Endothermic Reactions - Mixing Crystals of Barium Hydroxide and ...

CHEMISTRY
Endothermic Reactions - Mixing
Crystals of Barium Hydroxide and
Ammonium Isothiocyanate
Introduction
When the two crystalline substances - Ba(OH) 2 ×8HO and NH 4 SCN are thoroughly
mixed in a flask, a heat absorbing reaction is developed:
Ba(OH)2 × 8H2O + 2NH4SCN
Ba(SCN)2 + 2NH3 + 10H2O
The gaseous substance - Ammonia (NH 3 ) , formed in this reaction, can be easily
detected due to its pungent smell.
If the flask, which feels very cold to the touch, is set on a board covered with a thin
layer of water, the flask and the board freeze solidly together.
In this experiment we follow temperature changes occurring during the mixing of
crystalline barium hydroxide octahydrate and ammonium Isothiocyanate. We
experiment the freezing together of the reaction flask and the board.
Equipment
• A temperature sensor
• 2 grams of Ba(OH) 2 ×8H 2 O
• 4 grams of NH 4 SCN.
• A wooden or a plastic board, approximately 5cm x 5cm.
• A 10ml glass flask.
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CHEMISTRY
• A 10 cm glass rod.
• A MultiLog
Equipment Setup Procedure
1. Connect the MultiLog to the serial port of the computer.
2. Connect the temperature sensor to the I/O 1 port of the MultiLog
3. Assemble the equipment as illustrated in figure 1Abelow.
4. Turn the MultiLog on. Set the MultiLog up according to the setup specified below.
You can set up the MultiLog in two ways: Use the keypad of the MultiLog or select
the Control Panel from the Logger menu.
MultiLog Set Up
• Input 1: Temperature
• Rate: 1/sec.
• Samples: 200.
Figure 1A
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Experimental Procedure
1. Pour some water on the board till it is partly covered with a thin layer.
2. Weigh 2 grams of Ba(OH)2×8H2O in a 10 ml glass flask.
3. Insert the temperature sensor into the barium hydroxide crystals in the flask.
4. Weigh 4 grams of ammonium Isothiocyanate.
5. Start the MultiLog by pressing the Run/Stop key in the MultiLog’s keypad. You
can also start the MultiLog using the DB-Lab software: Press the Run button from
the Control Panel, or the (Run) button from the left toolbar.
Wait till readings from the temperature sensor are stable.
6. Add the weighed NH4SCN to the flask containing Ba(OH)2×8H2O.
7. Put the flask on the board covered with the thin layer of water.
8. Mix thoroughly the substances in the flask with the glass rod.
9. Follow changes in temperature in the flask, registered on the monitor, till no
further changes in temperature are observed.
10.When the temperature is stabilized, stop the logging.
11.Try to remove the flask from the board, as demonstrated in figure 1B.
figure 1B
Data Analysis
1. Calculate the change in temperature obtained in the process. Calculate the time
needed to reach the final temperature value: Use the grid to find the appropriate
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CHEMISTRY
values: choose View\Display from the menu. Mark the option grid in the Option
Display Window.
2. Calculate the heat of reaction: C- water capacity; ∆t - temperature change.
Water specific heat capacity at 25°C is 4.18 (J/g×°C).
3. Summarize the results obtained with the board.
An example of the graph obtained in this experiment, is shown in the next page:
Questions
1. What changes in the temperature in the flask did you observe? Explain your
results.
2. What kind of chemical reaction occurred in the flask?
3. Explain the results you obtained with the board.
Further Suggestions
1. Change the relative amounts of - Ba(OH) 2 ×8H 2 O and NH 4 SCN and follow the
temperature change in each case.
2. Perform an additional endothermic reaction: Dissolution of KNO 3 in water (25
grams in 50ml water).
3. Follow the rate of ammonium release in the reaction, using a pressure sensor. In
this way the reaction rate can be measured.
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Advantages of Using the MultiLog in
Studying Endothermic Reactions
1. Real time measurements of changes in temperature.
2. A very simple experimental system that enable the student to perform various
experiments, leading to improved comprehension of the process and the meaning
of heat of reaction.
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