Flameless ration heaters (FRHs) are nontoxic, self-contained heating units that do not product a flame. FRHs were developed by the United States Army for use by soldiers in the field to heat their food rations. The use of flameless ration heaters in this demonstration provides an interesting, real-world application of thermochemistry.
- Exothermic reactions
- Heat of reaction
Iron filings, Fe, 1 g
Magnesium ribbon, Mg, 2-cm strip
Nitric acid, HNO3, 3 M, 2 mL
Phenolphthalein indicator solution, 2 mL
Silver nitrate solution, AgNO3, 0.1 M, 1 mL
Sodium chloride, NaCl, 2 g
Sodium hydroxide solution, NaOH, 0.1 M, 1 mL
Erlenmeyer flask, 250-mL
Document camera (optional)
Filter funnel and filter paper
Flameless ration heaters, 2
Foam cup, insulated (optional)
Forceps or tongs
Magnet, cow or rare-earth type
Test tubes, large (25 x 150 mm), 4
The Flameless Ration Heater is considered nontoxic and nonhazardous when used as described. The inner pad in the FRH consists of a supercorroding metal alloy embedded in a polyethylene matrix. The alloy consists of magnesium and about 5 mole percent iron. Handle the inner pad with forceps or tongs. Do not handle with bare hands. Magnesium metal is flammable and burns with an intense flame. Nitric acid is corrosive and a strong oxidizing agent; it is toxic by ingestion and inhalation. Sodium hydroxide solution is a skin and eye irritant. Silver nitrate solution will stain skin. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.
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 solution remaining from the reaction of the FRH pad with water is strongly basic and should be neutralized according to Flinn Suggested Disposal Method #10.
Part A. Demonstrate the Reaction of the FRH Pad with Water
- Obtain a FRH bag and display it to the class. Read the operating instructions and describe how the heater would be used in its intended application.
- Cut the bag and remove the inner pad. Examine the pad with a magnifying lens and describe its appearance. (Optional) Place the pad under a ChemCam™ video camera to allow the class to see the magnified view.
Part B. Demonstrate the Composition of the FRH Pad
- Measure the mass of the inner metallic pad. Using scissors, cut a 3 cm2 section of the FRH pad and record its mass. Fill a 250-mL Erlenmeyer flask with 100 mL of water and measure its initial temperature. Add the FRH section to the water and stir. Record the highest temperature that the water reaches. Note: The Erlenmeyer flask is used to allow students to view the reaction as it occurs. To achieve more accurate measurements, however, the reaction should be carried out in an insulated foam cup.
- (Optional) Calculate the amount of heat of reaction per gram of the pad. Multiply the heat of reaction per gram by the mass of the entire pad to calculate how much heat would be evolved when the entire pad is activated.
Part C. Demonstrate the Reaction of Magnesium with Water
- Suspend the inner pad vertically and bring a strong magnet near the pad. (The particles in the pad are attracted to the magnet and will move toward the magnet. Carry out a parallel test with iron filings. Magnetism is a characteristic physical property of iron.)
- Filter the mixture from Step 3 through a funnel fitted with a filter paper cone. Collect two 10-mL fractions of the filtrate in large test tubes (25 x 150 mm).
- Place a few drops of filtrate from the first test tube into a deflagration spoon and hold the spoon in a Bunsen burner flame. Describe the color of the flame. (The flame should burn bright yellow-orange. Carry out a parallel flame test with sodium dissolved in water. A yellow-orange flame is characteristic of sodium ions.)
- To the remaining filtrate in the first test tube, carefully add 10 drops of 3 M nitric acid, followed by 3 drops of 0.1 M silver nitrate. Describe the observations. (The filtrate is clear. Addition of silver nitrate gives a cloudy white mixture. Carry out a parallel test with a solution of sodium chloride dissolved in water. The formation of a white precipitate indicates the presence of chloride ions.)
- Add 2–3 drops of phenolphthalein indicator solution to the filtrate in the second test tube. Describe the observations. (The filtrate turns red-violet. Carry out a parallel test with sodium hydroxide solution. The red-violet color indicates the presence of a strong base.)
- Cut a 1 cm2 section of the FRH inner pad. Using tongs, hold the pad in a Bunsen burner flame behind a safety shield. The burning metal emits UV light that may harm the eyes—do NOT look directly at the flame. (The pad burns with a brilliant light and an intense flame. Sparks fly off in several directions. Carry out a parallel test with a magnesium ribbon. The bright, intense flame is characteristic of magnesium metal.)
