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Lab One Grades Lab Two Grades
16.516.5
18.5
19.5
21
21
21.25
21.55
23.25
23.6523.75
24
25.15
25.25
16.919.25
21.5
23.25
23.5
24.25
24.5
25.25
25.25
25.526.4
26.5
27.75
28.65
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Chemistry 314
Experiment 5: Boiling Points
of Mixtures
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Objectives
Measure the boiling point of a mixture of
volatile liquids
Measure the boiling point of a mixture of
immiscible liquids
Use the observations to determine
physical and chemical properties of the
various liquids
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Vapor Pressure of Liquids
The pressure of a gas in equilibrium with its liquid
is the vapor pressure of the liquid.It depends on the tendency of liquid to convert into
a gas.
DHvap: the energy necessary to convert a mole of
a pure liquid into a mole of gasalways positive why?
The equilibrium vapor pressure, P, of a pure liquidat external pressure, Pex, increases with
temperature until P = PexAt this point, the liquid boils; that is, the boilingpointof a pure liquid is the temperature at whichP = Pex.
IfPex
=1 atm, the boiling temperature is thenormal boiling point.
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A liquid mixture can consist of several volatile
substances.
The vapor pressure of a liquid mixture is the sum
of the vapor pressures of the individualcomponents.
With increasing temperature, the vapor pressure of
each component in a liquid mixture increases.
When the total vapor pressure of a mixture
reaches Pex, the mixture boils.
Vapor Pressure of Liquid Mixtures
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Four Physical-Chemical Relationships
Related to Boiling Point
I. Clausius-Clapeyron EquationRelates the vapor pressure and temperature of a pure liquid
II. Dalton's Law of Partial Pressures
Relates thepartial pressures of each gas in a mixture
III. Raoult's Law
Treats the vapor pressure of a solvent with added solute
IV. Henry's LawRelates the vapor pressure of a volatile solute to its equilibrium
concentration in solution
Also treats the vapor pressure of a solution with volatile solute
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Clausius-Clapeyron
The Clausius-Clapeyron Equation relates the vapor
pressure of a pure compound, P, to temperature, T:ln (P1/P2) = - (DHvap/R)( 1/T1 - 1/T2 ) (1)
Pn is the equilibrium vapor pressure at temperature TnDHvap is the enthalpy change for the vaporization process
(liquid gas)Ris the universal gas constant (8.31 J/molK)
The equation incorporates three assumptions:
(a) DHvap is essentially independent of temperature.
(b) The volume of a mole of liquid, compared to that of amole of vapor, is insignificant.
(c) The vapor behaves as an ideal gas.
Since DHvap is positive, vapor pressure always increases astemperature increases.
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Clausius-ClapeyronThe vapor pressure for any one liquid
increases exponentially with increasing T
The vapor pressure of one liquid
compared to another depends inversely
on DHvap
The boiling point of any liquid depends on
the external pressure
If we plot ln P vs. 1/T, a line is
obtained
If the normal boiling point and
DHvap is known, the pressure at
any other T can be trivially
calculated
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If two or more gases (which do not react with each
other) are enclosed in a vessel, the total pressureexerted by them is equal to the sum of their partialpressures
In a mixture composed of two substances, A and B,
PA + PB = Ptot (2)PA is the vapor pressure of A
PB is the vapor pressure of B
and Ptot is the total vapor pressure of the mixtureConsequence:
A mixture boils when:
PA + PB = Ptot = Pext
Daltons Law
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When a nonvolatile solute is added to a volatile
solvent, the vapor pressure of the solution will belower than that of the pure solvent
The net vapor pressure is proportional to the solventsmole fraction
PA = cAPA
(3)PA is the vapor pressure of the solvent in thesolution
cA is the mole fraction of solvent (solvent molesdivided by total moles solution)
PA is thepure solvent's vapor pressure at the same
temperature
Equation 3 represents "ideal behavior
Raoults Law is accurately obeyed if the mole fractionof the solvent is near 1 (the solvent is nearly pure)
Raoults Law
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When a nonvolatile solute is added to a
solvent, the vapor pressure of the solution
will be lower than that of the pure solvent
Why?
Raoults Law
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At equilibrium, there is a balance
between the rate at which
molecules evaporate and
condense
rate of vaporization = kvap x SA
rate of condensation = kcon x PA
At equilibrium, kcon x PA = kvap x SA
PA = (SA) kvap/kcon
But when we add solute
molecules, some fraction of the
surface becomes unavailable for
evaporation while remaining
available for condensation
Assuming that the solute is evenly
distributed, the fraction of surface
area still available is cA
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Boiling Behavior of Solutions with a Non-volatile Solute
The boiling point of a solution containing non-volatile
solutes is higher than the boiling point of a pure solvent.
Why?
According to Raoult's Law, vapor pressure of the solution is
less than vapor pressure of the pure solvent (at constant
temperature) because csolvent < 1.
Thus, a higher temperature is required for P to reach Pext
Raoults Law
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Raoults Law
1 Mole fraction of B 0
Raoults Law also governs the
vapor pressure of mixtures oftwo or more volatile
components
The vapor pressure of each is
determined by its mole fractionand the vapor pressure of the
pure component
The total vapor pressure
always lies between the vaporpressures of the pure
components, and is determined
by the mixture composition
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Deals with the dissolution of soluble gases
in a liquid solvent
The vapor pressure of a dissolved solute is
proportional to its concentration
pA = kH[A] (4)
pA = kccAp
A= k
mm
AHenrys Law holds for any sufficiently dilute
solution
Henrys Law
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The vapor pressure is proportional to the
concentration, as in Raoults Law, but theproportionality constant is different
Why?
Henrys Law
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How does the chemical
environment of the
solvent compare to the
chemical environment
of the pure liquid?
How does the chemical
environment of the
solute compare to the
chemical environment
of the pure gas?
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Raoults Law and Henrys Law
Note that both Laws are inplay in any real mixture.
The vapor pressure of nearly
pure solvents is described by
Raoults Law, and that of very
dilute solutes is described byHenrys Law.
In between those two
regimes, the vapor pressures
of most real solutions differdramatically from ideal
behavior particularly if the
two components have
significant attractive or
repulsive forces
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A mixture consisting of two separate liquid phases
insoluble in one anotheris not governed by theselaws
As each liquid exists in its own pure state, eachcontributes its pure vapor pressure to the totalpressure of the system
The vapor pressure of the mixture is the sum of thevapor pressures of each component
The mixture boils when the total vapor pressure reachesPex
Hence, the mixture always boils at a lower temperaturethan eithercomponent!
The boiling point does not depend on relative quantitiesof immiscible phases, since the vapor pressure of acomponent only depends on its being present
Mixtures of Immiscible Liquids