The Heat of Crystallization of Several Hydrated Salts
By: Michael E.



My science display board at the regional science fair.
PURPOSE

    The purpose of this experiment was to determine the heat of crystallization of several hydrated salts:  Sodium Acetate Trihydrate, Sodium Borate Decahydrate, Sodium Thiosulfate Pentahydrate, and Sodium Phosphate Monobasic Dihydrate.  The heat of crystallization is the amount of heat liberated when the compounds go from a liquid to a solid state at a given temperature.

    I became interested in this experiment when using a heat pad while skiing.  The mechanism by which the heat release was totally unknown to me.  As a result, I was fascinated yet puzzled how this phenomenon happened.

    The information gained from this experiment will help me understand how the phenomenon took place and possibly lead to the development of a better heat pad.

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HYPOTHESIS

    I hypothesize that Sodium Acetate Trihydrate would be the best hydrated salt for developing a heat pad.

    I base my hypothesis on information gained during research on the Internet and at the Selah Junior High library.  I found that the melting points of these compounds were in the following range of 45-80°C.  Theoretically, all of these hydrated salts could be melted in boiling water and would crystallize at a high enough temperature value that sufficient heat would be released to benefit a person.

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EXPERIMENT DESIGN

The constants in this study were:

    The responding variable was the change in the calorimeter water temperature.

    The responding variable would be measured in degrees Celsius using an Electronic/Digital Thermometer.  From these values I could decide which hydrated salt would be the best candidate for use in a heat packet.

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MATERIALS
Quantity Item Description
6 Vinyl Sheets
1 Thermos (or Calorimeter)
1 Stopwatch
1 Electronic/Digital Thermometer
1 Magnetic Stir
1 Magnetic Stir Bar
1 Styrofoam Lid
1 Boiling Pan
1 Stove Burner
1 Analytical Scale
1 Scissors
1 Vinyl Seam Sealer
1 Forceps
1 Stainless Steel Disc (1 inch in Diameter)
1 Plastic Beaker (500 ml)
1 Spatula
1 500 g Bottle of Sodium Phosphate Monobasic Dihydrate
1 500 g Bottle of Sodium Borate Decahydrate
1 500 g Bottle of Sodium Acetate Trihydrate
1 500 g Bottle of Sodium Thiosulfate Pentahydrate

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PROCEDURES

1. Construct a calorimeter (see-Calorimeter Procedures).
2. Cut one vinyl packet with approximate dimensions of 2½ x 6 inches from one large vinyl packaging sheet.
3. Heat seal three of the edges with a seam sealer.
4. Place the vinyl packet into a plastic beaker on the analytical scale.
5. Tare the scale.
6. Measure exactly 25.0 grams of Sodium Acetate Trihydrate into the packet.
7. Place the round stainless steel disc into the packet.
8. Seal the fourth side of the vinyl packet with the vinyl seam sealer.
9. Fill the boiling pan ¾ full and place it onto the stove burner.
10. Place the vinyl packet into a boiling pan and boil the packet until all of the crystals have melted (or approximately 10-12 minutes).
11. After all of the crystals have melted, cut one corner off of the packet.
12. Squeeze all of the air out of the vinyl packet and immediately re-seal the packet.
13. Place the vinyl packet back into the boiling pan and set the temperature on med-hi.
14. Place the thermos onto the analytical scale.
15. Tare the scale, place two ice cubes and water until the amount of water (including the ice cubes) reaches 400 grams.  This results in a water temperature of about 10°C.
16. Place a 1-inch, round magnetic stir bar into the thermos and place the thermos back onto the magnet stirrer.
17. Turn the magnetic stirrer on to a setting of five.
18. After the ice cubes have melted and the electronic thermometer reads around 10° C (plus or minus 4°), remove the boiling pan from the stove burner and immediately take the vinyl packet of sodium acetate trihydrate and place it into the calorimeter.
19. Start the stopwatch and begin to take temperature measurements from the electronic thermometer every 30-seconds.
20. After five minutes have passed, begin to take measurements every minute.
21. Take temperature measurements for 10 minutes or until there is no temperature change.
22. Remove the Vinyl packet.
23. Click the Stainless steel disc.
24. As soon as you see small crystals forming, place the vinyl packet back into the thermos and start the stopwatch.
25. Again, take temperature measurements every 30-seconds until you reach five minutes.
26. After five minutes have passed, begin to take measurements every minute.
27. Take temperature measurements for 10 minutes or until there is no temperature change.
28. Repeat steps 2-22 using Sodium Phosphate Monobasic Dihydrate.
29. Repeat steps 2-22 using Sodium Borate Decahydrate.
30. Repeat steps 2-22 using Sodium Thiosulfate Pentahydrate.
31. Repeat steps 2-22 using 50-g of Sodium Phosphate Monobasic Dihydrate.
32. Repeat steps 2-22 using 50-g of Sodium Borate Decahydrate.
33. Repeat steps 2-22 using 50-g of Sodium Thiosulfate Pentahydrate.

