chapter 5. liquids and solids 2012 general chemistry i intermolecular forces liquid structure 5.1...
TRANSCRIPT
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Chapter 5. LIQUIDS AND SOLIDS
2012 General Chemistry I
INTERMOLECULAR FORCES
LIQUID STRUCTURE
5.1 The Origin of Intermolecular Forces5.2 Ion-Dipole Forces5.3 Dipole-Dipole Forces5.4 London Forces5.5 Hydrogen Bonding5.6 Repulsions
5.7 Order in Liquids5.8 Viscosity and Surface Tension
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INTERMOLECULAR FORCES (Sections 5.1-5.6)
5.1 The Origin of Intermolecular Forces5.1 The Origin of Intermolecular Forces
Phase: uniform in both chemical composition and
physical state
- Condensed phase: simply a solid or liquid phase
- Condensed phases form when attractive intermolecular forces between molecules pull them together; repulsions dominate at even shorter separations.
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- Intermolecular forces are weak compared with bonding forces, but boiling points and sublimation points depend on their strength.
- All interionic and almost all intermolecular forces can be traced to the coulombic interaction between charges.
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1/r : between ions (ionic bonding)
- Distance dependence of potential energy of interaction
1/r2 : between ions and dipoles1/r3 : between stationary dipoles1/r6 : between rotating dipoles
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TABLE 5.1 Interionic and Intermolecular Interactions
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5.2 Ion-Dipole Forces5.2 Ion-Dipole Forces
Hydration: attachment of water molecules to ionic solute particles is an example of ion-dipole interaction.
H2O
This is the attractive force between ions and polar molecules in liquid or solid phase.
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The potential energy of ion-dipole interactions (~15 kJ mol-1)
z = the charge number of the ion = the electric dipole moment of the polar molecule
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Water of crystallization: smaller and highly charged cations strongly attract polar water
molecules in the solid phase.
- Note hydrated salts of Li and Na vs. anhydrous salts of K, Rb, Cs, and NH4
+
- Note BaCl2 · 2H2O vs. anhydrous KCl (Ba2+; 136 pm, K+; 138 pm)
E.g. Na2CO3.10H2O compared with K2CO3
(Na+; 102 pm, K+; 138 pm)
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5.3 Dipole-Dipole Forces5.3 Dipole-Dipole Forces
Between stationary polar molecules in the liquid phase (~2 kJ mol-1)
Between rotating polar molecules in the gas phase (~0.3 kJ mol-1)
This is the attractive force between polar molecules.
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Self-Test 5.1B
Which will have the higher boiling point, 1,1-dichloroetheneor trans-1,2-dichloroethene?
Solution
1,1-dichloroethene is polar, whereas trans-1,2-dichloroetheneis nonpolar:
C CCl
Cl
H
HC C
Cl
H
H
Cl
Hence, dipole-dipole (as well as London) forces existin 1,1-dichloroethene, giving it the higher boiling point.
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5.4 London Forces5.4 London Forces
London force (induce dipole-induced dipole force, dispersion force) ~2 kJmol-1: it exists between all molecules but is the only interaction between nonpolar molecules.
- Attractive interactions due to instantaneousfleeting dipole moments
- Fluctuation of the electron distribution in one molecule→ temporary dipole → secondtemporary dipole in the other molecule → ···
Time
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Polarizability is the ease with which molecular electron clouds can be distorted: increases with number of electrons.
- Potential energy (strength) of the London interaction is given by
- A large linear molecule is more likely to have stronger London interactions (and hence a higher boiling point) than a smaller or nonlinear one.
C5H12; mobile liquid C15H32; viscous liquidC18H38; waxy solid
Examples
Alkanes
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- Halogens: gases (F2, and Cl2); liquid (Br2); solid (I2)
- Rod-shaped (pentane; Tb = 36 oC) vs. spherical (2,2-dimethylpropane; Tb = 10 oC)
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TABLE 5.2 Melting and Boiling Points of Substances
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Dipole-induced dipole interaction between a polar molecule and a nonpolar molecule (~2 kJ mol-1)
– Van der Waals interactions is the collective namefor dipole-dipole forces between rotating polar molecules, London forces, anddipole-induced dipole forces
Allied intermolecular interactions
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EXAMPLE 5.2
Explain the trend in the boiling points of the hydrogen halides:HCl, -85 oC; HBr, -67 oC; HI, -35 oC.
