Liquids & Solids

1. Liquids & Solids • Chapter 10

2. Heat of Fusion/Vaporization • H2O(s) ----> H2O(l) Hfo = 6.02 kj/mol • H2O(l) ----> H2O(g) Hvo = 40.7 kj/mol • From the Ho values above, which two states are most similar? • How do the attractive forces between the molecules compare in these two states to the third state?

3. Three States of Matter

4. Intramolecular within the molecule covalent bonding ionic bonding Intermolecular between molecules dipole-dipole forces hydrogen bonding London Dispersion Forces Types of Bonding When ice changes to liquid and then to vapor, the intramolecular forces (covalent bonds) stay intact, only the weaker hydrogen bonds between molecules weaken and break.

5. Intermolecular Forces • Forces between (rather than within) molecules. • dipole-dipole attraction: molecules with dipoles orient themselves so that “+” and “” ends of the dipoles are close to each other. (1 % as strong as covalent or ionic.) • hydrogen bonds: dipole-dipole attraction in which hydrogen is bound to a highly electronegative atom. (F, O, N)

6. Electrostatic interaction of two polar molecules.

7. The polar water molecule and hydrogen bonds among water molecules.

8. The boiling points of the covalent hydrides of the elements in Groups 4A, 5A, 6A, & 7A.

9. Instantaneous and induced dipole moments between nonpolar molecules -- London Dispersion Forces.

10. London Dispersion Forces • relatively weak forces that exist among noble gas atoms and nonpolar molecules. (Ar, C8H18) • caused by instantaneous dipole, in which electron distribution becomes asymmetrical. • the ease with which electron “cloud” of an atom can be distorted is called polarizability.

11. Some Properties of a Liquid • Surface Tension: The resistance to an increase in its surface area (polar molecules). A sphere has the maximum volume for the minimum surface area.

12. Some Properties of a Liquid • Capillary Action: Spontaneous rising of a liquid in a narrow tube. Viscosity: Resistance to flow (molecules with large intermolecular forces).

13. Some Properties of a Liquid • Cohesive forces exist between molecules of a liquid. Adhesive forces exist between the liquid and its container.

14. Types of Solids • Crystalline Solids: highly regular arrangement of their components [table salt (NaCl), pyrite (FeS2)]. • Amorphous solids: considerable disorder in their structures (glass).

15. Representation of Components in a Crystalline Solid • Lattice: A 3-dimensional system of points designating the centers of components (atoms, ions, or molecules) that make up the substance.

16. Representation of Components in a Crystalline Solid • Unit Cell: The smallest repeating unit of the lattice. • simple cubic -- 1 atom/cell • body-centered cubic -- 2 atoms/cell • face-centered cubic -- 4 atoms/cell

17. Three cubic unit cells and the corresponding lattices.

18. Simple Cubic Cell • 1 atom per cell • side length (do) = 2 r do = 2 r

19. Body-Centered Cell • 2 atoms per cell • Body diagonal = do3 = 4r • do2 -- diagonal through the base of cube. • 4r do • do2

20. Face-Centered Cell • 4 atoms per cell • Face diagonal = do2 = 4r • do2 -- diagonal through the face of cube. • 4r do • do

21. Face-centered cubic unit cell.

22. Bragg Equation • Used for analysis of crystal structures and to calculate the distance between planes in crystals. • n = 2d sin  • d = distance between atoms • n = an integer •  = wavelength of the x-rays

23. Reinforcement or cancellation of X-rays.

24. Reflection of X-rays of wavelength  from a pair of atoms in two different layers of a crystal.

25. Types of Crystalline Solids • Atomic Solid: contains atoms at the lattice points (diamond). • Ionic Solid: contains ions at the points of the lattice that describe the structure of the solid (NaCl). • Molecular Solid: discrete covalently bonded molecules at each of its lattice points (sucrose, ice).

