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Solutions
Unit 12:Properties of Solutions
Dr. Jorge L. AlonsoMiami-Dade College –
Kendall CampusMiami, FL
Textbook Reference: •Chapter # 14•Module # 2
CHM 1046: General Chemistry and Qualitative Analysis
Solutions
Solutions
• Solutions (soln) are homogeneous mixtures of two or more pure substances.
• The solvent (solv) is present in greatest abundance.
• All other substances are solutes (solu).
Volumetric flask
{PrepASolu}
Solutions
Solutions• Solutions are homogeneous mixtures
of two or more pure substances.• In a solution, the solute (< 50%) is
dispersed uniformly throughout the solvent (> 50%).
Homogeneous Heterogeneous
Solutions
Student, Beware!: solution vs. reaction
Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved.
• Dissolution is a physical change—you can get back the original solute by evaporating the solvent.
• If you can’t, the substance didn’t dissolve, it reacted.
Mg (s) + 2 HCl (aq) H2 (g) + MgCl2 (aq)Cu(NO3)2 (s) Cu(NO3)2 (aq) H2O
H2O
Solutions
How Does a Solution of Salts in Water Form?
NaCl (s) Na+ (aq) + Cl- (aq)dissociation
+ H20
The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.
Example: {NaCl + H2O*}
Solutions
Solubility in water
Covalent Compounds: many are insoluble gases, polar covalent are mostly soluble.
Ionic Compounds: many are soluble.SOLUBILITY RULES: for Ionic Compounds (Salts)1. All salts of alkali metals (IA) are soluble.2. All NH4
+ salts are soluble.
3. All salts containing the anions: NO3-, ClO3
-, ClO4-, (C2H3O2
-) are soluble.
4. All Cl-, Br-, and I- are soluble except for Ag+, Pb2+, and Hg22+ salts.
5. All SO42- are soluble except for Pb2+, Sr2+, and Ba2+.
6. All O2- are insoluble except for IA metals Ca2+, Sr2+, and Ba2+ salts.{Soluble metal oxides form hydroxides: CaO Ca 2+ + 2OH-}
7. All OH- are insoluble except for IA metals, NH4+ & slightly soluble Ca 2+ Ba2+ & Sr2+
6. All salts containing the anions: CO32-, PO4
3-, AsO43-, S2- and SO3
2- are insoluble except fro IA metals and NH4
+ salts. 7. For salts containing the anions not mentioned above (e.g., CrO4
2-, Cr2O72-, P3-,
C2O42- etc.) assume that they are insoluble except for IA metals and NH4
+ salts, unless, otherwise informed.
H2O
Elements: mostly insoluble solids, liquids & gases.
Solutions
Energetics of Solutions Heats (Enthalpy) of Solution (Hsoln)
Exothermic solution process
NH4NO3 (s) NH4+ (aq) + NO3
-(aq)
CaCl2 (s) Ca 2+(aq) + 2Cl- (aq)
H2O
Endothermic solution process
CaCl2+ H2O
NH4NO3
+H2O
Hsoln = - 81.3 kJ/mol
+ Heat
Hsoln = + 25.7 kJ/mol
Heat +H2O
Hot & Cold Packs
Solutions
(3) Formation of Ion-dipole
interactions
How Does a Solution Form? (3 events)
If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal.
H Lattice Energy
+788 kJ/mol Endothermic
NaCl
Crystal Lattice
H- bonding
H2OHvap
+41 kJ/mol Endothermic
Energy of Solution Hsoln = + or -
(1) Separation of solute molecules
(2) Separation of solvent molecules
Exothermic
Solutions
Energy Changes in SolutionThe enthalpy change of the overall process depends on H for each of these steps.
{EnerSoln}
- +
Solutions
Exothermic solution process
Endothermic solution process
NH4NO3 (s) NH4+ (aq) + NO3
-(aq)
Hsoln = + 25.7 kJ/mol
CaCl2 (s) Ca 2+ (aq) + 2Cl- (aq)
Hsoln = - 81.3 kJ/mol
H2O
H2O
Why do Endothermic Reactions occur Spontaneously?
