Transcript Solute - John A. Ferguson Senior High School

CHM 1046: General Chemistry and
Qualitative Analysis
Unit 12:
Properties of Solutions
Dr. Jorge L. Alonso
Miami-Dade College –
Kendall Campus
Miami, FL
Textbook Reference:
•Chapter # 14
•Module # 2
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
Solutions
{PrepASolu}
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
H2O
Cu(NO3)2 (s)
H2O
Cu(NO3)2 (aq)
Mg (s) + 2 HCl (aq)

H2 (g) + MgCl2 (aq)
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.
Solutions
• If you can’t, the substance didn’t dissolve, it reacted.
How Does a Solution of Salts in
Water Form?
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 (s)
{NaCl + H2O*}
+ H20
dissociation
Na+ (aq)
+ Cl- (aq)
Solutions
Solubility in water
Elements: mostly insoluble solids, liquids & gases.
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.
HO
{Soluble metal oxides form hydroxides: CaO 2
Ca 2+ + 2OH-}
7.
6.
7.
All OH- are insoluble except for IA metals, NH4+ & slightly soluble Ca 2+ Ba2+ & Sr2+
All salts containing the anions: CO32-, PO43-, AsO43-, S2- and SO32- are insoluble
except fro IA metals and NH4+ salts.
For salts containing the anions not mentioned above (e.g., CrO42-, Cr2O72-Solutions
, P3-,
C2O42- etc.) assume that they are insoluble except for IA metals and NH4+ salts,
unless, otherwise informed.
Energetics of Solutions
Heats (Enthalpy) of Solution (Hsoln)
Exothermic solution process
CaCl2 (s)
H2O
Hot & Cold Packs
Ca 2+(aq) + 2Cl- (aq) + Heat
Hsoln = - 81.3 kJ/mol
Endothermic solution process
Heat + NH4NO3 (s)
H2O
CaCl2
+ H2O
NH4NO3
+H2O
NH4+ (aq) + NO3-(aq)
Hsoln = + 25.7 kJ/mol
Solutions
How Does a Solution Form? (3 events)
(3) Formation of
Ion-dipole
interactions
(1) Separation of
solute molecules
NaCl
Crystal
Lattice
(2) Separation of
solvent molecules
H2O
H Lattice
Energy
Hvap
+788 kJ/mol
Endothermic
+41 kJ/mol
Endothermic
Exothermic
H- bonding
Energy of Solution Hsoln = + or If an ionic salt is soluble in water, it is because the ion-dipole
Solutions
interactions are strong enough to overcome the lattice energy
of
the salt crystal.
Energy Changes in Solution
The enthalpy change of the overall process depends on H for each of these steps.
{EnerSoln}
-
+
Solutions
Why do Endothermic Reactions
occur Spontaneously?
Exothermic solution process
CaCl2 (s)
H2O
Ca 2+ (aq) + 2Cl-
(aq)
Hsoln = - 81.3 kJ/mol
Endothermic solution process
NH4NO3 (s)
H2O
NH4+ (aq)
+
NO3-(aq)
Hsoln = + 25.7 kJ/mol
Solutions
Concentration of Solutions
(General)
• Unsaturated
{UnsatSoln}
 Less than the maximum amount of
solute for that temperature is
dissolved in the solvent.
• Saturated
 Solvent holds as much solute as is
possible at that temperature.
 Dissolved solute is in dynamic
equilibrium with solid solute
particles.
Solubility of NaCl in H2O = 35.9 g/100 mL (25 °C)
NaC2H4O2 = 76 g/100 ml (0°C)
Solutions
Concentration of Solutions
{*SuperSat1}
{*SuperSat2}
(General)
• Supersaturated
 Solvent 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.
Solutions
Factors Affecting Solubility
A. Solids and Liquids:
1. Polarity
2. Temperature
B. Gases :
1. Molar Mass
2. Pressure – Henry’s Law
3. Temperature
Solutions
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.
hydrophobic waterhating, nonpolar,
(hydrocarbon chain)
hydrophilic waterloving, polar,
Solutions
[Know molecular shapes and polar molecules]
{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.
Polar heads
Non-Polar tails
Polar
NonPolar
Solutions
[Know molecular shapes and polar molecules]
Factors Affecting Solubility of
Solids and Liquids: (1) Polarity
Glucose (which has
hydrogen bonding)
is very soluble in
water, while…..
…..cyclohexane
(which only has
dispersion
forces) is not. Solutions
Factors Affecting Solubility of
Solids and Liquids: (1) Polarity
• Vitamin A is soluble in nonpolar compounds
(like fats).
• Vitamin C is soluble in water. {*Iodine + Water then + CCl4 }
Solutions
Factors Affecting Solubility of Solids
and Liquids: (2) Temperature
Generally, the
solubility of solid
solutes in liquid
solvents
increases with
increasing
temperature.
Solutions
Factors Affecting Solubility of
Gases in Solution: (1) Molar Mass
•In general, the solubility of gases in water increases
with increasing molar mass.
Which gas
dissolves best
in water?
Why?
• Larger molecules
have stronger
London
dispersion
forces.
g-MM
28 g/n
28 g/n
32 g/n
40 g/n
84 g/n
Solutions
Factors Affecting Solubility
of Gases in Solution:
(2) Pressure
• 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).
{Henrys Law}
Sg
α Pg
Solutions
Factors Affecting Solubility of Gases
in Solution: (2) Pressure
Henry’s Law: {movie}
Sg
α Pg
Sg = kPg
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;
@ 250C, PN2= 0.75 atm, SN2= 0.00053 M.
What is SN2 when PN2 = 1.50 atm?
Pg
Sg
Solutions
Ans.= 0.00106 M
Factors Affecting Solubility of
Gases in Solution: (3) Temperature
• 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.
Solutions
Colligative Properties
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?
Pure Solvent
Solution:
Solvent + Solute
Solutions
Colligative Properties
• Properties that depend only on the
CONCENTRATION (number of solute particles
present), not on the identity of the solute particles.
1. Vapor pressure lowering
PA = XA  PA
2. Boiling point elevation
Tb = Kb  m
3. Freezing point
depression
Tf = Kf  m
4. Osmotic pressure
Pure solvent
Solvent + Solute
 = 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
Mass Percentage (%)
(parts per Hundred)
mass of solu A in solution
100 or 102
Mass % of A = total mass of solution  hundred
50. g of NaCl in 100.mL of H2O
50.g
 10 2  0.333  10 2  33.3%
150.g
Parts per Million (ppm)
mass of solu A in solution
ppm = total mass of solution  106
million
0.0005 g of NaCl in
100.mL of H2O
0.0005g
 106  0.000005  106  5 ppm
100.g
Parts per Billion (ppb)
mass of solu A in solution
9

