Calorimetry & Cooling Curves: Stearic Acid and Paraffin Wax

Categories: Chemistry

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Brief

You are a newly appointed technical assistant at a large chemical plant, Chemical Equip. As part of your induction period and to progress in your role, you have to demonstrate skills in a range of practical procedures and techniques. Part of your role is to ensure equipment is calibrated, and equipment and chemicals are safety checked.

You need to demonstrate your ability to use a calorimeter and associated equipment to obtain data to determine, analyze, and evaluate the rate of cooling of substances.

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You will need to present evidence of your practical skills analysis and evaluation in a report.

Calibration of Thermometers

Test	Temperature recorded (°C)
Liquid thermometer	Digital Thermometer
Ice water	Boiling water
Test 1	0.0	100.0
Test 2	0.0	99.0
Test 3	0.0	101.0
Test 4	0.0	100.0
Average	0.0	100.0

Introduction

Calorimetry is used to observe and determine the melting point of metals to understand their properties and limitations.

Apparatus/Techniques

Ensure the thermometer does not touch the sides or bottom of the boiling tube.

We used a liquid thermometer but could have used a digital thermometer connected to a computer.

Procedure

Use a clamp stand to hold the liquid thermometer off the side of the glass to measure the temperature of the wax and the acid, ensuring the glass is not heated during melting.
Remove them from the water bath, then place them in a boiling tube holder.
Place the thermometer into the boiling tube and hold it in place using a clamp stand and clamp, so the thermometer isn't touching the bottom or sides of the boiling tube.
Record the starting temperature after the temperature drops, then measure every minute.
Record the temperature every minute until it reaches room temperature.

Results

Substance	Time (min)	Temperature (°C)
Steric Acid	0	81
	1	76
	2	70
	13	50
	14	65
	15	64
	16	64
	17	64
	18	63
	19	63
	20	63
	21	62
	22	61
	23	61
	24	60
Paraffin Wax	0	64
	13	49
	14	49
	15	49
	16	49
	17	48
	18	48
	19	47
	20	47
	21	46
	22	45
	23	44
	24	41
	25	39
	26	38

Calculations

Rate of cooling point a (Steric Acid):

Temperature change = 81°C → 70°C = Decrease of 11°C

Time taken 0 min to 2 min = 2 minutes

Rate of cooling = Temperature change / Time taken = 11°C / 2 min = 5.5°C/min (2sf)

Rate of cooling point b (Steric Acid - Melting and Freezing Point):

Temperature change = 65°C → 63°C = Decrease of 2°C

Time taken 7 min to 20 min = 13 minutes

Rate of cooling = Temperature change / Time taken = 2°C / 13 min = 0.15°C/min (2sf)

Rate of cooling point c (Steric Acid):

Temperature change = 57°C → 37°C = Decrease of 20°C

Time taken 26 min to 35 min = 9 minutes

Rate of cooling = Temperature change / Time taken = 20°C / 9 min = 2.2°C/min (2sf)

Rate of cooling point a (Paraffin Wax):

Temperature change = 64°C → 54°C = Decrease of 10°C

Time taken 0 min to 3 min = 3 minutes

Rate of cooling = Temperature change / Time taken = 10°C / 3 min = 3.3°C/min (2sf)

Rate of cooling point b (Paraffin Wax - Melting and Freezing Point):

Temperature change = 51°C → 49°C = Decrease of 2°C

Time taken 6 min to 19 min = 13 minutes

Rate of cooling = Temperature change / Time taken = 2°C / 13 min = 0.15°C/min (2sf)

Rate of cooling point c (Paraffin Wax):

Temperature change = 44°C → 33°C = Decrease of 11°C

Time taken 26 min to 33 min = 7 minutes

Rate of cooling = Temperature change / Time taken = 11°C / 7 min = 1.6°C/min (2sf)

Results Analysis

Patterns and Trends:

The rate of cooling before the state change in stearic acid was 5.5°C/min, which is different from the rate after the state change, which was 2.2°C/min.

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For paraffin wax, the rate of cooling before the state change was 3.3°C/min compared to 1.6°C/min after the state change. This indicates that both substances have different rates of cooling, with a faster rate before the state change than after. This is because when a substance changes from one state to another, heat is a form of energy, and when it is given to particles, it gives them freedom. When particles have enough energy to break the bonds forcing them in a rigid structure, they become a liquid with a more free structure. The transition from liquid to gas is similar, but at a higher temperature because more energy is needed to break the bonds completely.

Additionally, both substances exhibited a similar overall pattern. There was a rapid decrease in temperature, followed by a stabilization during the state change, with little or no decrease in temperature. After the state change, there was a steady decrease until reaching room temperature.

It's worth noting that the temperature at which the state change occurred differed between the two substances. Steric acid underwent a state change at 63°C, while paraffin wax changed at 49°C. This variation highlights the different properties of these substances.

Comparison against Published Results and Class Values:

The data I calculated for steric acid's melting point was 65°C, while the published results were 69°C, indicating a difference of 4°C. This suggests possible errors in my experiment. Impurities in the sample or contamination during the experiment could have caused this discrepancy. The use of a mercury thermometer and visual judgment may also introduce human error.

For paraffin wax, the calculated melting point was 51°C, compared to the provided results of 64°C, a significant difference of 13°C. This suggests a large error in the paraffin wax experiment. It's important to acknowledge that my steric acid results, with a 5.8% error, were within the class average of 5%-10% error, indicating that the steric acid sample may not have been pure or could have been contaminated during experimentation.

In contrast, the paraffin wax results, with a percentage error of 20.3%, deviated significantly from the class average, indicating a potential fault in the experimental method or sample purity.

Evaluation of Procedure and Results:

There were clearly some errors in the experiment, as indicated by the high percentage errors compared to published results. Possible sources of error include impure samples, contamination, or human error. Using a data logger for more frequent and accurate temperature measurements would improve the results. Obtaining fresh samples for each experiment to minimize contamination is advisable.

The use of an old liquid thermometer with limited precision could also contribute to errors. Using electronic thermometers connected to a computer for real-time data recording would enhance accuracy and reduce human error. Moreover, repeating the experiment multiple times and averaging the results would provide more reliable data and help identify anomalies.

Calorimetry & Cooling Curves: Stearic Acid and Paraffin Wax. (2024, Jan 10). Retrieved from https://studymoose.com/document/calorimetry-cooling-curves-stearic-acid-and-paraffin-wax