GenChem: Paradichlorobenzene Lab
Name_____________________ Hr___
Cooling Behavior of a Pure Substance
Introduction:
Although there are only about 114 elements known to man, there are over 6 million different
compounds that have been studied. Each year several hundred thousand new compounds are
reported. Most of these substances, both elements and compounds, exist as solids at ordinary
temperatures. Only a few of them are liquids or gases at normal temperatures. Each has a
characteristic set of properties which serve to give it identity. One of these properties, the
melting point, is the temperature at which the substance both melts and freezes.
In this experiment you will study the cooling behavior of a pure substance: paradichlorobenzene (C6H4Cl2). This solid substance will be heated slightly above its melting point. Then,
time and temperature data will be recorded every 30 seconds as the heat energy is slowly
released from the compound. This lab will also allow you to collect data using a netbook and
temperature sensor and to print a computer-generated graph.
Procedure:
1) Get and turn on one of the computers. Connect a Go Link to the computer, and then connect a
temperature sensor to the Go Link. Launch the program called Logger Pro.
2) Fill a 400ml beaker four-fifths full of water. Heat the water on a hot plate to 35-40°C for a
cooling bath. Carefully place the beaker on the table.
3) Get a test tube of liquid paradichlorobenzene (PDCB) from your teacher. Insert the temperature
sensor into the test tube and wait for the temperature reading to stabilize.
4) Click the green Collect button in the upper right corner of the graph and insert the test tube into
the water. Gently stir the test tube with the temperature sensor until the substance freezes. Do
not try to remove the sensor from the solid.
5) Continue recording the temperature of the substance until it reaches 40-45°C for several
readings. Then click the red Stop button in the upper right of the graph.
6) Underneath the drop-down menu labeled “Page”, click the button with a blue A on it. This will
Autoscale the graph and makes it easier to work with.
7) Click the drop-down menu labeled “File” and then click “Print Graph”. Type your names into the
box labeled “Name”. (If you are not able to type your name, click the toggle button labeled “Print
Footer” then type your name into the box.) Then click ok and print the graph.
8) After printing the graph, increase the heat on the hotplate until the PDCB has just melted.
Remove the temperature sensor, turn off the hotplate, and return your test tube of PDCB to your
teacher.
Names________________________________________________ Hr___
Data Analysis:
1) On your graph:
a) Create a descriptive title
b) Label the area which represents liquid paradichlorobenzene (PDCB) and the area which
represents solid + liquid PDCB.
2) According to your data, what is the melting point (the temperature at which PDCB
solidified)? ___________
3) Use a resource to look up the actual melting point of paradichlorobenzene: _________
4) Calculate the total heat energy removed from the PDCB during the cooling and freezing
phases using the following information:
Heat of fusion = 125.4 J/g
(show work and units)
Specific heat of liquid = 1.00 J/gOC
For the next 2 responses, include the following terms/concepts: absorb energy, release energy,
molecular motion, heat, temperature, water bath, phase change
5) Describe and explain the behavior of the molecules of PDCB at the plateau.
6) Describe and explain the behavior of the molecules of PDCB within the temperature
change portion of the graph.