H-bonding IMF Dominate London IMF

Lecture TP Which of the following cannot form H-bonds between themselves? 1. 2. 3. 4. 5. 6. 7. 8. 9. Ammonia, NH3 Methanol, CH3OH Ethanol, CH3CH2OH Dimethyl ether, CH3OCH3 1 and 3 1 and 4 2 and 4 All of the above can form hydrogen bonds with themselves Not sure Why is NH3 > PH3 < AsH3 < SbH3? • • • • • H-bonding IMF dominate London IMF Summary: Three kinds of IMF: London, H-bond, dipole-dipole Practice: Boiling points and enthalpy of vaporization Vapor pressure: Liquid-vapor equilibrium Vapor pressure and boiling point • Phase diagrams Hydrogen bonding 1 Ice Water O H⋯O angle is 180° H⋯O bond is longer than O H bond 2 TP Which of the following cannot form H-bonds between themselves? 1. 2. 3. 4. 5. 6. 7. 8. 9. Ammonia, NH3 Methanol, CH3OH Ethanol, CH3CH2OH Dimethyl ether, CH3OCH3 1 and 3 1 and 4 2 and 4 All of the above can form hydrogen bonds with themselves Not sure Why is FH > NH3? Why is OH2 > FH > NH3? TP Why does FH boil at a higher temperature than NH ? 1. 2. 3. 4. 5. 6. 7. 8. FH is more polar The electron cloud of FH is larger FH forms more H bonds F is more electronegative than N 1 and 2 1, 2, and 3 1, 2, 3, and 4 Not sure 3 Intermolecular forces (IMF) summary There are three kinds of Coulomb interactions between neutral particles, in order of decreasing importance: If hydrogen bonds are possible, they are the dominant IMF. Otherwise, London forces are the dominant IMF. TP Identify the compound with the smallest energy of vaporization kJ/mol . 1. 2. 3. 4. 5. water ethanol dimethyl ether Hexane Not sure Dipole-dipole interaction is important only when molecules of similar London IMF have very different polarities. TP Identify the compound with the largest energy of vaporization kJ/mol . 1. 2. 3. 4. 5. water ethanol dimethyl ether Hexane Not sure When London IMF is very large, it dominates C H : 69℃ C H : 98℃ H O: 100℃ C H : 126℃ 4 TP The correct order of boiling point of CH3Br, CH2Br2, CH3Cl, and CH2Cl2 lowest to highest is … 1. 2. 3. 4. 5. 6. 7. CH2Br2 CH2Cl2 CH3Br CH2Cl2 CH2Br2 CH3Cl CH3Cl CH3Br CH2Cl2 CH3Br CH3Cl CH2Br2 CH3Cl CH3Br CH2Br2 some other order Not sure CH3Cl CH3Br CH2Br2 CH2Cl2 CH2Cl2 Review: Units of pressure 1 Pa 1 / Review: Units of pressure 1 Pa Review: Units of pressure 1 Pa 1 / 1 / 5 Review: Units of pressure 1 Pa 1 / 1 / 1 Review: Units of pressure 1 Pa 1 / 1 1 bar 100 kPa exactly 1 atm 101.325 kPa exactly / 1 760 Torr exactly Equilibrium vapor pressure Equilibrium vapor pressure What is going on at the surface at this point? What is going on at the surface now? 6 Equilibrium vapor pressure Equilibrium vapor pressure Vapor pressure must rise with temperature. Why is this so? How must pressure must change with temperature? Evaporation Vapor pressure rises with temperature. Why is a vapor pressure of ethanol greater? TP At 60℃, the vapor pressure of ethanol is greater than that of water. This is because … 1. 2. 3. 4. 5. 6. 7. water has smaller IMF than ethanol water has greater IMF than ethanol water has smaller London IMF water has greater London IMF water is more polar water is less polar Not sure 7 Evaporation Evaporation Vapor pressure rises with temperature. 𝑇 𝑇 60.0℃, 𝑃 Problem 15-21: A 0.750-gram sample of ethanol CH CH OH is placed in a sealed 400-mL container. Is there any liquid present when the temperature is held at 60℃? 