Materials Included In Kit

Additional Materials Required

Safety Precautions

The cold pack solid is ammonium nitrate. It is slightly toxic by ingestion and is a body tissue irritant. Ammonium nitrate in solid form is a strong oxidizer and may explode if heated or in contact with combustible materials. Avoid contact of the dry solid with combustible organic materials. Avoid contact of all chemicals with eyes and skin. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Wear chemical splash goggles and chemical-resistant gloves and apron. Please consult 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. The cold pack solutions generated in this experiment may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Two cold packs are provided for students to pass around and observe. Two cold packs are also included to use in Part A of the procedure. Other cold pack brands can be purchased or obtained from the athletic department and compared to the cold packs provided in this kit. Are all cold packs identical?
• The laboratory work for this inquiry-based experiment can reasonably be completed in one 50-minute lab period. Part A, obtaining information from the cold pack label, takes about 10 minutes. The actual experimental measurements for Part B take very little time—three independent trials are easily completed within 20 minutes.
• Class data may be collected in lieu of 2–3 runs per group. Averaging the class results will tend to eliminate outlying data.
• The most important element for success in an inquiry-based activity is student preparation. Sufficient time should be set aside for students to think through the measurements, how they will be made, the variables that will influence the measurements, and how the variables can be controlled. The PreLab Preparation section provides some leading questions to stimulate class discussion. The questions may be used as the basis of a small-group activity prior to lab or assigned as homework in preparation for lab. Encourage students to work together to devise a procedure for the calorimetry experiment.
• Temperature measurements may be made using digital thermometers, glass-bulb thermometers, or computer/calculator-interfaced temperature sensors. Digital thermometers are preferred over glass thermometers because they provide direct readings, update every second, and have a precision of ±0.1 °C. Glass thermometers are fragile and easily broken, especially if the solutions are vigorously stirred, as suggested in the Sample Procedure. In addition, the 1 °C divisions that are marked on most glass thermometers make them less precise (±0.5 °C) than digital thermometers. Never allow students to use a glass thermometer as a stirring rod.
• See the Supplementary Information in the Further Extensions section for directions on how to adapt the procedure to use a temperature sensor or probe for computer/calculator-interfaced data collection and analysis.
• Only four cold packs are included in the kit. These materials should be shared in Part A. The solid can be left in a weighing dish and the water left in a graduated cylinder.
• The minimum temperatures recorded in the Sample Data section were generally achieved within one minute after mixing and were usually stable for an additional minute. Both of these factors ensure that temperature is easily and precisely measured and that students will feel confident about their measurements.
• Two Styrofoam cups nested together provide better insulation and thermal stability than one cup. If two cups are used, students can easily run two trials without rinsing and drying the cup between trials. Simply have students interchange the actual solution cup and the bottom cup between measurements. In addition, it is recommended that students nestle the Styrofoam cup in a beaker for added stability when the thermometer is placed in the cup. Another option to use in this lab are small scale calorimeters, Flinn Catalog No. AP5928. The calorimeters are made from 2"-thick polystyrene and are only about $1.00 each.
• The volume of liquid is a variable that must be controlled in the experiment, but there is no obvious value that should be used. The minimum volume of water needed to ensure that the thermometer is completely immersed in liquid is about 20–30 mL. If too large a liquid volume is used, the observed temperature change will be small and less precise. Working with 5 g of cold pack solid and a water volume of 30 mL gives three significant figures in the measured temperature difference. It should be noted, however, that excellent results are also obtained when the reaction is carried out with 100 mL of distilled water.

Teacher Tips

• One of the more stubborn student misconceptions is the idea that if the reaction mixture gets cold, it must have lost heat, therefore the reaction must be exothermic. This misconception may be traced to a lack of understanding of the system versus the surroundings. The temperature change that is measured in a typical coffee-cup calorimeter experiment is that of the surroundings. A heat of solution experiment is probably more confusing on this point than a heat of neutralization or heat of combustion experiment, because water is involved in the reaction. Also, using the combined mass of the solute and the solvent in the heat equation to calculate the heat change tends to blur the traditional distinction between the reactants and products versus the solvent.
  
• Students should be familiar with the following definitions before beginning this activity: exothermic and endothermic reactions, heat and temperature, and the system versus the surroundings.
  
