ENERGY AND ENTROPY: PHASE CHANGES
INTRODUCTION:
As a liquid cools, its temperature drops. The entropy (degree of randomness or disorder) also
decreases. The continuous removal of energy from the liquid will eventually result in a phase
Enchange to a solid. For a pure substance, further removal of energy during the phase change
does not yield any further change in temperature. The entropy, however, continues to decrease.
When the phase change is complete, the temperature and entropy of the solid decrease as energy
continues to be removed.
In this experiment you will measure the temperature of sodium thiosulfate pentahydrate as it is
cooled to several degrees below its freezing temperature and then warmed to several degrees
above its melting temperature. The results of the experiment will permit you to determine the
freezing and melting temperatures of sodium thiosulfate pentahydrate and to interpret the changes
in energy and entropy.
Part 2: The Freezing of the Solid
1. Adjust the ring on the ring stand so that it is 10 cm above the top of the burner. See
Figure 3-1.
2. Place a 250 ml beaker three fourths full of tap water on the iron ring and gauze, and heat
the water to 85O C.
3. Also fill a 400 ml beaker three-fourths full of tap water for the cold water bath.
4. Obtain a test tube containing approximately 12 grams of sodium thiosulfate pentahydrate.
5. Clamp the test tube above the hot water bath as shown in Figure 3-1.
6. When the temperature of the hot-water bath reaches 85O C, turn off the burner, and
immerse the test tube in the hot-water bath. Occasionally stir the melting solid with one
of the thermometers.
7. When the temperature of the liquid sodium thiosulfate pentahydrate is approximately the
same as that of the hot-water bath, remove the test tube.
8. Record the temperature of the liquid sodium thiosulfate pentahydrate and the time in
the data table.
9. Immediately immerse the test tube in the cold-water bath and record the temperature of
the sodium thiosulfate pentahydrate at 15 second intervals.
10. When the temperature reaches 50O C, use forceps to add one or two seed crystals to the
test tube. Continue taking temperature readings every 15 seconds.
11. Continuously stir the liquid-solid in the test tube, until a constant temperature (between
45 and 50O C) is maintained. Do not try to move the thermometer when solidification
occurs.
12. Continue taking readings until the temperature of the solid is within five degrees of that
of the cold-water bath.
Part 2: The Melting of the Solid
1. Clamp the test tube once again above the hot-water bath as in Figure 3-1.
2. Heat the hot water to 60-65O C.
3. Turn the flame down as low as possible and move the burner so only the outer edge of the
bottom of the beaker is heated.
4. Adjust the position and size of the flame so the temperature of the hot-water bath remains
between 60 and 65O C.
5. Record the temperature of the sodium thiosulfate pentahydrate and the time in the data
table. The starting temperature should be less than 35O C.
ENERGY AND ENTROPY: PHASE CHANGES
6. Immediately immerse the test tube in the hot water bath.
7. Record the temperature at 15 second intervals. Use the thermometer as a stirring rod
when it becomes free of the solid.
8. Continue taking readings until the temperature of the sodium thiosulfate pentahydrate is
within five degrees of that of the hot-water bath.
9. Turn off the burner. Remove and rinse the thermometer.
10. Discard the contents of the test tube down the drain with plenty of water.
11. Clean all apparatus, wash your hands thoroughly at the end of this experiment and check
to see that the gas valve is completely turned off before leaving the laboratory.
ENERGY AND ENTROPY: PHASE CHANGES
DATA TABLE
C o o lin g
D a ta
T im e
(S )
0
15
30
45
60
75
90
105
120
135
150
165
180
195
210
225
240
255
270
285
300
315
330
345
360
375
390
Temp
(d e g. C )
O b s e rva tio ns
W a rm in g
D a ta
T im e
Temp
(s )
(d e g. C )
0
15
30
45
60
75
90
105
120
135
150
165
180
195
210
225
240
255
270
285
300
315
330
345
360
375
390
O b s e rva tio ns
ENERGY AND ENTROPY: PHASE CHANGES
Chemistry Energy/Entropy Lab Data Sheet 11-6-12
*Complete the following and answer the questions at the end regarding this weeks
Sodium Thiosulfate Experiment. Include it in your lab report then submit your lab report
to Moodle.
1.
2.
3.
4.
5.
6.
7.
Write a good quality focusing question for this experiment.
Write a correctly formatted hypothesis statement for this experiment.
What control group did you use for this experiment?
What was the independent variable?
What was the dependent variable?
Give at least 2 variables that needed to be controlled in this experiment.
Give at least one method by which you could have increased the quality of the
results of this experiment.
8. What is the accepted value for the freezing/boiling point of sodium thiosulfate
(remember you have the world at your fingertips!!)?
9. Construct a correctly labeled graph of BOTH THE WARMING AND COOLING
DATA using graph maker.
10. What was the freezing/boiling point of sodium thiosulfate according to YOUR
data and graph?
11. The formula for calculating % error is: (your result – accepted value) x 100
Accepted value
- what was your % error according to this formula?
12. What conclusion did you draw from your data……make sure you use numbers to
support your answer.
13. What was the purpose of the 2 “seed” crystals used in the cooling section of this
lab?
14. Explain what is happening to the entropy and energy of the thiosulfate during the
normal cooling phase and why.
15. Explain what was happening to the energy and entropy of the thiosulfate during
super cooling phase and why.
16. Explain what was happening to the energy and entropy of the thiosulfate during
the warming phase and why.