CALORIMETER CONSTRUCTION

1. Use a wide mouth thermos that can hold 500-550 ml of water.
2. Obtain a large piece of Styrofoam approximately 1 inch thick that is large enough to cover the top opening of the thermos.
3. Cut the Styrofoam into a circle until it is just large enough to use as a thermos lid.
4. Place the thermos on a magnetic stir and place a magnetic stir bar into the thermos.
5. Set the Styrofoam lid onto the thermos and poke the electronic thermometer probe through the styrofoam lid so that it will be submerged into the water with about one to two inches clearance from the bottom of the thermos.

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PROJECT LOG

Pre-Experiment Log

11-5-01
    Started to research for a title of science project, decided to search for a title in chemistry.
11-6-01
    Started to narrowed the search for my topic.
11-7-01
    Narrowed the topic to:  The heat of crystallization of some hydrated salts.
11-8-01
    Began research about heat of crystallization.
11-9-01
    Online research about crystallization and hydrated salts, very frustrated because there is not a lot of information to be found on the Internet about Sodium Acetate Trihydrate, Sodium Phosphate Monobasic Dihydrate, Sodium Thiosulfate Pentahydrate, and Sodium Borate Decahydrate.
11-13-01
    Online research about hydrate salts and started purpose and hypothesis.
11-14-01
    Online research worked on hypothesis and purpose.
11-15-01
    Worked on hypothesis and purpose.
11-16-01
    Finished hypothesis and started to work on experimental design.
11-19-01
    Finished purpose and worked on experimental design.
11-20-01
    Finished experimental design and started working on procedures.
11-21-01
    Worked on procedures.
11-27-01
    Worked on Procedures.
11-29-01
    Finished procedures and started to work on research report.
12-4-01
    Worked on Research Report.
12-6-01
    Worked on Research Report.
12-11-01
    Worked on Research Report.
12-13-01
    Worked on Research Report.
12-17-01
    Worked on Research Report.
 
Experiment Log

12-28-01 
     Started my experiment at Tree Top Inc.  I collected as much data as possible. I followed my procedures on Calorimeter Construction.  I then followed my experiment procedures.  I tested Sodium Acetate Trihydrate and Sodium Thiosulfate Pentahydrate for the 25-g packets.  The small stainless steel disc only worked in Sodium Acetate.  When flexed, it would not initiate the crystallization process for Sodium Thiosulfate Pentahydrate.  During the crystallization, Sodium Thiosulfate Pentahydrate became rock solid.

12-30-01
     I continued my experiment at Tree Top Inc.  I collected as much data as possible.  I followed procedures on Calorimeter Construction.  I then followed my experiment procedures. Continued to test 25-g packets, tested Sodium Phosphate Monobasic Dihydrate and Sodium Borate.  Found that Sodium Borate would not melt in boiling water, no information was found on Sodium Borate.  Sodium Phosphate Monobasic Dihydrate had a slow crystallization process compared to the other two salts.  Also, the small stainless steel disc did not start the crystallization process in Sodium Phosphate Pentahydrate.

12-31-01
     I continued my experiment at Tree Top Inc.  I collected as much data as possible.  I followed procedures on Calorimeter Construction.  I then followed my experiment procedures. Started to test 50-g packets, tested Sodium Acetate Trihydrate and Sodium Thiosulfate.

1-2-02
     I continued my experiment at Tree Top Inc.  I collected as much data as possible.  I followed procedures on Calorimeter Construction.  I then followed my experiment procedures. Continued to test 50-g packets, tested Sodium Thiosulfate.

