Lecture 9 ETH Zurich Chemistry PDF

Lecture #9, p. 1 Lecture 9: Announcements Today: Brown Ch. 13 Properties of Solutions 13.1 The Solution Process 13.2 Saturated Solutions and Solubility 13.3 Factors Affectin...

Lecture #9, p. 1 Lecture 9: Announcements Today: Brown Ch. 13 Properties of Solutions 13.1 The Solution Process 13.2 Saturated Solutions and Solubility 13.3 Factors Affecting Solubility 13.4 Expressing Solution Concentration 13.5 Colligative Properties Problem Set 8: Due before Exercise #9 tomorrow; upload on Moodle link Problem Set 9: Posted on Moodle; due before Exercise #10 next week Study Center: Wednesdays 18:00–20:00 in ETA F 5 Office Hours: Prof. Norris and Brisby, Thursdays 17:00–18:00 in LEE P 210 Five practice exams posted. Use wisely! Chemistry  Lecture #9, p. 2 Lecture 10 Next Week: Brown Ch. 14 Chemical Kinetics I 14.1 Factors That Affect Reaction Rates 14.2 Reaction Rates 14.3 Concentration and Rate Laws 14.4 The Change of Concentration with Time Chemistry  Red Thread Properties Where have we been? How do molecules interact? Intro Atoms Molecules Gases Solutions Thermo Chemistry  Red Thread Where are we going? Acid-Base Catalysis Properties Christmas! Kinetics Batteries Equilibrium Chemistry  Lecture #9, p. 3 Review In Lecture 8, we discussed intermolecular interactions and liquids Intermolecular interactions: due to forces between molecules Van der Waals interactions: dispersion and dipole–dipole Molecular dipoles Hydrogen bonding Ion–dipole interactions, polar solvents Relative strengths of intermolecular interactions Properties of liquids: viscosity, surface tension, vapor pressure Phase changes Heat of fusion, heat of vaporization, heat of sublimation Heating curves Phase diagrams, critical point, triple point Clausius–Clapeyron equation Chemistry  Lecture #9, p. 4 Review Chemistry  Lecture #9, p. 5 Today: Properties of Solutions Lecture 3: Solution = solvent + solutes Homogeneous mixture Solvation process Initiii We focused on solvent = water Open questions: When do substances dissolve? How much dissolves? Properties of solutions? Today: address these questions! Chemistry  Lecture #9, p. 6 Thermodynamics of Solutions Now we have our knowledge of thermodynamics ∆"!"#$ Define process: solvent + solute solution ∆"!"#$ ≡ Change in Gibbs free energy upon forming solution i.e., mixing solute uniformly in solvent We now know: Process spontaneous if ∆"!"#$ < 0 Meaning solute dissolves Can re-write: ∆"!"#$ = ∆&!"#$ − (∆)!"#$ At fixed # and $ Typically: ∆)!"#$ > 0 But: ∆&!"#$ = ? Depends on intermolecular interactions solute–solute Disorder increases by How do the interactions solute–solvent compare? mixing solute in solvent solvent–solvent Chemistry  Lecture #9, p. 7 ∆"!"#$? Consider breaking the solvation process down into three steps... (i) (solvent)n n・solvent ∆"!"#$%&' Separate solvent (ii) (solute)m m・solute ∆"!"#('% Separate solute ＋ (iii) n・solvent + m・solute solution ∆")*+ Mix separated solvent and solute (solvent)n + (solute)m solution ∆"!"#& Total enthalpy change With ∆"!"#& = ∆"!"#$%&' + ∆"!"#('% + ∆")*+ Chemistry  Lecture #9, p. 8 Enthalpy Changes During Solvation Implications? Chemistry  Lecture #9, p. 9 ∆"!"#$ Note: ∆&!"#$ can be ＋ or － And since ∆)!"#$ = ∆&!"#$ − ,∆-!"#$... If ∆&!"#$ < 0 If ∆&!"#$ > 0 f stilen Typically, solution forms for −"∆$!"#$ must be sufficiently any solute concentration negative to make ∆%!"#$ < 0 pure pure Otherwise, solute will not dissolve solvent solute And solute is then insoluble Solute is completely miscible Process will depend on " Solvation more likely at high " exothermic endothermic Use? Chemistry miscible mixable  Lecture #9, p. 10 Ammonium Nitrate Cold Packs ∆"!"