Prep: 15 mins | Activity: 10-15 mins
During this teacher-directed demonstration of an exothermic reaction, students observe the dehydration of a carbohydrate using concentrated sulfuric acid. Use the demonstration to illustrate topics such as reaction kinetics, organic reaction types, and energy or heat flow.
Relate heat to motion in a very simple demonstration. Ask students to quickly rub their hands together for 20 to 30 seconds. On the macroscopic scale, ask them to discuss what happened and why. Then ask if they think the same phenomenon may apply on the microscopic scale.
What is the energy or heat flow through an exothermic reaction?
Constructing Explanations
Physical Science: PS1 and PS3
Cause and Effect
Concentrated sulfuric acid is extremely corrosive and causes severe burns. Avoid acid contact with skin, eyes, and clothing.
The reaction performed in this demonstration produces dangerous sulfur dioxide fumes. It should be performed in a fume hood with adequate ventilation.
Know and follow all federal, state, and local regulations as well as school district guidelines for the disposal of laboratory wastes.
A carbon snake grows from the beaker and releases steam or a gas. The beaker appears to get hot since steam was released.
What evidence did you observe that a chemical reaction took place during the demonstration?
The formation of the carbon tube and steam being released
Was heat released or absorbed by the overall reaction during the demonstration? Explain the flow of heat during the reaction. (Consider asking chemistry students to explain the results based on bond energies. A discussion of entropy would also be appropriate.)
Heat was released. The contents of the beaker got hot and turned the water to steam. Heat was transferred to the environment from the reaction beaker. This was an exothermic reaction.
The dehydration of sucrose (C12H22O11) separates water from the carbohydrate, producing elemental carbon and water. Write a balanced chemical reaction for the dehydration of sucrose.
C12H22O11 →12C + 11H2O
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