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Boiling points of different mixtures
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Chemistry 314
Experiment 5: Boiling Points
of Mixtures
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
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.
DvapH: the energy necessary to convert a mole of a pure liquid into a mole of gas
always positive why? The equilibrium vapor pressure, P, of a pure liquid
at external pressure, Pex, increases with temperature until P = Pex
At this point, the liquid boils; that is, the boiling point of a pure liquid is the temperature at which P = Pex.
If Pex =1 atm, the boiling temperature is the normal boiling point.
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 individual
components.
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
Four Physical-Chemical Relationships
Related to Boiling Point
I. Clausius-Clapeyron Equation
Relates the vapor pressure and temperature of a pure liquid
II. Dalton's Law of Partial Pressures
Relates the partial pressures of each gas in a mixture
III. Raoult's Law
Treats the vapor pressure of a solvent with added solute
IV. Henry's Law
Relates the vapor pressure of a volatile solute to its equilibrium concentration in solution
Also treats the vapor pressure of a solution with volatile solute
I. Clausius-Clapeyron
The Clausius-Clapeyron Equation relates the vapor pressure of a pure compound, P, to temperature, T:
ln (P1/P2) = - (DvapH/R)( 1/T1 - 1/T2 ) (1)
Pn is the equilibrium vapor pressure at temperature Tn
DvapH is the enthalpy change for the vaporization process (liquid gas)
R is the universal gas constant (8.31 J/molK)
The equation incorporates three assumptions:
(a) DvapH is essentially independent of temperature.
(b) The volume of a mole of liquid, compared to that of a mole of vapor, is insignificant.
(c) The vapor behaves as an ideal gas.
Since DvapH is positive, vapor pressure always increases as temperature increases.
I. Clausius-Clapeyron The vapor pressure for any one liquid
increases exponentially with increasing T
The vapor pressure of one liquid
compared to another depends inversely
on DvapH
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
DvapH are known, the pressure at
any other T can be trivially
calculated
If two or more gases (which do not react with each other) are enclosed in a vessel, the total pressure exerted by them is equal to the sum of their partial pressures
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 mixture
Consequence:
A mixture boils when:
PA + PB = Ptot = Pext
II. Daltons Law
When a nonvolatile solute is added to a volatile solvent, the vapor pressure of the solution will be lower than that of the pure solvent
The net vapor pressure is proportional to the solvents mole fraction
PA = cAPA (3)
PA is the vapor pressure of the solvent in the solution
cA is the mole fraction of solvent (solvent moles divided by total moles solution)
PA is the pure solvent's vapor pressure at the same
temperature
Equation 3 represents "ideal behavior Raoults Law is accurately obeyed if the mole fraction
of the solvent is near 1 (the solvent is nearly pure)
III. Raoults Law
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?
III. Raoults Law
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 x 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
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, the vapor pressure of the
solution is less than the vapor pressure of the pure
solvent (at constant temperature) because csolvent < 1.
Thus, a higher temperature is required for P to reach Pext
III. Raoults Law
III. Raoults Law
1 Mole fraction of B 0
Raoults Law also governs the vapor pressure of mixtures of
two or more volatile
components
The vapor pressure of each is
determined by its mole fraction
and the vapor pressure of the
pure component
The total vapor pressure
always lies between the vapor
pressures of the pure
components, and is determined
by the mixture composition
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]
PA = kccA PA = kmmA
Henrys Law holds for any sufficiently dilute solution
IV. Henrys Law
The vapor pressure is proportional to the
concentration, as in Raoults Law, but the proportionality constant is different
Why?
IV. Henrys Law
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?
Raoults Law and Henrys Law
Note that both Laws are in
play in any real mixture.
The vapor pressure of nearly
pure solvents is described by
Raoults Law, and that of very dilute solutes is described by
Henrys Law.
In between those two
regimes, the vapor pressures
of most real solutions differ
dramatically from ideal
behavior particularly if the two components have
significant attractive or
repulsive forces
A mixture consisting of two separate liquid phases insoluble in one another is not governed by these laws
As each liquid exists in its own pure state, each contributes its pure vapor pressure to the total pressure of the system
The vapor pressure of the mixture is the sum of the vapor pressures of each component
The mixture boils when the total vapor pressure reaches Pex
Hence, the mixture always boils at a lower temperature than either component!
The boiling point does not depend on the relative quantities of immiscible phases, since the vapor pressure of a component only depends on its being present
Mixtures of Immiscible Liquids