5: Chapter Liquids and Solids: CONDENSED phases of matter. Van der Waal's equation: gas particles have size and some attractive force. High P, Low T: particles close enough - attractive effects become significant gas condenses to liquid (or deposits to solid) further cooling of liquid - freezes to solid. Attractive forces between particles: due to COULOMBIC attraction between: positive particle charge q+ r negative particle charge q+ qq separated by a distance r: Coulomb's Law: F (q+ )(q- ) r2 Energy = Force x Distance so Potential Energy of Attraction Ep = (q+ )(q- ) 40r Types of Forces INTRAmolecular force: covalent bonding between individual atoms in molecule or polyatomic ion. e.g., bond between: C and O in CO2 S and O in SO42INTERionic force: ionic bonding between individual ions in lattice of a crystal e.g., bond between: Mg2+ and Cl- in MgCl2 Mg2+ and SO42- in MgSO4 INTERmolecular forces: several types (see following..) between individual atoms, ions or molecules of a compound or mixture. e.g., forces between: separate CO2 molecules in CO2 H2O molecules and dissolved Na+, Cl- ions Warning! If you don't remember how to tell if a compound is covalent, polar covalent or ionic: GO BACK AND REVIEW CHAPTERS 2-3 REMEMBER: A molecule which is polar overall will have a NET DIPOLE. From now on we refer to this as a DIPOLE. Ion-Dipole Interaction Aqueous solutions of ionic compounds consist of ions with attached water molecules. HYDRATION: water molecules surround and bond to ions Water is a POLAR molecule - has a net DIPOLE:Each H2O molecule orients itself: O side is negatively charged (-) so is closest to a positive ion, for a negative ion, side with H's (+) would be closest Potential Energy of an Ion-Dipole Attraction: Ep - z NOTE: a 1/r2 dependence 2 r z = charge on the ion, = dipole moment of the polar molecule (e.g. water in this image) red is negative, blue positive. Which is the anion, cation? Ion Dipole interactions are WEAKER than ion-ion attractions. Size & charge of ion affects how many H2O molecules surround it HYDRATED salts exist : solids with H2O fixed in the lattice. Occur for Li+, Na+, Ba2+, La3+ (large charge/radius ratio) Shown using a "" symbol e.g. CuSO4 5(H2O)
O H
H
+
Dipole-Dipole Interaction Solid In a SOLID of polar molecules: dipoles line up: maximize attraction ; minimize repulsion. Potential energy of a DIPOLE DIPOLE interaction in a SOLID: Ep - 1 2 NOTE: a 1/r3 dependence r3 1 = dipole moment of one molecule, 2 = dipole moment of the other (if only one component this 2) Dipole-Dipole Interaction Gas In a GAS of polar molecules: molecules are far apart, rotating rapidly. No net overall effect? No! Molecules linger in orientations maximizing attraction, minimizing repulsion.
Imagine moving and spinning, but pausing here
-
+
-
+
+
Potential energy of a DIPOLE DIPOLE interaction in a GAS is: Ep - 1 2 NOTE: a 1/r6 dependence : K-M theory is not a bad approximation! 6 r IN A LIQUID the equation is also true but the molecules are closer. London Forces ***ALSO called dispersion forces Assumes existence of temporary dipoles: Nucleus of one atom attracted to electron cloud of another nearby molecule / atom TEMPORARY dipole induced as cloud gets distorted. Exact size and direction of temporary dipoles changes rapidly: - induces a complimentary change in nearby molecules - they still continue to attract. Think about a how a school of fish moves not perfect analogy but useful. POLARIZABITY: (see Chapter 3) Ability for electron cloud to be distorted. -bigger electron clouds = more polarizable molecule. -bigger molecule = higher dispersion force -more linear molecule = higher dispersion force (molecules can get closer) Potential energy of London forces is given by: Ep - 1 2 NOTE: a 1/r6 dependence : r6 London forces exist in ALL molecules but are the ONLY form of interaction in nonpolar molecules. Dispersion forces may be more dominant than dipole-dipole interactions if molecule is big enough "Velcro" Dipole Induced Dipole Forces TEMPORARY dipole-dipole dispersion interactions can be induced from proximity of a polar molecule:
This end distorts electron cloud Nonpolar species now temporarily polar
