Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Before students work on the Prelab Questions:
   1. Demonstrate how an FRH works for students.
   2. While wearing proper PPE, pass around a new, unopened FRH so students can see how it is packaged.
   3. Then, with another FRH, add water to the FRH, per the directions, sit upright and let students make observations. If desired, order a separate MRE and show students all the items that are included in the MRE and how the MRE is heated by the FRH.
   4. Cut a second, unused FRH open so students can see the packet and pouches containing the alloy inside the pouch.
      {12380_Preparation_Figure_3_How to open FRH}
2. The day of the lab, cut the FRH into quarters, like above. Each group will get one quarter of an FRH. If preferred, students can cut their own portion for lab.

Safety Precautions

Magnesium is a flammable solid. Avoid contact with flames and heat. The FRH is considered nonhazardous and nontoxic when used as described. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron, and observe prudent laboratory practices. Avoid contact of activated FRH with exposed skin-may cause burns. Avoid contact of all chemicals with eyes and skin. Follow all laboratory safety guidelines. Wash hands thoroughly with soap and water before leaving the laboratory. 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. The plastic packaging from the FRE can be placed in the trash. The solution remaining from the reaction of the FRH pad with water is basic and may be neutralized according to Flinn Suggested Disposal Method #10.

Lab Hints

• This laboratory activity was specifically written, per teacher request, to be completed in one 50-minute class period. It is important to allow time between the Prelab Homework Assignment and the Lab Activity. Prior to beginning the homework, show students how the heating element of an MRE works (See Prelab Preparation). Once students turn in the homework answers, tables and figures and their procedure, check it for safety and accuracy before implementation in the lab.
• Enough materials are provided in this kit for 24 students working in pairs, or for 12 groups of students. It is important to allow time between the Prelab Homework Assignment and the Lab Activity. Once students turn in the homework answers, figures, and procedure, check it for safety and accuracy before implementation in the lab.
• You can demonstrate another use of calorimetry by measuring the calories in various food items, such as a cheese puff or marshmallow. Flinn has a free write up online for this activity. Check out publication DC10861.
• To analyze the FRH further, check out the following video; Flameless Ration Heaters (VEL1305).
• Collecting data works best with a temperature sensor. TC1502 or AP8097 are both available from Flinn. Digital thermometers will also work. If using glass thermometers, remind students not to use the thermometers as stirring rods. When collecting data the sensor or thermometer should be placed in the calorimeter, stirring is not necessary.
• If using Styrofoam cups for a calorimeter, two Styrofoam cups nested together provide better insulation and thermal stability than one cup. For additional stability, place the cups in a glass beaker.

Teacher Tips

• This activity fits well into a thermochemistry unit.
• In the MRE meal kits, the food packet that is heated by the FRE weighs approximately 250 grams. As a teacher, you can add additional extension questions for students, such as calculating the temperature change of the food packet that would be heated. For example, an extension question could be: If all the energy is transferred to the MRE, what would the temperature of the MRE be (assuming the MRE has a specific heat capacity close to water, approximately 4.184 J/g•°C)?

  Using the data from the sample procedure below, if all energy was transferred to the food pouch, the following was calculated.
  
  –q_MRE = q_food
  36,244 J = (250g)(4.184J/g•°C)(ΔT)
  ΔT = 34.7 °C
  If we assume the food packet started at room temperature, approximately 23.0 °C, the final temperature of the food packet would be 57.7 °C (23.0 + 34.7).
  
  In reality, not all of the heat that is released goes directly into the food pouch. In another trial, a 250.0-gram food pouch was heated and its initial and final temperature was recorded. Another extension question you can ask students is: Using the following data, how much energy was lost to the surroundings during the MRE heating process?

  {12380_Tips_Table_3}

  q = mcΔT
  q = (250 g)(4.184 J/g•°C)(43.5 – 23.5 °C) = 20,920 J = 20,900 J
  Heat lost to the surroundings = 36,200 J – 20,900 J = 15,300 J

• Showing students what is inside an MRE is also a great way to engage students. If possible, heating an actual MRE meal is very engaging for students. You can purchase one separately. If any of the MRE will be eaten, open and display the MRE in a food safe room only. Do not use or store the MRE in a lab if it will be consumed.
• If students use a small amount of water when they run the procedure, the reaction can reach boiling temperature and stay at that temperature for a period of time. You can use this opportunity to discuss a heating curve and how to calculate the energy needed for the solution to boil. If wanted, as a class, you can calculate the energy used at the boiling plateau. The mass of the solution before and after would have to be weighed.
• The following student laboratory kits can be used for more hands-on activities involving thermochemistry: Designing a Hand Warmer—Inquiry Lab Kit for AP^® Chemistry (Flinn Catalog No. AP7654), Thermodynamics—Review Demonstration Kit for AP^® Chemistry (Flinn Catalog No. AP7328) and Thermodynamics—Enthalpy of Reaction and Hess’s Law—Classic Lab Kit for AP^® Chemistry (Flinn Catalog No. AP8832).

