Reminder: These notes are meant to supplement, not replace, the lab manual.
Determination of Melting Points
History and Application:
Since antiquity one of the simplest ways to identify an unknown substance was to
determine some physical property of that substance and compare it with physical
properties of known materials. Physical properties are observable properties that
are determined without changing the composition of the material. Common
physical properties include color, melting point, boiling point, hardness,
conductivity and density. In this lab the melting point will be used to help identify
a material and a mixed melting point will be used to assess the purity of
materials.
1.
Here is some terminology related to melting point.
Melting point: Strictly speaking, the temperature at which the solid and liquid
phases of a given compound or mixture have the same vapor pressure. In
practice, the temperature at which a substance changes from solid to
liquid. (Don’t say “temperature at which a substance melts” to define
melting point; melts is too close to melting to make a good definition.)
Melting point range: The interval from the temperature at which a solid sample
begins to melt and the temperature at which melting is complete.
Phase transition: When a material changes from one phase to another. Common
phase transitions are solid to liquid and liquid to gaseous.
2.
The reasons that chemists determine melting point ranges of compounds
include:
To identify unknown compounds by comparing experimental melting points with
melting points of known substances found in the chemical literature.
To determine the purity of samples. Pure samples will have narrow melting point
ranges, close to the literature value for that substance; impure samples
will have melting point ranges broader and lower than literature values.
(Note: The literature isn’t always right.)
To characterize new compounds (so that others will have a published melting
point range to compare with).
3.
Here are the structures of the three known compounds for this experiment, and
the eight other possible unknown compounds along with their literature melting
pointsi. (Draw all 11 structures in your notebook.)
naphthalene
(80.5oC)
acetanilide
(114.3 oC)
urea
(135 oC)
The structures of the other possible unknowns are:
p-nitrotoluene
(54.5 oC)
salicylic acid
(158 oC)
glutaric acid
pentanedioic acid
(99 oC)
2-methyl benzoic acid
benzoic acid
(107 oC)
(122.1 oC)
sulfanilamide
(butanedioic acid)
(165 oC)
succinic acid
3,5-dinitrobenzoic acid
(188 oC)
(205 oC)
4. Soluble impurities in a solid cause the solid to melt at a lower temperature. This is
true even if the impurity itself melts at a higher temperature. If a pure material
melts at 111-112oC and another pure material melts at 115-117 oC, then a 50:50
mixture of these materials will melt at a lower and broader range. A reasonable
expected range may be 95-109 oC. The exact range observed will depend on
how well the sample is mixed and the heating rate. This range cannot be easily
calculated, but can easily be experimentally determined. The fact that impurities
cause melting points to lower and broaden, allow experimentally determined
melting points to be used to assess the purity of samples. Melting points of
isolated or synthesized compounds will be taken throughout the rest of 2230L
and 2240L to assess the purity of the derived compounds.
5. Phase Diagrams explain the temperature of phase transitions (solid to liquid, liquid to
solid) of mixtures of materials. Below is an example of a phase diagram ii of tin
(Sn) and lead (Pb). Pure lead melts at 327 oC. Pure tin melts at 232 oC. A 50:50
mixture of tin and lead will start to melt at 183 oC and continue melting until
approximately 275 oC. Its melting point range will be 183-273 oC. The point
marked with an asterisk (*) is called the eutectic point. This is the lowest
temperature at which liquid may exist in the mixture. For this mixture the eutectic
point is at 183 oC with a composition of 62% tin and 38% lead.
6. Insoluble materials or materials with a very high melting point, such as sand or glass,
will have no impact on the melting point of a substance.
7. All melting point measurements using the Mel-temps apparatus need to be reported
as a melting point range not a single melting point. The range starts at the
temperature the first bit of liquid is observed. The range ends at the temperature
that all of the solid has disappeared. In order to get an accurate melting point
range it is necessary to have a slow rate of temperature increase.
8. A practical balance must be struck between allotting only a small amount of time for
obtaining a melting point range and a slow temperature increase of the Mel-temp
to obtain good data. A very slow heating rate (2o C/min) will give good data but
will take more than an hour to go from room temperature to 150oC. This is not
reasonable for a 2 hour and 50 minute lab period. If a 20oC/minute is used, it will
only take about 7 minutes, but the data will be very poor to useless. Often a
balance can be struck by ramping the Mel-Temp up quickly to a temperature 20
or 30 degrees under the literature value, then slowing the temperature increase
down to achieve good data. If the literature value is not known, a very quick
melting point can be taken to identify an approximate melting point range, then
with another sample, a slow melting point range is taken starting about 10
degrees below the anticipated value.
9. Detailed instructions on how to fill a capillary tube and take a melting point reading
can be found on this web siteii:
http://orgchem.colorado.edu/hndbksupport/meltingpt/mt.html.
If the capillary is overfilled a poor measurement will result. The depth of sample
in the tube should be no greater than 2 mm or about the thickness of a quarter.
10.Here is a procedure for identifying an unknown compound from melting point data.
First determine the melting point range of the unknown.
Compare the melting point range you determined with literature melting points of
known compounds. As a general guideline, consider any compound as a
possibility whose melting point comes within plus or minus 10° C of the
range of the unknown as a possible ID of the unknown (plus or minus 15°
C if the melting point range is above 200° C). The 10° C range allows for
thermometer inaccuracies, sample packing irregularities as well as
impurities in your unknown. As expertise is gained, these values may be
decreased by 5° C.
Mix a small sample of your unknown with a small sample of three of the possible
ID compounds, and determine the melting point ranges of these mixtures.
The mixture whose melting point range differs the least from the melting point
range of your pure unknown is most likely the same as your unknown.
11. If a set of melting point range data were provided, you should be able to predict the
identity of the material. You are not expected to memorize melting points
of materials.
12.
The behavior of samples when they melt:
Pure samples melt over a narrow range, close to the literature value of the
melting point (when the literature value is correct, which it usually is).
Samples containing soluble impurities have a melting point range that is broader
and lower than the melting point range of a pure sample. Example: Pure
substance, 121-122°; same substance when impure, 109-117°.
Samples containing insoluble impurities have a melting range similar to that of a
pure sample (except that you might see particles of the impurity floating
around after all of your compound has melted).
References
i CRC Handbook of Chemistry and Physics, 65th ed, CRC press, 1984, p c-65-c-575
ii Phase diagram http://www.chemguide.co.uk/physical/phaseeqia/snpb.html
(December 10, 2010)
iii Capillary tube filling and Mel-Temp use, University of Colorado, Boulder, Chemistry
and Biochemistry Department,
http://orgchem.colorado.edu/hndbksupport/meltingpt/mt.html.. (August 24,
2011)