Sugar Dehydration Without Sulfuric Acid

Equipment

60 mL syringe with nozzle cut off, scrap metal heat shield, mortar and pestle, pipette with bulb, match or butane lighter, aluminum foil and 100 mL beaker.

Reagents

Table sugar, potassium chlorate, 95% ethanol.

Presentation

1. Weigh out 15 g of table sugar (sucrose) and 4 g of potassium chlorate.
2. Place the sugar and potassium chlorate in the mortar and lightly grind until you produce a fine powder and the two components are mixed thoroughly.
3. Pull the plunger back into the syringe until it reaches the 20 mL mark.
4. Pour the powder mixture into the syringe to a depth of approximately 0.5 cm, add enough ethanol to thoroughly wet the powder. Tap the syringe plunger down on a solid surface to remove air bubbles.
5. Repeat step 4 until the syringe is full.
6. Cover the open end of the syringe with the scrap metal. Invert the syringe and place the scrap metal on a firm surface. Depress the plunger to further compress the mixture.
7. Lift the syringe off of the scrap metal and extrude the mixture from the syringe. You should end up with a column approximately 2-3 cm high resting on the scrap metal. If you are not going to use the demonstration within a few minutes, you should cover it with a small beaker to minimize the ethanol evaporation.
8. Place the mixture on a surface and remove any combustible material back about two feet. Laying down some aluminum foil will make any eventual cleanup easier.
9. Remove the beaker if there is one, add another dropper full of ethanol to the mixture and light it with the match or butane lighter.

Hazards

Potassium chlorate is a strong oxidizing agent. Caution, there are warnings that potassium chlorate should not be exposed to heat, shock or friction and that doing so may lead to fire or explosion, especially if there are oxidizable contaminants present such as sugar. This demonstration has been performed repeatedly without experiencing any such problems associated with the light grinding that is called for, but such possibilities should always be taken into account. There is a small amount of smoke and flame associated with these reactions. A room with even moderately decent ventilation will have no problem dealing with the amounts produced. The heat shield will get quite warm from the reactions and may cause burns if handled too soon upon completion of the demonstration.

Discussion

This demonstration consists of three reactions, each succeeding reaction is dependent upon the previous reaction for its activation energy. All of the reactions are exothermic.

CH3CH2OH(l) + 3O2(g) 2CO2(g) + 3H2O(g) + 1236 kJ	1
C12H22O11(s) + 8KClO3(s) 12CO2(g) + 11H2O(g) + 8KCl(s) + 2029 kJ	2
C12H22O11(s) 12C(s) + 11H2O(g) + 429 kJ	3

The energies that are shown for the three reactions are based upon the assumptions that the water and carbon being formed are in the gaseous and graphite forms respectively. There is evidence that these assumptions are not entirely accurate (2,3). From reaction 2, we can see that potassium chlorate reacts with the sugar, but there is only enough potassium chlorate to react with approximately 16% of the available sugar. This allows the remaining 84% of the sugar to react according to reaction 3, driven by the energy release of reaction 2. The gases that are produced during these reactions leave the solid carbon riddled with gas pockets, producing a light, airy carbon column.

References

1. Todd P. Silverstein and Yi Zhang J. Chem. Educ. 1998, Vol.75, p.748-749.
2. Shakhashiri, B.Z. Chemical Demonstrations, A Handbook for Teachers of Chemistry, University of Wisconsin Press: Madison, WI, 1983; Vol. 1, pp. 77-78.
3. Dahn, J.R.; Zheng, T.; Liu, Y.; Xue, J.S. Science 1995, 270, 590-593.