• Students may compare their results with the literature value for the heat of solution of ammonium nitrate. Assuming a literature value of 25.7 kJ/mole (CRC Handbook of Chemistry and Physics, 82nd Edition, Section 5, p. 105), the results in the Sample Data section (26.4 kJ/mole) give a calculated percent error of 3%.
  
• Some books represent the specific heat of water as c instead of s.

Further Extensions

Supplementary Information

The following instructions are provided for adapting Part B to the use of technology (computer-interfaced data collection and analysis).

1. Connect the interface (e.g., LabPro, CBL system) to a computer or calculator.
2. Plug a temperature sensor into the interface.
3. Open and set up a graph in the data collection software so that the y-axis reads temperature in degrees Celsius. Set the minimum and maximum temperature values at 0 degrees and 25 degrees, respectively.
4. The x-axis should be set for time in seconds. Set the minimum and maximum time values at 0 seconds and 180 seconds, respectively.
5. The time interval should be set so a temperature reading is taken every 10 seconds.
6. Obtain 30 mL of distilled water in a graduated cylinder and carefully transfer the water to an insulated foam cup nested in a 400-mL beaker for stability.
7. Place the temperature sensor in the water. Allow the probe to equilibrate at the initial temperature for 1 minute, then press Start to begin collecting temperature data.
8. Stir the water using a stirring rod and add the pre-weighed amount of cold pack solid.
9. Continue stirring the solution and collecting temperature data for three minutes (180 seconds).
10. The LabPro or CBL-2 interface will automatically stop collecting data at the preset maximum time value.
11. Print the computer-generated data table and graph, if possible, and use the data to complete the data table and the Post-Lab Calculations.

Answers to Prelab Questions

Consider the following questions and guidelines:

1. What information (data) is needed to calculate the enthalpy change for a reaction?

   In order to calculate the enthalpy change for a reaction, data for the three terms involved in the heat energy equation (q = m x s x ΔT) must be known or measured. The mass (m) is the mass of the solution after the cold pack solid has dissolved. The specific heat capacity (s) is assumed to be the same as the specific heat capacity of water (4.18 J/g•°C). The temperature change (ΔT) is equal to the difference between the final and initial temperatures (T_final – T_initial). Note: Assuming the specific heat capacity of the final solution is the same as that of water is a major source of error in the heat calculations. Generally speaking, the specific heat of an aqueous solution is calculated as the weighted average of the specific heat of the solute and water. For the final solution obtained in this experiment, the calculated specific heat is 3.83 J/g•°C.

2. Identify the possible variables in this experiment.

   Some of the important variables include: (1) the mass of the solute (cold pack solid); (2) the volume (mass) of the solvent; (3) whether all of the solute dissolves in the solvent; (4) the heat insulating properties of the reaction container; (5) how well the reaction mixture is stirred; (6) how stable the initial temperature reading is.

3. The independent variable in an experiment is the variable that is changed by the experimenter, while the dependent variable responds to (depends on) changes in the independent variable. Choose the dependent and independent variable for this experiment.

   In a calorimetry experiment, the mass of the solute in grams is the independent variable and will be varied in different trials. The temperature change that is produced depends on the mass of the solute and is thus the dependent variable in a calorimetry experiment.

4. What variables should be controlled (kept constant during the procedure)?

   The following variables should be held constant during the procedure: the volume (mass) of the solvent; the type of reaction container that is used (two insulating foam cups nestled one inside the other will provide better insulation than one cup); continuous stirring of the reaction mixture.

5. Discuss the factors that will affect the precision of the experimental results.

   Many factors will influence the precision of the results:

   • The precision of the balance used to measure the mass of solute.
   • The precision of the graduated cylinder used to measure the volume of solvent.
   • The precision of the thermometer used to measure the temperature of the reaction mixture.
   • The number of times the experiment is repeated to average the effects of random errors.
   • The type of vessel that is used as the calorimeter—how much heat is gained or lost by the calorimeter itself.

   
   The first three measurements should be made with the most precise glassware and equipment available in the lab—centigram balances (at least), appropriate size graduated cylinders, and digital thermometers, if possible. One important way to improve the precision of the experimental results is to average data obtained over several runs or trials. A minimum of 2–3 trials is recommended. Alternatively, class data may be averaged to eliminate outlying results.

