Properties of SolutionsProperties of Solutions

ClassificatioClassification of Mattern of Matter

Solutions are homogeneous mixtures

SoluteSoluteA solute is the dissolved substance in a solution.

A solvent is the dissolving medium in a solution.

SolvenSolventt

Salt in salt water Sugar in soda drinks

Carbon dioxide in soda drinks

Water in salt water Water in soda

Calculations of Solution Calculations of Solution ConcentrationConcentration

Mass percent - the ratio of mass (in grams) of solute to mass (in grams) of solution, expressed as a percent

100)(

)(x

gsolutionofmass

gsoluteofmasspercentMass

Calculations of Solution Calculations of Solution ConcentrationConcentration

Mass/volume (m/v) % - the ratio of mass (in grams) of solute to volume of solution (in mL), expressed as a percent

100)(

)()/%( x

mLsolutionofvolume

gsoluteofmassvm

Calculations of Solution Calculations of Solution ConcentrationConcentration

Volume/volume (v/v) % - the ratio of volume (in mL) of solute to volume of solution (in mL), expressed as a percent

( )%( / ) 100

( )

volumeof solute mLv v x

volumeof solution mL

Calculations of Solution Calculations of Solution ConcentrationConcentration

Mole fraction – the ratio of moles of solute to total moles of solution

BA

AA nn

nAoffractionMole

Calculations of Solution Calculations of Solution ConcentrationConcentration

Molarity (M) - the ratio of moles of solute to liters of solution

solutionofLiter

soluteofmolesMMolarity

Calculations of Solution Calculations of Solution ConcentrationConcentration

Normality (N) – moles of equilvalents/Liter of solution

_ _

_ _

moles of equivalentsNormality N

Liter of solution

Calculations of Solution Calculations of Solution ConcentrationConcentration

Molality (m) – moles of solute per kilogram of solvent

solventramloki

solutemolesmMolality

g

““Like Dissolves Like”Like Dissolves Like”

Fats BenzeneBenzene

SteroidsSteroids HexaneHexane

WaxesWaxes TolueneToluene

Inorganic Salts WaterWater

SugarsSugars Small alcoholsSmall alcohols

Acetic acidAcetic acid

Polar and ionic solutes dissolve best in polar solvents

Nonpolar solutes dissolve best in nonpolar solvents

Heat of SolutionHeat of SolutionThe Heat of Solution is the amount of heat energy absorbed (endothermic) or released (exothermic) when a specific amount of solute dissolves in a solvent.SubstanceSubstance Heat of Solution Heat of Solution

(kJ/mol)(kJ/mol)

NaOHNaOH -44.51-44.51

NHNH44NONO33 +25.69+25.69

KNOKNO33 +34.89+34.89

HClHCl -74.84-74.84

Steps in Solution FormationSteps in Solution Formation

H1 Expanding the solute

H2 Expanding the solvent

H3 Interaction of solute and solvent to form the solution

Separating the solute into individual components

Overcoming intermolecular forces of the solvent molecules

Enthalpy Changes in Solution

The enthalpy change of the overall process depends on H for each of these steps.

Start

End

EndStart

Why do endothermic processes sometimes occur spontaneously?

Some processes, like the dissolution of NH4NO3 in water, are spontaneous at room temperature even though heat is absorbed, not released.

Predicting Solution Predicting Solution FormationFormation

Solvent/ Solvent/

SoluteSoluteHH11 HH22 HH33 HHsol’nsol’n OutcomeOutcome

Polar/ Polar/

PolarPolar+ +

largelarge+ +

largelarge- -

largelarge+/-+/-

smallsmallSolution Solution

formsforms

Polar/ Polar/

NonpolaNonpolarr

+ + smallsmall

+ + largelarge

+/- +/- smallsmall

+ + largelarge

No No solution solution

formsforms

NonpolaNonpolar/ r/

NonpolaNonpolarr

+ + smallsmall

+ + smallsmall

+/- +/- smallsmall

+/- +/-

smallsmallSolution Solution

formsforms

NonpolaNonpolar/ r/

polarpolar

+ + largelarge

+ + smallsmall

+/- +/- smallsmall

+ + largelarge

No No solution solution

formsforms

Solubility TrendsSolubility Trends The solubility of MOST solids The solubility of MOST solids

increases with temperature. increases with temperature. The rate at which solids dissolve The rate at which solids dissolve

increases with increasing surface increases with increasing surface area of the solid. area of the solid.

