chapter 6 chemical potential in mixtures › staff › people › duesberg › asin... · chapter 7...
TRANSCRIPT
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 1
Chapter 6
Chemical Potential in
Mixtures
Partial molar quantities
Thermodynamics of mixing
The chemical potentials of liquids
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Concentration Units
• There are three major concentration units in use in thermodynamic descriptions of solutions.
• These are – molarity – molality – mole fraction
• Letting J stand for one component in a solution (the solute), these are represented by
• [J] = nJ/V (V typically in liters) • bJ = nJ/msolvent (msolvent typically in kg) • xJ = nJ/n (n = total number of moles of all species
present in sample)
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 3
PARTIAL MOLAR QUANTITIES
Recall our use of partial pressures:
p = xA p + xB p + …
pA = xA p is the partial pressure
We can define other partial quantities…
For binary mixtures (A+B): xA + xB= 1
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 4
PARTIAL MOLAR QUANTITIES: Volume
The contribution of one mole of a substance to the volume of a
mixture is called the partial molar volume of that component.
...,,, BA nnTpfV
AnTpA
An
VV
,,
...
B
B
A
A
dnn
Vdn
n
VdV
At constant T and p
In a system that contains at least two substances, the total value of
any extensive property of the system is the sum of the
contribution of each substance to that property.
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 5
Partial molar volume
Very Large Mixture of A and B
Add nA of A to mixture
When you add nA of A to a large mixture of A and B, the composition remains
essentially unchanged. In this case: VA can be considered constant and the volume
change of the mixture is nAVA.
Likewise for addition of B.
constn
VV
AnTpA
A
,,
The total change in volume is nAVA + nBVB . (Composition is essentially unchanged).
... BBAA nVnVV
Scoop out of the reservoir a sample containing nA of A and nB of B its volume is
nAVA + nBVB . Because V is a state function:
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 6
Partial molar volume
A different answer is obtained if we add 1 mol of water to a large volume
of ethanol.
3
,OH
OH cm18
2
2
Tpn
VVThe change in
volume is 18cm3
3
OH)CHCH(,,OH
OH cm14
232
2
nTpn
VVThe change in
volume is 14cm3
Example: What is the change in volume of adding 1 mol of water to a
large volume of water?
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Introduction to mixtures
Homogeneous mixtures of a
solvent (major component) and
solute (minor component).
Introduce partial molar property:
contribution that a substance makes
to overall property.
V = nAVA + nBVB
The partial molar volumes of water
and ethanol at 25C. Note the different
scales (water on the left, ethanol on
the right).
VA is not generally a constant: It
is a function of composition,
because the environment and
therefore their interaction of the
molecules changes with the
composition .
:
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Exercise Calculate the density of a mixture of 20 g of water and 100g of ethanol.
MM: EtOH: 46.07g/mol d=0.789 g/ml
• First calculate the mole fractions.
– 20 g H2O = 1.11 mol;
– 100 g EtOH = 2.17 mol
– xH2O = 0.34; xEtOH = 0.66
• Then interpolate from the mixing curve
– VH2O = 17.1 cm3 mol-1;
– VEtOH = 57.4 cm3 mol-1
• Then plug the moles and partial molar volume
– (1.11 mol)(17.1 cm3/mol) + (2.17 mol)(57.4 cm3/mol) =18.1 cm3 + 125 cm3 = 143.1 cm3
• Finally, the total mass is divided by the
total volume: 120 g/143 cm3 = 0.84 g/
cm3
Task: You mix 300 ml alcohol with a bottle of juice (water, 0.7 l) - Will you
really end up with for your x-mas party supply (1l)? -Of course not…
- How much do you have to mix to get 1 l of the same strength?
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 9
VA is not generally a constant: It is a function of composition, because
the environment and therefore their interaction of the molecules changes with the composition :
:
PARTIAL MOLAR QUANTITIES
The partial molar volume of a substance is the slope of the variation of the total volume of
the sample plotted against the composition. In general, partial molar quantities vary with
the composition, as shown by the different slopes at the compositions a and b. Note that
the partial molar volume at b is negative: the overall volume of the sample decreases as a is
added.
