The Carbon Soufflé

Introduction

Be a chemical gourmet—whip up a carbon soufflé! Your students will be amazed as they watch a yellow solid-liquid mixture turn brown, then black, expand out of the top of the beaker and solidify. The beaker becomes extremely hot and the odor of burnt sugar will fill the room.

Concepts

• Dehydration reaction
• Exothermic reactions

Materials

Safety Precautions

Sulfuric acid is severely corrosive to eyes, skin and other tissue; very considerable heat of dilution with water; mixing with water may cause spraying and spattering. Do not handle the carbon product with your hands; use tongs. The carbon product will contain unreacted sulfuric acid. Neutralize acid spills and the carbon product with sodium carbonate. The steam produced by the reaction can cause burns. Do not stand over the reaction vessel or inhale the steam produced. The reaction vessel will get extremely hot; allow the vessel to cool before handling. Perform this demonstration only in an efficient fume hood or a well-ventilated room. Wear chemical splash goggles, chemical-resistant apron and chemical-resistant gloves. Your carbon soufflé is not intended for consumption. It will contain corrosive sulfuric acid. 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. When the reaction is complete and the reaction vessel is cool, sprinkle the carbon product with sodium carbonate to help neutralize the remaining acid. Remove the carbon product from the reaction vessel using tongs and thoroughly rinse the carbon product under running water. Place the carbon lump inside a plastic bag. Seal the bag, then place it in the trash.

Procedure

1. All safety precautions must be followed. Perform this experiment only in a fume hood or in a well-ventilated room.
2. Add 60 grams of sucrose to a 250-mL borosilicate glass beaker.
3. Place the beaker on a layer of paper towels.
4. Using a 100-mL graduated cylinder, carefully measure 60 mL of concentrated sulfuric acid. (Any acid spills should be neutralized with sodium carbonate.)
5. Slowly pour the sulfuric acid into the beaker containing sucrose.
6. Stir briefly with a glass stirring rod. Leave the stirring rod inside the beaker. It will help support the column of carbon produced.
7. Stand back and observe. In a few minutes the solution starts to bubble and expand. Steam will be visible coming out of the mouth of the beaker. The beaker will get hot. Your carbon soufflé reaction takes 15 minutes from addition of the sulfuric acid to the hardening of the “soufflé.”
8. Allow the beaker to cool; then follow the cleanup and disposal procedure.

Student Worksheet PDF

12587_Student1.pdf

Teacher Tips

• Use the 9-minute period between the addition of the sulfuric acid and the start of the visual reactions to discuss the chemical reaction.

Answers to Questions

1. Describe what happened in this demonstration.

Concentrated sulfuric acid was added to a large amount of sucrose. The mixture was stirred. After several minutes, it began to bubble and “dry up.” The mixture began to expand upward, and steam was visible at the top of the beaker. After about 15 minutes the substance appeared to be completely dry, and there was a black column of carbon rising up out of the beaker.

2. Write a balanced chemical equation showing the dehydration of sucrose (C₁₂H₂₂O₁₁).

C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O

3. Sucrose is a form of stored energy used by plants. How does this demonstration show what happens to that energy when the “food” is consumed?

The dehydration represented the consumption of the sugar. As it occurred, energy was released in the form of heat. So much energy was released that the beaker grew hot and the carbon was forced to expand and rise out of the beaker. This shows not only the great amount of energy stored in sugar but the fact that it is all released when the food storing it is consumed.

Discussion

Plants combine carbon dioxide and water in the presence of chlorophyll and sunlight to produce food and oxygen. The food is stored energy for the plant and is in the form of sugars or carbohydrates. Sugars have a molecular formula of nC•H₂O (e.g., sucrose: C₁₂H₂₂O₁₁, glucose: C₆H₁₂O₆, arabinose: C₅H₁₀H₅. This stored energy is released when the food is consumed.

The Carbon Soufflé is a dramatic example of the amount of stored energy in sugar. Concentrated sulfuric acid is a strong dehydrating agent and will literally extract the water from the sugar and leave only carbon (reaction 1). Heat is generated during the dehydration step (–918.9 kJ/mol) and from the dilution of sulfuric acid (–40.6 kJ/mol). Some of the heat is used to convert water into steam.

Reaction 1:  C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O (–918.9 kJ/mol)
Reaction 2:  H₂SO₄(l) → H₂SO₄•nH₂O            (–40.6 kJ/mol)

References

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison, WI; 1983; Vol 1, p 77.

Summerlin, L. R.; Ealy, Jr., J. L. Chemical Demonstrations: A Source Book for Teachers; American Chemical Society: Washington DC, 1988; Vol 1. p 62.