Example: ccp Cl-: each octahedral hole surrounded by 6 Cl-

iv. One hole per each Cl-, one Na+ needed for each Cl-

C. Vapor Pressure1. Vapor Pressure = pressure of the vapor over a liquid at equilibrium

2. Equilibrium = molecules evaporating at the same rate as condensing

a. Dynamic = millions of individual molecules are changing state

b. Net change in gaseous molecules is Zero

3. Vapor Pressure depends on

Kinetic Energy = Temperature

4. Vapor pressure depends on Intermolecular Forces

a. Volatile Liquid = high vapor pressure

i. Small molecular weight

ii. Small intermolecular forces: no dipole or H-bonding

b. Non-volatile Liquid = low vapor pressure

i. Large molecular weight (high London Forces): oils

ii. Large intermolecular forces (H-bonding): water

butane

5. Quantitative description of Vapor Pressure

a. Heat of Vaporization = Hvap = heat added to free molecules to gas

b. Relationship of vap, Pvap, and T (in Kelvin)

c. Plots as a straight line: y = mx + b

CTR

HP vapvap

1

)ln( R = gas constantC = constant of the compound = “ln”

d. The Clausius-Clapeyron Equation: We can calculate Pvap at a new T

e. Example: Pvap(H2O) = 23.8 torr and Hvap = 43.9 kJ/mol at T = 25 oC. What is Pvap at T = 50 oC?

12,

,

12,,

22,

1,

11ln

11)ln()ln(

)ln()ln(

1

2

21

1

TTR

H

P

P

TTR

HPP

RT

HPC

RT

HP

vap

Tvap

Tvap

vapTvapTvap

TvapTvap

torrPtorr

P

torr

P

KKmolKJ

molJ

torr

P

TTR

H

P

P

TvapTvapTvap

Tvap

vap

Tvap

Tvap

7.9394.38.23

37.18.23

ln

298

1

323

1

/3145.8

/900,43

8.23ln

11ln

2,,,

,

12,

,

22

2

1

2

D. Changes of State1. States of matter can be reversibly changed without changing the substance

2. Changing states

a. Melting (Fusion) = Solid to Liquid (opposite is Freezing) [Hfus]

b. Vaporization = Liquid to Gas (opposite is Condensation) [Hvap]

c. Sublimation = Solid to Gas (opposite is Vapor Deposition)

q = mCiceT q = mCliqT q = mCgasT

2.09 J/goC 4.18 J/goC 2.01 J/goC

q = nHfus q = nHvap

Boiling Sublimation

The Heating Curve for Water

q = mCiceT q = mCliqT q = mCgasT

2.09 J/goC 4.18 J/goC 2.01 J/goC

q = nHfus q = nHvap

How much heat is required to heat 500g H2O at -20oC to steam at 250oC?

1. (500g)(2.09J/goC)(20oC) = 20,900 J

2. (500g)(1mol/18g)(6,020J/mol) = 16,700 J

3. (500g)(4.18J/goC)(100oC) = 209,000 J

4. (500g)(1mol/18g)(40,700J/mol) = 1,130,000 J

5. (500g)(2.01J/goC)(150oC) = 151,000 J

______________________________________________________________

6. Total = 20,900 + 16,700 + 209,000 + 1,130,000 + 151,000 = 1,527,600 J

E. Phase Diagrams1. Phase diagrams show us the temperatures and pressures of state changes

a. Critical Temperature = temp. above which substance is always a gas

b. Critical Pressure = pressure needed to make liquid at Critical Temp

c. Critical Point = intersection of critical temp. and critical pressure

d. Triple Point = single point where all three phases can coexist

2. Navigating in Phase Diagrams

3. Phase diagrams for other compounds: IodineWater is unusual

2. Applications of the Water Phase Diagram

a. Solid/Liquid Boundary has a negative slope: density of ice is less than liquid water (this is why ice floats on water)

b. Ice Skating: narrow skate blade applies large pressure + friction = ice melts to liquid. This lubricates the blade and allows smooth skating.

c. Elevation Effects: Lower pressure = lower boiling point

3. The CO2 Phase Diagram

a. Solid/Liquid boundary has positive slope (normal)

b. Solid is more dense than liquid

c. Triple point = 5.1 atm, -56.6 oC

d. Critical point = 72.8 atm, 31 oC

e. Solid sublimes directly to gas at 1 atm and 25 oC = Dry Ice

f. Fire Extinguisher = liquid CO2; high pressure makes it liquid

As it is opened to atmosphere, gaseous CO2 puts out fire (fog = water)