Chapter 13 Properties Of Solutions

Chemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.;and Bruce E. BurstenChapter 13Properties of SolutionsSolutions

Solutions Solutions are homogeneous mixtures of twoor more pure substances. In a solution, the solute is dispersed uniformlythroughout the solvent.Solutions

SolutionsThe intermolecularforces between soluteand solvent particlesmust be strong enoughto compete with thosebetween solute particlesand those betweensolvent particles.Solutions

How Does a Solution Form?As a solution forms, the solvent pulls soluteparticles apart and surrounds, or solvates,them.Solutions

How Does a Solution FormIf an ionic salt issoluble in water, it isbecause the iondipole interactionsare strong enoughto overcome thelattice energy of thesalt crystal.Solutions

Energy Changes in Solution Simply put, three processesaffect the energetics of theprocess: Separation of solute particlesΔH1( this is alwaysendothermic) Separation of solventparticles ΔH2 ( this too isalways endothermic) New interactions betweensolute and solvent ΔH3 ( thisis always exothermic)The overall enthalpy changeassociated with these threeprocesses :ΔHsoln ΔH1 ΔH2 ΔH3Solutions

The process solution formation can beeither endo or exothermic Hot packs use MgSO4 and cold packsuse NH4NO3 and water.Solutions

The solvent solute interactions must bestrong enough to make ΔH3 comparablein magnitude to ΔH1 ΔH2 So NaCl will not dissolve in nonpolarliquids as the attraction between theions and the nonpolar solvent will notcompensate for the energies required toseparate the ions.Solutions

Energy Changes in SolutionThe enthalpychange of theoverall processdepends on H foreach of these steps.Solutions

Why Do EndothermicProcesses Occur?Things do not tend to occurspontaneously (i.e., without outsideintervention) unless the energy of thesystem is lowered.Solutions

Why Do EndothermicProcesses Occur?Yet we know that insome processes,like the dissolutionof NH4NO3 in water,heat is absorbed,not released.Solutions

Enthalpy Is Only Part of the PictureThe reason is thatincreasing the disorderor randomness (knownas entropy) of a systemtends to lower theenergy of the system.Solutions

Enthalpy Is Only Part of the PictureSo even thoughenthalpy may increase,the overall energy ofthe system can stilldecrease if the systembecomes moredisordered.Solutions

The process occurring at a constanttemperature in which the randomness inspace or the entropy of the systemincreases tend to occur spontaneouslySolutions

Student, Beware!Just because a substance disappears when itcomes in contact with a solvent, it doesn’tmean the substance dissolved.Solutions

Student, Beware! Dissolution is a physical change—you can get back theoriginal solute by evaporating the solvent. If you can’t, the substance didn’t dissolve, it reacted. The above example is the reaction of nickel with HClresulting in the formation of NiCl2Solutions

Types of Solutions Saturated Solvent holds as muchsolute as is possible atthat temperature. Dissolved solute is indynamic equilibriumwith solid soluteparticles.Solutions

Types of Solutions Unsaturated Less than themaximum amount ofsolute for thattemperature isdissolved in thesolvent.Solutions

Types of Solutions Supersaturated Solvent holds more solute than is normallypossible at that temperature. These solutions are unstable; crystallization canusually be stimulated by adding a “seed crystal” orscratching the side of the flask.Solutions

Solubility, saturation and supersaturationAs you add more solute to a solutionBelow saturation Solvent is not yetholding themaximum amountof solute it can atthat temperature More solute couldstill dissolve – ifyou add more, it willmix inAt saturation point Solvent is holdingthe maximumamount of solute itcan at thattemperature If you add any moresolute, it will notdissolve (will fall tothe bottom –precipitate) Solubility oftenmeasured in gramsof solute per 100mL of solventBeyond saturation Solvent is holdingmore solute than itis able to at thattemperature –situation is unstable If you add anythingto the solution, theexcess (beyondsaturation point)will crystallize outSolutions

Factors Affecting SolubilitySolute Solvent Interaction Chemists use the axiom “like dissolveslike”: Polar substances tend to dissolve in polarsolvents. Nonpolar substances tend to dissolve innonpolar solvents.Solutions

The more similar theintermolecularattractions, the morelikely one substanceis to be soluble inanother.Solutions

Glucose (which hashydrogen bonding)is very soluble inwater, whilecyclohexane (whichonly has dispersionforces) is not.Solutions

Vitamin A is soluble in nonpolar compounds(like fats). Vitamin C is soluble in water.Solutions

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Gases in Solution In general, the solubility ofgases in water increaseswith increasing mass asthe attraction between thegas and the solventmolecule is mainlydispersion forces. Larger molecules havestronger dispersionforces.Solutions

Effect of Pressure The solubility of solids and liquids is notaffected by pressure. However the solubility of gases isgreatly affected by pressure.Solutions

Gases in Solution The solubility of agas in a liquid isdirectly proportionalto its pressure.Solutions

Henry’s LawSg α PgSg kPgwhere Sg is the solubility ofthe gas; k is the Henry’s lawconstant for that gas inthat solvent at thattemperature Pg is the partialpressure of the gasabove the liquid.Solutions

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Calculate the concentration of CO2 in a soft drink that is bottled with a partialpressure of CO2 of 4.0 atm over the liquid at 25 C. The Henry’s lawconstant for CO2 in water at this temperature is 3.1 10–2 mol/L-atm.Page 542:2Solutions

Calculate the concentration of CO2 in a soft drink after the bottle is openedand equilibrates at 25 C under a CO2 partial pressure of 3.0 10–4 atm.The Henry’s law constant for CO2 in water at this temperature is3.1 10–2 mol/L-atm.Solutions

TemperatureGenerally, thesolubility of solidsolutes in liquidsolvents increaseswith increasingtemperature.Solutions