Further Extensions

Alignment to the Curriculum Framework for AP^® Chemistry 

Enduring Understandings and Essential Knowledge
Atoms are conserved in physical and chemical processes. (1E)
1E1: Physical and chemical processes can be depicted symbolically, when this is done, the illustration must conserve all atoms of all types.
1E2: Conservation of atoms makes it possible to compute the masses of substances involved in physical and chemical processes. Chemical processes result in the formation of new substances, and the amount of these depends on the number and types and masses of elements in the reactants, as well as the efficiency of the transformation.

Chemical changes are represented by a balanced chemical equation that identifies the ratios with which reactants react and products form. (3A)
3A1: A chemical change may be represented by a molecular, ionic, or net ionic equation.
3A2: Quantitative information can be derived from stoichiometric calculations that utilize the mole ratios from the balanced chemical equations. The role of stoichiometry in real-world applications is important to note, so that it does not seem to be simply an exercise done only by chemists.

Chemical reactions can be classified by considering what the reactants are, what the products are, or how they change from one into the other. Classes of chemical reactions include synthesis, decomposition, acid-base, and oxidation-reduction reactions. (3B)
3B3: In oxidation-reduction (redox) reactions, there is a net transfer of electrons. The species that loses electrons is oxidized, and the species that gains electrons is reduced.

Chemical and physical transformations may be observed in several ways and typically involve a change in energy (3C)
3C1: Production of heat or light, formation of a gas, and formation of a precipitate and/or a color change are possible evidences that a chemical change has occurred.
3C2: Net changes in energy for a chemical reaction can be endothermic or exothermic.

Two systems with different temperatures that are in thermal contact will exchange energy. The quantity of thermal energy transferred from one system to another is called heat. (5A)
5A1: Temperature is a measure of the average kinetic energy of atoms and molecules.
5A2: The process of kinetic energy transfer at the particulate scale is referred to in this course as heat transfer, and the pontaneous direction of the transfer is always from a hot to a cold body.

Energy is neither created nor destroyed, but only transformed from one form to another. (5B)
5.B.1: Energy is transferred between systems either through heat transfer or through one system doing work on the other system.
5.B.2: When two systems are in contact with each other and are otherwise isolated, the energy that comes out of one system is equal to the energy that goes into the other system. The combined energy of the two systems remains fixed. Energy transfer can occur through either heat exchange or work.
5.B.3: Chemical systems undergo three main processes that change their energy: heating/cooling, phase transitions, and chemical reactions.
5.B.4: Calorimetry is an experimental technique that is used to determine the heat exchanged/transferred in a chemical system.

Breaking bonds requires energy, and making bonds releases energy. (5C)
5.C.2 The net energy change during a reaction is the sum of the energy required to break the bonds in the reactant molecules and the energy released in forming the bonds of the product molecules. The net change in energy may be positive for endothermic reactions where energy is required or negative for exothermic reactions where energy is released.

Learning Objectives
1.17 The student is able to express the law of conservation of mass quantitatively and qualitatively using symbolic representations and particulate drawings.
1.18 The student is able to apply conservation of atoms to the rearrangement of atoms in various processes.
3.1 Students can translate amount macroscopic observations of change, chemical equations, and particle views.
3.2 The student can translate an observed chemical change into a balanced chemical equation and justify the choice of equation type (molecular, ionic, or net ionic) in terms of utility for the given circumstances.
3.8 The student is able to identify redox reactions and justify the identification in terms of electron transfer.
3.11 The student is able to interpret observations regarding macroscopic energy changes associated with a reaction or process to generate a relevant symbolic and/or graphical representation of the energy changes.
5.2 The student is able to relate temperature to the motions of particles, either via particulate representations, such as drawings of particles with arrows indicating velocities, and/or via representations of average kinetic energy and distribution of kinetic energies of the particles, such as plots of the Maxwell-Boltzmann distribution.
5.3 The student can generate explanations or make predictions about the transfer of thermal energy between systems based on this transfer being due to a kinetic energy transfer between systems arising from molecular collisions.
5.4 The student is able to use conservation of energy to relate the magnitudes of the energy changes occurring in two or more interacting systems, including identifications of the systems, the type (heat versus work), or the direction of energy flow.
5.5 The student is able to use conservation of energy to relate the magnitudes of the energy changes when two nonreacting substances are mixed or brought in contact with one another.
5.6 The student is able to use calculations or estimations to relate energy changes associated with heating/cooling a substance to the heat capacity, relate energy changes associated with a phase transition to the enthalpy of fusion/vaporization, relate energy changes associated with a chemical reaction to the enthalpy of the reaction, and relate energy changes to PΔV work.
5.7 The student is able to design and/or interpret the results of an experiment in which calorimetry is used to determine the change in enthalpy of a chemical (heating/cooling, phase transition, or chemical reaction) at constant pressure.
5.8 The student is able to draw the qualitative and quantitative connections between the reaction enthalpy and the energies involved in the breaking and formation of chemical bonds.

