Cooling Rates and Melting Points: Stearic Acid vs. Paraffin Wax

Categories: Chemistry

Essay, Pages 10 (2270 words)

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Abstract:

When undertaking the practicals, appropriate and suitable equipment was chosen and calibrated in order to accurately and safely decipher the cooling rate of two substances. Accurate cooling curves were plotted and analyzed for initial and final cooling rates, melting points were estimated and compared to data book values, to determine the purity of the two substances. The initial cooling rate of stearic acid was ¹⁄₂ degrees Celsius, and the final cooling rate was ¹⁄₂ degrees Celsius. Stearic acid has a melting point of 69.

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3 degrees Celsius. Whereas paraffin wax's initial cooling rate was ¹⁄₄ degrees Celsius, and the final cooling rate was ¹⁄₄ degrees Celsius. Paraffin wax has a melting point between 46 to 68 degrees Celsius. The purity of the stearic acid and paraffin wax was checked by comparing both of their melting points to the data book values.

Introduction:

Calorimetry is the act of measuring the amount of heat energy released during a reaction in order to draw cooling curves of the results and determine if the reaction is exothermic (releases heat energy and the temperature drops) or endothermic (absorbs heat and the temperature rises).

During a change of state, the intermolecular bonds between the particles become weaker due to the addition of heat, in order to change a solid to a liquid (melting), or a liquid to a gas (evaporation). When you heat a substance, the internal energy it has increases due to the particles moving more, which increases their kinetic energy and, in turn, helps the bonds to break between the particles, leading to a change of state.

When particles cool, to change a gas to a liquid (condensing) and from a liquid to a solid (melting), the total internal energy of the particles decreases because the particles are moving around less, which decreases their kinetic energy and this leads to the intermolecular bonds reforming.

Many food companies use calorimeters in order to see how much energy is in their food and the number of calories present in that item. This is very important because food companies need to accurately know how many calories are in their item to correctly advertise to the consumer on the health implications that may come from their product. In chemical plants, they would also use calorimeters to test a small reaction to see how much heat is given off/lost before completing the full-scale reactions. This is very important to see if when the full reaction takes place the amount of heat given off or lost is a manageable amount or not. A Coffee cup calorimeter is used to measure the heat flow within a solution but cannot be used to measure reactions involving gases as they would just escape from the top of the cup. Another type of calorimetry is bomb calorimetry, which is used to measure the heat that comes off a reaction.

Stearic acid is used in many food products like butters and margarines. Stearic acid is also used in makeup/creams in order to allow them to spread easily/be creamy when applying. It is also used to harden and strengthen candles to allow the candles to stand up and melt on their own without needing support from any infrastructure. Stearic acid is tested with calorimetry because it has a high boiling point (383 degrees Celsius) due to the amount of saturated fatty acids it contains. This is a method of determining the substance's purity, which is essential for the food and cosmetic industry.

Paraffin wax is used to make fruits, vegetables, and sweets shiny, and it is also often used on the wrapper of hard cheese to preserve the cheese and allow it to remain hard. Paraffin wax is tested with calorimetry (it has a high melting point of 370 degrees Celsius), due to the amount of long-chain hydrocarbons it contains. Also, calorimetry is used to determine the purity of the substance, which is essential in the food industry.

Smith et al., using calorimetry techniques, found that stearic acid had a high melting point of 69.3 degrees Celsius, whereas paraffin wax was discovered in 1850, but in the 20th century, the demand has rapidly grown as the wax is being used in more and more products due to its features. It has a lower melting point of between 46 and 68 degrees Celsius.

My intent from this practical was to use calorimetry techniques to construct cooling curves and compare the rate of cooling of paraffin wax and stearic acid in order to compare their physical properties and uses in everyday life.

