Investigating exothermic reaction rates

Introduction

Heat packs are available in most grocery and drug stores to provide “fast-acting pain relief.” When the package surrounding the heat pack is opened, the heat pack slowly warms up, and stays warm for up to 12 hours. But how do they work? The production of heat requires an energy source. The energy that heats up the pack is generated by the reaction of iron in the heat pack with oxygen in the atmosphere. The pack stays at room temperature as long as it is protected from air. When the package is opened, oxygen (O2) is free to combine with the iron (Fe) in the pack to form iron oxide (Fe2O3), as shown in the chemical equation below:

Equation 1:

4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s) + heat energy
  • Fe is the chemical symbol for iron.
  • O2 is the chemical symbol for oxygen.
  • Fe2O3 is the chemical symbol for iron oxide (rust).

This is the same process by which iron is turned to rust, but in the heat pack, it occurs fast enough for the heat to be noticeable. Reactions that produce heat are referred to as exothermic.

The ingredients of most heat packs include iron powder, water, activated charcoal, vermiculite, and salt. The iron powder is the “active ingredient” that fuels the reaction. What are the other ingredients for? They promote the reaction and help to distribute the heat. In particular, the salt acts as a catalyst, the carbon helps disperse the heat, the vermiculite is used as an insulator for the purpose of retaining the heat, and water is required for the reaction. All of these ingredients are surrounded by a polypropylene bag that lets air in, but contains the moisture.

The reaction proceeds at a rate that is determined, in part, by the concentration of oxygen. In chemistry, the law of mass action states that the rate of a reaction is proportional to the product of the concentrations of the participating molecules. Thus, if the concentration of oxygen is increased, the rate of the reaction will also increase. As the reaction rate increases, so does the amount of heat produced. The Experimental Procedure of this science fair project presents two methods for varying the level of oxygen available to drive the reaction. To increase the concentration of oxygen, pure oxygen gas from a pressurized canister is gently blown over a heat pack. To decrease the concentration of oxygen, clear tape is used to cover either 50 percent or 75 percent of the air-permeable surface. You will use an infrared thermometer to measure the temperature of the heat pack. The infrared thermometer allows you to take the temperature of the heat pack just by pointing the thermometer at the hot surface.

 

Materials and Equipment

  • Oxygen canister, with regulator and hose. This can be purchased as part of a torch kit from most hardware stores. For example, the Benzomatic torch OX2550KC Cutting, Welding and Brazing Torch Kit (OX2550KC) has an oxygen canister with a regulator and hose. You will not be using the propane part of the torch, just the oxygen part.
  • Heat packs (15), must contain iron powder
  • Masking tape
  • Permanent marker
  • Ceramic bowls, matching (5)
  • Infrared (IR) thermometer; available online from retailers such as Amazon.com
  • Clear tape
  • Stopwatch
  • Lab notebook
  • An adult helper

Experimental Procedure

The goal of this experiment is to demonstrate that increasing the amount of oxygen available to react with the iron will increase the amount of heat produced by the exothermic reaction. There are two ways you will vary the oxygen for this experiment:

  1. Use the oxygen in the oxygen canister to increase the reaction rate.
  2. Use the clear tape to cover up a portion of the air-permeable surface of the heat pack, thus decreasing the rate of the reaction.

A third method is suggested in the Variations section, below, in which oxygen is produced by the action of the enzyme catalase.

Note: The reaction starts as soon as the wrapper is removed from the heat pack, so you should record the time at which you open each pack. Work as efficiently as possible, but because the reaction proceeds relatively slowly (it takes about a minutes to feel the heat after the pack is opened), you do not need to rush.

Preparing the Initial Setup

  1. Assemble the oxygen canister and regulator.
    1. Screw the oxygen canister into the regulator. Note that the oxygen tank may have a reverse thread. Turn the knob to check that oxygen flows. Close the oxygen valve.
  2. Place a heat pack next to each of the five ceramic bowls.
  3. Using the masking tape and permanent marker, label the bowls and the heat packs 1 to 5:
    • #1: 100% surface area open, air, no pure oxygen
    • #2: 100% surface area open, pure oxygen
    • #3: 50% surface area open, air, no pure oxygen
    • #4: 25% surface area open, air, no pure oxygen
    • #5: Unopened package
  4. Use the IR thermometer to measure the temperature of the unopened heat packs.
  5. Make three data tables for each heat pack, since you will need one table for each of the three trials.
  6. Record the time and temperature for each unopened heat pack.

Preparing the Heat Packs

In the procedure below, you will be recording the temperature of all five packs every few minutes.

  1. Open the wrapper for heat pack #1 and place it in bowl #1, with the air-permeable side facing up.
  2. Start the stopwatch. Use the time on the stopwatch for all of your readings, from the time you start until the experiment is complete. Restart the stopwatch for each new trial (not every time a new heat pack is opened) —there will be three trials. You can have your helper record the data as you take the temperatures.
  3. Record the time the wrapper was taken off.
  4. Open the wrapper for heat pack #2 and place it in bowl #2. Record the time.
  5. Open the wrapper for heat pack #3, and cover 50% of the perforated surface with clear tape. Place it in bowl #3. Record the time.
  6. Open the wrapper for heat pack #4, and cover 75% of its perforated surface with clear tape. Place it in bowl #4. Record the time.
  7. Place an unopened heat pack in bowl #5. This is a control to track room temperature.
  8. Make sure all of the heat packs have the air-permeable side facing up.
  9. Record the temperature of each of the heat packs every 2 minutes or so for the first 10 minutes.

Adding the Oxygen

In the next step, oxygen from the canister will be used to accelerate the reaction in heat pack #2. Record the temperature of heat pack #2 before and after each oxygen treatment. You will then record the temperature of all heat packs every 5 minutes.

  1. Open the valve on the oxygen canister until you have a gentle stream of oxygen.
  2. Gently blow the oxygen over the surface of heat pack #2 for 60 seconds. You can use the stopwatch that should still be running to see when 60 seconds have passed. Do not stop the stopwatch to do this.
  3. Take the temperature of all of the heat packs.
  4. Repeat steps 1-3 of this section every 5 minutes, for a total time of 90 minutes. Record all data in your lab notebook.
  5. Repeat the entire experiment two more times with new heat packs and average the results. Record the averages in your lab notebook.

Analyzing Your Results

  1. Graph your data: time in minutes on the x-axis and temperature on the y-axis.
    1. For precise time measurements, convert time from minutes and seconds to decimal minutes.
    2. Temperature for heat pack #2 may be “bumpy.”
  2. How long did it take for each sample to reach a steady temperature? How did changing the surface area of the perforated region affect the rate of heating? Which pack had the highest temperature?

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