- Fill two large test tubes with about 10 mL of water. Add 2–3 drops of phenolphthalein to each.
- Place a 2-cm strip of magnesium ribbon in each test tube. (No reaction occurs.)
- To the second test tube, add a pea-sized amount of solid sodium chloride. Compare the reaction to that in the first test tube. (Addition of sodium chloride increases the rate of reaction of magnesium with water. The mixture bubbles and turns pink, indicating the formation of hydrogen gas and basic magnesium hydroxide, respectively.)
- Ask students to complete the following sentence describing the reaction of the FRH pad with water: “Reaction of the metallic pad with water is an (exothermic/endothermic) reaction. Heat is (absorbed/released) by the metallic pad and produces a large temperature (increase/decrease) in the surrounding water.”
- Typical data for the amount of heat evolved in the reaction of the FRH pad with water: Reaction of 3.0 g of the FRH pad with 100 mL of water increased the temperature of the water from 25 °C to 50 °C. This corresponds to about 3500 joules of heat per gram of FRH. The mass of a FRH pad is approximately 17 g, so the amount of heat that would be liberated when the entire pad is activated is about 60 kJ. The amount of heat released in this reaction is not very reproducible, probably because the pad itself is not homogeneous.
- Lead students on a chemical detective mission as you demonstrate the composition and reaction of the FRH pad. After each step, ask students leading questions to deduce what materials are present and how they react. What two metals are present in the pad? How do you know? What is the role of sodium chloride? What is the nature of the productsthat are formed?
- A video of this demonstration, Flameless Ration Heaters, presented by Kathleen Dombrink, is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. Please visit the eLearning Web site at http://elearning.flinnsci.com for viewing information. The video is part of the Consumer Chemistry video package.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Disciplinary Core Ideas
MS-PS1.B: Chemical Reactions
MS-PS3.A: Definitions of Energy
HS-PS1.B: Chemical Reactions
Energy and matter
Cause and effect
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
MS-PS1-4. Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
MS-PS1-6. Undertake a design project to construct, test, and modify a device that either releases or absorbs thermal energy by chemical processes.
MS-PS3-3. Apply scientific principles to design, construct, and test a device that either minimizes or maximizes thermal energy transfer.
MS-PS3-4. Plan an investigation to determine the relationships among the energy transferred, the type of matter, the mass, and the change in the average kinetic energy of the particles as measured by the temperature of the sample.
MS-PS3-5. Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object.
The metallic pad in the Flameless Ration Heater is a composite material containing a magnesium–iron alloy and sodium chloride dispersed in a polyethylene matrix. According to the patent for its manufacture, the FRH is capable of increasing the temperature of 151 g of food approximately 55 °C—from an initial temperature of 21 °C, for example, to a final temperature of 76 °C. Some heat escapes the package in the form of steam and hydrogen gas, but the hydrogen gas dissipates rapidly and does not present a hazard. According to the patent, the hydrogen gas does not ignite, either with an open flame or with a spark. Reaction of the FRH pad with water involves oxidation of magnesium to form magnesium hydroxide and hydrogen, according to the following balanced chemical equation:
The heat of reaction for this highly exothermic reaction is –352 kJ/mole of Mg. Although thermodynamically favored, the reaction of magnesium with water is kinetically very slow in the absence of a promoter and/or catalyst. Elemental iron is added to the metal as a promoter—it initiates reaction at the magnesium surface by producing reactive intermediates. The reaction further requires a catalyst to increase the rate of the reaction and ensure a smooth, steady evolution of heat. Sodium chloride is added to the metal composite material to break down or destabilize the protective coating on the surface of the magnesium metal. Chloride ions replace hydroxide ions in the coating and produce channels that allow water to penetrate the reactive metal surface. Chloride ions redissolve in solution and thus function as a true catalyst.
Special thanks to Edmund J. Escudero, Summit Country Day School, OH for his ideas regarding Flameless Ration Heaters.
Cesa, I. ChemTopic™ Labs, Thermochemistry. Batavia, IL: Flinn Scientific, Inc., 2002. Vol 10, pp 75–77.
||Flameless Ration Heater
||Iron Metal Filings, 500 g
||Magnesium Metal Ribbon, 12.5 g, Approx. 45 Feet
||Nitric Acid Solution, 3 M, 500 mL
||Phenolphthalein Solution, 1%, 100 mL
||Silver Nitrate Solution, 0.1 M, 100 mL
||Sodium Hydroxide Solution, 0.1 M, 500 mL