- Electronegativity differences: HCl > HBr > HI
- Number of electrons and London forces: HCl < HBr < HI
→ not by dipole-dipole forces, but by London forces
Self-Test 5.2A
Account for the trend in boiling points of the noble gases,which increase from helium to xenon.
Solution
In the noble gases, only London forces need be considered:These increase as the number of electrons increases (sizeof the atom increases):
He (2)
B.Pt
Ne(10) Ar(18) Kr(36) Xe(54)
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5.5 Hydrogen Bonding5.5 Hydrogen Bonding
Some compounds are characterized by exceptionally high Tb due to hydrogen bonding: examples include NH3, H2O, HF – see Fig. 5.9.
London forces
Hydrogen bonding
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Hydrogen bonding: an attraction in which a hydrogen atom bonded to a small, strongly electronegative atom, specifically N, O, or F, is attracted to a lone pair of electrons on another N, O, or F atom. Intermolecular and intramolecule types exist.
- strong electrostatic interaction ~20 kJ mol-1
O…….H-O
linear but asymmetric (101 pm vs 175 pm)
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- Hydrogen fluoride, (HF)n - Acetic acid dimer (vapor)
- Protein folding
- DNA doublehelix
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Self-Test 5.3B
Which of the following molecules can take part in hydrogenbonding with other molecules of the same compound:(a) CH3OH; (b) PH3; (c) H-O-Cl?
Solution
(a) and (c), since these both have a H-O covalent bond:
O
H3C
H
H O
CH3
O
Cl
H
H O
Cl
(most likely)
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2013 General Chemistry I 21
Example 5.1
Identify the kinds of intermolecular forces that might arise between
molecules of each of the following substances:
179s
(a) NH2OH; (b) CBr4; (c) H2SeO4; (d) SO2
Solution
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2013 General Chemistry I 22
Example 5.5
Suggest, giving reasons, which substance in each of the following
pairs is likely to have the higher normal melting point (Lewis
structures may help your arguments):
179s
(a)HCl or NaCl; (b) C2H5OC2H5 (diethyl ether) or C4H9OH (butanol);
(c) CHI3 or CHF3; (d) C2H4 or CH3OH.Solution
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5.6 Repulsions5.6 Repulsions
-Intermolecular repulsions arise from the overlap of orbitals on neighboring molecules and the requirements of the Pauli exclusion principle.- They are important only at very short distances:
Ep
1
r12
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LIQUID STRUCTURE (Sections 5.7-5.8)
5.7 Order in Liquids5.7 Order in Liquids
- The liquid phase lies between the extremes of the gas and solid phases.
gas phase: moving with almost complete freedomminimal intermolecular forces
solid phase: locked in place by intermolecular forcesoscillate around an average location
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- In the liquid phase, molecules have short-range order but not long-range order.
- Water loses only 10% of hydrogen bonds uponmelting and the rest are continuously brokenand reformed.
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5.8 Viscosity and Surface Tension5.8 Viscosity and Surface Tension
Viscosity: resistance to flow, indication of the intermolecular force strength
- Water and glycerol: very viscous due to hydrogen bonding
- Hydrocarbon oils and grease: viscous due to tangling long chains
- Viscosity usually decreases with temperature due to higher energy of molecules.
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Viscosities of common liquids
Linear alkane chains inHeavy hydrocarbon oil
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Surface tension: the net inward pull, an indication of the intermolecular force strength
- Water: three times larger than many other
liquids, due to hydrogen bonds
- Mercury: more than six times that of water,
partially covalent
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Wetting: strong interactions of water with the materials’ surface. Water maximizes its contact with the materials by hydrogen bonding.
Capillary action: adhesive forces between a liquid and surface vs. cohesive forces within the liquid
– Meniscus: indication of the relative strength of adhesion and cohesion
H2O Hg
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2013 General Chemistry I 30
Example 5.21
Predict how each of the following properties of a liquid varies as
the strength of intermolecular forces increases and explain your
reasoning: (a) boiling point; (b) viscosity; (c) surface tension.
181s
Solution
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2013 General Chemistry I 31
Example 5.23
Predict which liquid in each of the following pairs has the greater
surface tension: (a) cis-dichloroethene or trans-dichloroethene;
181s
(b) benzene at 20 oC or benzene at 60 oC.
Solution
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2013 General Chemistry I 32
Example 5.25
Rank the following molecules in order of increasing
viscosity at 50 oC: C6H5SH, C6H5OH, C6H6.