Solutions
Concentration of Solutions (General)
• SaturatedSolvent holds as much solute as is
possible at that temperature.Dissolved solute is in dynamic
equilibrium with solid solute particles.
• UnsaturatedLess than the maximum amount of
solute for that temperature is dissolved in the solvent.
{UnsatSoln}
Solubility of NaCl in H2O = 35.9 g/100 mL (25 °C)
NaC2H4O2 = 76 g/100 ml (0°C)
Solutions
• SupersaturatedSolvent holds more solute than is normally
possible at that temperature.These solutions are unstable; crystallization can
usually be stimulated by adding a “seed crystal” or scratching the side of the flask.
{*SuperSat1} {*SuperSat2}
Concentration of Solutions (General)
Solutions
1. Polarity2. Temperature
Factors Affecting Solubility
A. Solids and Liquids:
B. Gases :1. Molar Mass2. Pressure – Henry’s Law3. Temperature
Solutions
{Water + Oil}
Factors Affecting Solubility of Solids and Liquids: (1) Polarity
• Chemists use the axiom “like dissolves like”:Polar substances tend to dissolve in polar solvents.Nonpolar substances tend to dissolve in nonpolar solvents.
[Know molecular shapes and polar molecules]
(hydrocarbon chain)hydrophobic water-
hating, nonpolar,
hydrophilic water-loving, polar,
Solutions
• Chemists use the axiom “like dissolves like”:Polar substances tend to dissolve in polar solvents.Nonpolar substances tend to dissolve in nonpolar solvents.
Factors Affecting Solubility of Solids and Liquids: (1) Polarity
[Know molecular shapes and polar molecules]
Polar heads
Non-Polar tails
Polar Non-Polar
Solutions
Glucose (which has hydrogen bonding) is very soluble in water, while…..
Factors Affecting Solubility of Solids and Liquids: (1) Polarity
…..cyclohexane (which only has dispersion forces) is not.
Solutions
• Vitamin A is soluble in nonpolar compounds (like fats).
• Vitamin C is soluble in water. {*Iodine + Water then + CCl4 }
Factors Affecting Solubility of Solids and Liquids: (1) Polarity
Solutions
Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.
Factors Affecting Solubility of Solids and Liquids: (2) Temperature
Solutions
Factors Affecting Solubility of Gases in Solution: (1) Molar Mass
• Larger molecules have stronger London dispersion forces.
•In general, the solubility of gases in water increases with increasing molar mass.
g-MM
28 g/n
28 g/n
32 g/n
40 g/n
84 g/n
Why?
Which gas dissolves best in water?
Solutions
Factors Affecting Solubility of Gases in Solution:
(2) Pressure
{Henrys Law}
• The solubility of liquids and solids does not change appreciably with pressure.
• The solubility of a gas (Sg) in a liquid is directly proportional to its pressure (Pg).
Sg Pg α
Solutions
Henry’s Law:
where
• Sg is the solubility of the gas;
• Pg is the partial pressure of the gas above the liquid.
• k is the Henry’s law constant for that gas in that solvent;
Sg = kPg
Factors Affecting Solubility of Gases in Solution: (2) Pressure
@ 250C, PN2= 0.75 atm, SN2= 0.00053 M.
What is SN2 when PN2 = 1.50 atm?Ans.= 0.00106 M
Pg
Sg
{movie}
Sg Pg α
Solutions
• The opposite is true of gases:Gases in carbonated
soft drinks are less soluble when warmed.
Warm lakes have less O2 dissolved in them than cool lakes.
Factors Affecting Solubility of Gases in Solution: (3) Temperature
Solutions
Colligative Properties
Pure Solvent Solution: Solvent + Solute
How does the presence of a solute change the properties of a solvent?
