Clarissa A. Gomez Group 4 CHE-3 CHM141L-A01 EXPERIMENT 1 PHYSICAL CONSTANTS OF SOLID AND LIQUID AND ORGANIC COMPOUNDS The first experiment deals with the effect of intermolecular forces of attraction in relationship with the melting point or boiling p oint of a certai n compound. The first part of the experiment in corporated the obtai ning of the melting point of an unknown solid. The melting point of a compound is the te mperature at which the sol id is in equilibrium with its liquid. A solid compound changes to a liquid when the molecules acquire enough energy to overcome the forces holding them together in an orderly crystalline lattice. For most organic compound s, these intermolecular forces are relative ly weak. The melting point range is defined as the span of temperature form which the crystal first begins to liquefy to the point at which the entire samp le is liquid. Most pure organic compounds melt over a narrow temperature range 1-2 0 C. 1 The presence of a soluble impurity always causes a decrease in the melting point expected for the pure compound and a broadening of the melting point range. In order to understand the effects of impurities on melting point behavior, we consider the eutectic point for two different fictitious organic compounds. A eutectic system is a Mixture of chemical compounds or elements that has a single Chemical composition that solidifies at a lower temperature than any other composition. On a Phase diagram (Figure 1) the Intersection of the eutectic temperature and the eutectic composition gives the eutectic point. The phase diagram at right displays a simple binary system composed of two components, A and B, which has a eutectic point. The phase diagram plots relative concentrations of A and B along the X-axis, and temperature along the Y-axis. The eutectic point is the point at which the liquid phase borders directly on the solid + phases (the solid forms of pure A and B), representing the minimum melting temperature of any possible alloy of A and B. The temperature that corresponds to this point is known as the e utectic temperatu re. 2 Figure 1: Phase Diagram
Because the melting point of a compound is a physical constant is useful in determining the identity of the
unknown compound. A good correlation between the experimentally measured melting point of an unknown
compound and the accepted melting point of a known compound suggests that the compound may be the same.
However, many different compounds have the same melting point. A mixture melting point is useful in confirming
the identity of an unknown compound. A small portion of a known compound, whose melting point is known from
the chemical literature, is mixed with the unknown compound. If the melting point of the mixture is the same as
that of a known compound, then the known and the unknown compounds are most likely identical. A decrease inthe melting point of the mixture and a broadening of the melting point range indicates that the compounds are
different.3
In preparing the sample, we pack the tube by pressing the end gently into a pulverized sample of the
crystalline material. Then to ensure good packing, we drop the capillary tube with the open end up through a 1-m
long piece of glass tubing and repeat it for several times until the crystals are transferred to the close end of the
capillary tube. However, there are other electrical instruments used in measuring melting points. One example is
the Thiele Tube. The Thiele tube (Figure2 a), named after the German chemist Johannes Thiele, is laboratory
glassware used to contain and heat bath oil. Such a configuration is commonly used in determining the melting
point of a substance. This is filled to the base of the neck with silicone oil or mineral oil. The capillary tube is
attached to thermometer so that the sample is located next to the middle of the thermometer bulb. The
thermometer is inserted into the oil and then the side arm of the Thiele tube is heated with a Bunsen burner
flame. Thomas-Hoover Uni-Melt device (Figure 2 b) is another instrument that contains silicone oil that is stirredand heated electrically. Silicone oil can be heated to temperatures up to 250
0C. With this device, up to seven
samples can be analyzed at one time. Another one is the Melt-Temp apparatus (Figure 2 c) consists of an
aluminum block that is heated electrically. The aluminum block can be heated easily to temperatures up to 4000C,
can tolerate temperatures up to 5000C for brief time periods. A thermometer up to three samples can be inserted
into the block at one time. A light and magnifier permit easy viewing of the sample(s).4
If the melting point of the compound is unknown, in order to save time an approximate MP is first
determined by heating the sample fairly rapidly. Once the approximate MP is known, a more careful determination
is made on a fresh sample. Note that once a sample has been melted, it may have decomposed slightly.
Contamination with decomposition product will change the MP of the sample, so a fresh sample must always be
used for each determination.5
Error in observed melting points often occurs due to a poor heat transfer rate from the heat source to thecompound. Sometimes slight changes, such as shrinking and sagging, occur in the crystalline structure of the
sample before melting occurs. To avoid these possible error, a large majority of pure organic compounds melt
neatly within a range of 1.5 ºC or melt with decomposition over a narrow range of temperature (2 ºC) at heating
rates below 0.5 ºC/min.3
Although many substances melt cleanly and can be melted, crystallized, and melted again
repeatedly without chemical decomposition, others chemically decompose before they melt, forming substances
of lower molecular weight. The temperature of decomposition is just as useful as the melting point in physically
characterizing a substance. Decomposition is usually signaled by a color change; for example, white substances
invariably start to turn brown near the decomposition temperature. The temperature at which the color change is
first observed signals that the substance is approaching the decomposition temperature at somewhat higher
temperature, liquid may form. At this temperature or at an even somewhat higher temperature, gas bubbles may
be seen if gaseous decomposition products are formed. All of these temperatures aid in characterizing a
substance, so all should be noted and reported.6
Some compounds decompose at or near their melting points.
