Calorimetry and Calibration Lab: Heat Measurement & Instrument Precision

Learning Aim: Undertake Calorimetry to Study Cooling Curves

Calorimetry and Calibration

Calorimetry is the scientific process used to measure the heat released or absorbed during a chemical reaction. By quantifying the heat change, we can determine whether a reaction is exothermic (heat-releasing) or endothermic (heat-absorbing) ¹.

The term "calorimetry" derives from the Latin word "calor" meaning heat, and the Greek word "metron" for measure.

Calibration involves aligning the readings of an instrument with those of a known standard to assess its accuracy. It is essential in scientific research to ensure the reliability of data. Accuracy refers to how closely the measured value matches the actual value ².

Scientific instruments, including thermometers, can become inaccurate over time due to factors such as aging and usage.

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This inaccuracy, often referred to as "drift," can lead to unreliable results.

Calibration of Two Types of Thermometers

This section covers the equipment used, risk assessment, and the setup for calibrating both digital and liquid thermometers safely.

Equipment:

• 500ml Beaker
• Bunsen Burner
• Digital Thermometer
• Analog Thermometer (alcohol-filled)
• Ice Cubes
• 1000ml Graduated Measuring Cylinder
• Wire Gauze
• Tripod Stand

Risk Assessment:

Procedure

The calibration process involves two methods: boiling water calibration and ice bath calibration, each using a different known standard for calibration.

Boiling Water Calibration

1. Place the digital scale on a level platform for accuracy. Turn it on and weigh the graduated measuring cylinder, recording the result. This measurement will be used to subtract the cylinder's mass from that of the water and cylinder combined.

2. Fill the measuring cylinder with 150ml of water, weigh it, and record the result. This step determines the mass of the water, ensuring that a similar mass of ice is used later to minimize inaccuracies.

3. Subtract the recorded mass of the empty cylinder from the combined mass of the cylinder and water. This provides the mass of water in grams.

4. Set up the Bunsen burner, connect it to a gas supply via the rubber tube, and place it on a heatproof mat under the tripod stand. Place the wire gauze on the tripod stand to ensure stability and safety.

5. Pour the water from the graduated measuring cylinder into the beaker and place the beaker on the wire gauze above the Bunsen burner. Turn the Bunsen burner collar to fully cover the air hole, producing a safety flame that confirms the burner is on but not being used for heating.

6. Insert the analog thermometer into the beaker and open the Bunsen burner collar to increase air flow, resulting in a hotter flame with complete combustion.

7. Wait for the water to start boiling and check if the analog thermometer reads 100 degrees Celsius. Record the thermometer reading and replace it with the digital thermometer to compare the readings.

8. Repeat the process of swapping and allowing the thermometers to cool three times. Calculate the mean of the results for each thermometer. This repetition under consistent conditions helps estimate result variability and improve accuracy, assuming no systematic errors are present.

Ice Bath Calibration

1. Calculate the mass of the water and measuring cylinder by subtracting the previously recorded mass of the empty cylinder (107g) from the combined mass of the 150ml water and cylinder (260g), resulting in a mass of 153g.

2. Empty the ice cubes into a beaker and add cool water. Stir the ice bath well using a glass rod and wait for 30 seconds to ensure temperature equilibrium.

3. Submerge the thermometers into the ice bath at three intervals, swapping between the two thermometers each time a stable reading is recorded. Ensure the bulb of the liquid thermometer or the tip of the digital thermometer is surrounded by ice cubes and positioned in the middle of the ice bath. This procedure ensures accuracy and measures the temperature of the ice bath, which remains a known constant at 0 degrees Celsius, used for calibrating the thermometers.

Results

Boiling Water Calibration

Mean reading = 104 Degrees Celsius

Mean Reading = 100.57 Degrees Celsius

Ice Bath Calibration

Mean reading = 1.33 Degrees Celsius

Mean Reading = 0.566 Degrees Celsius

In conclusion, the liquid thermometer has an average deviation of 1.33 degrees Celsius, while the digital thermometer has an average deviation of 0.566 degrees Celsius. This indicates that the digital thermometer is more accurate. Therefore, I will use the digital thermometer to record temperature data for my experiment, ensuring greater precision and reliability in the results.

Cooling of Paraffin Wax

This section outlines the equipment, risk assessment, and the setup for generating a time-temperature data table and a graph depicting temperature against time for the cooling of Paraffin wax.

Equipment:

• 500ml Beaker
• Bunsen Burner
• Digital Thermometer
• Scale
• Wire Gauze
• Tripod Stand
• Clamp Stand
• Clamps
• Heatproof Mat
• Paraffin Wax
• Paper Tray

Risk Assessment:

Procedure:

1. Zero the scale by placing a weighing boat on it and pressing the zero button before measuring 7.50 grams of paraffin wax. This step ensures the measurement records only the paraffin mass, not the weighing boat.
2. Pour the measured paraffin wax into a boiling tube using a spatula, ensuring all the wax is inside the tube, and clamp it onto the clamp stand.
3. Fill a 250ml beaker with 150ml of water and position it on the hot wire gauze over the tripod, situated above the Bunsen burner.
4. Adjust the clamp stand so that the boiling tube, attached to it, is submerged inside the water, ensuring the paraffin wax is fully covered. Clamp the digital thermometer above the boiling tube, with the probe surrounded by the paraffin wax.
5. Perform a safety check to ensure all equipment is stable and secure, and the Bunsen burner is resting on a heatproof mat under the tripod stand.
6. Turn on the gas valve for the Bunsen burner and ignite it at a safety flame by closing the air hole. This setup guarantees safe heating.
7. Once the paraffin wax has completely melted, turn off the Bunsen burner, start the timer, and record the temperature of the wax at 30-second intervals.