1. Vapor pressure?
2. Boiling point?
3. Freezing (melting) point?
4. Osmotic pressure?
Solutions
Colligative Properties• Properties that depend only on the
CONCENTRATION (number of solute particles present), not on the identity of the solute particles.
Pure solvent Solvent + Solute
1. Vapor pressure lowering
2. Boiling point elevation
3. Freezing point depression
4. Osmotic pressure
PA = XA PA
Tb = Kb m
Tf = Kf m
= MRT
Solutions
Ways of Expressing Concentrations of Solutions
(1) Mass Percentage (%)
(2) Parts per Million (ppm) & Parts per Billion (ppb)
(3) Mole Fraction (X)
(4) Molarity (M)
(5) Molality (m)
Solutions
ppm =mass of solu A in solution
total mass of solution 106
Parts per Million (ppm)
ppb =mass of solu A in solution
total mass of solution 109
Mass % of A =mass of solu A in solution
total mass of solution 100 or 102
Mass Percentage (%) (parts per Hundred)
50. g of NaCl in 100.mL of H2O %3.3310333.010g.150
g.50 22
0.0005 g of NaCl in 100.mL of H2O
ppm510000005.010g.100
g0005.0 66
0.000,000,5 g of NaCl in 100.mL of H2O
ppb510)105(10g.100
g5,000,000.0 999
Parts per Billion (ppb)
hundred
million
billion
Solutions
moles of substance Atotal moles in solution (A+B+∙∙∙)XA =
Mole Fraction (X)
You can calculate the mole fraction of either the solvent or of the solute — make sure you find the one you need for your calculations!
What are the mole fractions of methanol and water in a solution containing 128g CH3OH (MW=32.0) and 202 mL of H2O (MW=18.0)? Which is the solute and the solvent?
OH2.11g0.18
OH1
mL00.1
g00.1mL202OH? 2
22
OHCH00.4
g0.32
OHCH1g128OHCH? 3
33
263.02.1100.4
00.4
OHOHCH
OHCH
23
3OHCH3
737.0
2.1100.4
2.11
OHOHCH
OH
23
2OH2
1263.0737.0OHCHOH 32
Mole fraction of Solvent:Mole fraction of Solute:
Solutions
mol of soluteL of solution
M =
Molarity (M)
• Because volume is temperature dependent, molarity can change with temperature.
{The volume of most substances expand with increasing temperature}
M0.11
mL1000
L1mL202
g791.0
mL00.1g128
g0.32
OHCH1g128 3
OH 2
OH 2LOHCHL
OHCHM
3
3
What is the molarity of a solution containing 128g CH3OH (MW=32.0, d=0.791g/mL@25 C) and 202 mL of H2O (MW=18.0, d=1.00g/mL@25C)?
Solutions
moles of solutekg of solvent
m =
Molality (m)
Because both moles and mass do not change with temperature, molality (unlike molarity) is not temperature dependent.
What is the molality of a solution containing 128g CH3OH (MW=32.0, d=0.791g/mL@25 C) and 202 mL of H2O (MW=18.0, d=1.00g/mL@25C)?
m
gkg
g
gOHCH
g
8.19
10001
OH 202
0.321
128
2
3
OH 2
3
kg
OHCHm
Solutions
Problem: What is the molality (m) of a 2.0M NaCl solution at 250C if its density is 1.2 g/mL?
Changing Molarity to MolalityIf we know the density of the solution, we can calculate the molality from the molarity, and vice versa.
mol of soluteL of solution
M =mol of solutekg of solvent
m =
soln 1
22
L
NaClM
solv ?
2?
kg
NaClm
Lkg 1soln ?
NaClkgNaCl 2? = 1.8 m NaCl
solv 1.1
2?
kg
NaClm
NaCl 12.01000
1
1
0.58kg
g
kgg
OHNaCl 2.11000
1
1
2.1
1
10002
kgg
kg
mL
g
L
mL
Solutions
Colligative Properties• Properties that depend only on the number of
solute particles present, not on the identity of the solute particles.
