Abstract

Objective

The objective is to demonstrate how the rate of a chemical reaction can be altered by light.

Introduction

Some chemical reactions that are normally very slow can be dramatically sped up by adding energy from light. For example, a gaseous mixture of hydrogen and oxygen is stable at room temperature. Occasionally, a hydrogen molecule might collide with an oxygen molecule with enough energy for them to react, but this happens very rarely. The rate at which the molecules react might be measured in years, perhaps centuries. But if you shine an ultraviolet light on the mixture, oxygen and hydrogen molecules combine explosively to form water. The ultraviolet light in the aforementioned chemical reaction provided the activation energy required to stimulate the reaction of hydrogen and oxygen. In this photochemistry science project, you will study a different kind of light-stimulated chemical reaction. This reaction has an advantage—it actually occurs slowly enough for you to observe the changes caused by light, and it does not involve explosions!

Photochemical reactions are familiar to all of us. When your skin tans after exposure to sunlight, the tanning is the result of biological pathways that were stimulated by ultraviolet light. Your ability to see is the result of photochemical reactions that occur in the retina of your eye. And photosynthesis, where carbon dioxide is converted into starch, is completely dependent on sunlight. The reaction you will study involves the conversion of iodine to iodide in a solution of ammonium oxalate:

2 (NH₄)⁺ + C₂O₄^- + I₂ → 2 (NH₄)⁺ + 2 I^- + 2 CO₂

This equation states that two ammonium ions react with an oxalate ion and an iodine molecule to form ammonium ions, iodide ions, and carbon dioxide. Oxalate ions are oxidized to carbon dioxide, and iodine molecules are reduced to iodide. Since the iodine is colored anywhere from orange to black, depending on its concentration, and the iodide ion is colorless, the color of the reaction mix changes as the reaction proceeds. The reaction shown in Equation 1 occurs at a much faster rate when the reactants are exposed to light. In this photochemistry science project, you will use household materials to experiment with the affect of light on a chemical reaction. In the experimental procedure, you will test how different light sources affect the rate at which the reaction occurs.

Terms, Concepts, and Questions to Start Background Research

• Chemical reaction
• Hydrogen
• Oxygen
• Activation energy
• Photochemistry
• Photochemical reaction
• Ammonium ion
• Oxalate ion
• Iodine
• Iodide
• Carbon dioxide
• Oxidation
• Reduction
• Standards

Questions

• Based on your research, what are some ways to increase the rate of a chemical reaction, in addition to light?
• What are chemical oxidation and chemical reduction?
• What is the activation energy of a chemical reaction?
• What are some common chemical reactions that depend on light?
• What are the ingredients of tincture of iodine?

Bibliography

• Clark, J. (2002). The Effect of Catalysts on Reaction Rates. Retrieved December 7, 2009, from http://www.chemguide.co.uk/physical/basicrates/catalyst.html
• Michigan State University. (n.d.). Photochemistry. Retrieved December 7, 2009, from http://www.cem.msu.edu/~reusch/VirtualText/photchem.htm.

Materials and Equipment

Many of the following materials can be purchased from online suppliers, such as WARD's Natural Science at www.wardsci.com.

• Safety goggles, splash-proof
• Gloves, disposable (1 box)
• Masking tape
• Permanent marker
• Disposable centrifuge tubes with plastic caps; available from WARD's Natural Science, item # 18 V 0002
• Test tube rack; available from WARD's Natural Science, item # 18 V 4350
• Distilled water; available at most grocery and drug stores
• Pipets; available from WARD's Natural Science, item # 152091
• Tincture of iodine, 30 mL; available at most drug stores. You can also make a solution of 0.3 grams (g) of iodine crystals dissolved in 70 percent ethanol. Iodine crystals are sold in camping supply stores to purify water.
• Graduated cylinder, 10-mL; available from WARD's Natural Science, item # 18 V 1705
• Light sources (at least 4); possible sources include sunlight, candlelight, incandescent light, fluorescent light, neon light, LEDs, and any others you can think of.
• Optional: Scale accurate to 0.1 g; such as the Fast Weigh MS-500-BLK Digital Pocket Scale, 500 by 0.1 G, available from Amazon.com; desirable, but not required.
• Teaspoon measuring spoon, if you do not have access to a scale
• Oxalic acid; available from WARD's Natural Science, item # 9717506, or:
• Oxalic acid is also available from hardware stores, usually packaged as a "deck cleaner" or "rust remover."
• Bar Keepers Friend®; available in grocery or home goods stores, it is another source of oxalic acid.
• Beaker, 50-mL; available from WARD's Natural Science, item # 18 V 1000
• Clear ammonia, household; available at any grocery store
• Plastic spoon
• Aluminum foil
• Weighing paper (10 squares); make by cutting a piece of paper into 10 3-inch (in.) squares
• Timer
• Lab notebook
• Graph paper

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Experimental Procedure

This procedure will require some creative problem-solving since the ingredients and the equipment may vary, depending on your experimental setup. Varying the sources and concentrations of the reactants will provide many opportunities for further experimentation.

