Chem 1011 Dr. L. Dawe Lecture 12

Transcription

1 Intermolecular Forces - Summary February 3, 2010 Lecture Vaporization and Vapor Pressure The Process of Vaporization The Energetics of Vaporization Vapor Pressure and Dynamic Equilibrium The Critical Point: The Transition to an Unusual Phase of Matter 11.6 Sublimation and Fusion Sublimation Fusion Energetics of Melting and Freezing Recall: Heats of Reaction Different physical or chemical changes have their own enthalpy notation ( H) symbol Symbol H fus H solid H vap H cond H sol n H comb H f H decomp Representation H of fusion H of solidification H of vaporization H of condensation H of solution H of combustion H of formation H of decomposition Type of Change Physical (s l) Physical (l s) Physical (l g) Physical (g l) Physical (s aq) Chemical Chemical Chemical Positive or Negaive value Positive Negative Positive Negative Positive or Negaive Negative Negative (most) Positive (most) Heat is also stoichiometrically associated with phase changes. 1

2 Vaporization of Liquids Vaporization (evaporation): the passage of molecules from the liquid to the gaseous state. Intermolecular forces must be overcome in order for a sample to vaporize. Vaporization occurs more readily with: Increased temperature: more molecules have sufficient kinetic energy to overcome intermolecular forces. Only a small fraction of the molecules in a liquid have enough energy to escape. However, as the temperature increases, the fraction of the molecules with escape energy increases. The higher the temperature, the faster the rate of evaporation. Vaporization of Liquids Vaporization of Liquids: Vapour Pressure Vaporization (evaporation): the passage of molecules from the liquid to the gaseous state. Intermolecular forces must be overcome in order for a sample to vaporize. Vaporization occurs more readily with: Increased temperature: more molecules have sufficient kinetic energy to overcome intermolecular forces. Increased surface area: a greater proportion of the molecules are at the surface of the liquid. Decreased strength of intermolecular forces: less kinetic energy is required to overcome the forces of attraction. 2

3 The Energetics of Vaporization Enthalpy of vaporization ( H vap ): quantity of heat that must be absorbed in order to vaporize a liquid at constant temperature. Enthalpy of condensation ( H condensation ) is equal in magnitude but opposite in sign to H vap. H condensation = H vaporization Problem: Calculate the mass of water that can be vaporized with 155 kj of heat at 100 C. Given: Find: Conceptual Plan: Relationships: Solution: The Energetics of Vaporization 155 kj g H 2 O 1 mol H 2 O = 40.7 kj, 1 mol = g Check: Since the given amount of heat is almost 4 the H vap, the amount of water makes sense. The Energetics of Vaporization Problem: Calculate the amount of heat needed to vaporize 90.0 g of C 3 H 7 OH at its boiling point. ( H vap = 39.9 kj/mol) 3

4 Problem: Calculate the amount of heat needed to vaporize 90.0 g of C 3 H 7 OH at its boiling point. ( H vap = 39.9 kj/mol) 4

5 Vapor Pressure and Dynamic Equilibrium Vapor pressure: the pressure exerted by a vapor when it is in dynamic equilibrium with its liquid at a fixed temperature. liquid vaporization condensation vapor Liquid vaporizes in a closed container. Condensation begins. Rate of vaporization > rate of condensation. Equilibrium. Rate of vaporization = rate of condensation. Vapor Pressure and Dynamic Equilibrium Changing the container s volume disturbs the equilibrium. Initially, the rate of vaporization and condensation are equal and the system is in dynamic equilibrium. When the volume is increased, the rate of vaporization becomes faster than the rate of condensation. When the volume is decreased, the rate of vaporization becomes slower than the rate of condensation. Vapor Pressure and Dynamic Equilibrium Liquids with high vapor pressure at room temperature are volatile. Liquids with low vapor pressure at room temperature are nonvolatile. Vapor pressure increases with temperature. This is illustrated by vapor pressure curves. Vapor pressure curves: a graph of vapor pressure as a function of temperature. 5

6 Vapor Pressure Curves (a) Diethyl ether, C 4 H 10 O (b) Benzene, C 6 H 6 (c) Water, H 2 O (d) Toluene, C 7 H 8 (e) Aniline, C 6 H 7 N The normal boiling points are the temperatures at the intersection of the dashed line (P = 760 mmhg). The normal boiling point is related to vapor pressure and is lowest for liquids with the weakest intermolecular forces. Vapor Pressure and Boiling Point Boiling: a process in which vaporization occurs through a liquid. It occurs when the vapor pressure of a liquid is equal to barometric pressure. Normal boiling point: the temperature at which the vapor pressure of a liquid is 1 atm. It is the temperature at which the liquid boils in a container open to the atmosphere at 1 atm. Boiling points vary with barometric pressure. At an altitude of 1609 m (Denver, Colorado), barometric pressure is 630 mmhg and the boiling point of water is 95 o C. It takes longer to cook food under these conditions! Critical Point Critical Point: refers to the temperature and pressure at which a liquid and its vapor become identical. It is the highest temperature point on the vapor pressure curve. 6

7 Critical Point A gas can be liquefied only at temperatures below its critical temperature. If room temperature is above T c, then a gas can cannot be liquefied. Temperature must be lowered to below T c. Phase Diagrams Phase Diagrams: a graphical representation of the conditions of temperature and pressure at which solids, liquids and gases exist, either as single phases or states of matter or as two or more phases in equilibrium. Triple point: a condition of temperature and pressure at which three phases of a substance coexist at equilibrium. Phase Diagrams and Supercritical Fluids Above the critical point the state of matter has the high density of a liquid, but the low viscosity of a gas. It is called a supercritical fluid. 7

8 Sublimation Sublimation: the passage of molecules from the solid to the gaseous state. Deposition: the passage of molecules from the gaseous to the solid state. Enthalpy of sublimation: H sub = H fus + H vap Sublimation of solid iodine, and the deposition of the vapor to solid on the cooler walls of the flask. Melting (Fusion): the transition of a solid to a liquid that occurs at the melting point. The melting point and freezing point of a substance are identical. Heat of fusion ( H fus ) Fusion Energetics of Melting and Freezing Heat is also stoichiometrically associated with phase changes. H 2 O(l) H 2 O(g) H vap = 40.7 kj at 373 K H 2 O(s) H 2 O(l) H fus = 6.01 kj at K media_portfolio/text_images/031_changess tate.mov Note: For a reverse reaction, the sign of H is reversed. For example: H 2 O(g) H 2 O(l) H cond = kj at 373 K 8

9 Energetics of Melting and Freezing Heat is also stoichiometrically associated with phase changes. Phase changes are associated with changes in potential energy. Temperature changes are associated with changes in kinetic energy. Looking Ahead February 5, 2010 Lecture Heating Curve for Water Crystalline Solids: The Fundamental Types Molecular Solids Ionic Solids Atomic Solids 9