This decomposition is usually characterized by a darkening in the color of the compound as it melts. If the
decomposition and the melting occur over a narrow temperature range of 1-20C, the melting point is used for
identification and as an indication of sample purity. The melting points of such compound are listed in the
literature accompanied by d or decomp. Some compounds pass directly from solid to vapor without going through
the liquid phase, a behavior called sublimation. When sublimation occurs, the sample at the bottom of the
capillary tube vaporizes and recrystallizes higher up in the capillary tube. A sealed capillary tube is used to take the
melting point of a compound that sublimes at or below its boiling point. The literature reports the melting point
for these compounds accompanied by s, sub or subl .7
Table 1: Melting Point Determination
Trial 1 Trial 2
Initial Temperature0
C 129 128Final Temperature
0C 131 130
In the experiment, we recorded as the initial temperature at which the crystals first begin to melt and the
final temperature at which the last trace of crystals melts. Based on the data we gathered, the melting range of the
unknown substance is 128.5-130.50C. From the list of our researched compounds, the nearest possible identity of
the unknown compound is urea. We concluded this because urea has a melting point range of 132-1350C and
with a appearance of white crystals and white powder which is exactly the appearance of the unknown sample.
However, the temperature of the unknown is almost 20C lower than the real melting point range of urea.. With
this, we can say that the unknown substance is an impure sample of urea.
The second part of the first experiment deals with the boiling point determination. The boiling point of a
liquid is the temperature at which that liquid is converted to a gaseous state. Boiling point is formally defined
as the temperature at which the vapor pressure of the liquid becomes equal to the pressure at the surface of
the liquid. The boiling point of a liquid can change if the pressure at the liquid's surface changes. The normal
boiling point of a liquid is the constant temperature at 1 atmosphere of pressure at which the liquid changes
If a sample of a liquid is placed in an otherwise empty space, some of it will vaporize. As this happens, the
pressure in the space above the liquid will rise and will finally reach some constant value. The pressure under
these conditions is due entirely to the vapor of the liquid, and is called the equilibrium vapor pressure. The
phenomenon of vapor pressure is interpreted in terms of molecules of liquid escaping into the empty space
above the liquid. In order for the molecules to escape from the liquid phase into the vapor phase, the
intermolecular forces (have to be overcome which requires energy. Since the nature of the intermolecularforces is determined by the molecular structure, then the amount of energy required to vaporize the sample
also depends on the molecular structure. As the number of molecules in the vapor above the liquid becomes
larger, the rate of return of the molecules from the vapor to the liquid increases until the rate of return is
equals the constant rate of escape. This is the equilibrium condition and the corresponding concentration of
molecules in the vapor space gives rise to the equilibrium vapor pressure. At higher temperatures, the greater
average kinetic energy of the molecules in the liquid results in a greater constant rate of escape. Equilibrium is
established at higher temperatures, and so larger numbers of molecules are present in the vapor phase and
the pressure is higher. When the vapor pressure of a liquid is equal to the atmospheric (or applied) pressure
then boiling occurs. The temperature, at which this occurs, for a given pressure, is the boiling point.8
The
boiling point of a liquid is sensitive to atmospheric pressure, and varies directly with it. As atmospheric
pressure decreases, the boiling point will drop; at approximately normal pressure it will drop about 0.5
0
C foreach 10-mm Hg drop in pressure. At much lower pressures, close to 10 mmHg, the temperature will drop
about 100C when the pressure is halved.
9
At lower pressures, a boiling point nomograph or temperature-pressure alignment chart (shown below)
can be used to determine the boiling point. The basic principle is that a line through two known points on any
two different scales (A, B,C) can be used to read off the value on the third scale. There are two ways in which a
pressure nomograph can be used (i) to determine the boiling point at atmospheric pressure (760 mm Hg)
given the boiling point at a lower pressure and (ii) to determine the boiling point at a lower pressure given the
boiling point at atmospheric pressure. First, let's say we have a compound with a boiling point of 100o
C at
1mm Hg pressureTo know the boiling point at 760 mmHg, we need to draw a line from 100o
C on scale A (left
side, observed boiling point) to 1.0 mm Hg on scale C (right side, pressure "P" mm). We can then read off the
boiling point at 760 mm on line B, it is about 280o
