Glow Sticks
The effect of temperature on the rate of a chemical reaction is investigated.

Concepts
chemical change, energy, rate of reaction, fluorescence

Materials
  • Three glow sticks of the same color
  • ice
  • water
  • hot plate
  • Two 500 mL beakers

Procedure
1. Ask students to record observations about the glow stick before it is activated.
2. Activate one glow stick by bending it and ask the students to observe.
3. Activate the other two glow sticks. Compare intensities of the three glow sticks.
4. Place the second glow stick in the hot water (less than 70°C) and at the same time place the third in the ice-water bath. Keep the first at room temperature.
5. Allow the glow sticks to sit in the containers for the same period of time. Within a few minutes, students should observe a difference in the intensity of light from the glow sticks in the two containers.
6. Pull the glow sticks out of the containers and compare their intensities to the room-temperature glow stick.
7. Have students explain why the hot water and cold water glow sticks behaved the way they did. Have them also predict which glow stick will stop glowing first.
8. Let the students write a conclusion to the experiment. Be sure they relate what they know about the effect of temperature on molecules to what they observed in this experiment.

Explanation
A glow stick consists of a sealed plastic tube that contains 2 solutions. The glow stick is activated by bending the plastic tube which causes the glass vial to break so the two solutions can interact. When mixed, the two solutions interact, producing light.

The glass vial contains dilute hydrogen peroxide, and the other solution contains a fluorescent dye and phenyl oxalate ester. The ester and hydrogen peroxide react first, producing an intermediate compound that transfers energy to the dye molecule. The glow is visible as the excited-state dye molecule loses energy back to the ground state.
In general, the speed of a chemical reaction increases as the temperature increases. At a higher temperature, a larger fraction of the reacting molecules have sufficient energy to react upon collision; thus, at a higher temperature, the glow is brighter. The opposite happens at lower temperatures.

figure23.jpg
Figure 23: Light sticks in cold, room temperature, and warm water (left to right).

figure24.jpg
Figure 24: Cold, room temperature, and warm light stick (left to right).

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