1 Chapter 13 Physical Properties of Solutions Insert picture from First page of chapter

Copyright McGraw-Hill Types of Solutions

Copyright McGraw-Hill Types of Solutions Solution classifications based on amount of solute dissolved relative to the maximum: Saturated – maximum amount at a given temperature (This amount is termed the solubility of the solute.) Unsaturated – less than the maximum Supersaturated – more than a saturated solution but is an unstable condition

Copyright McGraw-Hill unsaturated supersaturated saturated heat

Copyright McGraw-Hill Conversion of a Supersaturated Solution to a Saturated Solution

Copyright McGraw-Hill A Molecular View of the Solution Process Factors that determine solubility Intermolecular forces present in the formation of a solution –Solute-solute interactions –Solvent-solvent interactions –Solute-solvent interactions

Copyright McGraw-Hill 20097

8 endothermic exothermic

Copyright McGraw-Hill Solubility can be predicted based on “like dissolves like” in terms of intermolecular forces. For example: water and methanol are both polar and dissolve in each other Two liquids that are soluble in each other in all proportions are term miscible. Ions readily dissolve in polar solvent due to solvation by the solvent molecules.

Copyright McGraw-Hill Predict whether Vitamin B 6 is water soluble or fat soluble.

Copyright McGraw-Hill Water soluble due to the presence of polar groups. polar groups

Copyright McGraw-Hill Energy and entropy –Exothermic processes are generally more favorable than endothermic processes –Solutions do form when the overall process is endothermic –Entropy (randomness or disorder) contributes to the solution process Entropy tends to increase for all process A solution is more disordered than the isolated solute and solvent

Copyright McGraw-Hill Concentration Units Molality (m) – number of moles of solute dissolved in 1 kg (1000 g) of solvent Percent by Mass – ratio of mass of solute to the mass of the solution times 100

Copyright McGraw-Hill Units analogous to percent by mass (part per hundred) to express very small concentrations Parts per million (ppm) Parts per billion (ppb) *masses must be expressed in the same units

Copyright McGraw-Hill Choice of units depends on the purpose of the experiment Mole fraction – used for gases and vapor pressures of solutions Molarity – commonly used since volumes of solutions are easy to measure Molality – temperature independent Percent by Mass – temperature independent and need not know molar masses Conversion between units requires the use of density if any mass to volume or volume to mass conversion in needed.

Copyright McGraw-Hill Determine the a) molality, b) percent by mass and c) the ppm concentrations of a solution prepared by dissolving g of glucose (C 6 H 12 O 6 ) in 255 g of water.

Copyright McGraw-Hill a) molality Molar mass of glucose = g/mol

Copyright McGraw-Hill b) percent by mass c) ppm (original slide typo!)

Copyright McGraw-Hill Factors that Affect Solubility Temperature –The solubility of solids may increase, decrease or remain relatively constant with increasing temperature –The solubility of gases decreases with increasing temperature Thermal pollution – a consequence of the relation between gas solubility and temperature

Copyright McGraw-Hill Temperature Dependence of the Solubility of Selected Solids

Copyright McGraw-Hill Pressure – significantly affects only the solubility of gases –Henry’s law – the solubility of a gas, c, is directly proportional to the pressure of the gas, P, over the solution where c, is in mol/L, k is Henry’s law constant with units of mol/L. atm and P is in atm.

Copyright McGraw-Hill Molecular View at Different Pressures P1P1 P2P2 P 1 < P 2

Copyright McGraw-Hill Colligative Properties Colligative properties depend only on the number of solute particles in solution and not the nature of the solute Vapor-Pressure lowering –Nonvolatile solute (no appreciable pressure) Vapor pressure of the solvent is decreased –Volatile solute (exhibit appreciable pressure) Vapor pressure is the sum of the individual pressures

Copyright McGraw-Hill –Raoult’s law – quantitative expression of the solution vapor pressure Nonvolatile solute (P 1 is the solution vapor pressure, P 0 is the vapor pressure of the pure substance, and  is the mole fraction.) Volatile solute (P T is the solution vapor pressure, and are vapor pressures of pure solution components) Volatile solute: not exam material

Copyright McGraw-Hill Calculate the vapor pressure of a solution made by dissolving 115 g of urea, a nonvolatile solute, [(NH 2 ) 2 CO; molar mass = g/mol] in 485 g of water at 25°C. (At 25°C,.)

