Basis Molecular of Surface Tension
surface tension
(J/m2) at 20 oC
substance
formula
diethyl ether
CH3CH2OCH2CH3
1.7 x 10-2
dipole-dipole; dispersion
ethanol
CH3CH2OH
2.3 x 10-2
H-bonding
butanol
CH3CH2CH2CH2OH
2.5 x 10-2
H-bonding; dispersion
H2O
7.3 x 10-2
H-bonding
Hg
48 x 10-2
metallic bonding
water
mercury
major force(s)
TDOB: Order the following in terms of surface tension:
benzene C6H6, 2-aminoethanol (CH2NHCH2OH), CH2I2, Hg
Perfluorohexane (C6F14)
Answers 29, 49, 51, 480, 12
Serianni Notre Dame
Capillary Action A Liquid Solid Interaction
stronger
adhesive
forces
stronger
cohesive forces
Capillarity: the rising of a liquid through a narrow
space against the pull of gravity
A-left- H2O/SiO2. water-glass forces water-water forces
B-right-Hg/SiO2. Hg-Hg forces > Hg-glass forces
Serianni Notre Dame
Contact Angle A Liquid Solid Interaction
Contact angle is
a measure of
liquid surface
free energy, solid
surface free
energy, and
liquid solid
interaction
energy
TDOB On which
surfaces would
you expect H2O
vs Hg to ball up:
glass, oil, gold,
teflon
Serianni Notre Dame
Viscosity A Liquid Liquid Interaction
Viscosity a
liquids
resistance to
flow.
At temperature
increases,
viscosity
decreases
temperature
(oC)
viscosity
(N.s/m2)*
20
1.00 x 10-3
40
0.65 x 10-3
60
0.47 x 10-3
80
0.35 x 10-3
*The units of viscosity are newton-seconds per square meter.
Long molecules
have higher
viscosity
Serianni Notre Dame
Viscosity A Liquid-Liquid Interaction
Viscosity is measured in units of poise, P, defined as 1 g
cm-1 s-1 (water has a viscosity of 1 cP).
Substances composed of small, nonpolar molecules
(such as gasoline and benzene) have low viscosities.
Polar molecules (such as glycerol) and molecules
composed of long chains of atoms (such as oil and grease)
have higher viscosities.
TDOB Order the above molecules along with water for
viscosity.
Serianni Notre Dame
Water Is Structured by Hydrogen Bonds
donor
acceptor
Four Hbonds per
molecule
in the solid
state;
fewer in
the liquid
state
liquid
solid
H-bonding responsible for many H2O properties including
1.
solvent power H bonding to solute
2.
high specific heat capacity, high heat of vaporization,
high surface tension high H20-H20 IMF
3.
high capillarity strong bond to OH terminated
surfaces
4.
density of liquid water vs ice - complex
Serianni Notre Dame
Enthalpy + Entropy Drives Solution Formation
Ionic solid dissolving in water
1. Solvent molecules attracted to surface ions.
2. Each ion is surrounded by solvent molecules.
3. Enthalpy (H) changes with each interaction broken
or formed.
4. Entropy strongly drives solution formation for
solid/liquid + liquid and is usually positive, i.e.
solution is less ordered than solid crystals. Proof
solubility increases with temperature.
Michigan Tech
Enthalpy + Entropy Drives Solution Formation
1. Solvent molecules attracted to surface ions.
2. Each ion is surrounded by solvent molecules.
3. Enthalpy (H) changes with each interaction broken or
formed.
4. The intermolecular force here is ion-dipole.
Michigan Tech
Enthalpy Changes for Solubility
Three enthalpy changes
1. Separation of solute
particles.
2. Separation of solvent
particle
3. Formation of new
interactions between
solute and solvent.
Michigan Tech
Enthalpy Changes for Solubility
Hsoln = H1 + H2 + H3
The enthalpy of
solution, Hsoln,
can be either
positive or
negative.
Hsoln (MgSO4)= -91.2
kJ/mol -->
exothermic rare
Hsoln (NH4NO3)= 26.4
kJ/mol -->
endothermic common
The only common substances which have exothermic heats
of solution have huge IMF with H2O and the gasses: SO32,SO42-, SeO42-,AsO42-, PO42Michigan Tech
Entropy Drives Spontaneous Endothermic
Processes This is the tough slide
Most salts have endothermic heat of solution but still have
somewhat soluble at 300K. These solubilities are driven by
entropy
Entropy is a measure of the number of microstates
(arrangements) in the system.
