General Chemistry I

Transcription

1 Liquids and Solids General Chemistry I Dr Rick Attrill Office MHMK 1405/5 Intermolecular Forces Properties of Liquids Crystal Structure X ray Diffraction by Crystals Types of Crystals Amorphous solids Phase Changes and Phase Diagrams 1 2 A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases Solid phase - ice Liquid phase - water Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular 41 kj to vaporize 1 mole of water (inter) 930 kj to break all O-H bonds in 1 mole of water (intra) Generally, intermolecular forces are much weaker than intramolecular forces. Measure of intermolecular force boiling point melting point H vap 3 H fus H sub 4

2 Intermolecular Forces Intermolecular Forces Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction 5 6 Dispersion Forces Intermolecular Forces Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction Intermolecular Forces Dispersion Forces Due to Temporary Fluctuating Dipoles A molecule approaches a molecule with an instantaneous dipole. The approaching molecule now has an induced dipole due to the proximity of the original temporary dipole. An instant later the electrons in the left hand molecule move to the other end. They will then repel the electrons in the right hand molecule. 7 If molecules are close enough together this movement of electrons can occur over a large number of molecules. 8

3 Dispersion Forces Continued Intermolecular Forces Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass. 9 What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH 4 CH 4 is nonpolar: dispersion forces. S SO 2 SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules. O O 10 Hydrogen Bond Intermolecular Forces The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H B or A H A A & B are N, O, or F Why is the hydrogen bond considered a special dipole-dipole interaction? Decreasing molar mass Decreasing boiling point 11 12

4 Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules Strong intermolecular forces Adhesion High surface tension Cohesion Properties of Liquids Water is a Unique Substance Viscosity is a measure of a fluid s resistance to flow. Strong intermolecular forces Maximum Density 4 0 C Density of Water High viscosity Ice is less dense than water 15 16

5 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. A unit cell is the basic repeating structural unit of a crystalline solid. lattice point At lattice points: Atoms Molecules Ions Unit Cell Unit cells in 3 dimensions

6 Shared by 8 Shared by 2 unit cells unit cells Closest Packing 1 atom/unit cell (8 x 1/8 = 1) 2 atoms/unit cell (8 x 1/8 + 1 = 2) 4 atoms/unit cell (8 x 1/8 + 6 x 1/2 = 4) 23 Close packed layer of spheres (layer A) Hexagonal closed packed (hcp) structure (ABA) Cubic close-packed (ccp) structure (ABC) 24

7 When silver crystallizes, it forms face-centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver. d = m V V = a 3 = (409 pm) 3 = 6.83 x cm 3 4 atoms/unit cell in a face-centered cubic cell m = 4 Ag atoms g x mole Ag x 1 mole Ag x atoms = 7.17 x g d = m V 7.17 x g = = 10.5 g/cm x cm Extra distance = BC + CD = 2d sin = n (Bragg Equation) 28

8 X rays of wavelength nm are diffracted from a crystal at an angle of Assuming that n = 1, what is the distance (in pm) between layers in the crystal? n = 2d sin n = 1 = = nm = 154 pm Types of Crystals Ionic Crystals Lattice points occupied by cations and anions Held together by electrostatic attraction Hard, brittle, high melting point Poor conductor of heat and electricity d = n 2sin = 1 x 154 pm 2 x sin14.17 = 77.0 pm 29 CsCl ZnS CaF 2 30 Types of Crystals Covalent Crystals Lattice points occupied by atoms Held together by covalent bonds Hard, high melting point Poor conductor of heat and electricity Types of Crystals Molecular Crystals Lattice points occupied by molecules Held together by intermolecular forces Soft, low melting point Poor conductor of heat and electricity carbon atoms diamond graphite 31 32

9 Types of Crystals Metallic Crystals Lattice points occupied by metal atoms Held together by metallic bonds Soft to hard, low to high melting point Good conductors of heat and electricity nucleus & inner shell e - Cross Section of a Metallic Crystal Types of Crystals mobile sea of e An amorphous solid does not possess a well-defined arrangement and long-range molecular order. Least Order A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing Evaporation Condensation Crystalline quartz (SiO 2 ) Non-crystalline quartz glass T 2 > T 1 Greatest Order

10 The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation H 2 O (l) H 2 O (g) Dynamic Equilibrium Rate of condensation = Rate of evaporation Before Evaporation At Equilibrium Molar heat of vaporization ( H vap ) is the energy required to vaporize 1 mole of a liquid. Clausius-Clapeyron Equation ln P = - H vap RT + C P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/K mol) The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm

11 The critical temperature (T c ) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure (P c ) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature. H 2 O (s) H 2 O (l) The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium Melting Freezing Molar heat of fusion ( H fus ) is the energy required to melt 1 mole of a solid substance

12 H 2 O (s) H 2 O (g) A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water Molar heat of sublimation ( H sub ) is the energy required to sublime 1 mole of a solid. Sublimation Deposition H sub = H fus + H vap ( Hess s Law)