MELTING POINT
DETERMINATION
Physical vs. Chemical Properties
• Chemical Properties – describe the
chemical interactions of a substance with
other compounds and involves chemical
change
• Physical Properties – can be measured
and observed without changing the
chemical composition of a substance (no
bond breaking!)
Physical Properties
A few examples used in laboratory:
• Melting Point
• Boiling Point
• Index of Refraction
• Density
• Specific rotation
• Solubility
Uses of Physical Properties
• Part of an identification procedure
• As a relative measure of purity
What is a melting point?
• Definition – the temperature at which the liquid
and solid phases exist in equilibrium without a
change in temperature.
• Heat is required for the phase change so heat
can be added or removed without changing the
temperature as long as matter is changing from
solid or liquid or the reverse.
• A melting point is also a function of
atmospheric pressure.
How is a melting point
reported?
• A melting point is reported as the
temperature range from the first
appearance of liquid to the disappearance
of all solid
• For pure compounds, the range should
not exceed 1oC.
Effect of Impurities
all liquid
Liquid + solid A
Liq +
solid B
Melting begins here – the
melt composition is that of
the eutectic
all solid
Mole fraction of B
Vapor Pressure of Mixture =
(Mole Fraction of A) * (VP of pure A) +
(Mole Fraction of B) * (VP of pure B)
Eutectic Mixture =
a mixture of substances with a
unique composition which exhibits a sharp
melting range as though it were a pure
compound – a constant melting mixture.
Therefore, a solid with a sharp melting
point (narrow melting range) is not always
one pure compound.
Impure compounds most often have
both –
• a wide melting range and
• a lower melting point than the pure
compound
WHY??
• Impurities decrease vapor pressure, which decreases melting point
(and increases boiling point.)
• Impurities “get in the way” of the intermolecular (between molecule)
bonds that would be holding the solid together.
• Therefore, impurities weaken the intermolecular bonds holding the
solid together, and consequently, it takes less energy to pull the
molecules apart (which means it will melt at a lower temperature).
Mixed Melting Point Technique
(Part of an identification procedure ...)
If you suspect you know the identity of a
solid –
(1) Thoroughly mix some of it with the
compound you think it may be.
(2) Determine the melting point.
If the two solids are the same - there will be
no depression of melting point and no
widening of the range.
Factors Affecting
Observed Melting Point
• Quantity of sample – Use the smallest
sample you can clearly observe.
• Particle size/Packing - Small particles
of reasonably uniform size – you may
need to grind the sample and pack it
without air pockets
• Rate of heating –1oC per minute
These can cause a lag time in heat transfer to
the solid or throughout the body of the solid!
Factors Affecting
Observed Melting Point (cont’d)
• Chemical Characteristics of Sample –
Some compounds decompose before melting –
color changes or gas evolution – and some
sublime.
• Purity of Sample
Thermometer Calibration
• Submersion line
• Graph of observed melting points of
known compounds comparing them to the
literature values
Steps for Melting Point (Range)
Determination:
• Perfect your technique. You know (Table of
Physical Properties) where pure benzoic acid
should melt. Carefully determine the melting
point of your purified sample. (Remember you
will need to use this information in both Expt I
and Expt II).
• Record the “unknown” letter in your notebook
and run a “fast” determination.
• Cool melting point apparatus 10-15oC below the
“approximate” melting point of the compound &
repeat. Report the melt as a range.
Completion …
You will not actually identify an unknown
by melting point. The goal is to get the
best/most accurate melting point.
How do you know you did?
• Include in your PROCEDURE:
– Instructions for operating the Electrothermal 9100
MP apparatus (pdf file on Moodle)
– Write instructions for how to prepare & run your
samples (see pg. 36 & handout)
– Figure 2.16 on pg. 37 in textbook (shows how to
properly prepare a MP sample)
• The data table @ bottom of the handout should go in
your DATA & OBSERVATIONS section.
• You will be doing the following…
1) Determining the MP of ONE standard (pure) compound.
2) Determining the MP of ONE unknown compound.
3) Determining the MP of your purified benzoic acid (from Exp. 1)
• Compounds to include in Table of
Chemical Properties:
–
–
–
–
Benzoic acid
Urea
Acetanilide
Naphthalene
‒ Salicylic acid
‒ 4-Nitrotoluene
‒ Benzophenone
‒ Biphenyl
‒ Trans-Cinnamic acid
‒ 3,5-Dinitrobenzoic acid
Remember:
• Weigh purified benzoic acid from last
week and record mass in NB!
• Finish Calculations and Discussion
sections for Exp. 1.
• Complete NB for Exp. 2.