Pure solvent Solvent + Solute
1. Vapor pressure lowering
2. Boiling point elevation
3. Freezing point depression
4. Osmotic pressure
PA = XA PA
Tb = Kb m
Tf = Kf m
= MRT
Solutions
1. Vapor Pressure (Pvap ) Lowering
Because of solute-solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.
Therefore, the vapor pressure of a solution (PA ) is lower than that of the pure solvent (PA).
{PureSolv}
{*Solution}PA = XA PA
Solutions
Raoult’s Law:
PA = XA P Awhere• XA is the mole fraction of solvent A
• P A is the normal vapor pressure of the pure solvent A at that temperature
• P A is the new vapor pressure of solution at that temperature
Consider: XA= 1 XA= 0.9 XA= 0.5
{VapPress.WaterVsEthGlycol}
Solutions
2. Freezing Point Depression & 3. Boiling Point Elevation
Nonvolatile solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.
f.pt. Pure solventf.pt. Solutionb.pt.
f.pt.
Solutions
Freezing Point Depression
• The change in freezing point is proportional to the molality of the solution :
Tf = Kf m
• Here Kf is the molal freezing point depression constant of the solvent.
Tf is subtracted from the normal freezing point of the solvent.
{f.pt. Equil} {f.pt. Lower}
Solutions
Boiling Point Elevation
The change in boiling point is proportional to the molality of the solution:
Tb = Kb 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.
Solutions
Freezing Point Depression & Boiling Point Elevation
Note that in both equations, T does not depend on what the solute is, but only on how many particles are dissolved.
Tf = Kf m
Tb = Kb m
Solutions
Osmosis• The movement of water from an area of high
concentration to lower concentration (diffusion) across a semipermeable membrane (SPM), until a homogeneous solution has been formed (equilibrium). The SPM allows smaller particles (water) to pass through, but blocks other larger particles.
• Diffusion of water across a SPM.
Diffusion• The movement of particles from an area of high
concentration to lower concentration until a homogeneous solution has been formed.
Solutions
OsmosisIn osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the area of lower solvent concentration (higher solute concentration).
{*OsmoPress}
SolventSolution
Solutions
{*OsmoSemiPerMemb}
Solution side (Low Conc. H2O)
Solvent side (High Conc. H2O)SPM
Solutions
Osmosis in Cells• If the water concentration outside the cell is more than that inside the cell, the solution is hypotonic (low in solute).
• Water will flow into the cell, and hemolysis results.
{EggOsmoPres}
H2O
H2O H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
High Water
Low Water
H2O
H2O
H2O
H2O
H2O
(high solute)
(low solute)
Solutions
Osmosis in Blood Cells• If the water
concentration outside the cell is lower than that inside the cell, the solution is hypertonic (high in solute).
• Water will flow out of the cell, and crenation results.
• Isotonic: equal concentrations.
H2O
H2OH2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
(salt water)
(salt water)
(salt water)
(salt
water)
High Water
(high solute)
(low solute)
H2O
H2O
H2O
H2O
Solutions
3. Osmotic Pressure• The pressure required to stop osmosis,
known as osmotic pressure, , is
nV
= ( )RT
where M is the molarity of the solution
V = n RT
= MRT
Solutions
Reverse Osmosis
The method of purifying liquid by pushing it through a semi-permeable membrane. Pressure is utilized to reverse the natural osmotic flow through a synthetic membrane so that pure water molecules pass through and impurities are flushed away.
{RevOsmo}
Membrane
H2O H2O H2O
Contaminated water
Reverse Osmosis {RevOsmo}
Membrane
H2O H2O H2OPure H2O
Contaminated water
Solutions
Determining Molar Mass from Colligative Properties
K
ΔT m
b
b
We can use the effects of a colligative property such as freezing point depression to determine the molar mass (MM) of a compound.
