Chapter 11 Intermolecular Forces and the Liquid State Lecture Notes PDF

Chapter 11. Intermolecular Forces and the Liquid State © 2018 Cengage Learning. All Rights Reserved.  Boiling Point Temperature at which Pvap = Patm (vapor pressure equals atmospheric pressure) – Normal boiling point: Pvap = 760 mm Hg (1 atm) As the strength of...

Chapter 11. Intermolecular Forces and the Liquid State © 2018 Cengage Learning. All Rights Reserved.  Boiling Point Temperature at which Pvap = Patm (vapor pressure equals atmospheric pressure) – Normal boiling point: Pvap = 760 mm Hg (1 atm) As the strength of IMFs increases, the boiling point increases At reduced external pressure, vapor pressure of water is equal to the external pressure at a lower temperature – Boiling occurs at a reduced temperature 2 © 2018 Cengage Learning. All Rights Reserved. Interactive Figure 11.2.3 - Explore the Boiling Point of a Liquid 3 © 2018 Cengage Learning. All Rights Reserved.  IMFs - Summary IMFs affect: – Enthalpy of vaporization (ΔHvap ) – Vapor pressure – Boiling point of liquids For a series of liquids, as strength of IMFs increases: – ΔHvap increases – Liquid vapor pressures decrease – Liquid boiling points increase 4 © 2018 Cengage Learning. All Rights Reserved. Example Problem 11.2.4 - Predict Boiling Point Using a Vapor Pressure–Temperature Plot 5 © 2018 Cengage Learning. All Rights Reserved.  Mathematical Relationship between Vapor Pressure and Temperature Clausius–Clapeyron equation expresses the mathematical relationship among vapor pressure (P), temperature (T), and strength of IMFs (related to ΔHvap)  ΔH vap ln P = + C RT – R - Ideal gas constant (R = 8.314 × 10–3 kJ/K·mol) – C - Constant of integration 6 © 2018 Cengage Learning. All Rights Reserved.  Clausius–Clapeyron (continued 1) Equation Can be written in the form of a straight-line equation:  ΔH vap 1 ln P = T  +C R   y = m x + b When vapor pressure is measured over a range of temperatures, the data can be used to determine the ΔHvap for a liquid – Plot of the natural log of vapor pressure (ln P) versus inverse temperatures (1/T) has a slope equal to –ΔHvap/R 7 © 2018 Cengage Learning. All Rights Reserved.  Exercise: Calculating ΔHvap for SO2 T (K) P (mm Hg) 220 81.6 230 147.4 240 253.6 250 417.7  ΔH vap  1  Given: ln P =   +C Find: ΔHvap R T  Make the plot: Completed and placed at the top  ΔH vap  ΔH vap Slope =  2993.8 K  1 = R kJ 8.3145  10 3 mol K ΔH vap = 24.9 kJ / mol 8 © 2018 Cengage Learning. All Rights Reserved.  Clausius–Clapeyron (continued 2) Equation Second form of this equation can be used to calculate vapor pressure of a liquid at a given T if the following values are given: – Enthalpy of vaporization of a liquid – Vapor pressure at another temperature P2  ΔH vap  1 1  ln =    P1 R  T2 T1  9 © 2018 Cengage Learning. All Rights Reserved. Example Problem 11.2.5 - Use the Two-Point Version of the Clausius–Clapeyron Equation 10 © 2018 Cengage Learning. All Rights Reserved. Intermolecular Forces and the Liquid State 11.3 Other Properties of Liquids 11 © 2018 Cengage Learning. All Rights Reserved.  Cohesive Forces Attractive forces that exist between molecules at the macroscopic level and affect the physical properties of a liquid – Also called bulk-scale forces – Result of IMFs Cohesive forces are important for: – Surface tension – Viscosity – Capillary action 12 © 2018 Cengage Learning. All Rights Reserved.  Surface Tension Measure of force required to break the surface of a liquid Not highly predictable based on other properties of liquids Results from different IMFs experienced by: – Molecules at the surface of a liquid Experience a net inward force – Molecules at the interior of a liquid Experience IMFs in all directions 13 © 2018 Cengage Learning. All Rights Reserved.  Factors Affecting Surface Tension Stronger intermolecular attractive forces result in a higher surface tension Raising the temperature of a liquid reduces its surface tension – Increase in average KE causes the molecules to move with a greater velocity, which makes it easier to stretch the surface 14 © 2018 Cengage Learning. All Rights Reserved. Table 11.3.1 - Surface Tension and Boiling Points for Some Common Liquids 15 © 2018 Cengage Learning. All Rights Reserved.  Measuring Surface Tension Use the contact angle – Angle between a horizontal surface and the tangent to the surface where a liquid droplet meets the horizontal surface – Greater the angle, stronger the surface tension 16 © 2018 Cengage Learning. All Rights Reserved.  Viscosity Measure of a liquid’s resistance to flow – Not highly predictable based on other properties of liquids Depends on IMF strength – Strong IMFs result in high viscosity Example – Honey is more viscous than H2O because of its strong IMFs 17 © 2018 Cengage Learning. All Rights Reserved.  Factors Affecting Viscosity Molecular structure of a liquid, particularly the length of molecules – Long, flexible molecules tend to become entangled and resist moving past one another as a liquid flows Temperature - Higher the temperature, lower the viscosity of the liquid – Increase in KE overcomes attractive forces, which enables molecules to easily move past each other 18 © 2018 Cengage Learning. All Rights Reserved.  Measuring Viscosity Time how long it takes for a certain amount of liquid to flow through a thin tube under gravitational force Drop a steel ball through the liquid, and track the time taken for it to drop More viscous liquids have greater times for both measurements 19 © 2018 Cengage Learning. All Rights Reserved.  Capillary Action Ability of a liquid to flow up a thin tube against the influence of gravity Cohesive forces within a bulk sample of liquid are overcome by adhesive forces – Adhesive forces: IMFs between a liquid and a surface such as glass or paper 20 © 2018 Cengage Learning. All Rights Reserved.  Cohesion and Adhesion Cohesion – Intermolecular attraction between like molecules – Example - Glue that holds molecules together Adhesion – Attraction between unlike molecules – Example - Glue that holds a bandage to the skin 21 © 2018 Cengage Learning. All Rights Reserved.