26. Three crystalline solids -- a) atomic solid, b) ionic solid, and c) molecular solid.

28. Packing in Metals • Model: Packing uniform, hard spheres to best use available space. This is called closest packing. Each atom has 12 nearest neighbors. • hexagonal closest packed (“aba”) • cubic closest packed (“abc”)

29. Closest packing arrangement of uniform spheres -- aba. This forms hexagonal closest packed -- hcp.

30. Atom in third layer lies over atom in first layer. 10_218 (a) (b) (a)) Top view Atoms arranged in aba pattern forming hexagonal closest packed (hcp) structure -- 2 atoms/cell.

31. Hexagonal closest packed structure -- central atom has 12 nearest neighbors.

32. Face-centered cubic is cubic closest packed (ccp). The spheres are packed in an abc arrangement.

33. Bonding Models for Metals • Electron Sea Model: A regular array of metals in a “sea” of electrons. • Band (Molecular Orbital) Model: Electrons assumed to travel around metal crystal in MOs formed from valence atomic orbitals of metal atoms. • Conduction Bands: closely spaced empty molecular orbitals allow conductivity of heat and electricity.

34. Representation of the energy levels (bands) in a magnesium crystal. 1s, 2s, & 2p orbitals are localized, but 3s & 3p orbitals are delocalized to make molecular orbitals.

35. Metal Alloys • 1. Substitutional Alloy: some metal atoms replaced by others of similar size. • brass = Cu/Zn Substances that have a mixture of elements and metallic properties.

36. Metal Alloys(continued) • 2. Interstitial Alloy: Interstices (holes) in closest packed metal structure are occupied by small atoms. • steel = iron + carbon • 3. Both types: Alloy steels contain a mix of substitutional (Cr, Mo) and interstitial (Carbon) alloys.

37. Substitutional Alloy Interstitial Alloy

38. Network Solids • Composed of strong directional covalent bonds that are best viewed as a “giant molecule”. • brittle • do not conduct heat or electricity • carbon, silicon-based • graphite, diamond, ceramics, glass

39. Network solid structure of diamond.

40. Semiconductors A substance in which some electrons can cross the band gap. • Conductivity is enhanced by doping with group 3a or group 5a elements. • n-type semiconductor -- doped with atoms having more valence electrons -- Phosphorus. • p-type semiconductor -- doped with atoms having fewer valence electrons -- Boron. • See Figure 10.31 on page 477 in Zumdahl.

41. Molecular Solids • molecular units at each lattice position. • strong covalent bonding within molecules. • relatively weak forces between molecules. • London Dispersion Forces -- CO2, I2, P4, & S8. • Hydrogen Bonding -- H2O, NH3, & HF.

43. Trigonal, Tetrahedral, & Octahedral Holes • Trigonal holes -- formed by three spheres in the same layer. • Tetrahedral holes -- formed when a sphere sits in the dimple of three spheres in an adjacent layer. • Octahedral holes -- formed between two sets of spheres in adjoining layers of closest packed structures.

44. Trigonal, Tetrahedral, and Octahedral holes.

45. Hexagonal & Cubic Closest Packed • 1 octahedral hole for each atom or ion. • 2 tetrahedral holes for each atom or ion. • Simple cubic and body-centered cubic are not closest packed structures!

46. The location (x) of a tetrahedral hole in the face- centered cubic unit cell. The S2- ions are closest packed with the Zn2+ ions in alternating tetrahedral holes.

47. The location (x) of an octahedral hole in the face- centered cubic unit cell. The Cl- ions have a ccp arrangement with the Na+ ions in all the octahedral holes.

48. Vapor Pressure • . . . is the pressure of the vapor present at equilibrium. • . . . is determined principally by the size of the intermolecular forces in the liquid. • . . . increases significantly with temperature. • Volatile liquids have high vapor pressures.

49. Vapor Pressure • Low boiling point • high vapor pressure. • weak intermolecular forces. • Low vapor pressure • high molar masses. • strong intermolecular forces.

50. Boltzman Distribution -- number of molecules in a liquid with a given energy versus kinetic energy at two different temperatures.