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10
PARTIAL MOLAR QUANTITIES
• We can extend the concept of partial molar properties to state functions, such as Gibbs energy, G. Or Chemical Pot m
G = nAGA + nBGB or G = nAmA + nBmB
m1 m2
Consider this single substance system:
dG = -m1 dn dG = +m2 dn
Total dG = 0 only if m1 = m2
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 11
The Partial Molar Gibbs Energy
Ap,T,nA
An
Gμ
The partial molar Gibbs energy is called the “chemical potential”
...
B
B
A
A
dnn
Gdn
n
GdG
G = nAµA + nBµB
At constant T and p
... BBAA dndndG mm
(At equilibrium dG = 0)
The implication of G = f ( p,T, ni ) is that we now write:
iidnVdpSdTdG m
... BdndnVdpSdTdG BAA mm
Fundamental
Equation of Chemical
Thermodynamics.
...,,, BA nnTpfG
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 12
The chemical potential of a substance is the slope of the total Gibbs energy
of a mixture with respect to the amount of substance of interest. In
general, the chemical potential varies with composition, as shown for the
two values at a and b. In this case, both chemical potentials are positive.
iidndG m
In case of constant
temperature and pressure
reduces to: iidnVdpSdTdG m
Chemical Potential = Partial Molar Gibbs Energy
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 13
The Wider Significance of the Chemical Potential
i
npTi
dnn
GVdpSdTdG
j,,
i
npSi
dnn
HVdpTdSdH
j,,
i
nVTi
dnn
ApdVSdTdA
j,,
i
nVSi
dnn
UpdVTdSdU
j,,
mi
jnpTin
G
,,
jnpSin
H
,,
jnVTin
A
,,
jnVSin
U
,,
All extensive thermodynamic properties change with composition!
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chem "chemical work"j j
j
dw dnm Example: E-chem cell:
Chemical Potential
iiadd dndGdw mmax,
In case of constant temperature
and pressure also:
j j
j
j j
j
j j
j
j j
j
dU TdS PdV dn
dH TdS VdP dn
dA SdT PdV dn
dG SdT VdP dn
m
m
m
m
, , , ,
, , , ,
j i j i
j i j i
i
i iS V n S P n
i iT V n T P n
U H
n n
A G
n n
m
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 15
2211 nnG mm
22112211 mmmm dndndndndG
The Gibbs-Duhem Equation
by differentiating:
At equilibrium: dG = 0 recall in case of constant temperature
and pressure => m1dn1 + m2dn2 = 0
22110 mm dndn
2
1
21or mm d
n
nd
The Gibbs-Duhem Equation. Mixtures
can not be change independentely: If
one is changed the other changes as
well…
A similar expression may be deduced
for all partial molar quantities
...2211 dndndG mm
(For binary systems)
iidndG m
iidnor m0
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 16
The Gibbs-Duhem Equation
0j j
j
SdT VdP n dm
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 17
THE THERMODYNAMICS OF MIXING
Imagine a system of two perfect gases in amounts nA and nB at equal T and p
are separated by a barrier. The initial total Gibbs energy of the system Gi is given by
Gi = nAµA + nBµB
m
m
p
pRTn
p
pRTnG
ln
ln
BB
AAi
The initial and final states considered in the
calculation of the Gibbs energy of mixing of
gases at different initial pressures.