The solubility of gases decreases The solubility of gases decreases with increases in temperature. with increases in temperature.

The solubility of gases increases The solubility of gases increases with the pressure above the with the pressure above the solution.solution.

Enthalpy Is Only Part of the Picture

Entropy is a measure of: • Dispersal of energy in

the system.• Number of microstates

(arrangements) in the system.

b. has greater entropy, is the favored state

(more on this in chap 19)

Therefore…Therefore…Solids tend to dissolve best when:

o Heated o Stirred o Ground into small particles

Gases tend to dissolve best when:o The solution is cold

o Pressure is high

Saturation of SolutionsSaturation of Solutions A solution that contains the maximum A solution that contains the maximum

amount of solute that may be dissolved amount of solute that may be dissolved under existing conditions is under existing conditions is saturatedsaturated. .

A solution that contains less solute than A solution that contains less solute than a saturated solution under existing a saturated solution under existing conditions is conditions is unsaturatedunsaturated. .

A solution that contains more dissolved A solution that contains more dissolved solute than a saturated solution under solute than a saturated solution under the same conditions is the same conditions is supersaturatedsupersaturated..

Degree of saturation

• SupersaturatedSolvent holds more solute than is normally

possible at that temperature.These solutions are unstable; crystallization can

often be stimulated by adding a “seed crystal” or scratching the side of the flask.

Solubility ChartSolubility Chart

Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

Why?• Larger molecules

have stronger dispersion forces.

Gases in Solution

• The solubility of liquids and solids does not change appreciably with pressure.

• But, the solubility of a gas in a liquid is directly proportional to its pressure.

Increasing pressure above solution forces more gas to dissolve.

Temperature• Higher temperature

drives gases out of solution.

Carbonated soft drinks are more “bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in them than cool lakes.

Henry’s LawHenry’s Law

The concentration of a dissolved gas in a solution is directly proportional to the pressure of the gas above the solution

kPCApplies most accurately for dilute solutions of gases that do not dissociate or react with the solvent

Yes CO2, N2, O2

No HCl, HI

Colligative Properties

• Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles.

• Among colligative properties areVapor pressure lowering Boiling point elevationMelting point depressionOsmotic pressure

Vapor Pressure

As solute molecules are added to a solution, the solvent becomes less volatile (=decreased vapor pressure).

Solute-solvent interactions contribute to this effect.

Raoult’s LawRaoult’s Law

The presence of a nonvolatile solute lowers the vapor pressure of the solvent. 0

solventsolventsolution PP Psolution = Observed Vapor pressure of the solution

P0solvent = Vapor pressure of the pure solvent

solvent = Mole fraction of the solvent

Liquid-liquid solutions in which Liquid-liquid solutions in which both components are volatileboth components are volatile

Modified Raoult's Law: Modified Raoult's Law:

00BBAABATOTAL PPPPP

P0 is the vapor pressure of the pure solvent PA and PB are the partial pressures

Colligative Properties of Electrolytes

Because these properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) show greater changes than those of nonelectrolytes.

e.g. NaCl dissociates to form 2 ion particles; its limiting van’t Hoff factor is 2.

The van’t Hoff Factor, The van’t Hoff Factor,

ii

Electrolytes may have two, three or more times the effect on boiling point, freezing point, and osmotic pressure, depending on its dissociation.