Results: Temperature vs. Time Data

Below is a table presenting time-temperature data for the cooling of Paraffin wax:

Calculations of the Rate of Cooling

Rate of Cooling:

The rate of cooling is determined by plotting a temperature versus time graph and drawing a cooling rate curve. To find the rate of cooling, we draw a tangent to the curve at specific points: Triangle A, representing the rapid temperature decrease during Paraffin wax's liquid cooling phase; Triangle B, where the temperature remains approximately constant, known as "thermal arrest"; and Triangle C, indicating the slope during solid cooling.

Cooling Curves:

Cooling curves are graphical representations depicting temperature changes over time. The initial point on the graph represents the starting temperature of the substance, which, in this case, is 87 degrees Celsius for Paraffin wax. The points where the graph plateaus indicate "thermal arrest," signifying a change in state – in this context, freezing. During this phase, the substance transitions from a solid to a liquid as it loses thermal energy, and intermolecular bonds strengthen with particles vibrating less.

Cooling of Stearic Acid

This section details the equipment, risk assessment, and setup for creating a table and a graph illustrating temperature changes over time for the cooling of Stearic Acid.

Equipment:

• 500ml Beaker
• Bunsen Burner
• Digital Thermometer
• Scale
• Wire Gauze
• Tripod Stand
• Clamp Stand
• Clamps
• Heatproof Mat
• Stearic Acid
• Paper Tray

Risk Assessment:

Procedure for Cooling of Stearic Acid

1. We begin by zeroing our scale, placing a weighing boat on it, and pressing the zero button to exclude the weighing boat's mass when measuring 4 grams of stearic acid. This step ensures accurate recording of the stearic acid's mass.
2. After recording the stearic acid's mass, carefully pour it into a boiling tube using a spatula to ensure all the substance is inside the tube. Clamp the boiling tube securely onto the clamp stand.
3. Prepare a 250ml beaker by filling it with 150ml of water. Position the beaker on the hot wire gauze, which is placed over the tripod stand above the Bunsen burner.
4. Adjust the clamp stand so that the boiling tube, attached to it, is fully submerged in the water, ensuring complete coverage of the stearic acid. Above the boiling tube, clamp the digital thermometer in such a way that its probe is surrounded by the stearic acid.
5. Turn on the gas valve for the Bunsen burner and ignite it, maintaining a safety flame by closing the air hole. Ensure the Bunsen burner is situated on a heatproof mat under the tripod stand, and confirm the stability and security of all equipment.
6. Once the stearic acid has fully melted, turn off the Bunsen burner, start the timer, and record the temperature of the stearic acid at 30-second intervals.

Results: Temperature vs. Time Data for Stearic Acid

Here is the table presenting time-temperature data for the cooling of Stearic acid:

Calculations of the Rate of Cooling

Rate of Cooling:

Calculating the rate of cooling involves examining the temperature changes at specific points: liquid cooling (A), thermal arrest (B), and solid cooling (C) for Stearic acid.

How the Rate of Cooling Relates to Intermolecular Forces and the State of the Substance:

During the setup phase, Paraffin wax is observed as a solid. When heat is applied, it undergoes a phase change known as melting. This process occurs as the internal energy of the solid increases, causing the particles within the substance to gain energy and vibrate more rapidly. Initially, this weakening of the bonds makes the wax "softer." Continued heating eventually provides enough energy to disrupt the Paraffin wax's structure, allowing the particles to break free from the intermolecular forces of attraction. This specific point is referred to as the melting point.

Upon allowing the liquid to rest, it gains viscosity and eventually solidifies, a process called freezing. This transformation occurs as energy is lost to the surroundings, causing the particles to lose energy and vibrate less.

These observations align with the principles of the particle model, a scientific theory that explains the properties of solids, liquids, gases, and their response to temperature changes (thermal energy).

Evaluation

In this section, I will evaluate the accuracy of the practical work conducted in calorimetry, specifically in relation to the analysis of the cooling curve.

Personal Competencies and Skill Evaluation in My Method:

One practical skill I improved upon in my method is zeroing the scale and ensuring it is placed on a level surface before measuring the mass of the stearic acid. This step helps maintain accuracy by excluding variables like the weighing boat's mass from the measurement.

However, one aspect I could have executed better is calibrating the scale by pressing the CAL button and using a calibrating weight. This additional step could have further enhanced the accuracy of my method.

Feedback Received from Peers and How It Aided My Method:

During the calibration of the liquid thermometer, a classmate pointed out that I did not ensure the measuring bulb was positioned at the center of the beaker filled with hot water. Instead, it was sitting at the bottom, unsecured by a clamp, with the bulb on the far-right side of the beaker. Upon receiving this feedback, I promptly repeated the boiling water calibration of my thermometer, rectifying the mistake and ensuring the accuracy of the calibration.

Analysis of My Results:

While my results were similar to those of my peers, I could have conducted further research on the rate of cooling for the substances online and compared my results to those collected by others. This additional step would have provided a better understanding of the representativeness of my results.