Popular in Course
Popular in Chemistry
This 14 page Class Notes was uploaded by Carmela Kilback on Wednesday September 9, 2015. The Class Notes belongs to CHEM 165 at University of Washington taught by Staff in Fall. Since its upload, it has received 27 views. For similar materials see /class/192603/chem-165-university-of-washington in Chemistry at University of Washington.
Reviews for H
Report this Material
What is Karma?
Karma is the currency of StudySoup.
Date Created: 09/09/15
Chemistry 165 241 Chapter 24 Optical and Electronic Materials Silicon Unlike carbon diamond structure only No graphitelike form because 1c bonds are weak due to larger size of Si atoms 117A for Si vs 077 A for C Similarly C02 is a gas while SiOz is a network solid There is no Si analog of benzene Preparation of Si Si022C gtSi2C02 0r ZSiCSiOZ gt3Si2CO SiOz is kept in excess to prevent the accumulation of SiC Further puri cation of Si is achieved by chlorination to SiCl4 which can be puri ed by distillation Reduction with Zn or Mg gives fairly pure Si which is then further purified for use in semiconductors by zone re ning Aside from semiconductors Si also has an extensive quotorganicquot chemistry based on the formation of Si C bonds by the quotdirectquot reaction of alkyl chlorides with Si in the presence of a Cu catalyst CH3 Clg 515 Cus 15 gt CH38iC13 CH3ZSiC12 CH33SiCl CH34Si The relative proportions of these four products can be varied by adjusting the conditions Careful hydrolysis leads to useful polymers called silicones MegSiCl igt MegSi O terminal group Me MeZSiClz H Zoe O Si0 chainforming group Me Me MeSiCl3 L Oquot O branchingampbridging group Silicone polymers have good thermal and oxidative stability excellent water repellency good dielectric properties and are resistant to ultraviolet irradiation By controlling Chemistry 165 242 molecular weight and the degree of crosslinking the physical properties can be controlled Germanium Tin and Lead Germanium is similar to silicon Tin and lead show more metallic behavior Lead is toxic and presents a significant human health hazard General trends in group IV elements get larger as we descend group Valence electrons are in orbitals with higher principal quantum number bonds between elements get weaker due to larger size e g SiSi bonds and Ge Ge bonds are weaker than CC due to reduced effectiveness of overlap less e density volume ionization potentials decrease as we descend group more metallic behavior oxidation state 11 becomes much more important eg CClz C12 gt CCl4 entirely to right quotSiClz C12 gt SiCl4 strongly to the right GeClz C12 2 GeCl4 slightly gt SnClz C12 ltgt SnCl4 about even PbClz C12 PbCl4 exist only under C12 pressure To form four bonds at 109 angles must mix 5 and 3p orbitals to make 4 equivalent MO s aka SP3 quothybridsquot Since ground state for atoms is 3132 must provide pronation energy This is recovered by forming m bonds instead of two Works less well as the bonds get weaker as we descend the group Lead would rather be PbH Chemistry 165 Semiconductors First consider bonding in metals and conduction of energy For example sodium metal Why does it conduct heat and electricity Consider 2 Na atoms each with one electron in a 35 orbital Make molecular orbitals for Na2 top and for Na3 bottom 0 antibonding 35 3s m bonding 4CD 3 a untibondins o o c o o CD u bondins For many atoms eg No get two bands Conduction I l bond GOGGO Energy Energy gap Eg Valence bond 01 metal b semlconducfor c insulator Chemistry 165 negative e ntype ptype semis 1 2 wave recti er o05l a Current blocked 0051 EH 5 Current flows 0051 c Half wave recti cation of alternating current 244 Chemistry 165 231 Chapter 23 Ceramic Materials Silicates The silicon oxygen bond is strong and many silicates are used for building materials such as bricks and cement Silicates are lattices built up from SiO4439 units along with cations Those with seperate SiO44 units are called orthosilicates The simplest orthosilicates are minerals such as Mg25i04 and FeZSiO4 Two of these units combined together make disilicates Combination with two trivalent cations leads to minerals such as Sc2SiO7 Other combinations of these units can form chains asbestos sheets talc and networks quartz or crystabolite See Fig 231 Replacement of some silicon atoms with aluminum gives a variety of aluminosilicates By varying elemental ratios in such lattices it becomes clear that there is an immense number of possible minerals Geochemistry deals with the study of this large number of minerals and rocks There are a few basic processes that contribute to the variety of materials in the Earth39s crust A very important property of minerals is the melting point Different materials crystallize from molten rock at different times during its cooling A second crucial property is solubility Solubility in water is important in seperating minerals Water can also react with minerals depending on the pH and the presence of either reducing or oxidizing conditions Evaporation of water precipitates minerals at different times according to their solubilities Compositional changes can also occur due to temperature and pressure increases deep in the Earth These reactions occur on geological timescales