2/10/2011 1 Chem 106 Thursday 10 Feb 2011 Colligative Properties overview Vapor Pressure of Liquid - effect of non-volatile solute (Raoult) Boiling Point Elevation Freezing Point Depression Osmotic Pressure
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2/10/2011 1
Chem 106 Thursday 10 Feb 2011 Colligative Properties overview Vapor Pressure of Liquid - effect of non-volatile solute (Raoult) Boiling Point Elevation Freezing Point Depression Osmotic Pressure
2/10/2011 2
Colligative Properties Overview
Colligative mean “holds together”. It refers to how solutes change the properties of the liquid based on the concentration of the solute (and not the shape or type).
solution
2nd phase (vapor, solid, or through a membrane)
solvent solute
The solute molecules or ions block the surface and “prevent escape” of the solvent molecules. The type of particle does not matter.
2/11/2009 3
Effects of non-volatile solutes: 1. Vapor pressure lowering (Raoult’s Law) 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure lowering
2/10/2011 4
Vapor pressure lowering by a non-volatile solute (Raoult’s Law)
0
solventsolventsolution PXP
0
solventsolventsolution PXP vapor pressure of solution due to the solvent molecules
mole fraction of solvent
vapor pressure of the pure solvent
= x
Recall that:
solutemolessolventmoles
solventmoles
solutionofmolestotal
solventmolesX solvent
solute moles solvent moles solvent moles
solution of moles total solvent moles
X solvent
2/10/2011 5
0
etheretherether PXP Chlmolethermol
ethermolX ether
0 ether ether ether P X P
Given data
Solve for mol Chl, then convert that to grams using molar mass.
0
ether
etherether
P
PX
Given
Now solve for mol Chl
985655.057.463
92.4560
mmHg
mmHg
P
PX
ether
etherether
Chl mol ether mol ether mol
X ether
0 ether
ether ether
P P
X
985008 . 0 57 . 463 92 . 456
0
mmHg mmHg
P P
X ether
ether ether
÷ ÷
-
ether
ether
ether ether
X X
ether mol Chl mol
ether mol ether mol X ether mol X
1
𝑋𝑒𝑡ℎ𝑒𝑟 ∗ 𝑚𝑜𝑙 𝑒𝑡ℎ𝑒𝑟 + 𝑋𝑒𝑡ℎ𝑒𝑟 ∗ 𝑚𝑜𝑙 𝐶ℎ𝑙 = 𝑚𝑜𝑙 𝑒𝑡ℎ𝑒𝑟
𝑚𝑜𝑙 𝐶ℎ𝑙 = 𝑚𝑜𝑙 𝑒𝑡ℎ𝑒𝑟 ∗ 1 − 𝑋𝑒𝑡ℎ𝑒𝑟
𝑋𝑒𝑡ℎ𝑒𝑟
2/10/2011 6
-
ether
ether
etherether
X
XethermolChlmol
ethermolethermolXethermolX
1
47.85
ChlmolChlmol
Ethg
EthmolEthgChlmol
X
XethermolChlmol
ether
ether
053554.0
985008.0
985008.01
12.74
18.260
1
-
-
Chl mol Chl mol Eth g
Eth mol Eth g Chl mol
X X
ether mol Chl mol ether
ether
053554 . 0 985008 . 0
985008 . 0 1 12 . 74
1 8 . 260
1
÷
-
´
÷ ÷
-
ChlgChlmol
ChlgChlmol 85.47
5.893053554.0
Chl g Chl mol
Chl g Chl mol 85 . 47 5 . 893
053554 . 0 ´
2/11/2009 7
Effects of (non-volatile) solutes: 1. Vapor pressure lowering (Raoult’s Law) 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure lowering
2/11/2009 8
Colligative Properties Overview
Colligative mean “holds together”. It refers to how solutes change the properties of the liquid based on the concentration of the solute (and not the shape or type).
solution
2nd phase (vapor, solid, or through membrane)
solvent solute
The solute molecules or ions block the surface and “prevent escape” of the solvent molecules. The type of particle does not matter.
