Why does dehydration of sugar by acid produce so much heat?

Why does the 'carbon tower' demonstration produce so much heat and vapor?

Concentrated sulfuric acid (H₂SO₄) is used to dehydrate sugar in this classic (and dangerous) demonstration. The sucrose (C₁₂H₂₂O₁₁) is dehydrated to carbon and water. The water produced by the sugar hydrates the sulfuric acid. The formation of strong hydrogen bonds between the acid and the water makes the process strongly exothermic- enough heat is released to vaporize the hydrated sulfuric acid. The vapor expands in pockets within the dehydrating sugar, making a porous black "tower" of carbon that rises out of the container.

Concentrated sulfuric acid is extremely dangerous. Choking sulfuric acid vapor will rise from the container. Do this demonstration in a fume hood. The reaction takes a little while to get started, so don't be tempted to dump more acid on the sugar if nothing seems to be happening. If you have a hood, use it.

For the procedure, refer to Bassam Shakhashiri's Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1, pp 77-78 (1983).

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2005 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 07/31/07.URL: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/carbon-tower.shtml