Sample Data

Part A. What Is an Instant Cold Pack?

The following information was obtained using an AFAS-COLD brand Chemical Cold Pack, manufactured by Afassco (Nevada).

1. Set two nested insulating foam cups in a 400-mL beaker for stability.
2. Measure 50.0 mL of distilled water in a graduated cylinder and add it to the insulating foam cup. Record the volume of water used in the data table.
3. Place a digital thermometer in the water in the cup and allow the thermometer and the water to adjust to room temperature for at least 2–3 minutes.
4. Tare a weighing dish on the balance. Add about 5 g of cold pack solid to the weighing dish. Measure and record the exact mass of solid to the nearest 0.01 g.
5. Measure the initial temperature of the water in the cup to the nearest 0.1 °C and record the value in the data table.
6. Stir the water in the cup with a stirring rod and add the massed amount of cold pack solid to the water. Make sure that all of the solid dissolves in the water.
7. Stir constantly and monitor the temperature of the solution. Record the lowest temperature that is reached.
8. Rinse the cup contents down the drain with plenty of excess water and dry the inside of the cup with a paper towel.
9. Repeat steps 1–8 at least once more. Record all data.

Answers to Questions

1. Calculate the heat energy change in joules when the cold pack solid dissolved in water in your experiment. Recall: q = m x s x ΔT, where s (specific heat of water) is equal to 4.18 J/g•°C.

   Trial 1. q = 35.37 g x 4.18 J/g•°C x (–11.8 °C) = –1740 J
   Trial 2. q = 34.78 g x 4.18 J/g•°C x (–11.0 °C) = –1600 J
   Trial 3. q = 34.89 g x 4.18 J/g•°C x (–11.1 °C) = –1620 J
   Note: The negative sign for the heat change indicates that the solution (the surroundings) lost heat energy when the solid dissolved. According to the law of conservation of energy, the amount of heat lost by the surroundings must be equal to the amount of energy gained by the system for the reaction to occur. The cold pack solid absorbed energy from the surroundings as it dissolved in water—it is an endothermic process. The heat change for an endothermic process is a positive quantity. See the Answers to Prelab Questions for a discussion of the error involved in using the specific heat of water in these calculations.

2. Calculate the energy change in joules per gram of solid for the cold pack solid dissolving in water.

   Trial 1. ΔH_soln = 1740 J/5.37 g = 324 J/g
   Trial 2. ΔH_soln = 1600 J/4.78 g = 335 J/g
   Trial 3. ΔH_soln = 1620 J/4.89 g = 331 J/g
   Average value = (324 + 335 + 331)/3 = 330 ±4 J/g
   Note: The heat of solution of ammonium nitrate dissolving in water is a positive quantity, equal in magnitude but opposite in sign to the heat change calculated in Question 1.

3. Calculate the energy change in units of kilojoules per mole of solid for the cold pack solid dissolving in water. To do this:
   1. Convert the heat energy change found in Question 1 to kilojoules.

      Trial 1. 1740 J x (1 kJ/1000J) = 1.74 kJ
      Trial 2. 1600 J x (1 kJ/1000J) = 1.60 kJ
      Trial 3. 1620 J x (1 kJ/1000J) = 1.62 kJ

   2. Convert the grams of solid used in the experiment to moles.

      Trial 1. 5.37 g x (1 mole/80.04 g) = 0.0671 moles
      Trial 2. 4.78 g x (1 mole/80.04 g) = 0.0597 moles
      Trial 3. 4.89 g x (1 mole/80.04 g) = 0.0611 moles

   3. Divide the energy change in kilojoules by the number of moles of solid to determine the energy change in units of kJ/mole. If more than one trial was performed, calculate the average value of the heat of solution also.

      Trial 1. 1.74 kJ/0.0671 moles = 25.9 kJ/mole
      Trial 2. 1.60 kJ/0.0597 moles = 26.8 kJ/mole
      Trial 3. 1.62 kJ/0.0611 moles = 26.5 kJ/mole
      Average value = (25.9 + 26.8 + 26.5)/3 = 26.4 ±0.3 kJ/mole

4. Using the result from Question 3c and the information obtained in Part A, calculate the number of kilojoules involved when the entire cold pack is activated. Recall (Part A, Question 6): The number of moles of solid in the instant cold pack is 0.336 moles.