2/10/2011 9
2/11/2009 10
Boiling point elevation by non-volatile solute
Example: “anti-freeze” prevents boil-over (and freezing) of engine coolant.
HO OH
HH H
H
ethylene glycol
HO OH
HH H
CH3
propylene glycol
H O O H
H H H
H
ethylene glycol (toxic)
H O O H
H H H
C H 3
propylene glycol
These compounds are used because they are very soluble in water, non-corrosive to metal parts, and relatively inexpensive.
2/11/2009 11
P (a
tm)
T (°C)
1.00
100
Vapor pressure of solution at T = 100 °C is LESS THAN vapor pressure of pure water.
110 So, to get the solution to boil, you have to heat it hotter.
Phase diagram of water
Solutions still require P = 1.00 atm to boil.
2/11/2009 12
Boiling elevation = ΔTbp
ΔTbp = Kbpmsolute
“molal boiling point elevation constant”
2/11/2009 13
Boiling point elevation by ionic compounds: The colligative effect depends on the NUMBER OF PARTICLES ONLY. So we take this into account by including the factor “i”. i = moles of particle per mole of solute i = 2 for NaCl = 3 for CaCl2
= 3 for H2SO4
= 1 for ethylene glycol
These can be molecules, or separate ions of ionic solute.
ΔTbp = iKbpmsolute
2/11/2009 14
2/11/2009 15
kg
ionsmol
AlClmol
ionsmolx
kg
AlClmol 328.04082.0
3
3
kg
ionsmol
CaBrmol
ionsmolx
kg
CaBrmol 30.0310.0
2
2
kg
ionsmol
NOAlmol
ionsmolx
kg
NOAlmol 36.0
)(
4)(090.0
33
33
kg
molecmol
glucmol
molecmolx
kg
glucmol 39.0139.0
A
B
C
D
kg ions mol
AlCl mol ions mol
x kg
AlCl mol 328 . 0 4 082 . 0 3
3
kg ions mol
CaBr mol ions mol
x kg
CaBr mol 30 . 0 3 10 . 0 2
2
kg ions mol
NO Al mol ions mol
x kg
NO Al mol 36 . 0 ) (
4 ) ( 090 . 0 3 3
3 3
kg molec mol
gluc mol molec mol
x kg
gluc mol 39 . 0 1 39 . 0
2/11/2009 16
Which aqueous solution (each 0.50 m) has the highest boiling point?
Eth
ylen
e gly
col
Chro
miu
m (I
II) n
itrite
Sodiu
m c
hloride
Cal
cium
sulfi
te
23
22
191. Ethylene glycol 2. Chromium (III) nitrite 3. Sodium chloride 4. Calcium sulfite
2/11/2009 17
Which aqueous solution (each 0.50 m) has the highest boiling point?
1. Ethylene glycol I = 1 2. Chromium (III) nitrite Cr(NO2)3 i = 4 3. Sodium chlorate NaClO3 i = 2 4. Calcium sulfite CaSO3 i = 2
(Please review ion charge, name, and formula.)
2/11/2009 18
Effects of (non-volatile) solutes: 1. Vapor pressure lowering (Raoult’s Law) 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure lowering
2/11/2009 19
Solute molecules or ions BLOCK access to the solid surface
2/11/2009 20
P (a
tm)
T (°C) ->
1.00
0.0
Phase diagram of water & solutions – melting point depression
Pure water + ice @0.0°C/1 atm
Solution (no ice) @0.0°C/1 atm
Must cool to -4°C (or whatever) to get ice again.
New triple point @lower temp/pressure
2/11/2009 21
P (a
tm)
T (°C) ->
1.00
0.0
Phase diagram of water and solutions – melting point depression
Liquid phase for pure liquid
Liquid phase for solution
2/11/2009 22
2/11/2009 23
Ice crystals in seawater are nearly fresh.