Temperature The opposite is trueof gases: Carbonated softdrinks are more“bubbly” if stored inthe refrigerator. Warm lakes haveless O2 dissolved inthem than cool lakes.Solutions

How to read a solubility curve1.How much KCl would be able to dissolvein 100 g of water at 50ºC?2. At 40ºC, exactly 64 g of an unknownsalt dissolved in 100 g of water.What is the likely identity of theunknown?3. At what temperature could you fullydissolve 80 g of NaCl in 200 g ofwater?4. At 60ºC, 30 g of KClO3 aredissolved in 100 g of water. Is thesolution undersaturated, saturatedor supersaturated?Solutions

How would this affect the marine life ifthe water temperature goes up .Solutions

Ways of Expressing Concentrationsof SolutionsSolutions

There are many ways to expressconcentration mathematically Weight percent mass of component per total mass(expressed as a percentage) Mole fraction moles of component per total moles(expressed as a value between 0 and 1) Molarity moles of solute per liter of solution Molality moles of solute per kg of solventSolutions

Mass Percentagemass of A in solution 100Mass % of A total mass of solutionSolutions

Parts per Million andParts per BillionParts per Million (ppm)For dilute solutionsppm mass of A in solutiontotal mass of solution 106Parts per Billion (ppb)For even more dilute solutionsppb mass of A in solutiontotal mass of solution 109Solutions

(a) A solution is made by dissolving 13.5 g of glucose(C6H12O6) in 0.100 kg of water. What is the masspercentage of solute in this solution?Solutions

A 2.5-g sample of groundwater was found to contain5.4 μg of Zn 2 What is the concentration of Zn 2 inparts per million?Solutions

Mole Fraction (X)moles of AXA total moles in solution In some applications, one needs the molefraction of solvent, not solute—make sure youfind the quantity you need! A in that case will be the solvent.Solutions

A solution is made by dissolving 13.5 g of glucose (C6H12O6) in 0.100 kgof water. What is the mass percentage of solute in this solution?b. A 2.5-g sample of groundwater was found to contain 5.4 μg of Zn 2 What is the concentration of Zn 2 in parts per million?Solutions

(a) Calculate the mass percentage of NaCl in a solution containing 1.50 gof NaCl in 50.0 g of water. (b) A commercial bleaching solutioncontains 3.62 mass % sodium hypochlorite, NaOCl. What is the massof NaOCl in a bottle containing 2500 g of bleaching solution?Solutions

Molarity (M)M mol of soluteL of solution You will recall this concentrationmeasure from Chapter 4. Because volume is temperaturedependent, molarity can change withtemperature.Solutions

Molality (m)m mol of solutekg of solventBecause both moles and mass do notchange with temperature, molality(unlike molarity) is not temperaturedependent.Solutions

Example (from p. 546)A commercial bleach solution contains 3.62 mass % NaOCl in water. Calculate(a)the molality,(b)the mole fraction of NaOCl,(c)the mole fraction of H2O in the solution.Solutions

Changing Molarity to MolalityIf we know thedensity of thesolution, we cancalculate themolality from themolarity, and viceversa.Solutions

A solution contains 5.0 g of toluene(C7H8) and 225 g of benzene andhas a density of 0.876 g/mL.Calculate the molarity of thesolution.Solutions

Colligative Properties Changes in colligative propertiesdepend only on the number of soluteparticles present, not on the identity ofthe solute particles. How would the ionic compounds andcovalent compounds behave .Solutions

Among colligative properties are Vapor pressure lowering Boiling point elevation Melting point depression Osmotic pressureSolutions

Vapor PressureBecause of solutesolvent intermolecularattraction, higherconcentrations ofnonvolatile solutesmake it harder forsolvent to escape tothe vapor phase.Solutions

Vapor PressureTherefore, the vaporpressure of a solutionis lower than that ofthe pure solvent.Solutions

Raoult’s Law The partial pressure exerted by solventvapor above the solution, PA, equals tothe product of the mole fraction of thesolvent in the solution, Xa, times thevapor pressure of the pure solvent.Solutions

Raoult’s LawPA XAP Awhere XA is the mole fraction of compound A P A is the normal vapor pressure of A atthat temperatureNOTE: This is one of those times when youwant to make sure you have the vaporpressure of the solvent.Solutions

Glycerin (C3H8O3) is a nonvolatile nonelectrolyte with a density of 1.26g/mL at 25 C. Calculate the vapor pressure at 25 C of a solution madeby adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressureof pure water at 25 C is 23.8 torr (Appendix B).Solutions

The vapor pressure of pure water at 110 C is 1070 torr. Asolution of ethylene glycol and water has a vapor pressure of1.00 atm at 110 C. Assuming that Raoult’s law is obeyed, whatis the mole fraction of ethylene glycol in the solutionSolutions

The people who attended today’sdiscussion please give me you namesor email it to me - today.Solutions

Only ideal solutions obey Raoult’s law.Real solutions best approximate ideal behavior when:a. Solute concentration is lowwhy?b. Solute and solvent have similar molecular sizeWhy?c. And they have similar type of intermolecularattractionsWhy?Solutions

When is Raoult’s law not obeyed:When the solution is not ideal.a. When the intermolecular forcesbetween the solvent-solvent andsolute-solute are stronger than theones between the solvent and solute.Then the vapor pressure would be higherthan predicted.Solutions

When the attraction between the solventand the solute is very strong then thevapor pressure would be lower than thatpredicted by the Raoul’s law.When do you expect a situation like thisto happen .Solutions

When the solvent and the solute startforming hydrogen bonds.Solutions

We will ignore these departures fromthe ideal solutions for this chapter whiledoing the numerical problems. But we still need to understand theconcept and be able to explain them inthe test.Solutions