Thermodynamic conditions for spontaneous reaction AG lt 0 may be present however reaction rates may depend on the presence of water or of solidsolid diffusion slow Aluminosilicates Replacement of one or more Si atoms in a silicate mineral gives aluminosilicates Talc is Mg3Si4O1OOH2 and is thus related to micas such as KMg3AlSi3010OH2 One of the Si atoms in the Si4O10439 building block has been replaced by Al so an additional Chemistry 165 23 2 potassium ion is required to balance the charge There are many different aluminosilicate minerals Clays Useful materials since prehistory Readily available and reversibly absorb water and other small molecules Structures are based on infinite sheets similar to micas If we start from A12Si4010OH2 a mica and replace every sixth Al3 with one Na and one Mg2 we obtain the clay called montmorillonite MgNaAl5Si40103OH6 See Fig 235 Zeolites These are aluminosilicates with large voids Very useful in ion exchange and catalysis applications Igneous Rocks Igneous rocks consist mostly of silicates and make up most of the crust of the earth As molten rock magma cools various aluminosilicate minerals crystallize from the melt Remember that Al can substitute for Si in silicate structures eg Si4O1o439 which is actually Si2052 subunits infinite sheets see 231 f If every fourth Si atom in the lattice is replaced with A1 we get AlSi301o39 where the ratio of Si Aloxygen is still 125 An infinite sheet structure still obtains but Al atoms are randomly distributed in the lattice and an alkali metal cation such as K or Na must be present for each Al As magma cools two different types of minerals crystallize Species composed of SiO44 tetrahedra with various cations such as CaA128i208anorthite are formed More SiO Si linkages can form leading to Na AlSi303 albite Albite has the basic formula of AlSi3 Og which has Si Aloxygen 12 So the structure is like that of quartz It turns out that the amount of Al is continuously variable and these two species form a nearly ideal solid solution of continuously variable composition Alternative pathways to crystallization as the magma cools lead to welldefined species in a discontinuous series Species based on SiO4439 tetrahedra such as MngiO4 and Fe28i04 crystallize first As more SiO Si bonds form minerals based on single chains SiOaz39 building blocks such as NaAlSizOe jadeite crystallize Ultimately layer structures based on SizO5239 or AlSi301o539 building blocks crystallize eg micas such as muscovite KA12AlSisO10OH2 Chemistry 165 233 Sedimentary Rocks These rocks are made up of materials which have been dissolved in water by weathering and then reprecipitated upon evaporation or a change in pH or other conditions For example limestone CaC03 precipitates from C02 saturated solutions in the presence of suitable amounts of calcium Aluminum oxides silicon oxides and aluminosilicates are all soluble to some extent in water particularly under acidic conditions So rocks dissolve eventually As in all other geochemical processes the observer needs to be patient The depostiondissolution of sediments is pH dependent For example Aluminum oxides are soluble in acidic or basic media but insoluble at neutral pH Another complication arises when redox chemistry is possible in that the redox potentials of common ions depends upon the pH of the solution see g 238 12 02 H 0 10 2 13 Fe Fe2 06 04 3 at O gt 02 a A ca 00 H 0 H2 2 02 2 0394 Fe I M Fe 06 Feb io7M Fe 08 Chemistry 165 234 Metamorphic Rocks Under very high pressures some interesting reactions occur slowlyi An example is the transormation of limestone to marble Both are calcium carbonate Metamorphism occurs at high temperatures but not high enough to melt the rocks Some rocks have been through several metamorphic transformations There are many cases of different crystalline forms with different densities eg SiOz exists as both quartz and crystobalite The higher density form is favored by higher pressures The conditions of temperature and pressure where both species coexist can be described by a line on a graph of P versus T analogous to the coexistence lines that we have previously seen in phase diagrams eg H200 I120g The slope of this line is given by dPdT ASAV AHTA V Similar condiditons apply if a chemical reaction occurs at high pressure eg CaMgCO32 s 23i02s lt3 CaMgSi206s 2C02g Dolomite quartz Diopside Higher pressure shifts the equilibrium to the left since there are no gases on the left hand side the entropy change in such a reaction is approximately equal to the standard entropy change corrected for the isothermal compression of 11 mole of gassee g 2310 AS z S AngRlnP At AG 0 T AHAS AH AS AngRlnP 1200 quot 1000 Sim RN10 quot 390 e 2 16294453013 5 Fez 9 15 FQSA 2S 3 39 I r A H B N a 600 5 P b CaMgC032s 2 SiOz quartz t CaMgSi206s 2 C02g 400 v 2000 2000 4000 6000 8000 10000 Pressure atm Chemistry 165 191 Chapter 19 Structure of Solids Oxtoby pages 693713 In order to appreciate the structures of ordered crystalline solids we need to understand symmetry elements which come in three types rotation axes 2fold 3fold etc re ection in a mirror plane inversion through a point inversion center Consider a cube each of these symmetry elements is present See Figure 193 in Oxtoby