   Amount of heat transfer = 26.4 kJ/mole x 0.336 moles = 8.87 kJ.
   Note: This amount of heat exchange should be sufficient to cool 45 mL of water in an instant cold pack from 25 °C to –22 °C!

5. Circle the correct choices in the following sentence to summarize the heat change that occurs when the commercial cold pack is activated:

   “When the white solid in the commercial cold pack dissolves in water, the pack feels (hot/cold) because the temperature of the solution (increases/decreases). Energy is (absorbed/released) from the surroundings during this reaction and the reaction is classified as (endothermic/exothermic). The sign of ΔH for the heat of solution is (positive/negative).”

References

This activity is from Flinn ChemTopic™ Labs, Volume 10, Thermochemistry; Cesa, I., Ed., Flinn Scientific: Batavia, IL, 2002.

Introduction

Instant cold packs are familiar first aid devices used to treat injuries when ice is unavailable. Most commercial cold packs consist of a plastic package containing a white solid and an inner pouch of water. Firmly squeezing the pack causes the inner pouch to break. The solid then dissolves in the water producing a change in temperature. Can the temperature change that occurs when the cold pack solid dissolves in water be measured and the heat change for this process determined?

Concepts

• Enthalpy change
• Heat of solution
• Calorimetry
• Dependent and independent variables

Background

The energy or enthalpy change associated with the process of a solute dissolving in a solvent is called the heat of solution (ΔH_soln). In the case of an ionic compound dissolving in water, the overall energy change is the net result of two processes—the energy required to break the attractive forces (ionic bonds) between the ions in the crystal lattice, and the energy released when the dissociated (free) ions form ion-dipole attractive forces with the water molecules. The process of a solute dissolving in water may either release heat into the aqueous solution or absorb heat from the solution.

Heats of solution and other enthalpy changes are generally measured in an insulated vessel called a calorimeter that reduces or prevents heat loss to the atmosphere outside the reaction vessel. When using a calorimeter, the reagents being studied are mixed directly in the calorimeter and the temperature is recorded both before and after the reaction has occurred. The amount of heat change occurring in the calorimeter may be calculated using the following equation: q = m x s x ΔT, where m is the total mass of the solution (solute plus solvent), s is the specific heat of the solution, and ΔT is the observed temperature change. The specific heat of the solution is generally assumed to be the same as that of water, namely, 4.18 J/g•°C.

Experiment Overview

The purpose of this inquiry-based experiment is to design and carry out a procedure to determine the enthalpy change that occurs when a “cold pack solid” dissolves in water.

Materials

Prelab Questions

Consider the following questions:

1. What information (data) is needed to calculate the enthalpy change for a reaction?
2. Identify the possible variables in this experiment.
3. The independent variable in an experiment is the variable that is changed by the experimenter, while the dependent variable responds to (depends on) changes in the independent variable. Choose the dependent and independent variables for this experiment.
4. What variables should be controlled (kept constant during the procedure) in this experiment?
5. Discuss the factors that will affect the precision of the experimental results.

Safety Precautions

The cold pack solid is slightly toxic by ingestion and is a body tissue irritant. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. What Is an Instant Cold Pack?

Complete the following activity to become familiar with the nature and amounts of materials in a commercial cold pack by filling in the table on the worksheet.

1. Obtain a label of a commercial cold pack and write the name of the solid used in the pack.
2. Read the warning information on the label and record any hazards associated with this product.
3. Using the known charges of ions, write the formula of the solid.
4. Calculate the molar mass of the solid.
5. Determine the total mass of the solid: Tare a large weighing dish or cup on the balance. Transfer the cold pack solid to the tared weighing dish. Record the mass of the solid to the nearest 0.01 g.
6. Calculate the number of moles of solid in the pack.
7. Measure the volume of water contained in the inner pouch.
8. Calculate the mass of water in the commercial cold pack (assume the density of water is 1.0 g/mL).

Student Worksheet PDF

13870_Student1.pdf