Colligative Properties overview Vapor Pressure of Liquid - effect of non-volatile solute (Raoult) Boiling Point Elevation Freezing Point Depression Osmotic Pressure
2/10/2011
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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).
2nd phase (vapor, solid, or through a membrane)
solution solvent
solute
The solute molecules or ions block the surface and “prevent escape” of the solvent molecules. The type of particle does not matter.
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Effects of non-volatile solutes: 1. Vapor pressure lowering (Raoult’s Law) 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure lowering
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Vapor pressure lowering by a non-volatile solute (Raoult’s Law)
0 Psolution X solvent Psolvent
vapor pressure of solution due to the solvent molecules
=
mole fraction of solvent
x
vapor pressure of the pure solvent
Recall that:
X solvent 2/10/2011
moles solvent moles solvent total moles of solution moles solvent moles solute 4
Given Given data
X ether 0 Pether X ether Pether
X ether X ether
Pether 0 Pether
mol ether mol ether mol Chl Solve for mol Chl, then convert that to grams using molar mass.
1 - X ether mol Chl mol ether X ether mol Chl 260.8 g Eth ´
÷ ÷
1 mol Eth 1 - 0.985008 ÷ 74.12 g Eth 0.985008
mol Chl 0.053554 mol Chl
0.053554 mol Chl ´ 2/10/2011
893.5 g Chl 47.85 g Chl mol Chl 6
Effects of (non-volatile) solutes: 1. Vapor pressure lowering (Raoult’s Law) 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure lowering
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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).
2nd phase (vapor, solid, or through membrane)
solution solvent
solute
The solute molecules or ions block the surface and “prevent escape” of the solvent molecules. The type of particle does not matter.
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Boiling point elevation by non-volatile solute HO
Example: “anti-freeze” prevents boilover (and freezing) of engine coolant.
OH
H H
H
H
ethylene glycol (toxic)
HO
OH
H H
H
CH3
propylene glycol
These compounds are used because they are very soluble in water, non-corrosive to metal parts, and relatively inexpensive.
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Phase diagram of water
Vapor pressure of solution at T = 100 °C is LESS THAN vapor pressure of pure water. Solutions still require P = 1.00 atm to boil.
P (atm)
1.00
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T (°C)
100 110
So, to get the solution to boil, you have to heat it hotter.
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Boiling elevation = ΔTbp
ΔTbp = Kbpmsolute “molal boiling point elevation constant”
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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
These can be molecules, or separate ions of ionic solute.
i=2 =3 =3 =1
for NaCl for CaCl2 for H2SO4 for ethylene glycol
ΔTbp = iKbpmsolute 2/11/2009
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C D
B A
0.082 mol AlCl3 4 mol ions 0.328 mol ions x kg mol AlCl3 kg 0.10 mol CaBr2 3 mol ions 0.30 mol ions x kg mol CaBr2 kg 0.090 mol Al ( NO3 )3 4 mol ions 0.36 mol ions x kg mol Al ( NO3 )3 kg
0.39 mol gluc 1 mol molec 0.39 mol molec x kg mol gluc kg 2/11/2009
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Which aqueous solution (each 0.50 m) has the highest boiling point? Ethylene glycol Chromium (III) nitrite Sodium chloride Calcium sulfite
23 19
lfi te su
or id
um al ci C
di um
ch l
)n So
(II I
gl le ne
hr om iu m
C
Et hy
2
e
itr ite
2
yc ol
1. 2. 3. 4.
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Which aqueous solution (each 0.50 m) has the highest boiling point? 1. 2. 3. 4.
Ethylene glycol I = 1 Chromium (III) nitrite Cr(NO2)3 Sodium chlorate NaClO3 i = 2 Calcium sulfite CaSO3 i = 2
i =4
(Please review ion charge, name, and formula.)
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Effects of (non-volatile) solutes: 1. Vapor pressure lowering (Raoult’s Law) 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure lowering
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Solute molecules or ions BLOCK access to the solid surface
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Phase diagram of water & solutions – melting point depression
Must cool to -4°C (or whatever) to get ice again.
Pure water + ice @0.0°C/1 atm Solution (no ice) @0.0°C/1 atm
P (atm)
1.00
New triple point @lower temp/pressure
0.0 2/11/2009
T (°C) -> 20
Phase diagram of water and solutions – melting point depression
Liquid phase for pure liquid Liquid phase for solution
P (atm)
1.00
0.0 2/11/2009
T (°C) -> 21
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Ice crystals in seawater are nearly fresh.
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Ice metamorphoses over a period of weeks and months, which eliminates salt impurities within the crystals.
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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”.
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
These can be molecules, or separate ions of ionic solute.
i=2 =2 =3 =1
for CsI for HCl for Ca(ClO4)2 for sucrose
ΔTfp = iKfpmsolute 2/11/2009
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Which aqueous solution (each 0.50 m) has the lowest freezing point? Sodium bromide Sodium chlorate Calcium chlorate Cobalt (III) nitrate
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te
or at e
(II I)
ch C
ob
al t
um al ci C
So
di um
ch l
br om di um So
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2 lo ra te
2
ni tr a
5
id e
1. 2. 3. 4.
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Which aqueous solution (each 0.50 m) has the lowest freezing point? 1. 2. 3. 4.
Sodium bromide NaBr i = 2 Sodium chlorate NaClO3 i = 2 Calcium chlorate Ca(ClO3)2 i = 3 Cobalt (III) nitrate Co(NO3)3 i = 4
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i=1 62.1
62.1
g/mol
T fp K fp m solute m solute
m solute
T fp K fp
T fp K
fp
- 1 . 155 o C - 1 . 86 C / m o
0 . 6210 m
0.6210 mol solute x 1kg H 2O kg solvent 293.0 g H 2O ´ 1000 g H 2O 0.6210 mol solute ´ 0.2930kg H 2O 0.18194 mol x kg solvent
msolute
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M
g 11.30 g 62.12 g / mol 62.1 g / mol mol 0.18194 mol
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Which one of the following is the most reasonable formula for this compound?
C5H2 Not enough H’s C4H14 Too many H’s C 2H 6O 2 C 3H 6O C3H8O Formula wrong for 62 g/mol
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C3 H8 O
C4 H1 4
C3 H6 O
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10
C5 H2
1. 2. 3. 4.
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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.
3. C3H6O
C 2H 6O 2 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…
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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
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Osmotic pressure = extra pressure that must be applied to the SOLUTION SIDE of semi-permeable membrane to EQUALIZE solvent flow.
2/11/2009
OSMOSIS = diffusion through a semi-permeable membrane.
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OSMOTIC Pressure = (atm) = iMsoluteRT i = Moles of particles per mole of solute
