CHM1046 Chapter 11 Intermolecular Forces Liquids and Solids Phase Homogeneous part of the system in contact with other parts of the system but separated from them by a well defined boundary Intermolecular Forces Intermolecular Forces Attractive forces between molecules o Much weaker o Ex 41 kJ to vaporize 1 mole of water Intramolecular Forces Hold atoms together in a molecule o Much stronger o Ex 930 kJ to break all O H bonds in 1 mole of water Measures of Intermolecular Force o Boiling point o Melting point o Hvap o Hfus o Hsub Polar Molecules and Dipole Moments Polar Molecule Electrons spend more time in the vicinity of one atom than the other atoms Dipole Moment The measure of the polarity of a bond In an H F molecule the electrons spend more time near the fluorine atom than the hydrogen because the fluorine is more electronegative Dipole Dipole Forces Attractive forces between polar molecules 1 Ion Dipole Forces Attractive forces between an ion and a polar molecule Dispersion Forces Attractive forces that arise as a result of temporary dipoles Polarizability The ease with which the electron distribution in the atom or CHM1046 induced in atoms or molecules o Applies to all o Usually increase with molar mass molecule can be distorted o Increases with Greater number of electrons More diffuse electron cloud Hydrogen Bond A special dipole dipole interaction between the hydrogen atom in a polar N H O H or F H bond and an electronegative O N or F atom Practice Problem 11 1 o What type s of intermolecular forces exist between the following pairs HBr and H2S Dipole dipole Dispersion Cl2 and CBr4 Dispersion I2 and NO3 Dispersion NH3 and C6H6 Ion induced dipole Dipole induced dipole Dispersion CH3OCH3 Yes CH4 No F2 Yes 2 Practice Problem 11 2 o Which of the following can form hydrogen bonds with water CHM1046 HCOOH Na Yes No Intermolecular Forces Strongest Weakest 1 Ion Dipole Ion polar molecule 2 Hydrogen bond O H F H or N H O N or F 3 Dipole dipole Polar polar 4 Ion induced dipole Ion nonpolar 5 Dipole induced dipole Polar nonpolar 6 Dispersion forces All Properties of Liquids Surface Tension The amount of energy required to stretch or increase the surface of a liquid by a unit area o Strong intermolecular forces high surface tension Cohesion The intermolecular attraction between like molecules Adhesion An attraction between unlike molecules Viscosity A measure of a fluid s resistance to flow o Strong intermolecular forces high viscosity Equilibrium Vapor Pressure The vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation Molar Heat of Vaporization Hvap The energy required to vaporize 1 mole of liquid at its boiling point 3 CHM1046 4 Vapor Pressure versus Temperature Practice Problem 11 7 o Diethyl ether is a volatile highly flammable organic liquid that is used mainly as a solvent The vapor pressure of diethyl ether is 401 mmHg at 18 C Calculate its vapor pressure at 32 C P2 657 mmHg Boiling Point Temperature at which the equilibrium vapor pressure of a liquid Normal Boiling Point The temperature at which a liquid boils when the external is equal to the external pressure pressure is 1 atm CHM1046 5 Critical Temperature Tc The temperature above which the gas cannot be made to liquefy no matter how great the applied pressure Critical Pressure Pc The minimum pressure that must be applied to bring about liquefaction at the critical temperature Solid Liquid Equilibrium The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium Molar Heat of Fusion Hfus The energy required to melt 1 mole of a solid substance at its freezing point CHM1046 Heating Curve Solid Gas Equilibrium Molar Heat of Sublimation Hsub The energy required to sublime 1 mole of a solid o Hsub Hfus Hvap Hess s Law Practice Problem 11 8 o Calculate the amount of energy in kilojoules needed to heat 346 g of liquid water from 0 C to 182 C Assume that the specific heat of water is 4 184 J g C over the entire liquid range and that the specific of steam is 1 99 J g C 985 kJ Phase Diagram Summarizes the conditions at which a substance exists as a solid liquid or gas 6 CHM1046 Phase Diagram of Water Phase Diagram of Carbon Dioxide 7 CHM1046 Effect of Increase in Pressure on the Melting Point of Ice and the Boiling Point of Water Chapter 12 Physical Properties of Solutions Solution A homogenous mixture of 2 or more substances Solute The substance s present in smaller amounts Solvent The substance present in the larger amount Saturated Solution Contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature Unsaturated Solution Contains less solute than the solvent has the capacity to dissolve at a specific temperature Supersaturated Solution Contains more solute than is present in a saturated solution at a specific temperature Three types of interactions in the solution process o Solvent Solvent Interaction o Solute Solute Interaction o Solvent Solute Interaction 8 CHM1046 9 Like dissolves like other Two substances with similar intermolecular forces are likely to be soluble in each o Nonpolar molecules are soluble in nonpolar solvents o Polar molecules are soluble in polar solvents CCl4 in C6H6 C2H5OH in H2O o Ionic compounds are more soluble in polar solvents NaCl in H2O or NH3 l Practice Problem 12 1 o Predict the relative solubilities in the following cases Bromine Br2 in benzene C6H6 0 D and in water 1 87 KCl in carbon tetrachloride CCl4 and in liquid ammonia NH3 Formaldehyde CH2O in carbon disulfide CS2 0 D and in D C6H6 nonpolar NH3 polar water Water Concentration Units solution Concentration The amount of solute present in a given quantity of solvent or CHM1046 Practice Problem 12 2 o A sample of 0 892 g of potassium chloride KCl is dissolved in 54 6 g of water What is the percent by mass of KCl in the solution 1 61 o Calculate the molality of a sulfuric acid solution containing 24 4 g of sulfuric acid in 198 g of water The molar mass of sulfuric acid 98 09 g Practice Problem 12 3 1 26 m Practice Problem 12 4 o The density of a 2 45 M aqueous solution of methanol CH3OH is 0 976 g mL What is the molality of the solution The molar mass of methanol is 32 04 g 2 73 m Practice Problem 12 5 o Calculate the molality of a 35 4 percent by mass aqueous solution of phosphoric acid H3PO4 The molar mass of phosphoric acid is 97 99 g 10 CHM1046 Temperature and Solubility 5 59 m
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