Post-Experiment Log

1-2-02
    Worked on Experiment Log.
1-17-02
    Started creating graphs.
1-18-02
    Continued creating graphs.
1-22-02
    Started typing up results.
1-26-02
    Finished typing results and started to type conclusion.
2-2-02
    Continued to type up conclusion and started typing abstract.
2-3-02
    Finished typing conclusion and abstract.
2-4-02
    Started reviewing
2-5-02
    Finished abstract and started typing table of contents.
2-6-02
    Started to collect all of my data.
2-7-02
    Finished typing all journal components. Printed all journal components off and placed them into journal binder.  Started to work on pro board.
2-8-02
    Continued to work on pro board.
2-9-02
    Continued to work on pro board.
2-10-02
    Completely finished science project!

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RESULTS

    The purposed of this experiment was to determine the heat of crystallization of several hydrated salts:  sodium acetate trihydrate, sodium borate decahydrate, sodium thiosulfate pentahydrate, and sodium phosphate monobasic dihydrate.
Sodium Borate Decahydrate
    After several attempts, Sodium Borate Decahydrate crystals would not melt in boiling water.  As a result, there was no way to determine the heat of crystallization for this compound.
Sodium Thiosulfate Pentahydrate
    Sodium Thiosulfate Pentahydrate turned rock solid during the crystallization process.  It exhibited a heat of crystallization of 35.2 cal/g for the 25-g packet and 36.8 cal/g for the 50-g packet with an average value of 36.0 cal/g (See graph 1).  This suggests that Sodium Thiosulfate Pentahydrate could be a good candidate to be used in heat pads.  However, the round stainless steel disc, when flexed, would not cause the Sodium Thiosulfate Pentahydrate solution to crystallize.  To encourage crystallization, a small hole was poked in one of the four corners so that a small amount of air could reach the liquid and start crystallization.
Sodium Acetate Trihydrate
    Sodium Acetate Trihydrate exhibited a heat of crystallization of 42.1 cal/g for the 25-g packet and 43.2 cal/g for the 50-g packet with an average value of 42.7 cal/g (See graph 2).  When flexed, the small stainless steel disc caused some of the Sodium Acetate Trihydrate to crystallize.  As a result, a small hole was not required to induce crystallization.
Sodium Phosphate Monobasic Dihydrate
    Sodium Phosphate Monobasic Dihydrate exhibited a heat of crystallization of 13.6 cal/g for the 25-g packet and 12.8 cal/g for the 50-g packet with an average value of 13.2 cal/g (See graph 3).  The round stainless steel disc, when flexed, did not induce crystallization.  As a result, a hole also had to poked in these packets. The crystallization process for Sodium Phosphate Monobasic Dihydrate appeared to be slower than Sodium Acetate Trihydrate and Sodium Thiosulfate Pentahydrate.
    Sodium Acetate Trihydrate exhibited the largest heat of crystallization of 42.7 cal/g followed by Sodium Thiosulfate Pentahydrate at 36.0 cal/g and Sodium Phosphate Monobasic Dihydrate at 13.2 cal/g  (See Averages Graph).  The specific heat also was determined for each of the liquid salts.  They were all the same with a Value of 16.0 cal/g/°C.
    Based on the observed results, it was obvious that Sodium Acetate Trihydrate would be the best hydrated salt for developing a heat pad.  Therefore, the total heat from a 100-g packet was measured to determine whether sufficient heat would be generated to warm a person’s hand.  The 100-g packet produced 4200 calories.  This appears to be adequate.
 
 


 
Graphs  Tables
Sodium Acetate Triydrate Sodium Acetate Trihydrate
Sodium Phosphate Sodium Phosphate
Sodium Thiosulfate Pentahydrate Sodium Thisulfate Pentahydrate

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CONCLUSION

    My hypothesis was Sodium Acetate Trihydrate would be the best hydrated salt for developing a heat pad.  The results indicate that my hypothesis should be accepted. Sodium Acetate Trihydrate exhibited the highest average heat of crystallization of 42.7 cal/g (See graph 2).  Also, Sodium Acetate Trihydrate was the only saturated liquid that would crystallize when the stainless steel disc was flexed in the vinyl packet.  A 100-g packet was shown to produce a total heat content of 4200 calories.  In conclusion, Sodium Acetate Trihydrate was an ideal hydrate salt to be used in heat pads because it met the following criteria:  it exhibited the highest heat of crystallization, it crystallized at around 50°C, and the crystallization could be controlled with flexing of the stainless steel disc.
 The other three hydrate salts would not make good candidates for heat pads because they either would not melt in boiling water or the crystallization process could not be induced on demand.

    For future research, I would try mixing two or more of the hydrated salts together to determine if this would enhance the heat of crystallization beyond that of Sodium Acetate Trihydrate.  I would also investigate if there is any way to modify the stainless steel disc or develop an alternative mechanism to cause crystallization on demand.

