Unit 9 Condensed Phases

1
Unit 9Condensed Phases

• Interparticle Forces
• Ionic Compounds
• regular array of cations and anions, crystal
  lattice
• forces are chemical bonds, ionic bonds
• no molecule
• high melting point and boiling point must
  break bonds
• insulator as solid, molten salts conduct
  electricity

2
Network Covalent Solids

• CO2 (s)?(g) -78oC SiO2 bp 2230oC
• No molecule, covalent bonds connect all atoms in
  structure
• 3-D network diamond
• 2-D network graphite
• 1-D network PTFE

3
Metallic Bonding

• Electron sea model - core cations occupy
  lattice sites, valence electrons float about
  them
• Band Theory delocalized molecular orbitals
  connect all metal atoms in lattice.
• Almost chemical bonding strength
• Valence electrons dont belong to any particular
  atom
• Electrical conductors
• alloys

4
Polar Molecules? Protocol

• Draw Lewis diagram
• Identify geometry using VSEPR
• Identify non-zero formal charges
• Find partial formal charges
• Bonding electrons attracted to more
  electronegative
• Does charge build up on one side of molecule -
  polar
• positive end and negative end
• Do bond polarities cancel out --- non-polar

5
Which molecules are polar?

• HCl
• O3
• PF3
• PF5
• CO2
• H2O

6
Forces Between Neutral Covalent
Molecules1. Polar Forces

• Solid O3
• Electrostatic forces between dipoles
• Much weaker than chemical bonds (smaller charges)
• Melt or boil, these forces break, not chemical
  bonds

7
Forces Between Neutral Covalent
Molecules2. Hydrogen Bonds

• Localized polar forces between
• 1. H covalently bonded to N, O, F
• 2. Lone pair on N, O, F
• Stronger than polar forces, weaker than chemical
  bonds
• H2O
• DNA bases

8
Forces Between Neutral Covalent
Molecules3. London Dispersion Forces (LDF)

• Polar forces between non-polar molecules due to
  temporary polarity during induced vibrations
• CO2
• Weaker than polar forces
• Everything has LDF, only matters if nothing else.

9
Interparticle forces in condensed phases

• Chemical Bonds
• 1. Network Covalent
• 2. () Ionic Bonding
• 3. (gt) metallic bonding
• gtgt
• Forces between neutral covalent molecules
• 4. Hydrogen Bonding
• 5. (gt) Polar Forces
• 6. (gt) London Dispersion Forces

10
Kinetic Molecular Theory

• Average kinetic energy directly proportional to
  absolute T
• 1/2mv2 3/2RT
• Kinds of motion
• Vibration
• Rotation
• Tranlsation

11
Kinetic theory explanation of mp and bp

• T

bp
mp
E
12
Effect on Physical Properties

• How does stronger intermolecular forces affect
• mp
• bp
• Heat of fusion
• Heat of vaporization
• How does larger molecular weight affect
• mp
• bp
• Heat of fusion
• Heat of vaporization

13
Vapor Pressure

• Pressure exerted by gas phase molecules that have
  escaped from the surface of a (s) or (l)
• Why does vaporization of water lower the
  temperature of the water?

14
Equilibrium Vapor Pressure

• Rate of vaporization rate of condensation
• All observables constant
• Equilibrium vapor pressure of one substance
  depends only on TEMPERATURE
• How does Peq of different substances depend on
• Strength of forces
• Molecular weight
• Temperature
• Relative humidity

15
Boiling Point

• How do you tell a liquid is boiling?
• Boiling point temperature at which equlibrium
  vapor pressure equals applied pressure
• Normal bp, applied pressure 1 atm.

16
Le Chatliers Principle

• If a system in dynamic equilibrium is affected by
  a stress which removes the system from an
  equilibrium state, the system will adjust in such
  a way as to relieve the stress and to establish a
  new equilibrium state.
• A liquid is in equilibrium with its vapor, what
  is the affect of
• An increase in V
• An increase in T

17
Phase Diagram of water

• phases
• Eq. v p of liquid
• Eq v p of solid
• (s) (l) equilibrium
• Normal boiling point
• Normal melting point
• Triple point
• Affect of P on melting point
• Boiling water at reduced P
• Freeze drying

P
T
18
Solutions and Solubility

• Solubility of A in B, maximum amount A that will
  dissolve in B
• Most likely when solute A and solvent B are
  similar in IMF, size
• Break A- - -A Break B- - -B Make A- -
  -B
• Exothermic if A- - -B stronger than A- - -A and
  B- - -B
• Almost impossible
• Best chance A- - -A, B- - -B, A- - -B same
  strength

19
Solubility of Gases

• Henrys Law, solubility of a gas in a liquid
  solvent directly proportional to partial pressure
  of gas above solution.
• Cmax K P
• Freeze Degassing
• Freeze liquid solution pure solvent freezes out
  first.

20
Raoults Law

• PA ?APAo
• PA equil. vapor pressure of A above a solution
• PAo equil. vapor pressure of pure A
• XA mole fraction of A in the solution
• Must be COLLIGATIVE concentration

21
Colligative Concentration

• Concentration of particles actually in solution
• What is mole-fraction of water in 1.0m CaCl2
• Colligative properties of acetic acid in benzene

22
Phase Diagram of water explainsFreezing point
depression, boiling point elevation

• Pure water
• (Volatile has vapor pressure)
• Non-volatile solute in water
• Freezing pt lower
• Boiling pt higher
• Volatile solute in water
• Freezing pt lower
• Boiling pt ????

P
T
23
More Colligative Properties

• Freezing Point Depression
• ?TF (TFo TF) kFm
• TFo freezing point of pure solvent
• TF freezing point of solution
• m molality of solute particles in solution
• kF freezing point depression constant of
  solvent
• Value does not depend on identity of solute
• Boiling Point Elevation (non-volatile solutes
  only)
• ?TB (TB TBo) kBm

24
Osmosis and Osmotic Pressure

• Semi-permeable membrane, allows solvent to pass,
  but not solute.
• Osmotic pressure, natural pressure pushing
  solvent through membrane (?)
• ? PX - PY
• ? MxRT

Semi-permeable membrane
Px
PY
Pure solvent
solution
25
Osmotic pressure causes ground water to rise in
tree trunk. What is the colligative
concentration required to drive water to top of a
100 foot tree?

• Answer

26
1.00 g each of NaCl, CaCl2, C6H12O6 dissolved in
100.0 g water. What is the vapor pressure of the
solution at 100oC? What is the freezing point
and the boiling point?

• ans

27
Pepsin is the principal digestive enzyme of
gastric juice. A 3.00 g sample of pepsin is
dissolved in enough water to make 10.0 mL of
solution. The osmotic pressure of the solution
is found to be 0.213 atm. What is the molecular
weight of pepsin?

• answer

28
A solution is prepared by mixing 100.0 g methanol
with 100.0 g propanol at 40oC. Find the mole
fraction of each component in the liquid phase
and the gas phase at 40oC. Equilibrium vapor
pressures at this T 303 torr for CH3OH
(methanol) and 44.6 torr for CH3(CH2)2OH
(propanol)

• Answer