Download Experiment #2 Melting Points

chemist. If you want to use the melting point
Experiment #2
Melting Points
to identify a solid compound that you have
isolated in the lab, you will need to compare
PURPOSE
its
To gain skill in lab procedure and measuring
techniques and to learn about the properties
of organic compounds.
melting
point
with
that
of
the
true
compound. Melting points are listed in various
sources of scientific data, as referenced on
the Chem Info page or in the physical data
OBJECTIVES
tables on this organic chem site from the
1. Determine several melting points
(mp's).
University of Colorado:
2. To see how mixing different compounds
changes their melting points.
(http://orgchem.colorado.edu/cheminfo/chem
3. To determine the effectiveness of
using melting points in determining
compound purity
Table of Physical Constants and Hazards of
Chem Info
info.html)
Some Organic and Inorganic Compounds
(http://orgchem.colorado.edu/hndbksupport/
MATERIALS
physdata.html)
• Mel-Temp, chemicals Time:~2 hour
How to set up the Mel-Temp Apparatus.
BACKGROUND
http://orgchem.colorado.edu/hndbksupport/
meltingpt/mtset.html.
Melting points of pure compounds are
Melting Points
easily
also recorded in handbooks, such as the
measured physical property of solid-state
Handbook of Chemistry and Physics (CRC) or
organic
the Merck Index.
Melting
points
compounds
are
useful
an
in
product
characterization and purity determination.
Measurements of the melting point of
Pure, crystalline solids have a characteristic
a solid will provide you with information about
melting point, which is expressed as the
the purity of the substance. Pure, crystalline
temperature range over which the solid
solids melt over a very narrow range (melting
range) of temperatures, whereas mixtures
melts to become a liquid. The transition
between the solid and the liquid is so sharp
melt
over
a
broad
temperature
range.
for small samples of a pure substance that
Mixtures also tend to melt at temperatures
melting points can be measured to ±0.1o C.
below the melting points of the pure solids.
Most solid substances prepared in the
Typically it is no more than ±1o C.
organic laboratory are initially impure. These
Since melting points are relatively
impurities
easy and inexpensive to determine, they
substance.
are handy identification tools to the organic
affect
In
a
the
melting
sample
that
point
of
a
contains
a
mixture of two compounds, each component
1
usually lowers the melting point of the
other, giving an observed melting point
Note:
range that is lower and broader than the
a) the temperature at which the powder first
melting point of either component.
We
Mel-Temp
b) the temperature at which the sample is
apparatus for measuring the melting point
completely liquid. (This is the melting
of
range, which we call a melting point.
our
will
use
compounds.
the
collapses or shows some liquid, and
The
powdered
compound is pushed into a small glass tube
Always report a melting range)
with a closed end called a capillary tube
Remember:
and
♦
packed
as
demontrated
by
the
instructor.
the sample should be heated through the
melting range at a rate of 1 o C or less per
minute.
Procedure
♦
Pay close attention and tell your partner
when the melting begins and ends.
I. Determine the melting point of two
pure compounds and a mixture. Melting
points are best determined using a finely
♦
Turn off the apparatus and let it cool.
♦
Capillaries can not be reused.
divided powder. Grind the sample using a
Always put used capillaries in
mortar and pestle to ensure homogeneity.
the glass waste container!
Fill and pack to a height of about 2-3 mm
one capillary tube each with:
II. Determine the melting point of a solid
unknown. Your unknown is one of the
1. benzophenone,
following
2. napthalene
compounds:
cinnamic
acid,
acetanilide or benzoic acid. Samples of all of
3. a 1:1 mixture of benzophenone AND
these compounds are available in the lab.
napthalene.
From
Put the 1-2 tubes into the Mel-Temp
the
unknown,
found
melting
decide
which
point
of
of
the
your
listed
apparatus closed end down. Make sure that
compounds it might possibly be. Identify your
you
unknown based on the best match.
can
see
the
samples
through
the
magnifying glass. Set the voltage to zero
List any important observations you
and turn on the Mel-temp. Turn the voltage
make while performing the experiment. For
to 45 and observe both the samples and
example,
temperature reading as you heat.
compound when it melts and any other visible
(Never set the voltage to more than 70)
changes occurring
2
describe
the
appearance
of
a
CH 102
Experiment #2
Melting Points
Name_________________________
Questions
1. How fast do you heat the sample in the Mel-temp when determining a
melting point?
2. If you heat too fast, will your observed melting point be higher or lower
than the true value? Explain.
3. Will the melting point of a mixture of two compounds be higher or lower
than the melting point of the pure compounds?
3