181s
Solution
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Chapter 5. LIQUIDS AND SOLIDS
2012 General Chemistry I
SOLID STRUCTURES
THE IMPACT ON MATERIALS
5.9 Classification of Solids5.10 Molecular Solids5.11 Network Solids5.12 Metallic Solids5.13 Unit Cells5.14 Ionic Structures
5.15 Liquid Crystals5.16 Ionic Liquids
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SOLID STRUCTURES (Sections 5.9-5.14)
5.9 Classification of Solids5.9 Classification of Solids
Crystalline solid: a solid in which the atoms, ions, or molecules lie in an orderly array with crystal faces
Amorphous solid: one in which the atoms, ions, or molecules lie in a random jumble
quartz amorphous silica
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Classification of Crystalline Solids According to the bonds that hold their atoms, ions, or molecules in place:
Molecular: assemblies of discrete molecules held in place by intermolecular forces
Metallic: consisting of cations held together by a sea of electrons
Ionic: built from the mutual attractions of cations and anions
Network: consisting of atoms covalently bonded to their neighbors throughout the extent of the solid
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2013 General Chemistry I 37
Example 5.35
Classify each of the following solids as ionic, network, metallic, or
molecular: (a) quartz, SiO2; (b) limestone, CaCO3; (c) dry ice, CO2;
(d) sucrose, C12H22O11; (e) polyethylene, a polymer with molecules
consisting of chains of thousands of repeating –CH2CH2- units.
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Solution
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5.10 Molecular Solids5.10 Molecular Solids
Molecular solids consist of molecules held together by intermolecular forces; physical properties depend on the strengths of those forces.
Amorphous molecular solids: as soft as paraffin wax
Crystalline molecular solids:
- sucrose: numerous hydrogen bonds between OH
groups account for high melting point at 184 oC
- ultrahigh-density polyethylene: smooth yet tough
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- ice
water benzene
Molecular Solids and Liquids: Melting andFreezing
Most substances increase in density on freezing:water is an important exception. Ice at 0 oC isless dense than water at 0 oC due to a moreopen hydrogen-bonded structure.
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2013 General Chemistry I 40
Example 5.33
Glucose, benzophenone (C6H5COC6H5), and methane are examples of
compounds that form molecular solids. The structures of glucose and
benzophenone are given here. (a) What types of forces hold these
molecules in a molecular solid? (b) Place the solids in order of
increasing melting point.
184s
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2013 General Chemistry I 41
184sSolution
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5.11 Network Solids5.11 Network Solids
Network solids are characterized by a strong covalent bond network throughout the crystal: they are very hard and rigid, with high Tm and Tb
E.g. Two common allotropes (forms of an element that differ in the way in which the atoms are linked) of carbon have very different network structures.
Diamond, with an sp3 hybrid -bonding framework, is one of the hardest substances
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- Ceramic materials: noncrystalline inorganic oxides, great strength
Graphite has flat sheets of sp2 hybrid -bonds with weak bonding between sheets. It conducts electricity parallel to the sheet, and is soft and slippery
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5.12 Metallic Solids5.12 Metallic Solids
In metallic solids, the cations are bound together by their interaction with the sea of the electrons that they have lost.
Close-packed structure: the spheres stack together with the least waste of space
- Hexagonal close-packed structure (hcp): packed with the sequence of ABABAB···
Coordination number: the number of nearest neighbors of each atom
Coordination number = 12(3 plane below + 6 own plane + 3 plane above): this is the maximum.
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- Cubic close-packed structure (ccp): packed with the sequence of ABCABC··· Coordination number of ccp = 12
Occupied space in a ccp:
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Holes: the gaps (interstices) between the atoms in a crystal
- Tetrahedral hole: a dip between three atoms is directly covered by another atom
- Octahedral hole: a dip in a layer coincides with a dip in the next layer
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5.13 Unit Cells5.13 Unit Cells
Unit cell: the smallest unit that, when stacked together repeatedly without any gaps and without rotations, can reproduce the entire crystal.
- Face centered cubic (fcc, cubic F)
- Body centered cubic (bcc, cubic I)
- Primitive cubic (cubic P)
Cubic F Cubic I Cubic P
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Bravais lattices: 14 basic patterns of unit cell in 3D crystalline systems; P = primitive; I = body-centered; F = face-centered; C = with lattice point on two opposite faces; R = rhombohedral
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Unit cells are characterized bylengths a, b, c and angles , ,
- Cubic unit cells
- Face centered cubic (fcc, cubic F)
- Body centered cubic (bcc, cubic I)
- Primitive cubic (cubic P)
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Self-Test 5.4A
How many atoms are there in a primitive cubic cell?