351 Torr 78.3℃, 𝑃 760 Torr We know that at 60.0℃, 𝑃 351 Torr How to proceed? Evaporation Evaporation Problem 15-21: A 0.750-gram sample of ethanol CH CH OH is placed in a sealed 400-mL container. Is there any liquid present when the temperature is held at 60℃? Problem 15-21: A 0.750-gram sample of ethanol CH CH OH is placed in a sealed 400-mL container. Is there any liquid present when the temperature is held at 60℃? At 60.0℃, 𝑃 At 60.0℃, 𝑃 𝑛 𝑛 351 Torr ?, calculate from molar mass ?, calculate from ideal gas law, using actual pressure, 𝑃 𝑛 𝑛 𝑅𝑇/𝑉 351 Torr ?, calculate from molar mass 𝑛 ?, calculate from ideal gas law, using actual pressure, 𝑃 𝑛 𝑅𝑇/𝑉 If 𝑛 𝑛 , liquid will remain. If 𝑛 𝑛 , liquid will have completely evaporated. 8 Evaporation Evaporation Problem 15-21: A 0.750-gram sample of ethanol CH CH OH is placed in a sealed 400-mL container. Is there any liquid present when the temperature is held at 60℃? Problem 15-21: A 0.750-gram sample of ethanol CH CH OH is placed in a sealed 400-mL container. Is there any liquid present when the temperature is held at 60℃? 351 Torr At 60.0℃, 𝑃 𝑛 0.0163 mol 0.00675 mol 𝑛 At 60.0℃, 𝑃 𝑛 𝑛 Since 𝑛 Evaporation 351 Torr 0.0163 mol 0.00675 mol 0.00675 mol 𝑛 0.0163 mol, liquid will remain. Liquid-vapor equilibrium Problem 15-21: A 0.750-gram sample of ethanol CH CH OH is placed in a sealed 400-mL container. Is there any liquid present when the temperature is held at 60℃? 351 Torr At 60.0℃, 𝑃 𝑛 0.0163 mol 𝑛 0.00675 mol 𝑛 𝑛 0.0163 mol 𝑛 0.00675 mol 0.0096 mol 9 Vapor pressure and boiling point Boiling means “bubbles”. Bubbles are pure vapor of the liquid. Bubbles form only if they are able to hold back the liquid walls. This means bubbles only exist if the vapor pressure is the same as the pressure acting on the liquid surface. Vapor pressure and boiling point TP Bubbles in a pot of boiling water contain … 1. 2. 3. 4. 5. 6. Only O2 𝑔 and N2 𝑔 Only H2O 𝑔 Only air H2O 𝑔 and air All of the above equally None of the above TP The order of normal boiling points lowest to highest is … Substance The normal boiling point is the temperature at which bubbles form at 1 atm. What do you predict for relative boiling points of these substances? 1. 2. 3. 4. 5. 6. Vapor pressure at 25℃, kPa Acetone, CH3C O CH3 30.8 Diethyl ether, CH3CH2 2O 71.7 Ethanol, CH3CH2OH 7.87 Water, H2O 3.17 Normal 1 atm boiling point/℃ 100 diethyl ether acetone ethanol ethanol acetone diethyl ether acetone diethyl ether ethanol ethanol diethyl ether acetone something else Not sure 10 Vapor pressure and boiling point Relative normal boiling point, Tb The normal boiling point is the temperature at which bubbles form at 1 atm. The normal boiling point is the temperature at which bubbles form at 1 atm. What do you predict for relative boiling points of these substances? Substance Vapor pressure at 25℃, kPa Normal 1 atm boiling point/℃ Acetone, CH3C O CH3 3: 30.8 3: 56 Diethyl ether, CH3CH2 2O 4: 71.7 4: 35 Ethanol, CH3CH2OH 2: 7.87 2: 78 Water, H2O 1: 3.17 1. 100 What do you predict for relative vapor pressures of these substances at 200°C? TP The substance with the lowest vapor pressure substances at 200°C is … 1. 2. 3. 4. 5. CH4 NH3 HCl N2 Not sure Phase diagrams Lines of 𝑃 versus 𝑇 at which different phases are present at the same time. That is, values of 𝑃 and 𝑇 at which different phases are in equilibrium. 11 Phase diagrams What about the intersection marked “A” Triple point Changes of phase Liquid tert-butyl alcohol, C CH3 3OH, can boil and freeze at the same time... http://goo.gl/4K1SR CO2 gas at 1 atm, 25℃ is heated to 1 atm, 35℃; then compressed to 75 atm, 35℃; then cooled to 75 atm, 25℃; then expanded to 65 atm, 25℃. 12 TP CO2 gas at 1 atm, 25℃ is heated to 1 atm, 35℃; then compressed to 75 atm, 35℃; then cooled to 75 atm, 25℃; then expanded to 65 atm, 25℃. At this point, the CO2 will be a … 1. 2. 3. 4. 5. gas supercritical fluid liquid solid Not sure 13