Safety:

Check the gas tap for any leaks before turning the gas on.
Check all equipment before use in order to prevent anything breaking during the practical. If any glass breaks during the practical, make sure to inform your teacher immediately and clear up the glass safely without causing any harm to yourself or others in the laboratory.
Wear appropriate clothing and always wear goggles when participating in the practical, especially around boiling substances.
Check the CLEAPSS Hazards for both substances that you are using in order to see the safety advice, if any of the substances contact any part of your body (e.g., skin and eyes).
COSHH = Control of Substances Hazardous to Health
CLEAPSS = Consortium of Local Education Authorities for the Provision of Science Services.
Card 38B for stearic acid states that the substance is not currently classified as hazardous, and its chemical formula is ^{CH₃(CH₂)¹⁶COOH}. To dispose of this substance, just place it in the normal waste as there are currently no specific guidelines on where to dispose of this substance.
Card 45B for paraffin wax states that the substance is not currently classified as hazardous, but when using the substance, wear eye protection, avoid inhaling the fumes where possible, and always ensure the lab you are working in is well-ventilated. To dispose of this substance, just place it in a normal waste bin as there are currently no specific guidelines on where to dispose of this substance.

Calibrating Thermometers: (Liquid and Digital)

Equipment:

2 x same-size test tubes
2 x long liquid thermometers
Equal quantities of paraffin wax and stearic acid (20ml)
Bunsen burner tripod
Bunsen burner
Stop clock
Gauze mat for Bunsen burner
Matches to light Bunsen burner (make sure these are not damaged/wet)
250 ml beaker of boiling water
2 x 100ml beakers
Ice

Cold Method:

A 100ml beaker was filled with ice.

A digital thermometer was inserted into the beaker (making sure it does not touch the side).

The temperature of the ice was recorded 3 times 1 minute apart. (The temperature of the ice was -5^oC at the start).

Record the results in a table and work out the correction factor for the experiment using the average temperature.

Repeat for both types of thermometer.

Hot Method:

A 100ml beaker was half-filled with water.

A Bunsen burner, tripod, and a gauze mat were set up (making sure to check the gas tap for any leaks before turning it on).

A digital thermometer was inserted into the boiling water (making sure it did not touch the side).

Once the water has reached boiling, the temperature was recorded 3 times 1 minute apart (The temperature was 80^oC at the start).

Record the results in a table and work out the correction factor for the experiment using the average temperature.

Repeat for both types of thermometer.

Method (For both heating and cooling – constructing a cooling curve)

A Bunsen burner tripod with a gauze mat and a Bunsen burner underneath was set up with the tube connected to the gas tap. The gas tap was checked first for any leakages to prevent accidents.
A 250ml beaker of boiling water was placed onto the gauze mat on a Bunsen burner. Then a test tube of each of the desired substances (20ml of stearic acid and paraffin wax) was placed into the water with a long liquid thermometer and heated until the substance reached its melting point.
Once the substances reached their melting point, both tubes were removed from the water to allow them to cool.
Each tube was clamped into a clamp stand. Each test tube had a calibrated long liquid thermometer inside, but it was not touching the sides (as this would make the results inaccurate).
The temperature of the paraffin wax and stearic acid was recorded every minute for 20 minutes.
The temperatures of both substances were recorded in a table of results. The results were adjusted using the calibrated correction factor for the liquid thermometer.

Calculating the Gradient using a Tangent:

First, draw your graph using your results from your experiment (temperature vs. time taken) - (See Appendix 1).

Secondly, draw a tangent on the initial cooling (steepest part of the graph) and extend it all the way to the x-axis and the y-axis. Draw a second tangent on the final cooling phase of the graph and again extend it to both axes.

Use the tangents to calculate gradients, which represent the rate of cooling.

Calculate the gradient using the equation: m = (y₂ - y₁) / (x₂ - x₁) - (see both Appendix 1 and 2).

By calculating the gradient, it enables the finding of the different rates of cooling at different points on the graph and also helps determine how quickly the temperature changes, so the properties of both substances can be compared.

