Transcript 幻灯片 1 - Sun Yat-sen University

Chapter 12
Liquids, Solids and
Materials
Gases, Liquids and Solids
high temperatures
low temperatures
Kinetic Energy
Attractive Intermolecular Forces
Ionic Forces
Ion-Ion
- +
- + + - +
+
e.g. NaCl(s)
Ion-Dipole
Ion-ion forces are very
strong and produce
high boiling points
and melting points.
e.g. NaCl dissolved in water
Ions form strong intermolecular
forces with polar molecule water.
Ionic Liquids
Intermolecular Interactions
• Three important intermolecular interactions: Dispersion Forces (non-polar
molecules), Dipole-Dipole Forces (polar molecules) and Hydrogen Bonding
Forces (molecules with F-H, O-H, N-H bonds).
Dispersion Forces (van der Waals Forces)
• The electrons on one atom are attracted
to the nucleus on a neighbouring atom.
• This creates an “instantaneous” (i.e.
temporary) dipole on the first atom.
• The instantaneous dipole on the first
atom then induces an instantaneous
dipole on the second atom.
• The two induced dipoles attract each
other.
Dispersion Forces (van der Waals Forces)
• Dispersion Forces are proportional to a molecule’s polarizability.
• The polarizability, the ease with which the e- cloud can be deformed, is
approximately proportional to the number of electrons and molecular weight.
• For example, the noble gases are spherical, non-polar atoms……
Element
Helium
Neon
Argon
Krypton
Xeon
Molecular weight
4.0
20.2
39.9
83.9
131.3
Number of e2
10
18
36
54
Boiling point/K
4
27
87
121
166
• As the molecule (atom) increases in size, the boiling point increases.
• Tbp is a good measure of the strength of intermolecular forces. A higher Tbp
indicates stronger intermolecular forces.
• Dispersion Forces also depend upon the molecular shape, e.g., Tbp(npentane) = 36oC and Tbp(neopentane) = 9oC
Dipole-Dipole Forces
In liquids of polar molecules, oppositely
charged ends of the molecules tend to
attract each other, causing partial alignment.
For molecules of roughly equal MW’s
(i.e. with similar Dispersion Forces),
the molecule with the higher dipole moment
will have a higher boiling point due to greater
Dipole-Dipole forces.
Propane, MW = 44
H
H
H
H
C
H
Acetonitrile, MW = 41
H
C
C
H
H H
0
No Dipole Moment, Tbp = -42 oC
H
C
C
N
H
 = 3.9 Debye (D)
Large Dipole Moment, Tbp = +82 oC
Hydrogen Bonding
• Hydrogen bond is a special intermolecular interaction between the H atom
in a polar N-H, O-H or F-H bond and an electronegative O, N or F atom.
Properties of Liquids
Viscosity
• Viscosity is the resistance of a liquid to flowing.
• High viscosity liquids (e.g. molasses, motor oil) flow slowly. Low
viscosity liquids (e.g. water, gasoline) flow quickly.
• When a liquid flows down a tube, the molecules slide over one another.
The higher the forces of attraction, the less of a tendency the liquid will have
to flow, and the higher will be the viscosity.
Surface Tension
• A molecule in the bulk liquid is stabilized by the attraction
of the other liquid molecules. A molecule on the surface
will be at a higher energy than one in the bulk of the liquid.
• Surface tension is a measure of the strength of
intermolecular attractions which pull on molecules at the
surface of a liquid.
Surface Tension
• The stronger intermolecular forces, the higher surface tension.
• Surface tension is responsible for the rise of a liquid such as H2O in a thin
glass capillary. This also gives rise to a meniscus when water or an aqueous
solution is in a pipette or a burette.
Adhesion
Adhesion is an
attraction between
unlike molecules
Cohesion is the
attraction between
like- molecules
Cohesion
• It is because of its high surface tension that water tends to “bead” up on a
waxy surface. That’s because a sphere gives the minimum ratio of surface
area to volume.