Science Practices
1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
2.1 The student can justify the selection of a mathematical routine to solve problems. (Appropriateness of selected mathematical routine.)
2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
2.3 The student can estimate numerically quantities that describe natural phenomena.
3.1 The student can pose scientific questions.
3.3 The student can evaluate scientific questions.
4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question.
4.2 The student can design a plan for collecting data to answer a particular scientific question.
4.3 The student can collect data to answer a particular scientific question.
4.4 The student can evaluate sources of data to answer a particular scientific question.
5.1 The student can analyze data to identify patterns or relationships.
5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
6.1 The student can justify claims with evidence.
7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.

Answers to Prelab Questions

1. Write a balanced chemical reaction for the magnesium portion of the flameless ration heater in the MRE reacting with water to form hydrogen gas and magnesium hydroxide. Include states of matter.

   Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)

   1. Write the oxidation states above each element in the reaction.
      {12380_PreLabAnswers_Reaction_1}
   2. Identify and explain what is oxidized and what is reduced in the reaction above.

      Magnesium solid is oxidized. Each magnesium loses two electrons and goes from a 0 oxidation number to a +2. Hydrogen is reduced in this reaction. The hydrogen in water goes from a +1 oxidation state to a 0 oxidation state in the hydrogen gas.

   3. Draw a particulate drawing for the reaction of magnesium and water.
      {12380_PreLabAnswers_Figure_4}
2. When 5.37 g of ammonium nitrate, NH₄NO₃, is dissolved in 30.0 mL of distilled water in a polystyrene calorimeter, the temperature of the water changes from 19.9 °C to 8.1 °C.
   1. Was this an endothermic or exothermic process and why?

      Endothermic, the temperature of the water decreased.

   2. What was the energy change in joules when the ammonium nitrate dissolved in the water? (Assume the specific heat of the solution is 4.184 J/g•°C.)

      q = mcΔT
      q = (35.37 g)(4.184 J/g•°C)(8.1 – 19.9 °C) = –1746 J = –1.746 kJ. 1.746 kJ was absorbed by the solution.

   3. The heat of solution for ammonium nitrate is 25.7 kJ/mol. Calculate the heat of solution (ΔH_soln) for the trial above and calculate the percent error.

      (5.37 g NH₄NO₃) x (1 mole NH₄NO₃/80.04 g NH₄NO₃) = 0.0671 moles NH₄NO₃
      1.746 kJ/0.0671 mole NH₄NO₃ = 26.02 kJ/mol
      (26.02 – 25.7) / 25.7 x 100 = 1.25% error

3. An unknown gray metal cube is heated in boiling water for five minutes and then placed into a polystyrene calorimeter with 175.7 mL of room temperature water. The student collects the following data:
   {12380_PreLab_Table_1}
   {12380_PreLab_Table_2}
   1. How much heat did the water absorb? Show all work.

      q = mcΔT
      q = (175.7 g)(4.184 J/g•°C)(27.8 – 25.5°C) = 1691 J were absorbed.

   2. What is the specific heat capacity and identity of the metal? Show all work.

      q_water = –q_metal
      1691 J = –(60.0g)c(27.8 – 99.4°C)
      c = 0.394 J/g•°C
      Since the metal is gray, the only metal that has the same color and similar specific heat, would be zinc.

