mixture boiling point

7/30/2019 mixture boiling point

1/19

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


2/19

Chemistry 314

Experiment 5: Boiling Points

of Mixtures


3/19

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


4/19

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.


5/19

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


6/19

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


7/19

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.


8/19

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


9/19

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


10/19

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


11/19

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


12/19

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


13/19

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


14/19

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


15/19

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


16/19

The vapor pressure is proportional to the

concentration, as in Raoults Law, but theproportionality constant is different

Why?

Henrys Law


17/19

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?


18/19

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


19/19

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

Documents

mixture boiling point