10
ppb =
total mass of solution
billion
0.000,000,5 g of NaCl
in 100.mL of H2O
Solutions
0.000,000,5g
9
9
9
 10  (5  10 )  10  5 ppb
100.g
Mole Fraction (X)
XA =
moles of substance A
total moles in solution (A+B+∙∙∙)
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?
Mole fraction of Solvent:
Mole fraction of Solute:
 1CH 3OH 
? CH 3OH  128g
  4.00CH 3OH
32
.
0
g


 CH OH
3
CH 3OH
4.00


 0.263
CH 3OH  H 2 O 4.00  11.2
 1.00g  1H 2 O 
? H 2 O  202 mL 
  11.2H 2 O

1
.
00
mL
18
.
0
g



H O 
2
H 2 O
11.2

 0.737
CH 3OH  H 2 O 4.00  11.2
 H O  CH OH  0.737  0.263  1
2
3
Solutions
Molarity (M)
M=
mol of solute
L of solution
• Because volume is temperature dependent,
molarity can change with temperature.
{The volume of most substances expand with increasing temperature}
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@25C)?
 1CH 3OH 
128g

CH 3OH
32.0g 

M

 11.0M
L CH 3OH  L H 2O 
  1L 
 1.00 mL 
128g 0.791g   202 mL H 2O   1000 mL  Solutions




 
Molality (m)
m=
moles of solute
kg of solvent
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@25C)?
 1CH 3OH 

128g 
CH 3OH
32.0 g 


 19.8 m
m
 1kg 
kg H 2O

202g H 2 O
 1000g 
Solutions
Changing Molarity to Molality
If we know the density of the solution, we can calculate the molality
from the molarity, and vice versa.
Problem:
What is the molality (m) of a 2.0M NaCl solution at 250C if its
density is 1.2 g/mL?
M=
mol of solute
L of solution
2NaCl
2M 
1L soln
m=
mol of solute
kg of solvent
2NaCl
?m 
? kg solv
? kg soln  1L  1000mL 1.2 g  1kg   1.2kg NaCl  H O
2



