MELTING POINTS and BOILING POINTS

 

Melting points

melting point

is a characteristic physical property

like MM, boiling point, density

 

pure substance will melt over a narrow range

usually <1oC

ex. 125-125.5oC

impure substance will melt over a lower, wider range

 

Melting points are used for three reasons.

1. To help characterize (identity) an unknown sample.

2. Recorded for new compounds for future identity by

others.

3. To determine the purity of a compound.

(A solid is considered pure, if its melting point does not

rise after recrystallization.)

 

Factors that determine melting point.

1. Intermolecular forces.

Covalent compounds most melt < 300oC

London forces

dipole-dipole attraction

H- bonding

Ionic compounds have much higher melting

pts., ex. NaCl 800oC

Ionic forces

 

 

 

 

 

 

Factors that determine melting point. (continued)

2. Larger molecules have higher melting points

 

methane ethane propane

CH4 CH3CH3 CH3CH2CH3

-184oC -172oC -190oC

 

butane pentane

CH3CH2CH2CH3 CH3CH2CH2CH2CH3

-135oC -131.5oC

 

3. More symmetrical compounds will have higher

melting points. (solid arrangement more orderly)

 

neopentane

pentane isopentane CH3

|

CH3CH2CH2CH2CH3 CH3CHCH2CH3 CH3-C-CH3

| |

CH3 CH3

m.p. m.p. m.p.

-129.72oC -159.9oC -16.55oC

lowest highest

 

however,

branching lowers boiling points

b.p. b.p. b.p.

36.1oC 27.85oC 9.5oC

highest lowest

 

 

 

 

 

Melting Points of Impure Compounds

 

MELTING POINT COMPOSITION CURVE

actually observed

 

| | liquid X + Y |

| | |mp Y

complete mp X| | |

melting | | |

| | |

first drop | | |

of liquid | | ET | eutectic

| | | point

| | solid X + Y | sharp

| | | melting pt

| | |

mol % X 100 75 60 0

mol % Y 0 25 40 100

 

X with Y with

impurity impurity

 

not all binary mixtures form eutectic points

some form more than one

 

Adding successive amounts of an impurity to a pure

substance causes its melting point to decrease in proportion

to the amount of impurity.

 

ET = eutectic point liquid & solid phases have the same

% composition

 

 

Melting Points of Impure Compounds

 

PHASE DIAGRAM for MELTING in a TWO-COMPONENT SYSTEM

 

| homogeneous |

| liquid X + Y solution |

| |tY mp Y

mp X tx| |

| liquid liquid |

| solution solution|

| + solid X + solid Y| ET - eutectic

| | point

| solid X + Y |

| |

mol %X 100 80 60 40 20 0

mol %Y 0 20 40 60 80 100

X with Y with

impurity Y impurity X

 

ET eutectic point - sharp melting point

X & Y & their solutions are in equilibrium

liquid saturated with Y, because 60% X & 40% Y

 

consider mixture of 75% X & 25% Y

temperature below ET mixture is solid X & solid Y

at ET the solid begins to melt

as X begins to melt, Y dissolves in X,

lowering the melting pt.

as more X begins to melt, one begins to see liquid

once all X melts composition of mixture becomes

uniform & mixture melts sharply at point M

melting range somewhere above ET to tM (melting pt of mixture)

Because in practice it is very difficult to detect ET

because it is difficult to detect when an infinitesimal

amount of the solid has started to melt

Recrystallization

enriches predominant compound

results in - higher melting point

-        decreased melting point range

 

impurity must be soluble to depress m.p. (f.p.)

 

insoluble impurity (ex. sand, charcoal) will not depress

m.p.

 

impurity does not need to be solid

 

Mixed m.p. can be used to identify (characterize) compounds

X & Y have same m.p., ex. = 125oC

Is unknown X or Y?

 

Mix X + unknown

Y + unknown

 

m.p. that is lowered is not unknown

X + unknown lowered 122-124oC

Y + unknown same as X & Y 124.5-125oC

unknown is Y

 

Apparatus

Mel-Temp fig. 4.3 p.69

electrically heated Al block that accommodates up to

3 cappillaries

heat rate can be controlled

with a special thermometer can be used up to 500oC

(far above useful limit of silicone oil 350oC)

 

 

 

Filling m.p. capillaries

Grind sample into fine powder

Fill capillary

form small pile

force capillary down into pile

Compress sample

drop down 2 ft length of glass tubing into hard surface

Height of sample 2 to 3 mm

 

Sealed Capillaries

some samples sublime

use capillary or seal small end of a Pasteur pipette

add sample

seal other end while aspirating

 

 

 

 

 

 

 

 

 

 

Determining m.p.

The rate of heating is the most important factor in obtaining

accurate melting points

Heat no faster than 1oC/minute (video 2oC/minute)

Too fast melting point will be too high.

Always record melting point range not melting point

 

 

 

Exp. 1 Calibration of the Thermometer

 

2. Melting Points of Pure Urea and Cinnamic Acid

 

3. Melting Points of Urea-Cinnamic Acid

1:4, 1:1, 4:1

 

4. Unknowns - Identify 1 unknown

 

Part 2. Boiling Point

 

Boiling Point

is a characteristic property, like m.p., MM, density

requires more material

with only a few microliters boiling point range is

difficult to determine

need at least 1-2 mL

less affected by impurities so not as good an indicator of

purity

affected by the same forces as m.p.

ionic attraction

H-bonding

dipole-dipole interactions

London forces

 

 

 

 

 

 

 

 

 

 

 

Factors that affect boiling point

Structures and boiling point

Molar mass

normal saturated hydrocarbons

b.p. increases with MM

-162o CH4 to 330oC n-C19H40

(n-heptane MM = 100, b.p. near 100oC (98.4oC))

Branching lowers boiling point

neopentane

pentane isopentane CH3

|

CH3CH2CH2CH2CH3 CH3CHCH2CH3 CH3-C-CH3

| |

CH3 CH3

b.p. b.p. b.p.

36.1oC 27.85oC 9.5oC

more points of spherical

attraction molecule

less points of

attraction

Boiling point as intermolecular attractions increase

London forces weakest

dipole-dipole interactions

H-bonding

ionic attractions strongest

 

Boiling point as a function of pressure

boiling point (of a pure liquid) is the temperature at which

vapor pressure = atmospheric pressure

boiling point increases as atmospheric pressure increase

and vice versa

altitude of 14,000 ft 81oC

sea level (760 torr) 100oC

death valley or pressure cooker higher

Thermometer

immersion line (3 in 76 mm from bottom of bulb)

the thermometer will record accurate temperature

if immersed to this line.

 

If thermometer breaks immediately inform instructor

Hg vapors are toxic

 

Prevention of Superheating

boiling sticks & boiling stones (chips)

do not use sticks in reaction mixtures

 

Apparatus & Techniques

best by distillation will do in Chapter 5

constant boiling point is no guarantee of purity

there are many constant boiling point azeotropes

ex. 95% ethanol 5% water

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In a reaction tube

 

 

 

thermometer adaptor

 

thermometer

 

 

 

 

refluxing reaction tube

vapor

 

liquid (0.3 mL)

boiling chip

 

No part of the thermometer should touch the reaction tube.

 

Boiling liquid refluxes 3 cm up

condenses & drips down

the thermometer

-        but does not boil out of the apparatus

-         

Droplets of liquid must drip from the thermometer

in order to heat the Hg thoroughly

 

b.p. = highest temperature recorded by the thermometer

that is maintained for 1 minute