Solution
In a cubic P cell there are only eight corner atoms,hence the total number of atoms per unit cell is:8 x 1/8 = 1.
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EXAMPLE 5.3
The density of copper is 8.93 gcm-3 and its atomic radius is 128 pm.Is the metal (a) close-packed or (b) body-centered cubic?
(a) Fcc (ccp) and hcp cannot be distinguished by density only.
For 4 atoms in a fcc cell,
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(b) For 2 atoms in a bcc cell,
close-packed cubic (fcc)
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Self-Test 5.5A
The atomic radius of silver is 144 pm and its density is10.5 g cm-3. Is the stucture face-centered cubic (fcc;close packed) or body-centered cubic (bcc)?
Solution
We begin by assuming fcc;
d =4M
83/2NAr3=
4 x (107.9 g mol-1)
83/2 x (6.022 x 1023 mol-1) x (1.44 x 10-8 cm)3
= 10.6 g cm-3. Hence silver has fcc structure.
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2013 General Chemistry I 56
190sExample 5.47
One form of silicon has density of 2.33 gcm-3 and crystallizes in a cubic
Lattice with a unit cell edge of 543 pm. (a) What is the mass of each unit
cell? (b) How many silicon atoms does one unit cell contain?
Solution
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2013 General Chemistry I 57
Example 5.49
What percentage of space is occupied by close-packed cylinders of
length l and radius r ?
190s
Solution
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5.14 Ionic Structures5.14 Ionic Structures
- Ionic structures are (like metallic structures) close packed, with anions forming a slightly expanded close-packed structure with smaller cations occupying some of the enlarged holes in the expanded lattice.
- Smaller tetrahedral hole for small cations,larger octahedral hole for somewhat bigger cations
Coordination number: the number of ions of opposite charge immediately surrounding a specific ion
Radius ratio ()
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Rock-salt structure of NaCl
Has Cl– ions forming an fcc structure (expanded ccp) and Na+ ions occupying all the octahedral holes
Found for a number of minerals having ions of the same charge number, including NaCl, KBr, RbI, MgO, CaO, AgCl
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- Rock-salt (fcc) structure: (6,6) coordination, the coordination numbers of the cations and the anions are both 6.
- Common whenever the cations and the anions have very different radii; the cations can fit into the octahedral holes in a fcc array of anions; 0.4 < < 0.7
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Cesium chloride structure
- Cl– ions form an expanded primitive cubic array and Cs+ ions occupy large cubic holes (bcc).
- The radii of the cations and anions are similar with > 0.7.
r(Cs+) = 167 pm, r(Cl-) = 181 pm; = 0.923
- It has (8,8) coordination and is less common.Examples: CsBr, CsI, TlCl, TlBr
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Zinc-blend (sphalerite) structure, < 0.4
- NiAs: strong Ni–As covalent characterhcp As with Ni in all octahedral holes(6,6)-coordination
- ZnS: expanded ccp array of S2– and small Zn2+ in half of the tetrahedral holes: (4,4)-coordination
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Self-Test 5.6A
Predict (a) the likely structure and (b) the coordinationtype of ammonium chloride. Assume that theammonium ion can be approximated as a sphere with aradius of 151 pm.
Solution
Radius ratio, =Radius of smaller ion
Radius of larger ion
=151 pm
181 pm= 0.834
This indicates (a) a cesium chloride structure with (b) (8,8)-coordination.
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Self-Test 5.7B
Estimate the density of cesium iodide from its crystalstructure.
Solution
r(Cs+) = 170 pm; r(I-) = 220 pm
Length of diagonal, b = 170 + 2(220) + 170 pm = 780 pm
Length of side a = 780/3 = 450 pm12
CsI has a cesium chloride (bcc) type structure.
Hence unit cell volume is = 9.11 x 107 pm3 or 9.11 x 10-23 cm3
(1 pm3 = 10-30 cm3)
Each bcc unit cell has one Cs+ ion and one I- ion,Density = mass/volume =
(132.91 + 126.90) g mol-1
(6.022 x 1023 mol-1)9.11 x 10-23 cm3 = 4.74 g cm-3
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2013 General Chemistry I 65
192s
Example 5.53
Calculate the number of cations, anions, and
formula units per unit cell in each of the
following solids:
(a) the cesium chloride unit cell
Solution
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2013 General Chemistry I 66
(b) The rutile (TiO2) unit cell
192s
(c) What are the coordination numbers of the ions in rutile?
