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How can I calculate the heat released from the dehydration of sugar by sulfuric acid?

I need to know how much heat is released when you mix 6 grams of KClO3, 2 grams of C12H22O11, and a drop of 18M H2SO4 in kJ. All I'm interested in is the heat released from the burning of the sugar (the reaction between C12H22O11 and H2SO4). Thanks.

There are two reactions here; the oxidation of sucrose by chlorate, and the dehydration of sucrose by sulfuric acid. The reaction with chlorate produces the most heat, and the second reaction is really only to provide a little localized heat for the first reaction to get started. But you specifically say it's the second reaction you're interested in, so I'll concentrate on that:

C12H22O11(s) + 11 H2SO4(l) --> 12 C(s) + 11(H2SO4.H2O)

Since you're adding only a drop of H2SO4 to a large excess of sugar, the H2SO4 is the limiting reagent, and you'll have to measure precisely to figure out how much heat the reaction evolves. Follow these steps:

  1. Look up the heats of formation for all reactants and products in the above reaction from a general chemistry text or, if you must use the Web, at the NIST WebBook. You'll probably have to make a trip to the library to get the enthalpy of formation for the sulfuric acid monohydrate.
  2. Use Hess's Law to calculate the enthalpy of reaction. The number you get will be the number of kilojoules of heat released per 11 moles of sulfuric acid added.
  3. Convert the exact amount of H2SO4 to kilojoules of heat released. If you weighed the sulfuric acid, you'll convert g H2SO4 to mol H2SO4 to kJ heat.

I've run this reaction many times as a vivid demonstration of an exothermic reaction. It is a violent and dangerous reaction. It releases an enormous amount of heat in a very short time, and will sometimes go off in a flash as you're mixing the ingredients. The molten salts the reaction sprays into the air can cause serious burns and permanent damage to the eyes. Please don't try this at home!

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How can I calculate the heat released from the dehydration of sugar by sulfuric acid?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/dehydration-of-sugar.shtml