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Duesberg, Chemical Thermodynamics
After mixing each gas exerts a partial pressure pJ, where pA + pB = p. The
final G is given by
The Thermodynamics of mixing
m
m
p
pRTn
p
pRTnG
BBB
AAAf
ln
ln
ifmix GGG
NA, p, T NB, p, T
pA+ pB = p,T
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 19
mmp
pRTn
p
pRTn ln ln BBAA
mmp
pRTn
p
pRTnGG B
BBA
AAif ln ln
p
pRTn
p
pRTn B
BA
A lnln
Gmix
The Thermodynamics of mixing
NA, p, T NB, p, T
pA+ pB = p,T
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 20
mixG = nRT (xA ln xA + xB ln xB) mixG is always negative, since x<1
The Thermodynamics of Mixing
The Gibbs energy of mixing of two
perfect gases and (or of two liquids
that form an ideal solution). The
Gibbs energy of mixing is negative
for all compositions and
temperatures, so perfect gases mix
spontaneously in all proportions.
Writing xA for the mole fraction of
component A
nA = xA n therefore pA /p = xA,
(same for any component), so
n
n
n
n A
x
A
p
pRTn
p
pRTnGmix
BB
AA lnln
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21
Gas mixtures What About the Enthalpy of
Mixing?
G = H – TS
Compare
Gmix = nRT {xAln xA+ xB ln xB}
Smix = − nR {xAln xA+ xB ln xB}
Therefore
H mix= 0
All due to the effect of entropy
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 22
)ln ln (
ΔΔ
BBAA
,,
mixmix
BA
xxxxnR
T
GS
nnp
The Entropy of Mixing
which is positive as xA and xB are <1.
Perfect gases mix spontaneously in all proportions
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Duesberg, Chemical Thermodynamics
Exercise: at 25C, calculate the Gibbs energy change when the
partition is removed
(cannot used the previous one as different starting pressures)
pRTpRT
pRTnpRTnG
NH
BBBAAAi
ln1)3ln(3
lnln
22
00
00
mm
mm
BfBBAfAAf pRTnpRTnG ,
0
,
0lnln mm
mol 1mol, 3 BA nn
pppp BA 1, 3 pp f 2 pppp BfAf 21
,23
, ,
21
21
3
3
ln4ln1ln3
lnln
23
,,
RTRT
RTnRTnGGGp
p
Bp
p
Aifmix
BfAf
kJ8.6)69,0(4298/314.8 KKJGmix
pRTpRT NH 210
230 ln1)ln(3
22 mm
KT
KJR
298
/314.8
69.0ln 21
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24
Chemical Potential of Liquids
We need to know how the Gibbs energy of a liquid varies
with composition in order to discuss properties of liquid
mixtures (like solutions).
Pure Liquid
= A
*
APMixture
= A = B
Vapor Pressure =
P
P
AAART*
ln* mm
For vapor phase:
APPartial Pressure =
P
P
AAART ln mm
For vapor phase:
At equilibrium…
)()( ** lg AA mm
At equilibrium…
)()( lg AA mm
P
P
AAART*
ln* mm
For liquid phase:
P
P
AAART ln mm
For solution:
Combine these expressions…
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25
Total Vapour Pressure of an ideal solution
*
* lnA
AAA
P
PRT mm
*
AAA PxP AAA xRT ln* mm
This serves to define an ideal solution if true for all values of xA
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Duesberg, Chemical Thermodynamics
For SOME pairs of liquids, RAOULT'S LAW that
pA = xA pA* is obeyed, so (xi= moluar fraction, * for pure substance)
RAOULT'S LAW
µA(l) = µA*(l) +RT ln pA/pA
*
pA = xA pA*
µA(l) = µA*(l) + RT ln xA .
This defines an IDEAL SOLUTION)
BAtotal PPP
**
BBAAtotal PxPxP
)( ***
ABBAtotal PPxPP
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27
Ideal Mixtures
• Two types of molecules are randomly distributed
• Typically, molecules are similar in size and shape
• Intermolecular forces in pure liquids & mixture are similar
• Examples: benzene & toluene, hexane & heptane
In ideal solutions, the partial vapor pressure of component
A is simply given by Raoult’s Law:
*
AAA PxP
vapor pressure of pure A mole fraction of A in solution
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 28
The Measurement of Vapor Pressure of Solutions
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 29
Similar liquids can form an
ideal solution obeying
Raoult’s Law
Raoult's law
A pictorial representation of the molecular
basis of Raoult's law. The solute hinder the
escape of solvent molecules into the vapour, but
do not hinder their return
Large spheres = solvent molecules
small spheres = solute molecules.
