Intermolecular Forces and Liquids and Solids Chapter 11
- Slides: 47
Intermolecular Forces and Liquids and Solids Chapter 11 1 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.
A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a welldefined boundary. 2 Phases Solid phase - ice Liquid phase - water 2
Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular • 41 k. J to vaporize 1 mole of water (inter) • 930 k. J to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force Generally, intermolecular forces are much weaker than intramolecular forces. boiling point melting point DHvap DHfus DHsub 3
Intermolecular Forces Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 4
Intermolecular Forces Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction 5
Interaction Between Water and Cations in solution 6
Intermolecular Forces Dispersion Forces Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction 7
Induced Dipoles Interacting With Each Other 8
Intermolecular Forces Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond an electronegative O, N, or F atom. A H…B or A H…A A & B are N, O, or F 9
Example 11. 2 Which of the following can form hydrogen bonds with water? CH 3 OCH 3 CH 4 F 2 HCOOH Na+
Example 11. 2
Example 11. 1 What type(s) of intermolecular forces exist between the following pairs? (a) HBr and H 2 S (b) Cl 2 and CBr 4 (c) I 2 and (d) NH 3 and C 6 H 6
Total attraction pre • Hydrogen bond mi – Dipole-dipole – Dispersion V T t en r e f f i s d e e g k i a *L ck a p l e n n a ch • Dipole-dipole – Dispersion • Dispersion um ba sic 13
van der Waals forces • Dipole-dipole • Dipole-induced dipole • Dispersion 14
Intermolecular Forces Dispersion Forces Continued 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. 15
Differences in Melting and Boiling point • Melting and boiling points increase as the number of electrons in a molecule increases • More electrons = larger molar mass 16
Why is the hydrogen bond considered a “special” dipole -dipole interaction? Decreasing molar mass Decreasing boiling point 17
Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Strong intermolecular forces High surface tension 18
Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules Adhesion Cohesion 19
Properties of Liquids Viscosity is a measure of a fluid’s resistance to flow. Strong intermolecular forces High viscosity 20
3 -D Structure of Water is a Unique Substance Maximum Density 4 0 C Density of Water Ice is less dense than water 21
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. Crystalline quartz (Si. O 2) Non-crystalline quartz glass A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing 22
An Arrangement for Obtaining the X-ray Diffraction Pattern of a Crystal. 23
Reflection of X-rays from Two Layers of Atoms Extra distance = BC + CD = 2 d sinq = nl (Bragg Equation) 24
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 Cs. Cl Zn. S Ca. F 2 25
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 carbon atoms diamond graphite 26
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 water benzene 27
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 Cross Section of a Metallic Crystal nucleus & inner shell emobile “sea” of e- 28
Types of Crystals 29
Phase Changes Least Order Greatest Order 30
Effect of Temperature on Kinetic Energy T 2 > T 1 31
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 32
Measurement of Vapor Pressure Before Evaporation At Equilibrium 33
Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of a liquid at its boiling point. Clausius-Clapeyron Equation ln P = - DHvap RT + C P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8. 314 J/K • mol) Vapor Pressure Versus Temperature 34
Alternate Forms of the Clausius-Clapeyron Equation At two temperatures or 35
Example 11. 7 Diethyl ether is a volatile, highly flammable organic liquid that is used mainly as a solvent. The vapor pressure of diethyl ether is 401 mm. Hg at 18°C. Calculate its vapor pressure at 32°C.
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. 37
The critical temperature (Tc) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure (Pc) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature. 38
The Critical Phenomenon of SF 6 T < Tc T > Tc T ~ Tc T < Tc 39
Solid-Liquid Equilibrium 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. 40
Molar heat of fusion (DHfus) is the energy required to melt 1 mole of a solid substance at its freezing point. 41
Heating Curve 42
Solid-Gas Equilibrium H 2 O (s) H 2 O (g) Molar heat of sublimation (DHsub) is the energy required to sublime 1 mole of a solid. DHsub = DHfus + DHvap ( Hess’s Law) 43
Example 11. 8 Calculate the amount of energy (in kilojoules) needed to heat 346 g of liquid water from 0°C to 182°C. Assume that the specific heat of water is 4. 184 J/g · °C over the entire liquid range and that the specific heat of steam is 1. 99 J/g · °C.
A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water 45
Phase Diagram of Carbon Dioxide At 1 atm CO 2 (s) CO 2 (g) 46
Effect of Increase in Pressure on the Melting Point of Ice and the Boiling Point of Water 47
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