Solution
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2013 General Chemistry I 67
192s
Example 5.61
Graphite forms extended two-dimensional layers.
(a)Draw the smallest possible rectangular unit
cell for a layer of graphite. (b) How many carbon
atoms are in your unit cell? (c) What is the
coordination number of carbon in a single layer
of graphite?
(a) Solution
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2013 General Chemistry I 68
190s
Example 5.63
If the edge length of a fcc unit cell of RbI is 732.6 pm, how long would
the edge of a cubic single crystal of RbI be that contains 1.00 mol RbI?
Solution
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THE IMPACT ON MATERIALS(Sections 5.15-5.16)
5.15 Liquid Crystals5.15 Liquid Crystals Liquid crystals are substances that flow like viscous liquids, but their molecules lie in a moderately orderly array.
- mesophase: an intermediate state of matter with the fluidity ofa liquid and some of the molecular order of a solid
- responsive to changes in temperature and electric fields
- isotropic vs. anisotropic (due to ordering of rodlike molecules)
- p-azoxyanisole, a long and rodlike liquid crystal molecule
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2013 General Chemistry I 70
Three classes of liquid crystals according to structure
- Nematic phase: the molecules lie together, all in the same directionbut staggered.
- Smectic phase: the molecules line up like soldiers on parade and form layers.
- Cholesteric phase: the molecules form ordered layers, but neighboringlayers have molecules at different angles and so the liquid crystal has a helical arrangement of molecules.
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- Lyotropic LC: ordering effects induced by a solvent. The molecules are
amphiphiles (surfactants), with hydrophilic and hydrophobic parts in one molecule
Uses: watches, LCD, thermometers, ···
- Thermotropic LC: made by melting solids. They have long rod shaped Molecules. Example: p-azoxyanisole
Example: sodium lauryl sulfate
Two classes of liquid crystals according to method of preparation
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- LCD television or monitor
Layers of a liquid crystal in a nematic phase lie between the surfaces of twoglass or plastic plates.
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2013 General Chemistry I 73
190s
Example 5.65
Why do long hydrocarbon molecules that do not have multiple bonds,
such as decane, CH3(CH2)8CH3, not form liquid crystals?
Solution
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2013 General Chemistry I 74
190sExample 5.67
When long surfactant molecule having a polar headgroup and a
nonpolar “tail” are placed into water, micelles are formed in which
the nonpolar tails aggregate, with the polar headgroups pointing
out toward the solvent. Inverse micelles are similar but have the
nonpolar regions pointing outward. How can inverse micelles be
placed?
Solution
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5.16 Ionic Liquids5.16 Ionic Liquids
- A liquid at room temperature is likely to be a molecular substance.There will be nonionic, weak intermolecular interactions and it will have a relatively high vapor pressure.
Ionic liquids: These are characterized by a relatively small anions (BF4
–) + large organic cation (e.g. 1-butyl-3-methylimidazolium ion), preventing crystallization.
Low vapor pressure, novel solvent properties: reducing pollution
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X-RAY DIFFRACTION
The Technique
- interference: When two or more waves pass through the same region,interference is observed as an increase (constructive) ora decrease (destructive) in the total amplitude of the wave.
- diffraction: interference between waves that arises when there is anobject in their path
- Regular layers of atoms in a crystal giving a diffraction pattern
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- Why x-rays?
The separation between layers of atoms in a crystal ~ 100 pm
corresponding to the x-ray region
Experimental Techniques
- Powder diffraction technique: a monochromatic (single-frequency)beam of x-rays is directed at a powdered sample spread on asupport.
- Bragg equation 2d sin =with the angles , to the spacing d, for x-rays of wavelength
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- Single-crystal diffraction technique
1) Growing a perfect single crystal of the sample (~ 0.1 mm)
- very challenging!
2) Placing the crystal at the center of a four-circle diffractometer
3) Fourier synthesis (conversion) into the locations of the atoms
- raw data including intensities and angles of the diffraction
- description of the atomic locations, bond lengths, and angles
big anglediffraction
Narrowspacing of10 bases
per turn ofhelix
X-Raydiff. pattern
of DNAHelix witha regularpitch and
radius