Exploring Light Sticks

Introduction

Magical light sticks can be used to teach students chemistry. Explore the effects of a catalyst and changing the pH of lightsticks with these “illuminating” demonstrations.

Concepts

• Catalyst
• Conjugate acid–base pair
• pH control of a reaction
• Chemiluminescence

Background

The first demonstration illustrates how a catalyst affects the light stick reaction. The second demonstration qualitatively shows how pH affects the kinetics of the reaction.

Materials

Safety Precautions

Sodium salicylate is slightly toxic by ingestion and a body tissue irritant. Dilute sodium hydroxide and hydrochloric acid are body tissue irritants. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Follow all laboratory safety guidelines and 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.Combine the light stick solutions treated with sodium salicylate, 0.1 M hydrochloric acid, and 0.1 M sodium hydroxide. Check the pH of this combined solution and neutralized with acid or base as needed according to Flinn Suggested Disposal Methods #24b or #10, respectively. The resulting neutral solution should be filtered to remove any possible broken glass particulate and then disposed of down the drain with excess water. The glass particulate should be placed in the broken glass container.

Procedure

Part A. Catalyst

1. Obtain two light sticks. One will be used as a control and one will demonstrate the effect of a catalyst.
2. Activate the two light sticks by bending them until a cracking sound is heard. Gently shake each light stick to achieve a uniform mixture (see Figure 1 in the Discussion section).
3. Carefully cut open one light stick near one end using very sharp scissors or a utility knife.
4. Transfer the liquid contents of the light stick into a 13 x 100 mm test tube.
5. Repeat steps 3 and 4 for the second light stick.
6. Note the temperature of the contents of the test tube, either with a thermometer or qualitatively by feeling the outside of the test tube.
7. Add 0.15 g of sodium salicylate into one of the test tubes and mix.
8. Have students note all observations and compare the control to the treated sample, including checking the temperature again.
9. If desired, immediately after the light stick reaction has completed, ignite a wood splint.
10. Direct the ignited splint into the test tube that has been treated with sodium salicylate. (The flame should extinguish with the CO₂ that is produced from the reaction.)

Part B. pH Optimization

11. Keep the control from the first demonstration and obtain two more light sticks. The additional two light sticks will demonstrate the effect of pH on the reaction.
12. Activate the two new light sticks (see Figure 1 in the Discussion section) and gently shake each to achieve a uniform mixture.
13. Carefully cut open one end of each light stick using very sharp scissors.
14. Pour the liquid contents of each light stick into individual test tubes.
15. To the first test tube add 10–30 drops of 0.1 M HCl. Mix the contents of the test tube well.
16. The second test tube is the control from Part A and no modifications will be made.
17. To the third test tube add 10–30 drops of 0.1 M NaOH.
18. Observe the rate of reaction in each test tube and compare the amount of light produced from each of the three reactions.
19. 19. To the first test tube that had the 0.1 HCl added to it, add 10–30 drops of 0.1 NaOH. Is the bright emission recovered?
20. (Optional) If desired add 10–30 drops of 0.1 M HCl to the third test tube.

Student Worksheet PDF

12023_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration five times: 20 light sticks, 25 mL of 0.1 M hydrochloric acid solution and 30 mL of 0.1 M sodium hydroxide.
• This demonstration is more effective in a darkened room; however, do not compromise safety. Leave the lights on while cutting the light sticks
• This activity may be extended by testing other bases, such as sodium bicarbonate, for efficacy.
• Temperature also plays a role in light stick reaction kinetics—as the light stick is cooled, the reaction slows down and the light stick glow is dimmed. As the light stick is warmed to room temperature the reaction rate will increase.

Sample Data

Answers to Questions

1. Why is a control used in experiments and demonstrations?

   A control is used as a reference to verify that a change occurs in response to a specific variable and not to other factors such as temperature.

2. What is the effect of sodium salicylate on the light stick reaction?

   Sodium salicylate is a catalyst that speeds up the reaction, and increases the brightness.

3. How does changing pH affect the reaction time of the light sticks?

   If the pH is lower (more acidic), the reaction will be slower. If the pH is higher (more basic), the reaction will be faster.

4. In the case of a commercial light stick, a specific amount of sodium salicylate is added. How much sodium salicylate would be too much? Why?

   Too much sodium salicylate would be at the point where the reaction time is so short the glow would not last long enough to satisfy consumers.

5. Traditional emergency flares used to be found in all road hazard kits. The flares were lit and burned out after a few hours. Discuss the advantages and disadvantages of replacing road flares with light sticks.

   Flares burn at incredibly hot temperatures which can be dangerous. Flares are also not safe around flammables. Light sticks generate light energy and are safe to touch. Light sticks may also be used around flammables. Flares have a much better shelf life and are brighter than commercial light sticks.