   3. Draw a diagram of the calorimeter set up with the metal cube. Label the equipment in your diagram. Add arrows to represent the transfer of heat.
      {12380_PreLabAnswers_Figure_5}
4. In lab, you will have to analyze a portion of a flameless ration heater, also known as an FRH. These FRHs are used in MREs, Meals Ready to Eat. Each group will be given approximately 2.5 g of the FRH powder to analyze. 2.5 g is approximately ¼ of a FRH in an MRE. Assuming each FRH contains 10.0 g of the reacting mixture, write a general step-by-step procedure to determine the amount of energy released in an entire FRH.
   1. Think safety first. Make sure you have the proper PPE available to perform this lab (i.e., goggles, apron and gloves).
   2. Make a list of the equipment and glassware needed for this lab.
   3. Number the steps in your procedure; remember to be as detailed as possible, from set-up to clean-up.
   4. Draw the necessary data table(s) in your notebook for data collection during the lab.
   5. Draw the set up(s) you will be using in the lab. Label all equipment.
   6. Write the reaction(s) occurring in your procedure. Label the reaction with the step it is occurring in your procedure.

Sample Data

Equipment needed for lab: 50 mL graduated cylinder, temperature probe and interface and calorimeter.

1. Obtain a quarter FRH pouch from your instructor.
2. Cut the pouch open and weigh the contents. Record mass in the data table.
3. Measure out 25 mL of distilled or deionized water. Record the exact volume in the data table.
4. Pour the water into the calorimeter.
5. Measure the initial temperature of the water.
6. Add the FRH mixture to the water. Close the calorimeter lid.

Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)

7. Monitor and record the highest temperature (This can be done with a thermometer or temperature probe).
8. Calculate the energy released by the mixture.

   q = mcΔT
   q = (27.3g)(4.184 J/g•°C)(89.9 – 22.3°C) = 7,720 J

9. Calculate how much energy is released from the entire FRH heating component.

   7,720 J/2.13 grams of mixture x 10 grams = 36,200 J for 10 grams of MRE Mixture

   {12380_Data_Figure_7}

References

College Board, The. 2014. “AP Chemistry Course and Exam Description, rev. ed.” NY: The College Board. Accessed June 7, 2017. http://media.collegeboard.com/digitalServices/pdf/ap/ap-chemistry-course-and-exam-description.pdf

Kainthla, Sesock, & Tinker. “Air-Activated Ration Heaters.” Accessed August 10, 2017. http://nsrdec.natick.army.mil/library/00-09/r08-106.pdf

Introduction

Put your chemistry skills to commercial use! From instant cold packs to flameless ration heaters and hand warmers, the energy changes accompanying physical and chemical transformations have many consumer applications. Some of these energy changes occur during physical processes, while others occur by chemical reactions. In this lab, you will analyze how much energy is released in a MRE (Meals Ready to Eat).

Concepts

• Enthalpy change
• Exothermic vs. endothermic
• Commercial science
• Calorimetry
• Oxidation and reduction

Background

Changes in energy occur around us every day. Natural gas is burned to cook our meals and heat our homes. In the summer, our houses are cooled by air conditioning units using electricity. In this lab, the chemical reaction occurring in MRE’s (Meals Ready to Eat) will be observed and analyzed. MRE’s are used in the military to provide hot food to soldiers in locations where using a stove or oven is not possible. They are also used by the Red Cross to help people who have been affected by natural disasters.

In this lab, the chemical reaction behind what makes an MRE work and the energy released will be studied. We will be investigating the thermochemistry behind the reaction. Thermochemistry is the study of heat changes that accompany a physical process or a chemical reaction—heat may be either absorbed or released. Heat is defined as the energy transferred from one object to another due to a difference in temperature. In this lab, the temperature change that accompanies the heat transfer will be measured.

The amount of heat transferred in these processes depends on a difference in a quantity called enthalpy, represented by the symbol H. The enthalpy change for a physical process or a chemical reaction is defined as the heat change that occurs at constant pressure. This is convenient, because most of the reactions that we carry out in the lab are in flasks or containers that are open to the atmosphere and take place at a constant pressure equal to the external pressure.

Equation 1 shows the equality between the change in enthalpy (ΔH) of a system and the amount of heat transferred, symbolized by q, for a reaction carried out at constant pressure.

The amount of heat (q) transferred to a substance or object depends on three factors: the mass (m) of the object, its specific heat (c) and the resulting temperature change (ΔT). See Equation 2. The specific heat of water is 4.184 J/g•°C.

When salt solutions are made in lab, the specific heat of the solution is typically assumed to be the same as water’s specific heat. While the specific heat capacity of various salt solutions can differ from the 4.184 J/g•°C, we will be using this estimation in lab.

