Cookin' with Sugar
Students learn from health and nutrition class that sugar is an abundant source of energy. Chemistry class confirms this but offers only mathematical proof. This demonstration will amaze your students with the remarkable amount of energy stored in sugar as they watch a yellow solid-liquid mixture turn brown, then black, release steam, expand and solidify. The beaker becomes extremely hot and the odor of burnt sugar fills the room.

Chemical Concepts:

• Dehydration reaction
• Exothermic reactions

Procedure:

1. All safety precautions must be followed. Perform this demonstration only in a fume hood.
2. Add 50 grams of sucrose to a 250 mL Pyrex beaker.
3. Place the beaker on several layers of old newspaper.
4. Using a 100 mL graduated cylinder, carefully measure 50 mL of concentrated sulfuric acid. (Any spills should be neutralized with sodium hydrogen 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 reaction will be complete in a few minutes but allow an additional 15 minutes for the carbon column to harden.
8. Allow the beaker to cool: then follow the cleanup and disposal procedure.

Hints:
You will have a few minutes between the addition of the sulfuric acid and the start of the visible reaction to discuss the chemical reaction.

Discussion:
In photosynthesis, energy from the sun is collected and stored in sugar molecules made from water and carbon dioxide with the aid of chlorophyll; oxygen is given off as a by-product of the process. The captured energy in the sugar molecules is the fuel source for all living things.

This reaction dramatically demonstrates the amount of stored energy in sugar. Concentrated sulfuric acid is a powerful dehydrating agent and will rip water molecules from the sugar (C₁₂H₂₂O₁₁) and leave only carbon (Reaction 1). Heat is released by the dehydration step (-920 kJ/mol) and from the dilution of sulfuric acid (-40 kJ/mol) (Reaction 2). Some of the heat is used to convert water to steam.

Reaction 1: (-920 kJ/mol)
C₁₂H₂₂O₁₁(s) -> 12C(s) + 11H₂O

Reaction 2: (-40 kJ/mol)
H₂SO₄(l) -> H₂SO₄·nH₂O

Safety Precautions:
Sulfuric acid is severely corrosive to eyes, skin, and other tissue. It also has a huge heat of dilution with water; mixing with water may cause boiling and splattering. 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 hydrogen 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 well ventilated room. Wear chemical splash goggles, chemical-resistant apron, and chemical-resistant gloves. The carbon product should not be handled by students at any time. It will contain corrosive sulfuric acid.

Disposal:
When the reaction is complete and the reaction vessel is cool, generously sprinkle the carbon product with sodium hydrogen 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. Dispose according to instructions from a reputable handbook or safety manual.

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

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison, WI; 1983; Vol 1, pp 77-78

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