• The high surface tension of water also causes molecules on the surface to
pack very closely together. This is why some insects can “walk on water”.
Structure of Solids
• A crystalline solid possesses rigid and long-range order. In a crystalline
solid, atoms, molecules or ions occupy specific (predictable) positions
• An amorphous solid does not possess a well-defined arrangement and
long-range molecular order. Irregular (disordered) arrangement of atoms (or
molecules) are found in an amorphous solid.
Crystalline solid, e.g. Quartz (SiO2)
Amorphous solid e.g. Silica Glass (SiO2)
The Crystalline Lattice
• Lattice is the three dimensional array of points repeating periodically.
• Lattice point can be atoms, molecules and ions.
• Unit cell is the basic repeating structural unit of a crystalline solid.
lattice point
unit cell
3-D lattice
arrangement
• The unit cells stacked in 3-D space describe the bulk arrangement of atoms of
the crystal. The unit cell is given by its lattice parameters, the length of the cell
edges & the angles between them, while the positions of the atoms inside a unit
cell are described by the set of atomic positions measured from a lattice point.
The Crystalline Lattice and Type of Cell
primitive cell
body-centred
face-centred
The 14 Bravais Lattices
X-Ray Crystallography
BC + CD = 2d sinq = nl
2d sinq = nl
(Bragg Equation)
Types of Solids
• There are four classifications of solids, depending on the type of bonds that
are present:, Ionic Solids, Metallic Solids, Molecular Solids and Covalent
Network Solids.
Ionic Solids
• Lattice points occupied by cations and anions held together by electrostatic
attraction
• Hard, brittle, high melting point, poor conductor of heat and electricity
• Examples: All typical salts, e.g. NaCl, Ca(NO3)2, MgBr2
CsCl
ZnS
CaF2
Metallic Solids
• Lattice points occupied by metal atoms held together by metallic bond
• Soft to very hard, low to very high melting point, excellent thermal and
electrical conductivity, malleable and ductile
• Examples: All metals, e.g. Cu, Fe, Sn, Au, Ag
Bonding due to delocalized valence
electrons (shown in blue)
Strength of bonding varies between
different metals, resulting in wide
range of physical properties
Molecular Solids
• Lattice points occupied by molecules held by intermolecular forces
• Fairly soft, moderately low melting point (usually <
200oC), poor thermal and electrical conductivity
• Examples: Argon , CH4, CO2, H2O
Covalent Network Solids
• Lattice points occupied by atoms connected in network of covalent bonds
• Hard, high melting point, poor conductor of heat and electricity
• Examples: Diamond (C), Quartz (SiO2)
Each carbon is
connected to 4 others
by covalent bonds
carbon atoms
diamond
graphite
Coordination Network Solids
• Coordination polymer is a metal coordination compound where a ligand
bridges between metal centres, where each metal centre binds to more than
one ligand to create an infinite array of metal centres e.g. a polymer.
• More conventionally, coordination polymer is reserved for compounds
where the metals are bridged by multi-dentate ligands, such as cyanide or
carboxylates.
• The majority of common halides and oxides are coordination polymers.
Dicopper(II) tetracarboxylate building block
[Cu3{1, 3, 5-C6H3-(COOH)3}2(H2O)x]
Crystal Engineering
• Crystal engineering is the design and synthesis of molecular solid-state
structures with desired properties based on an understanding and
exploitation of intermolecular interactions.
• Crystal engineering relies on non-covalent bonding to achieve organization
of molecules and ions in the solid state. Much of the initial work on purely
organic systems focused on the use of hydrogen bonds, though with the
more recent extension to inorganic systems, the coordination bond has also
emerged as a powerful tool.
• Other intermolecular forces such as p…p, halogen…halogen and Au…Au
interactions have all been exploited in crystal engineering studies, and ionic
interactions can also be important.
• Polymorphism is the phenomenon wherein the same chemical compound
exists in different crystal forms. It is one of the most exciting branches of the
subject partly because polymorphic forms of drugs may be entitled to
independent patent protection if they show new and improved properties
over the known crystal forms.