Transcript Document
Chapter 12 Intermolecular Forces: Liquids, Solids, and Phase Changes 12-1 Intermolecular Forces: Liquids, Solids, and Phase Changes 12.1 An Overview of Physical States and Phase Changes 12.2 Quantitative Aspects of Phase Changes 12.3 Types of Intermolecular Forces 12.4 Properties of the Liquid State 12-2 ATTRACTIVE FORCES electrostatic in nature Intramolecular forces bonding forces These forces exist within each molecule. They influence the chemical properties of the substance. Intermolecular forces nonbonding forces These forces exist between molecules. They influence the physical properties of the substance. 12-3 Phase Changes exothermic sublimation vaporizing melting solid liquid freezing endothermic 12-4 gas condensing Table 12.1 A Macroscopic Comparison of Gases, Liquids, and Solids State Shape and Volume Compressibility Ability to Flow Gas Conforms to shape and volume of container high high Liquid Conforms to shape of container; volume limited by surface very low moderate Solid Maintains its own shape and volume almost none almost none 12-5 Figure 12.1 Heats of vaporization and fusion for several common substances. 12-6 Figure 12.2 12-7 Phase changes and their enthalpy changes. Figure 12.3 A cooling curve for the conversion of gaseous water to ice. 12-8 Quantitative Aspects of Phase Changes Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average Ek as the most probable speed of the molecules changes. q = (amount)(molar heat capacity)(T) During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in Ep, as the average distance between molecules changes. q = (amount)(enthalpy of phase change) 12-9 Figure 12.4 12-10 Liquid-gas equilibrium. Figure 12.6 Vapor pressure as a function of temperature and intermolecular forces. 12-11 Figure 12.7 A linear plot of vapor pressure- temperature relationship. The Clausius-Clapeyron Equation ln P = -Hvap1 C R T P2 -Hvap 1 1 ln = R T2 T P1 1 12-12 SAMPLE PROBLEM 12.1 The vapor pressure of ethanol is 115 torr at 34.90C. If Hvap of ethanol is 40.5 kJ/mol, calculate the temperature (in 0C) when the vapor pressure is 760 torr. PROBLEM: PLAN: Using the Clausius-Clapeyron Equation We are given 4 of the 5 variables in the Clausius-Clapeyron equation. Substitute and solve for T2. SOLUTION: P2 -Hvap 1 1 ln = P1 R T2 T1 760 torr ln 115 torr = -40.5 x103 J/mol 8.314 J/mol*K T2 = 350K = 770C 12-13 34.90C = 308.0K 1 1 T2 308K Figure 12.8 Phase diagrams for CO2 and H2O. CO2 12-14 H 2O Figure 12.10 Periodic trends in covalent and van der Waals radii (in pm). 12-15 12-16 12-17 Figure 12.11 Polar molecules and dipole-dipole forces. solid liquid 12-18 THE HYDROGEN BOND a dipole-dipole intermolecular force A hydrogen bond may occur when an H atom in a molecule, bound to small highly electronegative atom with lone pairs of electrons, is attracted to the lone pairs in another molecule. The elements which are so electronegative are N, O, and F. H hydrogen bond acceptor .. O .. O .. .. .. .. F .. hydrogen bond donor hydrogen bond acceptor hydrogen bond acceptor 12-19 H .. .. N .. F .. hydrogen bond donor H .. N hydrogen bond donor Figure 12.12 12-20 Dipole moment and boiling point. Figure 12.13 12-21 Hydrogen bonding and boiling point. Polarizability and Charged-Induced Dipole Forces distortion of an electron cloud •Polarizability increases down a group size increases and the larger electron clouds are further from the nucleus •Polarizability decreases left to right across a period increasing Zeff shrinks atomic size and holds the electrons more tightly •Cations are less polarizable than their parent atom because they are smaller. •Anions are more polarizable than their parent atom because they are larger. 12-22 Figure 12.14 Dispersion forces among nonpolar molecules. separated Cl2 molecules 12-23 instantaneous dipoles Figure 12.15 Molar mass and boiling point. 12-24 Figure 12.16 Molecular shape and boiling point. fewer points for dispersion forces to act more points for dispersion forces to act 12-25 Figure 12.17 Summary diagram for analyzing the intermolecular forces in a sample. INTERACTING PARTICLES (atoms, molecules, ions) ions present ions only IONIC BONDING (Section 9.2) ions not present polar molecules only DIPOLE-DIPOLE FORCES ion + polar molecule ION-DIPOLE FORCES nonpolar molecules only DISPERSION FORCES only H bonded to N, O, or F HYDROGEN BONDING polar + nonpolar molecules DIPOLEINDUCED DIPOLE FORCES DISPERSION FORCES ALSO PRESENT 12-26 Figure 12.20 The H-bonding ability of the water molecule. hydrogen bond donor hydrogen bond acceptor 12-27 The Unique Nature of Water •great solvent properties due to polarity and hydrogen bonding ability •exceptional high specific heat capacity •high surface tension and capillarity •density differences of liquid and solid states 12-28