#$ > 0 Chemistry  Lecture #9, p. 11 How Much Dissolves? If solute dissolves in solvent, limit may exist for how much Solubility = max amount of solute dissolvable in solvent at a given temperature Any solute added beyond solubility concentration won’t dissolve Ex’s: Chemistry  Lecture #9, p. 12 Saturated Solution At or beyond maximum concentration of solute Note: at the solubility concentration, ∆"!"#$ = 0 Dynamic equilibrium is established: dissolve solvent + solute solution crystallize Any solute joining the solution is balanced by dissolved solute crystallizing If we change conditions, solubility concentration changes Solute dissolves or crystallizes until new dynamic equilibrium established Chemistry  Lecture #9, p. 13 Whatfavorshighsolubilityconcentrations Factors Affecting Solubility? Strong solute–solvent interactions favor high solubility concentrations Solids: Ions dissolve more easily in polar solvents Liquids: Polar liquids dissolve more easily in polar solvents Non-polar liquids are immiscible in polar solvents immiscible ≡ not mixable General rule: Substances with similar intermolecular interactions tend to be soluble in each other “like dissolves like” Chemistry  Lecture #9, p. 14 Other Factors Affecting Solubility? Pressure: Solubility of a gas in a liquid depends on !! Iiiiiii iiiit c !! ≡ gas partial pressure Gas solubility #! = %" !! %" ≡ Henry’s law constant Henry’s law ic andT Temperature: Ion solubilities in H2O increase with T ⇒ ∆##$%& > 0 Consider Gas solubilities in H2O decrease with T ⇒ ∆##$%& < 0 Trically Chemistry a a  Lecture #9, p. 15 Becauseconcentration is animportantparameterforsolutions weneedwaystoquantify wealreadylearned morefraction and molarity Others Expressions for Concentration? Qualitatively: “dilute” or “concentrated” Mole moles solute mass solute ! ≡ Mass % ≡ × 100 fraction total moles total mass moles solute mass solute " Molarity ≡ ppm ≡ × 10' PITP liters solution total mass Ii # Molality ≡ moles solute kg solvent ppb ≡ mass solute total mass × 10( Iii Chemistry hi i it aiiii.EE ntiti i t fsoiteiean solute ftp.i.tt fiiie n  Lecture #9, p. 16 Sohowdothepropertiesofliquidschange as we addsolute How Does Solute Affect Properties of the Solvent? Colligative properties Solute affects properties of liquid If change depends only on the amount of solute ⇒ “colligative” Effects bp boilingpain A. Boiling-point elevation #$%& ∆#!" = #!" − #!" "'() ) ≡ molality Eimples of solute = ' (! ) Iii (! ≡ molal bp elevation 1 for non-electrolytes constant ' = Number of ions produced for electrolytes ' ≡ van’t Hoff Ex: ' = 2 for NaCl factor ' = 3 for CaCl2 Chemistry at n etat stier especially Ex tip guff10 00 1 perliterofHao ng  Lecture #9, p. 17 Whydoestheboilingtemperaturechangewithsolute How Does Solute Affect Properties of the Solvent? Colligative properties B. Vapor-pressure lowering Recall: a liquid boils when its !./* = ! Consider: volatile solvent + non-volatile solute *+,- If the solution is an ideal solution ⇒ #$%& !./* = ,#$%.-&0 3 !./* Raoult’s law Vapor pressure mole fraction Vapor pressure Or in words: of solution = of solvent ⋅ of pure solvent Chemistry In solution  Ideal Gas versus Ideal Solution? What are our assumptions? Ideal gas No intermolecular interactions ⇒ !# =%&' Ideal solution solute–solute all assumed All intermolecular interactions are identical ⇒ solvent–solvent to be the solute–solvent same solvent–solvent If solute–solvent < ⇒ #$%&, /30+/% !./* #$%&, 4/$+%0 > !./* solute–solute solvent–solvent If solute–solvent > ⇒ #$%&, /30+/% !./* #$%&, 4/$+%0 < !./* solute–solute Chemistry  Lecture #9, p. 18 Ideal solution Whatassumptions are involved Let'scompare to idealgases Ideal Gas versus Ideal Solution? What are our assumptions? Ideal gas No intermolecular interactions ⇒ !