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Example
Chapter 7 : Slide 30
• A solution is prepared by dissolving 1.5 mol C10H8 in 1.00 kg
benzene. The v.p. of pure benzene is 94.6 torr at this
temperature (25oC). What is the partial v.p. of benzene in the
solution?
• Solution: We can use Raoult’s law, but first we need to know
the mole fraction of benzene.
• MM benzene = 78.1 g/mol, so 1.00 kg = 12.8 mol.
• xbenz = 12.8 mol / (12.8 mol + 1.5 mol) = 0.895
• pbenz = xbenz p*benz = (0.895)(94.6 torr) = 84.7 torr
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31
Deviations from Raoult’s Law
Methanol, ethanol, propanol
mixed with water. Which one
is which? (All show positive
deviations from ideal behavior)
CS2 and dimethoxymethane: Positive
deviation from ideal (Raoult’s Law) behavior. trichloromethane/acetone: Negative
deviation from ideal (Raoult’s Law) behavior.
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32
Raoult and Henry
1 as * AAAA xPxP
0 as , AAHAA xkxP
Raoult’s law
Henry’s law constant: *
, AAH Pk
The Henry’s law constant reflects the intermolecular interactions between
the two components.
Solutions following both Raoult’s and Henry’s Laws are called ideal-dilute
solutions.
Henry’s behavior:
Henry’s law (Raoult’s Law)
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33
Ideal-dilute solutions Raoult’s law generally describes well solvent vapour pressure when
solution is dilute, but not the solute vapour pressure
Experimentally found (by Henry) that vp of solute is proportional to its mole fraction, but proportionality constant is not the vp of pure solute.
Henry’s Law
pB = xBKB
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 34
Ideal-Dilute Solutions (Henry’s Law)
For real solutions at low concentrations i.e. xB << xA the vapor pressure of
a the solute is proportional to its mole fraction but the proportionality
constant is not pA* but some empirical constant KB
pB = xBKB Henry’s Law
In a dilute solution, the solvent molecules are in an
environment that differs only slightly from that of the
pure solvent. The solute particles, however, are in an
environment totally unlike that of the pure solute.
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35
Gas solubility
KH/(kPa m3 mol1
)
Ammonia, NH3 5.69
Carbon dioxide, CO2 2.937
Helium, He 282.7
Hydrogen, H2 121.2
Methane, CH4 67.4
Nitrogen, N2 155
Oxygen, O2 74.68
Henry’s law constants for gases dissolved in water at 25°C
Concentration of 4 mg/L of oxygen is required to support aquatic life,
what partial pressure of oxygen is required for this?
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Application-diving
Table 1
Increasing severity of nitrogen narcosis symptoms with depth in feet and
pressures in atmospheres.
Depth P Total P N2 Symptoms
100 4.0 3.0 Reasoning measurably slowed.
150 5.5 4.3 Joviality; reflexes slowed; idea
fixation.
200 7.1 5.5 Euphoria; impaired concentration;
drowsiness.
250 8.3 6.4 Mental confusion; inaccurate
observations.
300 10. 7.9 Stupefaction; loss of perceptual
faculties.
Gas narcosis caused by nitrogen in normal air dissolving into nervous
tissue during dives of more than 120 feet [35 m] Pain due to expanding or contracting trapped gases, potentially leading to
Barotrauma. Can occur either during ascent or descent, but are potentially
most severe when gases are expanding.
Decompression sickness due to evolution of inert gas bubbles.
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 37
The experimental partial vapour
pressures of a mixture of
chloroform (trichloromethane) and
acetone (propanone) based on the
data in Example 7.3. The values of
K are obtained by extrapolating the
dilute solution vapour pressures as
explained in the Example.