Discussion

Light sticks have become very popular—what was once a specialty item is now a grocery store seller. Many of the original light stick–type apparatus and U.S. patents were military based. The light sticks were considered a safer light source around flammables because there was not a spark source. Light sticks are sold today as a safe alternative light source as well as for entertainment. Light sticks are used in many home preparedness kits, emergency packs and for activities, such as hiking, camping and scuba diving.

A light stick is a chambered vessel containing two sections. The outside of the light stick is semipliable plastic and inside the plastic tube are two different reactants. To prevent the chemical reaction from occurring until desired, one reactant is put inside a glass ampule. Individual commercial formulations vary but the two main reactants are hydrogen peroxide and a phenyl oxalate compound such as trichlorophenyl oxalate (TCPO) with a fluorescent dye added to give different colors. To activate a light stick, the plastic chamber is flexed (see Figure 1). The outer plastic tube bends but the inner glass ampule does not and will break. The distinctive cracking noise heard upon activating a light stick is the inner glass breaking.

Upon mixing, the trichlorophenyl oxalate (TCPO) reacts with the hydrogen peroxide producing trichlorophenol, carbon dioxide and energy (see Equation 1). The energy released in the reaction is from the decomposition of the high energy but unstable C₂O₄ intermediate (see Equation 1). The dye molecules harness this reaction’s energy, elevating the dye molecules to an excited state. The chemical energy produced in the reaction is then transformed into light energy. The dye releases the energy by the emission of light— chemiluminescence. Chemiluminescence is light energy that is released from molecules that have gained chemical energy. Dyes used in commercial light sticks have a light emission wavelength in the visible range so they may be seen. In the military, light sticks use dyes that may release light emissions in the visible and infrared ranges. The brightness of a light stick is a good indicator of how fast the chemical reaction is proceeding. The brighter the light, the faster the reaction rate. The light stick reaction is catalyzed by base. A catalyst is a substance that can speed up a chemical reaction—many times it alters the mechanism or reaction pathway to reduce the activation energy required. Catalysts are not consumed in the reaction itself and therefore a single catalyst molecule can initiate many reactions before the limiting reactant is consumed. Addition of a basic catalyst to a light stick will increase the rate at which light energy is produced. Faster energy production allows dye molecules to more quickly absorb and release the energy as light. A good deal of research has been conducted in determining the best catalyst for the light stick reaction. Many light sticks use sodium salicylate as the base catalyst of choice. The catalyst amount that is added to the light sticks is carefully controlled. The light stick reaction needs to occur fast enough that the light emissions are visible but not so fast that the reaction expires too quickly. By varying light stick chemical concentrations the brightness and length of time of the reaction changes. Some companies sell a very bright 5-minute light stick while other light sticks have longer reaction times. The grocery store variety will usually continue to release light for 4–12 hours. The addition of sodium salicylate to the light stick contents tremendously speeds up the reaction. The catalyst speeds the reaction to the point that foaming may be observed from the rapid evolution of the carbon dioxide produced in the reaction (Equation 1). A burning splint inserted into the test tube will be extinguished, indicating the presence of CO₂. Since the light stick reaction is catalyzed by a base, the pH of the reaction is very important. The salicylate ion from sodium salicylate is a weak base. Since the base does not ionize 100%, both the salicylate ion (C₇H₅O₃^–) and HC₇H₅O₃ are present in light sticks to some degree. HC₇H₅O₃ and C₇H₅O₃^– are a conjugate acid–base pair—substances that differ by the presence of one H⁺ unit (see Figure 2). In solutions (such as light sticks) that contain both C₇H₅O₃^– and HC₇H₅O₃, the pH of the solution determines whether more C₇H₅O₃^– and HC₇H₅O₃ are present. In a solution at pH 3, there is an equal concentration of both C₇H₅O₃^– and HC₇H₅O₃ (the pK_a of HC₇H₅O₃ is 3.0). Above this pH, C₇H₅O₃^– predominates, below this pH, HC₇H₅O₃ dominates. Because C₇H₅O₃^– catalyzes the reaction, the pH must be carefully controlled. The pH is engineered so that the concentration of C₇H₅O₃^– is high enough to observe light emission, but low enough so the reaction does not go too fast. In this demonstration, addition of NaOH to the light stick mixture causes the conversion of HC₇H₅O₃ into C₇H₅O₃^– via the following neutralization reaction (Equation 2). This increases the rate of the light stick reaction and the brightness of the initial light emission observed.

Addition of HCl to the light stick causes the conversion of C₇H₅O₃^– into HC₇H₅O₃ via the reverse neutralization reaction (Equation 3). This decreases the rate of the overall light stick reaction and the brightness of light emission observed. Because the components of the conjugate acid–base pair can be interconverted by addition of acid or base, the brightness of the light stick reaction can be controlled with pH.

References

Special thanks to Tom Kuntzleman, Spring Arbor University, Spring Arbor, MI, for sharing this activity with Flinn Scientific.

Kuntzleman, T. S.; Comfort, A.; Baldwin, B. W. J. Chem. Ed., 2009, 86, 64–67.