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RESEARCH REPORT

Introduction

    During the 1800’s, a German chemist named Robert Wilhelm Bunsen invented the calorimeter.  Bunsen was born in Göttingen, Germany on March 31, 1811 and died on August 16, 1899.  Calorimetry is used mainly to determine the Latent Heat, Specific Heat, and Heat of Crystallization.  A calorimeter consists of an insulated container and usually a liquid like water, a stirring device, and a thermometer.

Sodium Acetate Trihydrate  (C2H3NaO)

    Sodium Acetate Trihydrate is a clear granule or crystal.  It has a slight acetic acid odor and is efflorescent in warm air.  The solubility for Sodium Acetate Trihydrate is 76 gm/100-mls water at 0°C with a density of 1.45 g/ml.  The melting point of Sodium Acetate is 58°C and becomes anhydr at 120°C.  Sodium Acetate Trihydrate is used in photography, in chemistry to eliminate effect of strong acids, foot warmers, and as milk-bottle warmers.

Sodium Borate Decahydrate (B4Na2O7)

    Sodium Borate Decahydrate has White, Gray, Bluish or Greenish White Streaked Crystals. It is a hard odorless crystals, granules, or cryst powder and is efflorescent in dry air.  It has a melting point of 75°C and a density of 1.73 g/ml.  At 320°C, Sodium Borate Decahydrate becomes anhydr.

Sodium Phosphate Monobasic Dihydrate (H2NaO4P)

    Sodium Phosphate has White crystalline powder.  It is very soluble in water and has a density of 1.92 g/ml.  It has a melting point of 60°C.   Sodium Phosphate Monobasic Dihydrate is used in baking powders and in boiler water treatments.

Sodium Thiosulfate Pentahydrate (Na2S2O3)

    Sodium Thiosulfate has odorless monoclinic colored crystals.  The solubility for Sodium Thiosulfate Pentahydrate is 79g/100-ml water at 4°C and is efflorescent in warm dry air.  Sodium Thiosulfate Pentahydrate has a melting point of 48°C and a density of 1.75 g/ml. Sodium Thiosulfate Pentahydrate can dissolve silver salts and because of this, its main use is in developing film.  It is also used in chrome tanning leather and in Chemical Manufacturing. Sodium Thiosulfate Pentahydrate can dissolve in water and acts as a mild reducing agent.

Thermo-Pad

    A Thermo-pad typically consists of a packet, usually made out of vinyl or plastic. Inside of the packet is sodium acetate, water, and a small stainless steel disc.  When Thermo-pads are in there liquid state, they are supersaturated solutions.  In order to create heat, the small metal disc must be flexed.  After flexing the disc, the packet begins to crystallize.  The Crystallization process always occurs at 54°C.  The Thermo-pad can produce heat for up to 6 hours.  This is because when the crystallization process starts, not all of the liquid begins to crystallize.  Throughout the crystallization process, part of the heat pad is still melted.  As the heat pad cools down, the melted section of the heat pad turns into crystals.  Thus, heat is given off for 10 minutes to 1 hour.

Heat of Crystallization

    Heat of crystallization is defined as the amount of heat that must be removed from one gram of a liquid as it freezes with no change in temperature. Heat of crystallization is measure as calorie per gram (cal/g).

Chemical Salt

    Chemical salt is a compound that results when a base neutralizes an acid. During this reaction, the metal in the base displaces the hydrogen in the acid. 

Calorimetry

    Calorimetry is any of several apparatuses for measuring quantities of absorbed or evolved heat or for determining specific heats.  The apparatus used in heat measurement is called a calorimeter. The measurement given by the most common type of calorimeter depends upon the temperature change in a fixed quantity of water when heat is transferred between the water and an exothermic or endothermic process.  However, If the temperature difference is not too large, then the heat transferred is equal to the heat capacity of the water times the mass of the water times the change in temperature. The precision of this method of heat measurement depends on the belief that all the heat transferred in the process passes into or out of the water in which the temperature difference is measured.  All of this should happen without any heat being lost to the environment and none being absorbed by the walls of the container.