Excess Functions:
We discuss properties of real solutions in terms excess functions
XE. The excess entropy for example is defined as:
S E = mix S- mix S ideal.
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 38
SUMMARY • Partial molar quantities and the Gibbs-Duhem equation. Tells us how
chemical potentials vary with composition of a mixture. • Chemical potentials µ of liquids are accessed via µ for the vapor in
equilibrium.
• Raoult's Law, Henry’s Law
• Real and ideal gases activity
• In general µ = µs + RT ln a.
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 39
Colligative Properties
Colligative properties are the properties of dilute solutions that
depend only on the number of solute particles present.
They include:
1. The elevation of boiling point
2. The depression of boiling point
3. The osmotic pressure
All colligative properties stem from the reduction of the solvents m by
the presence of the solute.
µA(l) = µA*(l) + RT ln xA
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40
Colligative properties
Chemical potential of a solution (but not vapour or solid) decreases by a factor (RTlnxA) in the presence of solute
Molecular interpretation is based on an enhanced molecular randomness of the solution
Get empirical relationship for FP and BP (related to enthalpies of transition)
mKT
mKT
bb
ff
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Boiling-point elevation Tb = KbbB
Freezing-point depression Tf = KfbB
where bB is the molality of the solute B in the solution
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 42
The Elevation of Boiling Point
µA*(g) = µA
*(l) + RT ln xA
Want to know T at which:
Presence of a solute at xB causes an increase in the boiling temp from
T * to T * + T where:
T = KxB H
RTK
vap
2*
T = K bb b is the molality of the solute (proportional to xB).
Kb is the ebullioscopic constant of the solvent.
Identical arguments lead to: T = K f b where Kf is the cryoscopic constant.
The Depression of Freezing Point
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 43
Examples:
7.8(b) Calculate the cryoscopic and ebullioscopic constants of
naphthalene.
7.10(b) The addition of 5.0 grams of a compound to 250 grams of
naphthalene lowered the freezing point of the solvent by 0.780 K.
Calculate the molar mass of the compound.
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 44
Osmosis – for Greek word “push” Spontaneous passage of a pure solvent into a solution separated from it
by a semi-permeable membrane (membrane permeable to the solvent,
but not to the solute)
Osmotic pressure – P – the pressure that must be applied to the solution
to stop the influx of the solvent
Van’t Hoff equation:
P = [B] R T
where [B] = nB/V
Osmometry - determination of molar mass by measurement of
osmotic pressure – macromolecules (proteins and polymers)
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 45
Solvent A with chemical
potential mA*(p)
Semipermeable
Membrane
Height
Proportional
to Osmotic
Pressure P
Solution
Osmosis
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 46
It is assumed that the van’t Hoff equation is only the first term of a virial-
like expression:
P = [B] R T {1 + B[B] + . . . }
Osmosis
EXAMPLE: Using Osmometry to determine the molar mass of a macromolecule.
Osmotic pressures of PVC in cyclohexanone are given below. The pressures are given in
terms of heights of solution (with r = 0.980 g cm-3) in balance with the osmotic pressure. Determine the molar mass of the polymer.
c / (g L-1) 1.00 2.00 4.00 7.00 9.00
h / cm 0.28 0.71 2.01 5.10 8.00
c is the mass concentration
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 47
Activities How can we adjust previous equations to account for deviations
from ideal behavior?
1. solvent
2. solute
Solvent activity:
General form of the chemical potential of a real OR ideal solvent:
mA = mA* + RT ln (pA/pA*)
Ideal solution – Raoult’s law is obeyed:
mA = mA* + RT ln xA i.e xA = pA/pA*
Real solution – we can write:
mA = mA* + RT ln aA
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 48
aA is the activity of A – essentially an “effective” mole fraction
aA = pA/pA*
As xA 1 (solute concentration is nearing zero), aA xA,
aA = gA xA gA 1 and aA xA
Solvent activity
Illustration: The vapor pressure of 0.5 M KNO3 (aq) at 100oC is 749.7 Torr, what is the
activity of water in the solution?