Based on the law of conservation of energy, the amount of heat released by the system must be equal to the amount of heat absorbed by the surroundings. The sign convention in Equation 3 reveals that the heat change occurs in the opposite direction. 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. Enthalpy changes can either release heat or absorb heat, but the amount of heat exchanged between the calorimeter and the outside surroundings should be minimal. A simple calorimeter can be set up like Figure 1. The military has recently created a new MRE heating source that is air activated, not water activated. Currently, a small amount of water must be added to the FRH (Flameless Ration Heater). With the new air activated FRH, the following reaction occurs: 2Zn(s) + O₂(g) → 2ZnO(s) ΔH_f = –1.28 kcal/g Zn In addition to heat being released when this reaction occurs, the reaction is a reduction–oxidation reaction, also known as a redox reaction. The oxidation states of the zinc metal and oxygen change from the reactant side to the product side: The zinc loses electrons and is oxidized. The oxygen gains electrons and is reduced. See Figure 2 for a particulate drawing of the zinc/oxygen reaction. In this lab, you will be analyzing a water activated MRE. The water activated MRE is made of a magnesium, iron and salt granular mixture. The MRE mixture is mainly magnesium. It contains an alloy of magnesium-iron with 95% magnesium. When the magnesium reacts, the reaction produces heat and hydrogen gas.

Experiment Overview

The purpose of this activity is to complete the homework assignment prior to lab to promote conceptual understanding of calorimetry and enthalpy. You will first review and analyze a calorimetry experiment in the homework assignment. After analyzing your homework assignment, you will then design a procedure to analyze a flameless ration heater used in an MRE.

Prelab Questions

Complete the following homework set and write a lab procedure to be approved by your instructor prior to performing the lab. Along with your procedure, you will turn in any graphs, tables or figures you were asked to create in this homework set, and answers to the questions. Use a separate sheet of paper if needed.

1. Write a balanced chemical reaction for the magnesium portion of the flameless ration heater in the MRE reacting with water to form hydrogen gas and magnesium hydroxide. Include states of matter.
   1. Write the oxidation states above each element in the reaction.
   2. Identify and explain what is oxidized and what is reduced in the reaction above.
   3. Draw a particulate drawing for the reaction of magnesium and water.
2. When 5.37 g of ammonium nitrate, NH₄NO₃, is dissolved in 30.0 mL of distilled water in a polystyrene calorimeter, the temperature of the water changes from 19.9°C to 8.1°C.
   1. Was this an endothermic or exothermic process and why?
   2. What was the energy change in joules when the ammonium nitrate dissolved in the water? (Assume the specific heat of the solution is 4.184 J/g•°C.)
   3. The heat of solution for ammonium nitrate is 25.7 kJ/mol. Calculate the heat of solution (ΔH_soln) for the trial above and calculate the percent error.
3. An unknown gray metal cube is heated in boiling water for five minutes and then placed into a polystyrene calorimeter with 175.7 mL of room temperature water. The student collects the following data:
   {12380_PreLab_Table_1}
   {12380_PreLab_Table_2}
   1. How much heat did the water absorb? Show all work.
   2. What is the specific heat capacity and identity of the metal? Show all work.
   3. Draw a diagram of the calorimeter set up with the metal cube. Label the equipment in your diagram. Add arrows to represent the transfer of heat.
4. In lab, you will have to analyze a portion of a flameless ration heater, also known as an FRH. These FRHs are used in MREs, Meals Ready to Eat. Each group will be given approximately 2.5 g of the FRH powder to analyze. 2.5 g is approximately ¼ of a FRH in an MRE. Assuming each FRH contains 10.0 g of the reacting mixture, write a general step-by-step procedure to determine the amount of energy released in an entire FRH.
   1. Think safety first. Make sure you have the proper PPE available to perform this lab (i.e., goggles, apron and gloves).
   2. Make a list of the equipment and glassware needed for this lab.
   3. Number the steps in your procedure; remember to be as detailed as possible, from set-up to clean-up.
   4. Draw the necessary data table(s) in your notebook for data collection during the lab.
   5. Draw the set up(s) you will be using in the lab. Label all equipment.
   6. Write the reaction(s) occurring in your procedure. Label the reaction with the step it is occurring in your procedure.
5. Consult your instructor for appropriate disposal procedures.

Safety Precautions

Magnesium is a flammable solid. Avoid contact with flames and heat. The FRH is considered nonhazardous and nontoxic when used as described. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron and observe prudent laboratory practices. Avoid contact of activated FRH with exposed skin-may cause burns. Avoid contact of all chemicals with eyes and skin. Follow all laboratory safety guidelines. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.