Go = Ho - T So
Whenever Go < 0, reaction is spontaneous in the forward
direction
Exothermic processes are almost always spontaneous since
|Ho| >> |T So|. Note for exothermic processes, solubility
decreases with increasing temperature; for example nearly
all gasses
Endothermic processes are only spontaneous if So>>0. Note
for endothermic processes, solubility increases with
increasing temperature; for example NaCl and sucrose
Thermo of T vs Solubility tough slide
Go = Ho - T So = -RTlnK
Or
K = exp(- Ho/RT + So/R)
K = equilibrium constant = conc product/conc reactant
= [products]/[reactants] but liquids and solids are not
included. This will be explained in CH 17
K vs T: For exothermic Ho<0, as T then K so solubility
K vs T: For endothermic Ho>0, as T then K so solubility
For salt solubility: AB(s) + H2O(l) A+(aq) +B-(aq)
so K = [A+][B-]
For gas solubility: B(g) + H2O(l) B(aq)
so K = [B(aq)]/[B(g)]
Temperature vs Solubility
Generally, the solubility of solid solutes in liquid solvents
increases with increasing temperature since the
solubilities are endothermic
Generally, gasses have solubilities which decrease with
increasing temperature since they are exothermic.
Michigan Tech
TDOB: Assessing Entropy Change
Does the entropy of the system increase or decrease?
O2(g) + H2O(l) O2(aq)
Na+(aq) + Cl-(aq) NaCl(s) (precipitation)
CH3CH2OH(l) + H2O(l) CH3CH2OH/H2O mixture
water vapor reacts
with excess solid
sodium sulfate to
form the hydrated
form of the salt.
Michigan Tech
IMF and Solubility
For liquid-liquid solutions, to have the most favorable heat of
solution, solute-solvent interactions should to be at least
as strong as solute-solute an solvent-solvent interaction
thus like dissolves like
Polar molecules dissolve in polar solvent especially
hydrogen bonded molecules dissolve in solvents which
make H bonds
Non-polar molecules dissolve in non-polar solvents Michigan Tech
IMF and Solubility Example
Example: ethanol in water
The stronger the intermolecular attractions between solute
and solvent, the more likely the solute will dissolve.
Ethanol is complete soluble in H2O because Hsoln ~ 0 since
it is just exchanging CH3CHOH to CH3CHOH H-bond for
CH3CHOH to HOH H-bonds
Note solubility in H2O will decrease as the solute molecules
get larger if it only has one H-bonding group (see previous
table) since H2O will not give up its H-bonds for IMF with
the non-polar groups.
Michigan Tech
IMF Solubility and Examples
Glucose
(which has
hydrogen
bonding) is
very soluble
in water.
Cyclohexane
(which only
has
dispersion
forces) is
not watersoluble.
Michigan Tech
TDOB IMF and Solubility Examples
Which is soluble in H2O vs fat
Vitamin A is soluble in nonpolar compounds (like
fats).
Vitamin C is soluble in water.
Michigan Tech
TDOB IMF and Solubility Examples
Which is soluble in H2O vs fat
Ionic Solubility
The greater the charge on an ion and the smaller the size of
the ion, the larger the heat of hydration
For ionic substances in which both the anion and the cation
have a rare gas configuration (ex LiF, MgCl2, CsCl), the
solubility increases with decreasing lattice energy which can
be predicted from Coulombs law.
The addition of colvalent character to the ionic bonds
strengths the bonds and decreases the solubility. A cation
which does not have a rare gas configuration (ex AgCl) will
have covalent character in its bonds.