Dissociation Equations and Dissociation Equations and the Determination of the Determination of ii

NaCl(s)

AgNO3(s) MgCl2(s)

Na2SO4(s)

AlCl3(s)

Na+(aq) + Cl-(aq)

Ag+(aq) + NO3-(aq)

Mg2+(aq) + 2 Cl-(aq)

2 Na+(aq) + SO42-

(aq)Al3+(aq) + 3 Cl-(aq)

i = 2

i = 2

i = 3

i = 3

i = 4

van’t Hoff Factor

One mole of NaCl in water does not really give rise to two moles of ions.

van’t Hoff Factor

Some Na+ and Cl− reassociate as hydrated ion pairs, so the true concentration of particles is somewhat less than two times the concentration of NaCl.

The van’t Hoff Factor

• Reassociation is more likely at higher concentration.

• Therefore, the number of particles present is concentration dependent.

Boiling Point Elevation and Freezing Point Depression

Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

Boiling Point Elevation and Freezing Point Depression

In both equations, T does not depend on what the solute is, but only on how many particles are dissolved.

Tb = Kb i m

Tf = Kf i m

Boiling Point ElevationBoiling Point Elevation

Each mole of solute particles raises the boiling point of 1 kilogram of water by 0.51 degrees Celsius.

Kb = 0.51 C kilogram/mol

soluteb mKiT

m = molality of the solution

i = van’t Hoffvan’t Hoff factor

Boiling Point Elevation

The change in boiling point is proportional to the molality of the solution:

Tb = Kb i m

where Kb is the molal boiling point elevation constant, a property of the solvent.

Tb is added to the normal boiling point of the solvent.

Freezing Point DepressionFreezing Point Depression

Each mole of solute particles lowers the freezing point of 1 kilogram of water by 1.86 degrees Celsius.

Kf = 1.86 C kilogram/mol

solutef mKiT

m = molality of the solution

i = van’t Hoffvan’t Hoff factor

Freezing Point Depression

• The change in freezing point can be found similarly:

Tf = Kf i m

• Here Kf is the molal freezing point depression constant of the solvent.

Tf is subtracted from the normal freezing point of the solvent.

Freezing Point Depression and Boiling Point Elevation Constants,

C/m

Solvent Kf Kb

Acetic acid 3.90 3.07

Benzene 5.12 2.53

Nitrobenzene 8.1 5.24

Phenol 7.27 3.56

Water 1.86 0.512

Osmotic PressureOsmotic Pressure

The minimum pressure that stops the osmosis is equal to the osmotic pressure of the solution

Osmotic Pressure

• The pressure required to stop osmosis, known as osmotic pressure, , is n

V = i( )RT = iMRT

where M is the molarity of the solution

If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic.

i i

Molar Mass from Colligative Properties

We can use the effects of a colligative property such as osmotic pressure to determine the molar mass of a compound.

K

Suspensions and Suspensions and ColloidsColloids

Suspensions and colloids are NOT solutions. Suspensions: The particles are so large that they settle out of the solvent if not constantly stirred.

Colloids: The particles intermediate in size between those of a suspension and those of a solution.

Types of ColloidsTypes of Colloids

ExamplesExamples DispersinDispersing g

MediumMedium

Dispersed Dispersed

SubstancSubstancee

Colloid TypeColloid Type

Fog, aerosol spraysFog, aerosol sprays GasGas LiquidLiquid AerosolAerosol

Smoke, airborn germsSmoke, airborn germs GasGas SolidSolid AerosolAerosol

Whipped cream, soap Whipped cream, soap sudssuds

LiquidLiquid GasGas FoamFoam

Milk, mayonnaiseMilk, mayonnaise LiquidLiquid LiquidLiquid EmulsionEmulsion

Paint, clays, gelatinPaint, clays, gelatin LiquidLiquid SolidSolid SolSol

Marshmallow, StyrofoamMarshmallow, Styrofoam SolidSolid GasGas Solid FoamSolid Foam

Butter, cheeseButter, cheese SolidSolid LiquidLiquid Solid Solid EmulsionEmulsion

Ruby glassRuby glass SolidSolid SolidSolid Solid solSolid sol