2/11/2009 24
Ice metamorphoses over a period of weeks and months, which eliminates salt impurities within the crystals.
2/11/2009 25
Arctic explorers say that “one-year ice is good for nothing, two-year ice is OK to cook with and only three-year ice will do for tea”.
http://www.jeanlouisetienne.fr/poleairship/EN/images/encyclo/imprimer/14.htm
2/11/2009 26
Freezing Point depression elevation = ΔTfp
ΔTfp = Kfpmsolute
“molal freezing point depression constant”
2/11/2009 27
Melting point depression by ionic compounds: The colligative effect depends on the NUMBER OF PARTICLES ONLY. So we take this into account by including the factor “i”. i = moles of particle per mole of solute i = 2 for CsI = 2 for HCl = 3 for Ca(ClO4)2
= 1 for sucrose
These can be molecules, or separate ions of ionic solute.
ΔTfp = iKfpmsolute
2/11/2009 28
Which aqueous solution (each 0.50 m) has the lowest freezing point?
Sodiu
m b
rom
ide
Sodiu
m c
hlora
te
Cal
cium
chlo
rate
Cobal
t (III
) nitr
ate
5
37
22
1. Sodium bromide 2. Sodium chlorate 3. Calcium chlorate 4. Cobalt (III) nitrate
2/11/2009 29
Which aqueous solution (each 0.50 m) has the lowest freezing point?
1. Sodium bromide NaBr i = 2 2. Sodium chlorate NaClO3 i = 2 3. Calcium chlorate Ca(ClO3)2 i = 3 4. Cobalt (III) nitrate Co(NO3)3 i = 4
2/11/2009 30
fp
fp
solute
solutefpfp
K
Tm
mKT
mmC
C
K
Tm
o
o
fp
fp
solute 6210.0/86.1
155.1
-
-
molOHkgsolventkg
solutemolx
OHg
OHkgOHg
x
solventkg
solutemolmsolute
18194.02930.06210.0
1000
10.293
6210.0
2
2
22
62.1 62.1 g/mol
molgmolgmol
g
mol
gM /1.62/21.62
94181.0
30.11
fp
fp solute
solute fp fp
K
T m
m K T
m m C
C
K
T m
o
o
fp
fp solute 6210 . 0
/ 86 . 1
155 . 1
-
-
mol O H kg solvent kg
solute mol x
O H g O H kg
O H g
x solvent kg
solute mol m solute
18194 . 0 2930 . 0 6210 . 0 1000
1 0 . 293
6210 . 0
2
2
2 2
´
´
mol g mol g mol g
mol g
M / 1 . 62 / 2 1 . 62 94 181 . 0 30 . 11
i = 1
Which one of the following is the most reasonable formula for this compound?
2/10/2011 31
C5H2
C4H14
C3H6O
C3H8O
10 1011
131. C5H2
2. C4H14
3. C3H6O 4. C3H8O
C2H6O2
Too many H’s
Not enough H’s
Formula wrong for 62 g/mol
2/10/2011 32
3. C3H6O C2H6O2
Turns out that C5H2 does have at least one energy minimum as calculated by the PM3 semi-empirical method. This molecule might exist in the gas phase at close to Absolute Zero out in space somewhere…
This is the formula for ethylene glycol, which is possible to synthesize by rather easy methods in the lab, in contrast to the molecule shown below.
2/11/2009 33
Effects of (non-volatile) solutes: 1. Vapor pressure lowering (Raoult’s Law) 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure lowering
2/11/2009 34
Osmotic pressure = extra pressure that must be applied to the SOLUTION SIDE of semi-permeable membrane to EQUALIZE solvent flow.
OSMOSIS = diffusion through a semi-permeable membrane.
2/11/2009 35
OSMOTIC Pressure = (atm) = iMsoluteRT
Molarity of solute
Gas Constant = 0.08206 L-atm/(K-mol)
Absolute Temperature (Kelvin, K)
i = Moles of particles per mole of solute
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