Hot Wax

Introduction

The temperature at which paraffin (candle wax) melts is only about 55 °C. When the melted wax solidifies, however, it releases heat and may cause severe skin burns. How much heat is released when “hot wax” solidifies?

Concepts

• Phase changes
• Heat of fusion
• Calorimetry

Materials

Safety Precautions

Exercise care when working with hot water baths and hot melted wax. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Paraffin may be disposed of according to Flinn Suggested Disposal Method #26a.

Procedure

Part A. Melting Point of Paraffin

1. Fill a 400-mL beaker about one-half full with water and add two boiling stones. Heat the water to 90 °C on a hot plate.
2. Add about 100 mL of cold tap water (15–20 °C) to a polystyrene cup and nest the cup inside a second polystyrene cup. Place the nested cups inside a 250-mL beaker on a ring stand.
3. Obtain about 10 g of paraffin or candle wax shavings in the large test tube. Have students record the precise mass on the worksheet.
4. Holding the test tube with a clamp, place the test tube in the hot water bath and insert a digital thermometer into the paraffin. When the temperature is about 80–85 °C, remove the test tube from the hot water bath and clamp the test tube to the ring stand.
5. Measure and record the precise temperature of the melted paraffin and immediately lower the test tube into the cold water bath in the cup. Start timing. Carefully stir the paraffin with the digital thermometer and measure the temperature of the paraffin every 30 seconds for 10 minutes or until the temperature is about 40 °C (whichever comes first). Have students record all temperature and time measurements on the worksheet.
6. Have students plot the cooling curve data for paraffin on the worksheet and estimate and record the melting point of paraffin.

7. Put the test tube containing the solidified paraffin back into the hot water bath (step 4).
8. Empty the water from the cup and switch the positions of the nested polystyrene cups so that the “clean” cup is now on top.
9. Add 125 mL of cold tap water (15–20 °C) to a graduated cylinder. Measure and record the precise volume on the worksheet. Carefully pour the cold water into the cup and place a clean digital thermometer in the cold water.
10. When the paraffin (step 4) has melted, remove the test tube from the hot water bath and clamp the test tube to the ring stand. Dry the test tube with paper towels.
11. Observe the melted paraffin. When the first traces of solid appear, measure and record the initial temperature of the cold water and immediately immerse the test tube into the cold water bath. Note: The temperature of the paraffin should be very close to its estimated melting point when the test tube is inserted into the cold water bath.
12. Carefully stir the paraffin as it has all solidified and the temperature of the cold water is stable (3–4 minutes). Measure and record the final temperature of the water.

Student Worksheet PDF

12814_Student1.pdf

Teacher Tips

• For best results, schedule at least 30 minutes for this demonstration—20 minutes to perform the experiment and 10 minutes for data analysis and discussion. This kit contains enough chemicals and disposable materials to perform the demonstration as written seven times: 30 g of paraffin, 10 g of boiling stones, 14 polystyrene cups and 2 borosilicate glass test tubes. The paraffin wax and test tubes are reusable—simply use the same sample from one class period to another.
• Remove the fused paraffin from the test tube by heating the test tube under hot running water and pouring out the hot wax. Rinse the test tube twice with acetone to remove any residual wax and wash the test tube well with soap and water.
• The heat of fusion may be calculated in Joules per mole or calories per gram. The literature value is reported in calories per gram.
• There are two main sources of experimental error in this demonstration. Both will lead to a calculated heat of fusion higher than the theoretical value. (1) If the temperature of the paraffin is above the melting point when it is placed into the cold water bath (step 11), some of the temperature increase in the cold water bath will be due to temperature equilibration, not to the heat released as the liquid wax solidifies. (2) Similarly, if the paraffin is allowed to remain in the water bath after it has completely solidified, that is, to a temperature below the melting point, some of the measured temperature change will again be due to temperature equilibration.
• Paraffin wax is used for passive energy storage in “phase-change wallboard,” a new energy-efficient type of building material. Phase-change wallboard contains paraffin wax embedded in gypsum (drywall). The paraffin wax undergoes reversible phase changes when heated or cooled, absorbing or releasing large amounts of heat in the process. The wallboard thus helps to maintain a constant temperature in a room or building. Consider how this phase-change material might be used in a mild climate area. During the day, when the outside temperature rises, the solid paraffin melts and absorbs (stores) the excess heat energy. This cools the building and reduces the need for air conditioning. The reverse occurs at night. When the outside temperature decreases, the liquid paraffin solidifies and releases the “stored” heat energy, thus reducing the need for heating the building.
• Crushed ice may be needed to reduce the tap water temperature to the desired 15–20 °C.

Sample Data

1. The temperature of a pure substance will remain constant at the melting point as long as both solid and liquid are present. Graph the cooling curve data (Part A) and estimate the melting point of paraffin.

2. Calculate the heat absorbed by the cold water bath as the hot wax solidified:

Q = (121 g) x (5.1 °C) x (1 cal/g•°C) = 617 cal

3. Calculate the heat of fusion of paraffin.

ΔH_fusion = 617 cal/10.08 g = 61 cal/g

Discussion

The melting point of paraffin is 57–58 °C. This is consistent with a straight chain (normal) alkane having the formula C₂₆H₅₄ or C₂₇H₅₆. Most long-chain n-alkanes (> 20 C atoms) exhibit two very closely spaced phase transitions at or just below the melting point. (The second transition corresponds to two crystalline phases, α and β.) Each transition is associated with an enthalpy change. For n-C₂₇H₅₆, the phase transitions occur at 53 °C and 56.4 °C, and the corresponding enthalpy changes are 21.6 cal/g and 38.6 cal/g, respectively. The literature value for the “heat of fusion” as determined in this experiment should be 60.2 cal/g. The experimental value is 61 cal/g (1% error).

References

This kit was adapted from Solids and Liquids, Flinn ChemTopic™ Labs, Volume 11; Cesa, I., Editor; Flinn Scientific, Inc.: Batavia, IL (2005).