# =%&' IFL Ideal solution solute–solute all assumed All intermolecular interactions are identical ⇒ solvent–solvent to be the solute–solvent same solvent–solvent If solute–solvent > ⇒ #$%&, +./'+% #*+" #$%&, 0+$'%/ < #*+" solute–solute i.EEatse alities solvent–solvent If solute–solvent < ⇒ #$%&, +./'+% #*+" #$%&, 0+$'%/ > #*+" solute–solute Chemistry IEEE's  Lecture #9, p. 19 How Does Solute Affect Properties of the Solvent? Colligative properties C. Freezing-point depression ( ≡ molality itiven #$%& ∆"!" = "!" "'() − "!" of solute Note = − & '! ( '! ≡ molal fp Fp free Equation is for dilute solutions that are ideal depression constant Uses: Anti-icing treatments & ≡ van’t Hoff Automotive anti-freeze factor Ex: 25% ethylene glycol in H2O '! = 1.86 °C/( for H2O ∆"!" = −1. 1.86 °C/(. 5.37 ( = −10 °C Chemistry Thisantifreezesolutionis5.37molal T.IE  Lecture #9, p. 20 How Does Solute Affect Properties of the Solvent? Colligative properties D. Osmosis Two solutions separated by “semi-permeable” membrane “Semi-permeable” ≡ passes only solvent, not solute Place pure solvent on Solution on WHAT one side of membrane other HAPPENS? More solvent flows left to right until equilibrium established “Osmotic flow” tries to equalize concentrations on both sides “Osmotic pressure” ≡ pressure needed to counteract osmotic flow Chemistry insolute  Lecture #9, p. 21 As a student I neverreally understood osmosis Butits lessmagicalthan it seems It justhastodowithhow theflowof solventacrossthemembranecompares onthesolutionside thesolutegetsinthewayandunbalancesflowofsolventmolecules Osmosis How can we quantify osmotic pressure? Chemistry  Lecture #9, p. 22 Osmotic Pressure Given a solution at temperature T in a volume V... Π ≡ osmotic pressure Eiii Π# = %&'( $ ≡ molarity, "⁄# Π = %)'( % ≡ van’t Hoff factor Two solutions with same Π ⇒ Isotonic Iiii Π > Π!"# ⇒ Hypertonic Π < Π!"# ⇒ Hypotonic Note: common lab use of Π ⇒ Determine $$ Mithila Chemistry Ref referencesolution  Lecture #9, p. 23 Determining Molecular Weight Problem: A solution containing 0.150 g of an enzyme in 210.0 mL of solution has an osmotic pressure of 0.127 kPa at 25 °C. What is the molar mass of the enzyme? !! Chemistry  Lecture #9, p. 24 Antifreeze Proteins (AFPs) “A class of polypeptides produced by certain animals, plants, fungi, and bacteria that permit their survival in temperatures below the freezing point of water.” —wikipedia.org Insect antifreeze protein, Tenebrio-type “Unlike the widely used automotive antifreeze, ethylene glycol, AFPs do not lower freezing point in proportion to concentration. “Rather, they work in a non-colligative manner. “This phenomenon allows them to act as an antifreeze at concentrations 1/300th to 1/500th of those of other dissolved solutes. “Their low concentration minimizes their effect on osmotic pressure. “The unusual properties of AFPs are attributed to their selective affinity for specific crystalline ice forms and the resulting blockade of the ice-nucleation process.” —wikipedia.org Chemistry Uses kinetics instead of thermodynamics NEXT TIME kinetics  Lecture #9, p. 25 What We Learned Thermodynamics of solutions: ∆"!"#$ , ∆#!"#$ , and ∆$!"#$ Solubility, miscible, immiscible, saturated solutions Factors affecting solubility: polarity, temperature, pressure Henry’s law: solubility of gases in liquids Expressions for concentration Colligative properties: Boiling-point elevation Vapor-pressure lowering Ideal solution, Raoult’s law Freezing-point depression Osmosis and osmotic pressure Chemistry