Note from author: When I tested this procedure I found that the mixture of ammonia and iodine tincture, without the addition of oxalic acid, yielded a black-colored solution that was very sensitive to sunlight. As an option, try the procedure with and without oxalic acid.

Make a Set of Standards

1. First you will make a set of standards by which to compare the color changes that occur in the test samples. Put on your safety goggles and a pair of disposable gloves.
2. Use masking tape and the permanent marker to label six centrifuge test tubes, as follows:
1. 100%
2. 50%
3. 25%
4. 12.5%
5. 6%
6. 0%
3. Using the pipette, add 8 mL of distilled water to the tube labeled 100%.
4. Add 4 mL distilled water to the remaining tubes.
5. Add 20 drops of iodine into the tube labeled 100%.
6. Cap the test tubes and mix the contents of the tube by turning it upside down several times.
7. Use a clean pipet and the 10-mL graduated cylinder to transfer 4 mL from the 100% tube to the 50% tube.
8. Mix the contents of the 50% tube thoroughly.
9. Use a clean pipet to transfer 4 mL from the 50% tube to the 25% tube.
10. Mix the contents of the 25% tube thoroughly.
11. Use a clean pipet to transfer 4 mL from the 25% tube to the 12.5% tube.
12. Mix the contents of the 12.5% tube thoroughly.
13. Use a clean pipet to transfer 4 mL from the 12.5% tube to the 6% tube.
14. Mix the contents of the 6% tube thoroughly.
15. Do not add iodine to the tube labeled 0%.

Running the Experiment

1. Prepare areas with the light sources that you plan to use so that they are ready when needed in the remainder of the procedure. For example, find a good spot outdoors that receives direct sunlight, or use incandescent light, fluorescent light, LED light, etc.
2. Place a piece of weighing paper on the scale and weigh out 2.5 grams (g) of oxalic acid.
1. Use ¾ teaspoon (tsp.) of oxalic acid if a scale is not available.
3. Transfer the oxalic acid to the 50-mL beaker and add 25 mL of water.
4. When the crystals have dissolved, add 25 mL of clear ammonia.
5. Stir with a clean plastic spoon to mix the solution of ammonium oxalate.
6. Use masking tape and a permanent marker to label five new test tubes 1–5.
7. Wrap the five new test tubes in aluminum foil. These will be the test solutions, which will be exposed to various kinds of light.
1. These are wrapped at first to protect them from light until they are exposed later in the procedure.
8. Add 3 ml of iodine, measured with the 10 ml graduated cylinder, to the ammonium oxalate solution and mix it thoroughly with the plastic spoon.
9. Pour 4 mL of the iodine-ammonium-oxalate solution into each of the tubes labeled 1–5. Caution: Keep the lids of the tubes loose, as the reaction produces carbon dioxide gas.
10. Keep tube #1 in foil and protected from light.
11. Start the timer.
12. Remove the foil wrappers from tubes 2–5.
13. Place each of the four tubes in an area with a different light source. For instance:
1. Tube #2: Sunlight
2. Tube #3: Incandescent light
3. Tube #4: Fluorescent light
4. Tube #5: LED flashlight

Observing the Tubes

1. Record the change in color of each tube at varying times, such as 15 minutes, 30 minutes, 1 hour, and 4 hours.
1. Compare the color of each tube to the standards to estimate how much of the color has been lost due to light exposure.
2. Record your results in your lab notebook.
2. Make a numerical scale from 1 to 6 for the color of the standard solutions. Use this scale to graph your results. Graph time on the x-axis and color on the y-axis.
3. Pour the chemicals down the sink and rinse your materials and sink with water.
4. Wash and dry the plastic tubes.
5. Perform the entire procedure two more times. This shows that your results are repeatable.

Variations

• Vary the amounts of oxalic acid, ammonia and iodine. Record and explain your results.
• Try filtering the light with various colored filters.
• Try other light sources, such as open shade, a UV lamp, an infrared lamp, etc.

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

David B. Whyte, PhD, Science Buddies

This science project is based on the following sources:

• Woodrow Wilson Leadership Program in Chemistry. (n.d.). Photochemical Reaction: Ammonium Oxalate and Iodine. Retrieved December 7, 2009, from http://www.woodrow.org/teachers/ci/1986/exp32.html
• Thompson, R.B. (2008). Illustrated Guide to Home Chemistry Experiments. Sebastopol, O'Reilly, 2008. pp. 310–313.

Trademark

• Bar Keepers Friend® is a registered trademark of SerVaas Laboratories.

Last edit date: 2011-11-18 08:00:00