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