1L
 1mL  1000g 
 58.0 g  1kg 
  0.12kg NaCl

 1  1000g 
? kgNaCl  2NaCl 
2NaCl
?m 
1.1kg solv
Solutions
= 1.8 m NaCl
Colligative Properties
• Properties that depend only on the number of
solute particles present, not on the identity of the
solute particles.
1. Vapor pressure lowering
PA = XA  PA
2. Boiling point elevation
Tb = Kb  m
3. Freezing point
depression
Tf = Kf  m
4. Osmotic pressure
Pure solvent
Solvent + Solute
 = 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.
{PureSolv}
Therefore, the vapor pressure of
a solution (PA ) is lower than that
of the pure solvent (PA).
PA = XA  PA
Solutions
{*Solution}
{VapPress.WaterVsEthGlycol}
Raoult’s Law:
PA = XA P  A
where
• 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
Solutions
2. Freezing Point Depression &
3. Boiling Point Elevation
f.pt. Solution
Nonvolatile
solutesolvent
interactions
also cause
solutions to
have higher
boiling
points and
lower
freezing
points than
the pure
solvent.
f.pt. Pure solvent
f.pt.
b.pt.
Solutions
Freezing Point Depression
{f.pt. Equil}
{f.pt. Lower}
• 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.
Solutions
Tf is subtracted from the normal freezing point of the solvent.
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.
Solutions
Tb is added to the normal boiling point of the solvent.
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
Diffusion
• The movement of particles from an area of high
concentration to lower concentration until a
homogeneous solution has been formed.
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.
Solutions
Osmosis
In 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).
Solution
{*OsmoPress}
Solvent
Solutions
{*OsmoSemiPerMemb}
Solution side
(Low Conc. H2O)
SPM
Solvent side
(High Conc. HSolutions
2O)
• 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.
Osmosis in Cells
High Water
H2O
H2O
(low solute)
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Low Water
H2O
(high solute)
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Solutions
{EggOsmoPres}
Osmosis in Blood Cells
• If the water
concentration
outside the cell is
lower than that
HO
inside the cell,
(high solute)
(salt water)
the solution is
HO
hypertonic (high H O
HO
in solute).
HO
HO
• Water will flow
High Water H O
HO
out of the cell,
(salt
(low solute)
HO
and crenation
HO
water)
results.
HO
HO
• Isotonic: equal
Solutions
HO
HO
concentrations.
(salt water)
2
2
2
2
2
2
2
2
2
2
2
2
2
(salt water)
2
3. Osmotic Pressure
• The pressure required to stop osmosis,
known as osmotic pressure, , is
 V = n RT
n
 = ( V )RT
 = MRT
where M is the molarity of the solution
Solutions
Reverse Osmosis
{RevOsmo}
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.
Contaminated
water
Membrane
Solutions
Pure H2O H2O
H2O H2O
Determining Molar Mass from
Colligative Properties
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
where m 
m oles
kg solv
moles () 
#g
MM
ΔTb
m
Kb
0.357 0 C

 0.0711 m
0
5.02 C / m

 solu 
 0.0711
 0.0400 kg of CCl4   2.84 x 10 -3  solu
kg of CCl4 

 # g   0.250 g 
MM  
 
  88.0 g/
-3
    2.84 x 10  
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.
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.
 = MRT
moles
where M 
Liter
moles () 
#g
MM
 1 atm 
1.54 torr


 760 torr 
M

 8.28 x 10-3 /L
RT  0.082 L  atm 298 K 
K

 1L 
-7
M  8.28x10 -3  


5
.
00
mL

4
.14
x
10


L 1000 mL 
1g 

3
.50
mg
x

 #g 
1000
mg
  8.45x10 3 g
MM     
-7




4.14
x
10

 


Solutions
Colligative Properties & Molar Mass
Vapor pressure lowering:
PA = XA  PA
=




1



A

 #g  
 
 1 A  
 MW  B  A

 PA
Boiling point elevation:
Tb = Kb  m
= Kb 
Freezing point depression:
Tf = Kf  m = Kf 
Osmotic pressure:
 = MRT =
  #g  

 
MW

solu 

 Lso ln 


  #g  

 
MW
solu 

 Kgsolv 




  #g  

 
MW
solu 

 Kgsolv 




RT
Solutions
Colligative Properties of Electrolytes
Since these properties depend on the number of particles dissolved,
solutions of electrolytes (which dissociate in solution) should show greater
changes than those of nonelectrolytes.
Non-electrolyte: 1η C6H12O6 (s)
Electrolyte: 1η CaCl2 (s)
H2O
H2O
C6H12O6 (aq) 1 η molecules
Ca 2+ (aq) + 2Cl-
(aq)
3 η ions
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 …………….
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.
Solutions
• Therefore, the number of particles present is concentration
dependent.
The van’t Hoff Factor
We modify the previous equations for Colligative Properties by
multiplying them by the van’t Hoff factor,
• Equations
i
Vapor pressure lowering:
PA = i XAPA
Boiling point elevation:
Tb = i Kb  m
Freezing point depression:
Osmotic pressure:
Tf = i Kf  m
 = i MRT
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 Systems
Some molecules have a polar, hydrophilic (waterloving) end and a nonpolar, hydrophobic (waterhating) end.
H2O
HO
2
Polar
Head
Sodium
stearate is
one example
of such a
molecule.
H 2O
H 2O
NonPolar
Tail
H2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
Solutions
Soap Micelles in water solution
Colloids in Biological Systems
These molecules can aid in the emulsification of fats
and oils in aqueous solutions.
{Micelle}
Solutions
{Cotton Fabric Softener}
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
Energetics of Solutions
Heats of Solution (Hsoln)
Recyclable Hot Pack: Super Saturated Solution of Sodium Acetate
+
Na
(aq)
+ C 2H 3O 2
_
crystallization
(aq)
heating
NaC2H3O2(s)
∆Hsoln = - 67 kJ/n
Solutions
{HotPack Na Acet}