non l
-
+
Potential energy of dipole induced dipole forces: Ep - 12 2 NOTE: a 1/r6 dependence : r6
Hydrogen Bonding SPECIAL CASE of Dipole-Dipole Interaction. ONLY between H and F, H and O, or, H and N. F, O, N = small, highly electronegative atoms. Partial charges - and + on H-F, H-N, H-O are larger. Interaction is much more pronounced. *"Hydrogen Bond" a misnomer implies a chemical bond; just a very strong dipole-dipole interaction. H-BONDING critical in structure/properties of many substances: Methanol, water, ammonia, amino acids (thus proteins), cellulose (woody plants, trees). Comparing the forces: See Table 5.1 and related figures for H bonding, and different dependencies on r. Using these Interactions You will be asked to compare interactions in different compounds: STRONGER INTERMOLECULAR INTERACTIONS MEANS it is MORE DIFFICULT to SEPARATE the component atoms/ions MEANS HIGHER MELTING AND BOILING POINTS (and other effects.... see later) Example For these substances: helium isooctane (C8H18 in a straight chain) methane calcium carbonate glycerol (possesses three OH groups) sucrose (possesses six OH groups) determine order of increasing boiling point. Always use common sense and prior knowledge to help!
VISCOSITY -Resistance to flow. (honey, molasses...) "Flow": particles slide past each other. LARGER attractive forces mean HIGHER VISCOSITY -Measured with viscometer (narrow necked vessel). Units : poise. Examples: Molecules with: multiple H-bonding sites (eg. Glycerin) larger surface area (long chain hydrocarbons) What effect does temperature have on viscosity? SURFACE TENSION -Inward force that prevents a liquid's surface area expanding. Liquids seem to have a "skin" Consider attractive forces: in bulk - in all directions; at surface - only toward interior. -surface particles move toward bulk -minimize particles on surface Shape with minimum surface area = SPHERE small liquid drop is ~ spherical. large drop on a surface flattens due to gravity.
CAPILLARY ACTION COHESIVE FORCES: hold liquid together ADHESIVE FORCES: hold to liquid another surface Which dominates affects liquid's ability to `wet' surface. (water `beads' on waxed surface (e.g., car)) Water / glass: Adhesive water-glass force > cohesive water-water force. Water tries to maximize glass contact. Result: CONCAVE meniscus Mercury / glass: Cohesive mercury-mercury force > adhesive mercury-glass force. Mercury tries to minimize glass contact. Result: CONVEX meniscus CAPILLARY ACTION: Adhesive force > cohesive force. Liquid creeps up sides of tube until limited by gravity. Roots use this! Structure of Matter: Gas: Compressible, very low density, particles far apart, moving fast until they collide. NO order. Liquid: Very slightly compressible, high density, particles can move but are in contact (and interacting...) so can only move past each other slowly. SOME order but on on a very SHORT range. Solid: Incompressible, highest density, particles are locked into postion; can only wiggle. Order of this solid may be high, low or intermediate. CRYSTALS are solids with VERY long range orders. THE SOLID STATE X-ray DIFFRACTION can be used to examine the structure of substances CRYSTALLINE SOLIDS: well defined, ordered structure. Intermolecular forces constant throughout. can be cleaved Have sharp melting points Examples: table salt, ice Types of Crystalline Solids: METALLIC e.g., s and p block metals Li, Na, In, Pb cation lattice surrounded by electron `sea' : electrostatic interaction good electrical & thermal conductors malleable, ductile, lustrous, hardness and m.p., b.p. vary IONIC e.g., salts , hydrated salts: NaCl, Ca3(PO4)2, CuSO45(H2O) lattice of ions: interionic interactions poor electrical & thermal conductors unless dissolved in H2O hard, brittle, high m.p. & b.p. MOLECULAR e.g., covalent compounds, some elements: BeCl2, CH4, CO2, S8, P4, ice, glucose, napthalene lattice of atoms or molecules : one or more of: London, dipole-dipole, H-bonding interactions poor electrical & thermal conductors soft, low m.p. & b.p. (unless H-bonding or significant London force present) brittle if pure NETWORK (also called COVALENT) e.g., some elements, some compounds: B, C, black P, BN, SiO2 (quartz) lattice of atoms ALL covalently bonded! A GIANT molecule ! poor electrical & thermal conductors (except graphite) high m.p. & b.p., brittle AMORPHOUS SOLIDS no well defined structure Intermolecular forces vary through the sample. Shatter or break into jagged random lumps Have broad melting points Examples: glass, amorphous sulfur, rubber
