CHEMISTRY. CHM202 Class #2 CHEMISTRY. Chapter 10. Chapter Outline for Class #2

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1 CHEMISTRY Fifth Edition Gilbert Kirss Foster Bretz Davies CHM202 Class #2 1 Chemistry, 5 th Edition Copyright 2017, W. W. Norton & Company CHEMISTRY Fifth Edition Gilbert Kirss Foster Bretz Davies Chapter 10 Intermolecular Forces: The Uniqueness of Water 2 Chemistry, 5 th Edition Copyright 2017, W. W. Norton & Company Chapter Outline for Class # Solubility of Gases in Water 10.6 Vapor Pressure of Pure Liquids 10.7 Phase Diagrams: Intermolecular Forces at Work 10.8 Some Remarkable Properties of Water 3 3 1

2 Solubility of Gases in Water At lower temperatures gas dissolved in water has less kinetic energy, and dipoleinduced dipole interactions between solute and solvent molecules keep more gas dissolved. At higher temperatures molecules have more kinetic energy, which overcomes solute solvent interactions and reduces solubility. 4 4 Solubility of Gases in Water The solubility of a gas in a liquid such as water depends on the partial pressure of the gas in the air above the surface of the liquid. The amount of gas that dissolves in a liquid depends on the frequency and number of collisions that the gas molecules have with the surface of the liquid; the more collisions, the more gas molecules that may form intermolecular attractions with the solvent. 5 5 Henry s Law The quantitative relationship between gas solubility in a liquid and the partial pressure of the gas is known as Henry s Law. C gas = kh Pgas C gas represents the concentration (solubility) of a gas in a particular solvent. k H is the Henry s Law constant for the gas in that solvent. P gas is the partial pressure of the gas in the environment surrounding the solvent

3 Practice: Henry s Law Calculate the concentration of carbon dioxide in a soft drink that was bottled under a partial pressure of 5.0 atm CO 2 at 25 C. Collect, Organize, and Analyze: C gas = kh Pgas The Henry s Law constant for CO 2 is mol/(l atm). 7 7 Practice: Henry s Law Calculate the concentration of carbon dioxide in a soft drink that was bottled under a partial pressure of 5.0 atm CO 2 at 25 C. Solve: C ( 3.1 x 10-2 mol/l atm)( 5 atm ) = 1.61x10-1 gas = mol/l 8 8 Practice: Henry s Law Calculate the concentration of carbon dioxide in a soft drink that was bottled under a partial pressure of 5.0 atm CO 2 at 25 C. Think About It: Carbon dioxide is nonpolar and water is polar. One would expect a low concentration due to the weak dipole-induced dipole forces

4 Factors Affecting Vapor Pressure Temperature: Higher temperature = greater kinetic energy Surface area: Number of molecules in position to enter gas phase Intermolecular forces: Greater forces = greater kinetic energy needed for molecule to escape surface Vapor Pressure vs. Temperature Clausius Clapeyron Equation ln H 1 = + C R T vap ( Pvap ) 1 1 = P vap,t R T 2 2 T1 ln P vap,t1 H vap

5 Practice: Enthalpy of Vaporization Sulfur dioxide has a vapor pressure of mmhg at 21.0 C and a vapor pressure of mmhg at 44.0 C. What is the enthalpy of vaporization of sulfur dioxide? [R = J/(K mol)] Collect, Organize, and Analyze: We use the Clausius-Clapeyron equation. The temperatures must be converted to Kelvin and then place the values for the corresponding variable and solve for the enthalpy of vaporization Practice: Enthalpy of Vaporization Sulfur dioxide has a vapor pressure of mmhg at 21.0 C and a vapor pressure of mmhg at 44.0 C. What is the enthalpy of vaporization of sulfur dioxide? [R = J/(K mol)] Solve: T 1 = 21 C = K T 2 = 44 C = K mmhg H 1 1 ( ) vap ln = mmhg J/ K mol K K H vap H vap = J/mol = 24.9 kj/mol Factors Affecting Physical States Intermolecular forces: Strength of attractive forces compared to kinetic energy of atoms/molecules Temperature: Affects kinetic energy of atoms/molecules Pressure: Affects distance between atoms/molecules

6 Physical State and Phase Transformations Phase diagram: A graphical representation of how the stabilities of the physical states of a substance depend on temperature and pressure Equilibrium lines represent phase changes between solid/liquid, liquid/gas, and solid/gas under specific conditions (T, P). Supercritical region: fourth state of matter with properties intermediate between gas and liquid Features of Phase Diagrams Triple point: Temperature/pressure where all three phases of a substance coexist Critical point: Specific temperature/pressure at which the liquid and gas phases have the same density Supercritical fluid: A substance above its critical temperature and pressure Phase Diagram for Water

7 Phase Diagram for CO Practice: Phase Diagrams Describe the phase changes that occur when the temperature of CO 2 is increased from 100C to 200C at a pressure of 25 atm. Collect and Organize: We are given the phase diagram for CO 2 and need to determine the phase change(s) that occur as temperature is increased at a constant pressure of 25 atm Practice: Phase Diagrams Describe the phase changes that occur when the temperature of CO 2 is increased from 100C to 200C at a pressure of 25 atm. Analyze: Using the initial conditions ( 100C, 25 atm), we can identify the initial phase of CO 2 on the phase diagram. We can follow a horizontal line at a constant pressure of 25 atm to the final temperature of 200C, noting any phase changes that occur as we pass over the equilibrium lines between phases

8 Practice: Phase Diagrams Describe the phase changes that occur when the temperature of CO 2 is increased from 100C to 200C at a pressure of 25 atm. Solve: At the initial conditions ( 100C, 25 atm) CO 2 is a solid. As temperature is increased, we pass through the melting line (solid liquid) at a temperature of approximately 56C. Continuing horizontally at constant pressure, we pass through the liquefaction point (liquid gas) at a temperature near 0.0C. At the final temperature (200C), CO 2 is a gas Practice: Phase Diagrams Describe the phase changes that occur when the temperature of CO 2 is increased from 100C to 200C at a pressure of 25 atm. Think About It: The transition from solid-toliquid-to-gas is consistent with the increased kinetic energy of the molecules as temperature increases Properties of Water Surface tension: Energy needed to separate the molecules at the surface of a liquid Viscosity: Measure of resistance to flow of a liquid Related to intermolecular forces As interactive forces increase, surface tension and viscosity also increase. Related examples: meniscus, capillary action

9 Surface Tension Intermolecular forces holding molecules together Cohesive vs. Adhesive Forces: Meniscus a) Adhesive forces: interactions between unlike particles b) Cohesive forces: interactions between like particles Examples of Cohesive/Adhesive Forces Meniscus: Curvature of liquid surface due to adhesive and cohesive forces Concave: adhesive forces cohesive forces (e.g., water on glass) Convex: cohesive forces > adhesive forces (e.g., mercury on glass) Capillary action: The rise of a liquid up a narrow tube due to adhesive forces between the liquid and the tube and cohesive forces within the liquid

10 Viscosity Viscosity = resistance to flow. Factors: Intermolecular forces Molar mass Molecular shape Temperature Density Density of water decreases as it freezes. H bonding results in a cagelike structure in solid state; less dense than liquid state Water and Aquatic Life Importance of density: Lakes/rivers freeze from top down, allowing fish and aquatic life to survive below. As surface waters warm or cool, nutrient-rich bottom waters cycle to the surface; oxygen-rich surface waters cycle to the bottom