Results:

Time (minutes)	Temperature of stearic acid (Degrees Celsius)	Temperature of stearic acid with the calibration factor (degrees Celsius)	Temperature of paraffin wax (Degrees Celsius)	Temperature of paraffin wax with the calibration factor (degrees Celsius)
1	91	89.17	73	71.17
2	84	82.17	71	69.17
3	82	81.17	65	62.17
4	78	76.17	62	60.17
5	74	72.17	57	55.17
6	61	59.17	56	54.17
7	58	56.17	55	53.17
8	56	54.17	54	52.17
9	55	53.17	54	52.17
10	55	53.17	53	51.17
11	55	53.17	52	50.17
12	55	53.17	51	49.17
13	55	53.17	51	49.17
14	55	53.17	50	48.17
15	55	53.17	49	47.17
16	55	53.17	48	46.17
17	55	53.17	47	45.17
18	55	53.17	45	43.17
19	55	53.17	44	42.17
20	52	50.17	42	41.17

Calculating Initial and Final Cooling Rates using Tangents:

The rate of cooling can be calculated using the formula:

Rate of Cooling = (Final Temperature - Initial Temperature) / (Final Time - Initial Time)

For example, in the case of stearic acid, the initial cooling rate can be calculated as follows:

Steric acid initial cooling rate = (91°C - 81°C) / (2 min - 1 min) = 10 degrees Celsius/min

Similarly, you can calculate the initial and final cooling rates for paraffin wax.

Conclusion from Practical:

In our experiment, both substances cooled relatively quickly and changed state from a liquid to a solid. This transition occurred because the substances were removed from the heat source, causing the intermolecular bonds between the particles to decrease in distance and increase in strength, ultimately forming a solid state. The flat areas on the cooling curve represent the phase change from liquid to solid and the associated energy release. During this phase change, extra energy is released as the bonds form, temporarily slowing down the cooling process until the bonds are fully established in the solid state.

Notably, stearic acid reached a higher temperature of 91 degrees Celsius during heating but cooled to a temperature 10 degrees higher than paraffin wax during cooling. This observation suggests that stearic acid requires more energy to form intermolecular bonds in the solid phase compared to paraffin wax. This difference aligns with the melting points of the two substances; stearic acid has a melting point of 69.3 degrees Celsius, while paraffin wax's melting point falls within the range of 46 to 68 degrees Celsius. The varying energy requirements for breaking and forming bonds influence the cooling rates.

It is essential to determine the melting points from your graphs and compare them with values obtained during prior research.

Additionally, we can assess the purity of the substances by comparing the melting points obtained in our experiment with published results from other scientists. A common mathematical method for purity evaluation involves dividing our results by the published values online and multiplying by 100%. It's worth noting that substances repeatedly used in a school environment may have altered properties, potentially resulting in lower melting points.

To calculate the mathematical percentage purity, divide your purity values by the published ones and multiply the result by 100.

Evaluation for Practical:

The strength of the intermolecular forces affects the cooling and melting of any substance. Stronger intermolecular forces result in a higher melting point for the substance. The experiment results indicated that stearic acid heats and cools more quickly than paraffin wax. This difference can be attributed to stearic acid's higher melting point compared to paraffin wax, making it harder to reform the intermolecular bonds, causing a phase change from a liquid to a solid.

Comparing our cooling curves to online studies, we find some similarities and differences. Both curves show a steady initial decrease. However, our results reached a lower minimum temperature than those in the online study. While the cooling curve for paraffin wax is more consistent between our results and the online study, the stearic acid graphs exhibit significant differences, particularly in the minimum temperature reached.

Accuracy Considerations:

During the experiment, several factors could have affected the accuracy of the results. These include:

Not allowing the substances to reach their melting point before removing them from the heat source.
Allowing the thermometer to touch the sides of the test tube.
Room temperature fluctuations affecting the experiment.
Reading errors, such as parallax error, when interpreting the thermometer.
The choice of thermometer, with digital thermometers offering higher precision but potentially more fluctuation, and liquid thermometers being more affordable and convenient for school use.

To improve the accuracy, if the experiment were to be repeated:

Ensure that the substances fully melt (all intermolecular forces are broken) before recording temperatures.
Keep the thermometer upright and avoid contact with the test tube walls to prevent unreliable results.
Insulate the test tubes and use bungs to minimize the impact of room temperature fluctuations.
Pay careful attention to avoid parallax errors when reading the thermometers.
Consider using digital thermometers for increased precision, despite potential fluctuations.

It's important to note that liquid thermometers, while less precise, are more cost-effective and convenient for school environments due to their lack of reliance on batteries or power supplies.