The Unit Cell Introduction smallest complete repeat unit of crystal lattice - can often be shown in more than one way BUT must reflect the EMPIRICAL FORMULA of the compound ALL possible crystal structures summarized by 14 BRAVAIS LATTICES 6 parameters: length, width, height (a,b,c) ; angles between the 3 sides (,,) get increasingly complex; some have atoms at corners only; others also in the sides, or center of cell. You will NOT be tested on the Bravais Lattices! Interpreting a Unit Cell You can determine how many atoms of each element are in a unit cell if it is shown. (Imagine cutting them up, as if they were clay spheres.) Atoms at a VERTEX : 1/8 of the atom. Atom in a FACE : 1/2 of the atom. Atom in the CENTER : ALL of the atom Atom in the EDGE : of the atom
Example: Metallic Solids Two close packed structures are common: cubic close-packed (CCP) ; hexagonal close packed (HCP) BOTH have a COORDINATION NUMBER (number of nearest neighbors) of 12 Try this yourself with pennies:
ABABAB... Hexagonal C.P. (Hexagonal Lattice) Ionic Solids Three structures are found: Rock Salt (very common) Cesium Chloride (less common) Zinc Blende (very rare)
ABCABCABC...Cubic C.P. (Face Centered Cubic Lattice)
Which form the ionic solid takes is given by the Radius Ratio: = radius smaller ion radius larger ion 0.4 - 0.7, predicts rock salt > 0.7 predicts cesium chloride < 0.4 predicts zinc blende
Ionic Solids: Detail: Rock Salt Structure - anions form a face centered cubic arrangement; cations fit between the anions COORDINATION NUMBER is 6: "6,6 coordination" each anion is surrounded by 6 cations; each cation surrounded by 6 anions CsCl Structure - anions form primitive cubic arrangement; one cation at center - overall, body centered cubic. COORDINATION NUMBER is 8: "8,8 coordination" each anion is surrounded by 8 cations; each cation surrounded by 8 anions ZnS (Zinc Blende) Structure - anions form face centered cubic arrangement; four cations fit into some of the gaps (the regions shaded) COORDINATION NUMBER is 4: "4,4 coordination" each anion is surrounded by 4 cations; each cation surrounded by 4 anions Molecular Solids Each example is unique. Properties depend on intermolecular forces: here's two examples: Ultra High Density Polyethylene: Very orderly arrangement of long hydrocarbon chains in close packed cylinders: London forces very strong. Used in bulletproof vests. Ice: Hydrogen bonds cause the water molecules to form a cage: each O has four H neighbors, two covalently bonded, two hydrogen bonded. Very open network - so ice is less dense than water.
Network (Covalent) Solids Giant covalently bonded molecules. Again, each case is different. Diamond: Each atom is sp3 hybridized to bond to its four neighbors. Looks a little like ice- the covalent bond is very strong - diamonds are very hard! Graphite: Each atom is sp2 hybridized to bond to three neighbors. `Chicken wire' structure, 4th (weaker) bond is to next sheet. Sheets slip easily uses: lubricant, pencils.. Alloys Homogenous alloys: even mixture throughout. (examples: coin alloys, brass) Heterogeneous alloys: tiny crystalline phases, with different compositions (examples: solder, dental amalgam)
Substitutional: elements have similar atomic radii (examples: copper-zinc alloy used in coins) Liquid Crystals
Intersitial: one element is much smaller - fits into gaps. (examples: Steel -an iron and carbon alloy)
MESOPHASE: has properties between liquid and solid. Made from long rod-shaped molecules:
SMECTIC
NEMATIC
CHOLESTERIC
LCD's: An LC layer parallel to a glass plate only transmits light polarized one way. Apply potential difference - the LC's re-orient themselves; changes angle of polarization. LCD includes a polarizing filter to show effect.