Properties of Solutions. Classification of Matter Solutions are homogeneous mixtures.
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Transcript of Properties of Solutions. Classification of Matter Solutions are homogeneous mixtures.
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