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Copyright McGraw-Hill Boiling-Point Elevation (  T b ) –The boiling point of a solution, T b, of a nonvolatile solute will be higher than that of the pure solvent,. –Elevation is directly proportional to the molal concentration. –K b is the molal boiling-point elevation constant.

Copyright McGraw-Hill Effect of Vapor Pressure Lowering Effect on Boiling Point

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Copyright McGraw-Hill Freezing-Point Depression (  T f ) –The freezing point of a solution, T f, will be lower than that of the pure solvent,. –Depression is directly proportional to the molal concentration. –K f is the molal freezing-point elevation constant.

Copyright McGraw-Hill Effect of Vapor Pressure Lowering Effect on Freezing Point

Copyright McGraw-Hill Calculate a) the freezing point and b) the boiling point of a solution containing 268 g of ethylene glycol and 1015 g of water. (The molar mass of ethylene glycol (C 2 H 6 O 2 ) is g/mol. K b and K f for water are 0.512°C/m and 1.86°C/m, respectively.)

Copyright McGraw-Hill a) freezing point

Copyright McGraw-Hill b) boiling point

Copyright McGraw-Hill Osmosis - Selective passage of solvent molecules through a semipermeable membrane solventsolution solvent solution

Copyright McGraw-Hill Osmotic Pressure (  ) –Pressure required to stop osmosis –Directly proportional to molar concentration, M  = MRT where R is L. atm/mol. K and T is in kelvins

Copyright McGraw-Hill Solutions of Electrolytes –Dissociation of strong and weak electrolytes affects the number of particles in a solution –van’t Hoft factor (i) – accounts for the effect of dissociation

Copyright McGraw-Hill Modified Equations for Colligative Properties

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Copyright McGraw-Hill Formation of ion pairs affects colligative properties

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Copyright McGraw-Hill The freezing-point depression of a m MgSO 4 solution is 0.225°C. Determine the experimental van’t Hoff factor of MgSO 4 at this concentration.

Copyright McGraw-Hill One approach: Note, at this concentration the dissociation of MgSO 4 is not complete.

Copyright McGraw-Hill Ideal freezing point depression Compare ideal and real freezing point depression Another approach to the same problem:

Copyright McGraw-Hill A solution made by dissolving 25.0 mg of insulin in 5.00 mL of water has an osmotic pressure of 15.5 mmHg at 25°C. Calculate the molar mass of insulin. (Assume that there is no change in volume when the insulin is added to the water and that insulin is a nondissociating solute.)

Copyright McGraw-Hill Calculate the M of the solution

Copyright McGraw-Hill Calculate the moles of insulin Molar mass is ratio of grams to moles

Copyright McGraw-Hill Colloids A colloid is a dispersion of particles of one substance throughout another substance. Intermediate between a homogenous and heterogeneous mixture Range of particle size: 10 3 – 10 6 pm Categories –Aerosols –Foams –Emulsions –Sols –Gels

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Aerogel Lowest density solids known Good thermal, electric, sound insulators Copyright McGraw-Hill

Copyright McGraw-Hill A 2.5-kg brick supported by a 2-g piece of aerogel (source: Wikipedia)

Copyright McGraw-Hill Exhibit the Tyndall effect colloidsolution

Copyright McGraw-Hill Fog – A Familiar Manifestation of the Tyndall Effect

Copyright McGraw-Hill Many important colloids are aqueous and can be further classified. –Hydrophilic (water loving) –Hydrophobic (water fearing) Hydrophilic groups on the surface of a protein stabilize the molecule in water

Copyright McGraw-Hill Stabilization of Hydrophobic Colloidal Particles by Ion Adsorption

Copyright McGraw-Hill Removal of Grease by Soap (Sodium Stearate)

Copyright McGraw-Hill Emulsification – stabilization of an unstable colloid accomplished by the addition of an emulsifier or emulsifying agent Sodium glycoholate – a bile salt or biological emulsifying agent for ingested fats

Copyright McGraw-Hill Lecithin is a mixture of substances found in natural sources such as egg yolk. It contains molecules such asphosphatidylcholine (shown right) that have both hydrophobic (nonpolar) and hydrophilic (polar; charged) regions. Lecithin acts as an emulsifier. This is why egg yolk is used to make mayonnaise. The egg yolk is typically whisked with a small amount of vinegar or lemon juice (the aqueous phase). Vegetable oil is then added gradually while whisking. Mayonnaise is the resulting emulsion of vegetable oil and aqueous acid.