The cube is a repeating unit in several types of important crystal structures called the cubic system In all such crystal structures the unit cell or repeat unit has equal edge lengths a b and c and the angles between these edges 0L 3 and y are all 90 In general the volume of a unit cell is given by the formula V aboJl cos2 a cos2 3 cosZ 7 2 cosoc cosB cos y Note that when at B 7 90 this simplifies to V abc For an element whose crystal contains nC atoms per unit cell the calculated cell density is n a density p mass NR n M volume Vc N 0V The nearest neighbor separation in a simple cubic crystal is equal to the lattice parameter 11 so the atomic radius in that case is 112 For the bcc lattice the central atom in the unit cell touches each of the eight atoms at the corners of the cube but those corners do not touch one another as shown in Figure 1912 Oxtoby The nearest neighbor distance is calculated from the Cartesian coordinates of the atom at the origin 0 0 O and that at the cell center 11 2 112 112 B the Pythagorean theorem the distance between these points is 1122 1122 1122 11413 2 so that the atomic radius is 1 5 4 Figure 1913 Oxtoby shows that in an fcc crystal the atom at the center of a face such as at 0 11 2 11 2 touches each of the nei hborin corner atoms such as at 0 0 0 so that the nearest neighbor distance is 122 122 aJE 2 and the atomic radius is 54 The results for cubic lattices are summarized below Chemistry 165 192 Structural Properties of Cubic Lattices TI 3 0524 20680 0740 The ways in which the free volume is distributed among the atoms of a crystal are both interesting and important especially for the closepacked fcc structure Two types of sites can be identified call interstitial sites upon which the free volume in the unit cell is centered Octahedml sites are those that are surrounded at equal distances by six nearest neighbor atoms Figure 1915 Oxtoby shows that such sites lie at the midpoints of the edges of the fcc unit cell A cell has 12 edges each of which is shared by four unit cells so that they contribute three octahedral interstitial sites per cell In addition the site at the center of the unit cell is also octahedral so the total number of octahedral sites per fcc unit cell is four the same as the number of atoms in the unit cell With a bit of simple geometry we can calculate the size of an octahedral site in an fcc structure or more precisely the radius r of a smaller atom that would fit in the site without overlapping its neighboring atoms Figure 1915Oxtoby represents a cell face in which the length of the diagonal is 4r1 and the length of the cell edge is 2n 2r2 where r1 is the radius of the host atoms and r2 is the radius of the octahedral site From the gure J5 r a 1 4 a 2r2 2r1 2r2 211 a J5 T2 E a and the ratio of the octahedralsite radius to the host atom radius is 014611 51 0414 r 2 1 tar4 The second type of interstitial site known as a tetrahedral site lies at the center of the space defined by four touching spheres Such a site is formed in an fcc cell in the volume that lies between a corner atom and the three facecentered atoms nearest to it Chemistry 165 19 3 Geometrical reasoning like that for the octahedral site gives the result that the ratio of the radius of a tetrahedral site to that of a host atom is 51 o225 T 1 The fcc unit cell contains eight tetrahedral sites twice the number of atoms in the cell Interstitial sites are important when a crystal contains atoms of several kinds that differ considerably in their radii We shall return to this shortly when we consider the structures of ionic crystals Molecular Crystals review sections 47 and 52 Consider the prototypical polar molecule HCl 5 8 H Cl recall that 3 ranges from 0 gt 1 T T Q1 e5 charge Q2 e5 I I 127A There are various ways to bring the 2 molecules together to some close distance eg 5 A 5 5 8 5 worst Cl H HC H 5 H 8 next I I E C39s 0395 H H Cl 0 O I CI H CI good I CI H best Cl H Cl H What about nonpolar molecules What about atoms Consider 2 Kr atoms Chemistry 165 194 E inst dipole induced dipole This force is called the dispersion force aka London forces Quantitatively VR where C6 is a coefficient for each atom It is larger for more polarizable atoms Repulsive Forces At short distances the electron clouds begin to overlap in a repulsive fashion e density distorts Wellde ned quotsizesquot for atoms can be deduced Repulsive forces be ak Note R is sometimes denoted van der Waals radius a b are constants for each atom calculated from results on deviation of expt real gases from ideal gas behavior PE be BR general case For H82 88 J mol391 at ca 3 A vs 436 k mol391 for H2 Combination of Theory and Experiment More sophisticated treatment by LennardJones 2 variable parameters Vu4ell lul l J T well depth Lennard Jones Parameters for Atoms and Molecules Substance 039 A I I N k moll He 256 141 x 1022 0085 Ne 275 492 x 10 22 0296 Ar 340 1654 X 1021 0996 Kr 360 236 x 1021 142 Xe 410 306 X 1021 184 Hz 293 511 X 10 2 0308 02 358 1622 x 1021 0977 CO 376 1383 x 1021 0833 N2 370 1312 x 1021 0790 CH4 382 2045 x 10 21 1232 Chemistry 165 Van an moiquot IllTIllllllllllllll R I 191 A 010 Principles of XRay Diffraction gt constructive interference gt destructive interference