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. Ashley

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:

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.
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.
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?