Specific Heat

    Specific heat is defined as the heat in calories required raising the temperature of one gram of a substance one degree Celsius.  Specific heat is measured by calorie per gram. Heat capacity is the quantity of heat necessary to change the temperature of a unit mass 1°. The heat capacity of water is 1 calorie per gram per degree Celsius (1 cal/g/°C).  Hence, the specific heat of some additional substance relative to water will be numerically equal to its heat capacity; for this reason, specific heat is often applied when the heat capacity actually is meant.  Because the heat capacities of most substances vary with changes in temperature, the temperatures of both the specified substance and the reference substance must be known in order to give a precise value for the specific heat. The heat capacity of water at 15°C, this is a frequently used value. Specific heat is a dimensionless quantity.
    The law formulated by the French chemists Pierre Louis Dulong and Alexis Thérèse Petit states that the specific heats of the solid elements are inversely proportional to their atomic weights.  The specific heat multiplied by the atomic weight is approximately a constant quantity for all the solid elements. 

Super Cooled Liquids

    Super cooling is a process of retaining the liquid state of a substance at the temperature value it would normally freeze.  In meteorology, some meteorologists have found that small water droplets have been found to stay in liquid form in certain clouds.  In some of the most extreme cases, some supercooled clouds have been known to exist at –40°C.

Freezing Point

    Freezing Point, temperature at which a liquid congeals into the solid state at a given pressure (Freezing Point," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved.).  The freezing point of a pure, unmixed liquid is considered the same as the melting point of the same substance in the solid form.  It may be regarded as the temperature at which the solid and liquid states of the substance are in equilibrium.  If heat is applied to the mixture of the liquid and solid substance at its freezing, the temperature value of the substance remains constant until it has become completely liquefied.  This is because the heat is absorbed, not in warming the substance, but in providing the covert heat of fusion.  Similarly, if heat is taken from a mixture of liquid and solid substance at its freezing point, the substance will remain at the same temperature until it has become completely solid, because the substance, in its change from liquid to the solid state, gives off heat. Thus, the melting point or freezing point of a pure substance may also be defined as the temperature at which melting or freezing continues once it has started.

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BIBLIOGRAPHY

"Calorimetry." Columbia Encyclopedia. CD-ROM. New York: Columbia UP, 2001.
"Chemical, Salt." Columbia Encyclopedia. CD-ROM. New York: Columbia UP, 2001.
Equilibrium Crystallization. 12 Dec. 2001 <http://www.cci.unl.edu/Chemistry/DoChem075.html>.
FAQS about Thermo-Pad Heating Pads,. 30 Nov. 2001 <http://thermo-pad/fag.htm>. 
"Freezing Point." Microsoft Encarta Encyclopedia 2000. CD-ROM. Microsoft Corporation, 1999.
"Heat of Crystallization." Columbia Encyclopedia. CD-ROM. New York: Columbia UP, 2001.
Heat of Fusion. 11 Dec. 2001 <http://www.onlink.net/~bernas/fusion.htm>.
"Robert Wilhelm Bunsen." Microsoft Encarta Encyclopedia 2000. CD-ROM. N.p.: Microsoft Corporation, 1999.
"Sodium Acetate Trihydrate." The Merck Index, Twelfth Edition. Whitehouse Station, NJ: Merck & Co., Inc, 1996. 1470. 
"Sodium Borate Decahydrate." The Merck Index, Twelfth Edition. Whitehouse Station, NJ: Merck & Co., Inc, 1996. 1472. 
"Sodium Phosphate Monobasic Dihydrate." The Merck Index, Twelfth Edition. Whitehouse Station, NJ: Merck & Co., Inc, 1996. 1480. 
"Sodium Thiosulfate Pentahydrate." The Merck Index, Twelfth Edition. Whitehouse Station, NJ: Merck & Co., Inc, 1996. 1484.
"Specific Heat." Microsoft Encarta Encyclopedia 2000. CD-ROM. Microsoft Corporation, 1999.
"SuperCooling." Microsoft Encarta Encyclopedia 2000. CD-ROM. Microsoft Corporation, 1999.
The Heat of Fusion of Ice. 11 Dec. 2001 <http://chem.Isu.edu/htdocs/people/lgbutle/1202_Fall96/SupMat/LectDemo_HeatOfFusion.html>.

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Other Science Project I Have Completed:
WHICH WINDMILL BLADES ANGLE IS MOST EFFICIENT (1999)
The Effects Of Blade Size And Shape On The Electrical Output Of A Generator (2000)
The Effects of Series/Parallel something on Electrical output of a very large generator (2001).


If you have any questions or commnents you can E-mail my teacher Mr. McMillen, by clicking on the mad scientist head or this link................he will then forward your questions and comments to me, thanks!

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