A solution of an unknown nonvolatile electrolyte was prepared by dissolving 0.250g of the substance in 40.0 g of CCl4. The b. pt. of the resultant solution was 0.357 0C higher than that of the pure solvent. What is MM of solute? The Kb for CCl4 is 5.02 0C/m.
Tb = Kb m m 0.0711 m
0.357
0
0
/C02.5
C
solu10 x 2.84 CCl of kg 0.0400 CCl of kg
solu 3-
4
4
0711.0
g#
MM
g/ 88.0 10 x 2.84
g 0.250
3-
solvkg
moles m where
MM
g# )( moles
Solutions
Determining 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.
= MRT
Liter
moles M where
MM
g# )( moles
RT
M
/L 10x 8.28 K 982K
atm L 0.082
torr760
atm 1 torr1.54
3-
L 8.28x10 M 3-
7-10 x .144 mL 00.5
mL 1000
L1
g#
MM
g8.45x10
10x 4.14mg 1000
g 1x mg .503
3
7-
The osmotic pressure of a protein solution was measured @ 250C to be 1.54 torr. The solution contained 3.50 mg of protein dissolved in water to make 5.00 mL of solution. Determine the molar mass (MM) of the protein.
Solutions = MRT
Vapor pressure lowering:
Boiling point elevation:
Freezing point depression:
Osmotic pressure:
Tf = Kf m
PA = XA PA
Colligative Properties & Molar Mass
= PA
Tb = Kb m = Kb
= Kf
= RT
lnso
solu
LMW
g#
solv
solu
KgMW
g#
solv
solu
KgMW
g#
ABA
A
MWg
#
1
1
Solutions
Colligative Properties of ElectrolytesSince these properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) should show greater changes than those of nonelectrolytes.
You would expect a 1 M solution of CaCl2 to show three times the change in freezing point that a 1 M solution of C6H12O6, however …………….
CaCl2 (s) Ca 2+ (aq) + 2Cl- (aq)H2O
C6H12O6 (s) C6H12O6 (aq) H2O1η 1 η molecules
1η 3 η ions
Non-electrolyte:
Electrolyte:
Solutions
van’t Hoff Factor
One mole of NaCl in water does not really give rise to two moles of ions.
Some Na+ and Cl− reassociate for a short time, so the true concentration of particles is somewhat less than two times the concentration of NaCl.
• Reassociation is more likely at higher concentration.• Therefore, the number of particles present is concentration
dependent.
Solutions
The van’t Hoff FactorWe modify the previous equations for Colligative Properties by
multiplying them by the van’t Hoff factor, i• Equations
Vapor pressure lowering:
Boiling point elevation:
Freezing point depression:
Osmotic pressure: = i MRT
Tb = i Kb m
Tf = i Kf m
PA = i XAPA
Solutions
Colloidal Dispersion
homogeneous mixtures of particles larger than individual ions or molecules, but too small to be settled out by gravity.
Solutions
Tyndall Effect
The scattering of light by colloids.
Solutions
Colloids in Biological SystemsSome molecules have a polar, hydrophilic (water-loving) end and a nonpolar, hydrophobic (water-hating) end.
Sodium stearate is one example of such a molecule.
Non- Polar Tail
Polar Head
Soap Micelles in water solution
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2OH2O
H2OH2O
H2O
H2O
H2O
H2O
Solutions
Colloids in Biological SystemsThese molecules can aid in the emulsification of fats and oils in aqueous solutions.
{Cotton Fabric Softener}
{Micelle}
Solutions
Suspension:heterogenous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometre.[1] The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents.
Solutions
Solutions
Solutions
Solutions
Solutions
Solutions
2006 (A)
Solutions
Solutions
Solutions
Solutions
2003 A
Solutions
Na+(aq) + C2H3O2
_
(aq) NaC2H3O2(s)
Energetics of Solutions Heats of Solution (Hsoln)
crystallization
Recyclable Hot Pack: Super Saturated Solution of Sodium Acetate
heating
{HotPack Na Acet}
∆Hsoln = - 67 kJ/n
