Curriculum Notes 

This demonstration is usually performed when collision theory and factors that influences the rate of reactions (temperature) are being discussed. 

Allow about 5 minutes for this demonstration.

One day of lead time is required for this project.

Discussion 

This demonstration shows the effect of temperature on reaction rate.  Raising the temperature of a reaction mixture results in both more frequent and more energetic molecular collisions. Increasing the temperature increases the average kinetic energy of the molecules. At higher temperatures molecular collisions are more likely to have enough energy to form products. This increases the rate of reaction.  At lower temperatures, there are fewer collisions and most of the collisions that do occur between reactants do not have sufficient energy to form products.  Because at any temperature in a gas, liquid, or solution, there is a distribution of molecular speeds, some molecules will have sufficient energy that when they collide a reaction will occur. At reasonably cold temperatures reactions occur but there are fewer effective reactions compared to the system at higher energy.

In a chemiluminescent reaction such as this one, at elevated temperature, the increased reaction rate can be perceived as increased brightness of the light being given off by the light stick.  At cold temperatures, the light stick glows less brightly because in a given time period, fewer reactant molecules are colliding with sufficient energy to form the products. 

The chemical reaction in a light stick usually involves several different steps. A typical commercial light stick has a thin walled-glass ampule containing hydrogen peroxide solution floating in a solution of a phenyl oxalate ester with a fluorescent dye. During the reaction, an intermediate is produced which transfers energy to the fluorescent dye molecule.  When the dye molecule absorbs energy, the energy is used to raise electrons to an excited state.  When the dye molecule returns to the ground state, the excited electrons return to the ground state and energy in the form of light is emitted.  E = hv  Photons are emitted.

Here is the sequence of events that occur when the two solutions are combined:

• The hydrogen peroxide substitute one of the phenyl groups on the oxalate ester, producing a peroxyacid functional group.
• The peroxyacid substitues one of the other phenyl groups, resulting in an unstable cyclic peroxy compound.
• The cyclic peroxy compound decomposes to carbon dioxide.
• This decomposition releases energy to the dye.
• The electrons in the dye atoms jump to a higher level, then fall back down, releasing energy in the form of light.

Extensions of the demonstration

Discuss the effect that temperature has on the rate of chemical reactions.  Discuss how the energy of activation barrier is overcome. Ask students which light stick will use up the reactants first. Discuss the concept that the intensity of light is proportional to the reaction rate. The faster the rate of reaction, the sooner all the reactants are consumed and the reaction ceases.

Materials 

• 3 light sticks with strings tied to them
• an electric teapot with boiling water
• two 400 mL beakers
• a Dewar flask containing liquid nitrogen
• heat resistant gloves

Procedure 

• Dim the lights.
• Snap and shake the three light sticks so that they begin to glow.
• Place one of the light sticks in the beaker containing liquid nitrogen and another in the beaker containing the boiling water. The third light stick serves as a control. Leave the light sticks in their respective beakers until a change is observed.
• Withdraw the light sticks from their beakers using the attached strings. Compare their relative brightness. The light stick that was immersed in the boiling water should be brighter than the control stick at room temperature and the stick that was immersed in the liquid nitrogen should be dimmer than the control stick.

Safety Precautions 

• The demonstrator will be working around boiling water and liquid nitrogen in conditions of impaired visibility (dim light). Be careful, these substances can cause burns or frostbite.
• Light sticks have been known to burst under conditions of high temperatures. The demonstrator must wear goggles to protect eyes.  Students should be at least 3 meters away from the demonstration area.

Footnotes 

References

1 Bassam Z. Shakashiri, Lightsticks (2.2). Chemical Demonstrations, A Handbook for Teachers of Chemistry, volume.1 (Madison: The University of Wisconsin Press, 1983) pp. 146-152.

2. Bassam Z Shakhashiri, Lloyd G. Williams, Glen E. Dirreen, and Ann Francis, “Cool-Light Chemiluminescence,” J. Chem. Educ., Vol. 58, 1981, 70–72. The dependence of reaction rates on temperature is demonstrated with chemiluminescent light sticks. 

Prep. Notes 

Boil the water before the demonstration begins.