Chap 13: PROPERTIES of SOLUTIONS Describe the solution process in terms of solutesolvent interactions and thermodynamics Calculate solution concentrations in terms of molarity, molality, and percentaes Understand the influence of polarity, pressure, and temperature on solubility Understand and predict the effect of concentration on colliative properties REVIEW 4.5 13.1 TYPES of SOLUTIONS SOLUTION Homoeneous mixture of of at at least 2 components: the (in (in lesser quantity) and the SOLVENT (in (in reater quantity) SOLUTION STATE SOLVENT SEA WATER CLUB SODA 14-karat GOLD Pd HYDRIDE liquid liquid solid solid H 2 O H 2 O Au Pd NaCl CO 2 Cu H 2 AIR as N 2 O 2 SUGAR CORN OIL ETHYL ALCOHOL SOLUBLE Dissolves to to a sinificant extent in in a solvent more suar! NON 2 phases NON 2 phases more alcohol! INSOLUBLE Does not dissolve to to a sinificant extent in in a solvent MISCIBLE Liquids soluble in in all all proportions Opposite: IMMISCIBLE Derees of Solution Nitroen Potassium Chloride Calcium Carbonate Ascorbic Acid Ethyl Alcohol Sucrose = miscible FORMULA N 2 0.0019 KCl 29 CaCO 3 0.0012 C 6 H 8 O 6 33 C 2 H 5 OH SOLUBILITY /100 H 2 O C 12 H 22 O 11 179 An UNSATURATED solution contains less than the maximum amount of solute that can be dissolved under existin conditions SOLVENT SOLUTION A SATURATED solution contains the maximum amount of solute that can be dissolved SOLVENT SOLUTION A SUPERSATURATED solution temporarily contains excess dissolved solute Excess solid solute may come out of solution by CRYSTALLIZATION SOLVENT SOLUTION Pae 13-1
Like Dissolves Like CH 3 OH SOLUBILITY mol/k H 2 O CH 2 CH 2 CH 2 CH 2 OH 1.1 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH 0.058 water-like polar H-bondin hydrocarbon-like nonpolar dispersion Substances with with similar types of of interparticle forces dissolve in in each other. The Role of Interparticle Interactions The physical state of the solvent determines the physical state of the solution Forces established between solvent and solute must be comparable in strenth to interparticle forces disrupted in both solvent and solute ( 13.2) Ex: Gases are miscible with each other (dispersion forces only) Ex: Hydrocarbon liquids are miscible in each other (dispersion forces only) Ex: Low-MW alcohols are miscible with water (H-bondin predominates in both solvent and solute) 13.2-3 THE SOLUTION PROCESS Dissolvin Ionic Compounds Solute particles separate: Reduction of interparticle attractions (ion-ion, dipole-dipole, H- bonds, dispersion forces) amon solute particles ( H solute > 0) Solvent particles separate ( H solvent > 0) Solute particles and solvent particles mix ( H mixin < 0) When an ionic solid dissolves in water, the polar solvent removes ions from the crystal lattice: Solvated ion H soln = H solute H solvent H mixin H soln = H solute H solvation SOLVATION: The The process of of surroundin a solute particle with with solvent particles Dissolvin Covalent Compounds Covalent compounds do not dissociate: THE SOLUTION PROCESS, cont. As a solid dissolves, some solvated solute particles collide, associate, and recrystallize. As lon as rate soln > rate xtln, more solute dissolves. The solution process eventually reaches equilibrium: rate soln = rate xtln Solute (undissolved) solvent Solute (dissolved) The solution at this point is SATURATED, it contains the maximum amount of dissolved solute. SUPERSATURATED solutions [Solute] > [Solute] equil are unstable. Pae 13-2
