January 25th 2017 Chapter 12 HW Ch 11 12 due Feb 3rd Exam 1 2 weeks Chapters 11 12 13 SLW Lab Tutoring Wednesday 4 45 6 15 Library 722 Individual options tutoring siu edu A Kinetic Molecular View of the Three States 2 types of electrostatic forces Intramolecular or bonding forces are found within a molecule They influence the chemical properties of the substance Intermolecular or nonbonding forces are found between molecules They influence the physical properties of the substance Figure 5 1 Types of Phase Changes and Their Enthalpies endothermic sublimation melting vaporization freezing condensation deposition exothermic 12 2 Quantitative Aspects of Phase Changes We will quantify the heat involved in a phase change and examine the equilibrium nature of the process Heat Involved in Phase Changes Stage 2 Temperature C Stage 1 q n H vap Stage 3 Stage 4 Stage 5 q nCwater l T q n H fus H vap H fus q nCwater g T Heat Released q nCwater s T Sample Problem 12 1 PROBLEM Finding the Heat of a Phase Change Depicted by Molecular Scenes The scenes below represent a phase change of water Find the heat in kJ released or absorbed when 24 3 g of H2O undergoes this change Liquid There are 3 stages involved in this process 1 heating of the liquid to its boiling point 2 the phase change from liquid to gas 3 heating the gas to the final temperature Gas Sample Problem 12 1 SOLUTION mol H2O 24 3 g H2O x 1 mol H2O 18 02 g H2O 1 35 mol H2O For Stage 1 q n x Cwater l x DT 1 35 mol 75 4 J mol C 100 85 0 C 1527 J 1 53 kJ For Stage 2 q n DH vap 1 35 mol 40 7 kJ mol 54 9 kJ For Stage 3 q n x Cwater g x DT 1 35 mol 33 1 J mol C 117 100 C 759 6 J 0 760 kJ qtotal 1 53 kJ 54 9 kJ 0 760 kJ 57 2 kJ The Equilibrium Nature of Phase Changes Figure 12 4 Liquid gas equilibrium In a closed flask the system reaches a state of dynamic equilibrium where molecules are leaving and entering the liquid at the same rate Effects of Temperature and Intermolecular Forces on Vapor Pressure Vapor pressure increases as temperature increases Vapor pressure decreases as the strength of the intermolecular forces increases Quantifying the Effect of Temperature Clausius Clapeyron Equation 1 D H C ln P R T vap y m x b Where R universal gas constant 8 314 J mol K The two point form is used when the vapor pressures at two different temperatures are known P2 DHvap 1 1 ln P1 R T2 T1 Sample Problem 12 2 Applying the Clausius Clapeyron Equation The vapor pressure of ethanol is 115 torr at 34 9 C If DHvap of ethanol is 38 6 kJ mol calculate the temperature in C when the vapor pressure is 760 torr P1 115 torr T1 34 9 C 273 15 308 0 K DHvap 38 6 kJ mol SOLUTION ln P2 DHvap P1 R 760 torr ln 115 torr 1 1 T2 T1 38 6 x 103 J mol 8 314 J mol K 1 1 T2 308 0 K T2 352 K 273 15 79 C P2 760 torr T2 For a given compound a plot of the natural log of the vapor pressure ln P versus the inverse of the temperature 1 T gives a slope of 4890 K What is the enthalpy of vaporization for this compound R 8 314 J mol K a 4 89 kJ mol b 4 89 kJ mol c 40 7 kJ mol d 0 588 kJ mol e 40 7 kJ mol 1 D H C ln P R T vap y Hvap R m x b m 4890 K Hvap 4890 K 8 314 J mol K Hvap 40 7 kJ mol