For a real solution, we can write:
mA = mA* + RT ln xA + RT ln gA
Standard state is the pure liquid solvent at 1 bar
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 49
Solute activity approach ideal dilute (Henry’s law) behavior as xB 0
Ideal-dilute: pB = KB xB
mB = mB* + RT ln (pB/pB*)
= mB* + RT ln (KB /pB*) + RT ln xB
The second term on the rhs of the above equation is composition
independent, so we may define a new reference state:
mB
+ = mB* + RT ln (KB /pB*)
So that:
mB = mB+ + RT ln xB
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 50
Real solutes – permit deviations from ideal-dilute behavior
mB = mB+ + RT ln aB
Where aB = pB/KB and aB = gB xB
Note: As xB 0, aB xB and gB 1
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 51
Measuring Activity
Use the following information to calculate the activity and activity
coefficient of chloroform in acetone at 25oC, treating it first as a
solvent and then as a solute with KB = 165 Torr.
xC 0 0.2 0.4 0.6 0.8 1.0
pC / Torr 0 35 82 142 200 273
a 0 0.13 0.30 0.53 0.73 1.0
g 0.65 0.75 0.87 0.91 1.0
a 0 0.21 0.50 0.86 1.21 1.65
g 1 1.05 1.25 1.43 1.51 1.65
Chloroform regarded as solvent
Chloroform regarded as solute
a = p / p*
a = p / KB
g = a / xC
g = a / xC
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 52
Activities in terms of molalities, bB:
mB = mB+ + RT ln xB
mB+ = mB* + RT ln (KB /pB*)
For an ideal-dilute solute we had written in terms of mole fractions:
with
Molality in terms of mole fraction:
bB = nB / MA MA = nA * Mr(A)
xB = nB / (nA+nB) nB / nA
bB = nB / ( nA * Mr(A) )
bB = xB / Mr(A) xB = bB Mr(A)
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 53
Activities in terms of molalities, bB:
xB = bB Mr(A)
mB = mB+ + RT ln xB
mB = mB+ + RT ln bB + RT ln Mr(A)
mB = mB + RT ln bB
mB = mB at standard molality b = 1 mol kg-1
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Duesberg, Chemical Thermodynamics
Chapter 7 : Slide 54
To allow for deviations from ideality we introduce (in the normal way)
aB = gB bB (assuming unit-less)
Then:
mB = mB + RT ln aB
As bB 0, mB -∞
In other words, as a solution becomes increasingly diluted, the
solution becomes more stabilized – It becomes difficult to remove
the last little bit of solute.
Activities in terms of molalities, bB:
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55
Osmotic pressure
Van’t Hoff equation
MRTP
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56
Phase diagrams of mixtures
We will focus on two-component systems
(F = 4 ─ P), at constant pressure of 1 atm
(F’ = 3 ─ P), depicted as temperature-composition diagrams.
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57
Fractional Distillation-volatile liquids
Important in oil refining
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58
Exceptions-azeotropes Azeotrope: boiling without changing
High-boiling and Low-boiling
Favourable interactions between components
reduce vp of mixture
Trichloromethane/propanone
HCl/water (max at 80% water, 108.6°C)
Unfavourable interactions between
components increase vp of mixture
Ethanol/water (min at 4% water, 78°C)
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59
Liquid-Liquid (partially miscible)
Hexane/nitrobenzene as example
Relative abundances in 2 phases given by Lever Rule
n’l’ = n’’l’’
Upper critical Temperature is limit at which phase separation occurs. In thermodynamic terms the Gibbs energy of mixing becomes negative above this temperature
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60
Other examples
Water/triethylamine
Weak complex at low temperature disrupted
at higher T.
Nicotine/water
Weak complex at low temperature disrupted
at higher T. Thermal motion homogenizes
mixture again at higher T.
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Liquid-solid phase diagrams