Environmental Effects PHASE SOLID LIQUID GAS EFFECT on SOLUBILITY T > 0 P > 0 increases varies decreases Henry s Law (as solute): C solute = k H P solute where P solute is partial pressure of the as above the solution. no chane no chane increases 13.4 QUANTIFYING CONCENTRATION When describin a solution, we need a way to tell how much solute is present We refer to the solute s CONCENTRATION Concentrations may be described in terms of Molarity (M, mol/l) Molality (m, mol/k) Parts by mass (w/w) Parts by volume (v/v) Mole fraction (χ, mol solute /mol solute mol solvent ) [X] read as as the concentration of of X X MOLARITY MOLALITY Mass of of solute (mol) M = Volume of of solution (L) Moles of of solute (mol) m = Mass of of solvent (k) Accounts for differences in formula weihts: Equal volumes of 1M ( 1 molar ) lucose and 1M ethyl alcohol contain the same number of molecules Temperature independent M m for dilute solutions Concentrations Expressed as Percents 13.5 COLLIGATIVE PROPERTIES TYPE WEIGHT/WEIGHT PERCENT* UNITS Mass, SOLUTION UNITS Mass, FORMULA w solute w solution Properties that depend on on the number of of solute particles Depends on how much, but not what type of solute WEIGHT/VOLUME PERCENT Weiht, w solute v solution Contribution of electrolytes (ionic materials) is based on the effective number of ions produced VOLUME/VOLUME PERCENT * ppm = 10 10 6 w solute /w solute /w solution solution v solute v solution VAPOR PRESSURE LOWERING BOILING POINT ELEVATION FREEZING POINT DEPRESSION OSMOTIC PRESSURE Pae 13-3
Vapor Pressure Lowerin This colliative property is described by Raoult s Law: Solution vapor pressure decreases in proportion to the concentration of solute. Boilin Point Elevation The manitude of boilin point elevation increases with increasin solute concentration: mole fraction* * 5.4 P solvent = χ solvent P solvent P solvent =(1 χ solute ) P solvent P = χ solute P solvent T b = K b m EXAMPLES: Makin maple syrup Antifreeze Solvent K b, C/m CCl 4 5.03 CHCl 3 3.63 CH 3 CH 2 OH 1.22 H 2 O 0.51 Freezin Point Depression Osmosis The manitude of freezin point depression increases with increasin solute concentration: T f = K f m EXAMPLES: Saltin roads in winter Makin ice cream Antifreeze Diffusion of of a solvent throuh a semipermeable membrane from a more dilute solution to to a more concentrated one SEMIPERMEABLE MEMBRANE Small molecules (such as solvent) o throuh, lare molecules (such as hydrated ions) do not RESULT: PRESSURE due to one-way solvent flow Cell walls are semipermeable membranes Semipermeable Membrane SEMIPERMEABLE MEMBRANE Cl Na Na Cl PURE ELECTROLYTE SOLVENT SOLUTION Water flows uphill until pressure equalized Pae 13-4
Osmotic Pressure Ionic vs Covalent Compounds in Solution Pressure required to to stop osmosis 1 mol NaCl 1 mol lucose Π = MRT OSMOLARITY (mol particles/l) ~1 mol of Na ~1 mol of Cl ~2 mol of PARTICLES 1 mol of PARTICLES Colliative Properties of Non-Ideal Solutions Observed colliative properties are less than many calculated values most prominently for solutions of stron electrolytes. Ex: For 0.100 m NaCl, T f (obsd) = 0.372 C < T f (calcd) = 0.384 C Difference results from ion pair formation: Result: < 2 mol particles per mol NaCl Cl Na Similar behavior (stackin interactions) by certain nonelectrolytes: M obsd > M calcd van t Hoff Factor (i) The ratio of an observed colliative property to the value calculated assumin the substance to be a nonelectrolyte: Property( obsd ) i = Property( calcd ) It s concentration, chare dependent: Cmpd Sucrose NaCl K 2 SO 4 MSO 4 [X], m 10 1 10 2 10 3 0 1.87 2.32 1.21 1.94 2.70 1.53 1.97 2.84 1.82 2.00 3.00 2.00 Incorporated into colliative property equations: P = i(χ solute P solvent ) T b = i(k b m) T f = i(k f m) Π = imrt The effective number of of solute particles determines colliative properties. Pae 13-5