TDOB: Predict the relative solubilities of Fe2O3, NaCl, NaI,
Al2O3, MgCl2
Characteristics of Crystalline Solids
Heitz SUNY
Network Covalent Solids
Covalent Solids - contains a network of
covalent bonds that extend throughout
a crystalline solid, holding it firmly
together. Examples: Group IV and III-V
semiconductors, metal nitrides, and
many transistion metal oxides
Top - Diamond has each carbon bonded
to four other carbons in a tetrahedral
arrangement using sp3 hybridization
Bottom - Graphite has each carbon
bonded to three other carbons in the
same plane using sp2 hybridization
Heitz SUNY
Crystal Lattices
Crystals have repeating units
in multidimensional space
The repeating unit of the lattice is called the unit cell
Heitz SUNY
Unit Cells Repeat Unit
The simple cubic cell is the simplest unit cell and has
structural particles centered only at its corners
The body-centered cubic (bcc) structure
has an additional structural particle at the
center of the cube
The face-centered
cubic (fcc) structure
has an additional
structural particle at the
center of each face
TDOB: number of
atoms per unit cell
depend on crystal
structure
Heitz SUNY
Unit Cells Coordination Number
In a simple cubic structure,
the coordination number
is 6
The coordination number
and the number of atoms
per unit cell depend on
crystal structure
The coordination number is the number of nearest neighbor
particles to a single particle in the crystal
Heitz SUNY
Close-Packed Structures HCP is not cubic
Hexagonal close-packed
(hcp) arrangements occur
when the third layer covers
the tetrahedral holes. These
produce two-layer repeating
units
Cubic close-packed (ccp)
arrangements occur when
the third layer covers the
octahedral holes. These
produce three-layer
repeating units
Heitz SUNY
CH12 Part III
Cubic Unit Cells Geometry Review
1/30/2012
George Mason
Cubic Unit Cells Geometry Review
Space-filling representation of cubic unit cells
8 x 1/8 = 1
1 atom/cell
1/30/2012
8 x 1/8 + 1 = 2
2 atoms/cell
8 x 1/8 + 6 x 1/2 = 4
4 atoms/cell
George Mason
Cubic Ionic Crystals
CsCl
ZnS
CaF2
Lattice points occupied by cations and anions
Held together by electrostatic attraction
Hard, brittle, high melting point
Poor conductor of heat and electricity in solid form
Conduct electricity when melted
uwp
Ionic Radii and A+B- Crystal Type
r cation
r anion
Coord #
< .155
2
.155-.225
3
.225-.414
4
.414-.732
6
.732-1.0
8
ZnS
(zincblende)
NaCl
(sodium
chloride)
Cs Cl
(cesium
chloride)
The ratio of the ionic radii determines the coordination number
The coordination number determine the crystal type.
Note: ZnS is the diamond structure with the anions at the
uwp
internal sites
TDOB Predict the Structure of FeO
Cation
Al3+
Fe 2+
Fe 3+
Ca 2+
Anion
O 2Cl F-
Ionic radius (nm)
0.053
0.077
0.069
0.100
Answer:
r cation 0.077
0.140
r anion
0.550
based on this ratio,
--CN = 6
--structure = NaCl
0.140
0.181
0.133
uwp
Example AmXp Structure
For CaF2 :
r cation 0.100
0.8
0.133
r anion
For this ratio, coord # = 8 and structure = CsCl.
CsCl structure w/only half the cation sites occupied.
Only half the cation sites are
occupied since #Ca2+ ions =
1/2 # F- ions.
uwp
Diffraction Analysis
Formation of an x-ray diffraction pattern of the protein hemoglobin
Xray diffraction is the technique employed to determine the structure of
all crystals even proteins with 1000s of atoms
Hoeger
Diffraction Analysis
Simple analysis is intensity of a spot vs diffraction angle
The spot is bright when there is constructive interference of xray from
different layer of the crystal
Constructive interference occurs when the path length difference is an
integer number of wavelengths
Hoeger
Polymer Solids Usually Amorphous Solids
Molecular weight = 2000 to 5 million. Huge London forces
Flexible polymer tend to be high molecule weight but cross linking can
increase stiffness
Many polymers can be thermally molded or chemically synthesized in a
mold. They are usually amorphous so easy to get into any shape.
The random coil shape of a polymer chain
The chains interact via London
forces to form a coil to maximize
London forces
Small regions can have crystalline
order
In dilute solution, soluble polymers
uncoil decreasing viscosity.
Entropy Drives Spontaneous Endothermic
Processes This is the tough slide
Most salts have endothermic heat of solution but still have
somewhat soluble at 300K. These solubilities are driven by
entropy
Entropy is a measure of the number of microstates
(arrangements) in the system.
Go = Ho - T So
Whenever Go < 0, reaction is spontaneous in the forward
direction
Exothermic processes are almost always spontaneous since
|Ho| >> |T So|. Note for exothermic processes, solubility
decreases with increasing temperature; for example nearly
all gasses
Endothermic processes are only spontaneous if So>>0. Note
for endothermic processes, solubility increases with
increasing temperature; for example NaCl and sucrose
Thermo of T vs Solubility tough slide
Go = Ho - T So = -RTlnK
Or
K = exp(- Ho/RT + So/R)
K = equilbrium constant = conc product/conc reactant
= [products]/[reactants] but liquids and solids are not
included. This will be explained in CH 17
K vs T: For exothermic Ho<0, as T then K so solubility
K vs T: For endothermic Ho>0, as T then K so solubility
For salt solubility: AB(s) + H2O(l) A+(aq) +B-(aq)
so K = [A+][B-]
For gas solubility: B(g) + H2O(l) B(aq)
so K = [B(aq)]/[B(g)]