Student Worksheet for the Demonstration
Exothermic Dehydration of Sugar
Table sugar is a carbohydrate known by the scientific name
sucrose. Its formula is C12H22O11.
It is a disaccharide made of linked molecules of glucose
and fructose. A disaccharide is a carbohydrate with two
linked sugar molecules. Starches are composed of many
linked sugar molecules and are known as polysaccharides.
Carbohydrates are so named because their empirical formulas
may be written as a combination of carbon and whole number
multiples of molecules of water. Sucrose for
have its formula re-written as C12(H2O)11.
Concentrated sulfuric acid is 98% pure H2SO4 by mass. Its
concentration may be expressed as 18 mol/L. Sulfuric acid
is the chemical manufactured in the largest amount
worldwide. Annual production is likely near 160 million
tons. It is mainly used to dissolve ores in mining and
phosphate rock in the manufacture of fertilizers. Because
it is a diprotic acid (meaning it has two hydrogen ions, or
protons, per molecule) sulfuric acid provides the
equivalent of 36 mol/L for the concentration of
H+. This is 45
times the H+
concentration of 5% table vinegar. Besides being an acid,
also a powerful dehydrating agent due to its highly
exothermic enthalpy of solution.
The heat of reaction in this demonstration may be
calculated by a Hess’s Law combination of reactions:
+ 12O2 →
ΔH = –5641 kJ
C + O2 → CO2
ΔH = –393.5 kJ
Use Hess’s Law to combine the reactions given above to produce a reaction in which sucrose is converted completely into elemental carbon and water. (a) Write the resulting balanced chemical equation. (b) What is the overall change in enthalpy (ΔH)?
Forty grams of sucrose are used in the demonstration. How much heat does the process of converting sugar into carbon and water produce in this demonstration?
Consider the following physical process and its accompanying change in enthalpy: H2SO4(l) → H2SO4(aq) ΔH = –41 kJ
This equation shows the hydration of sulfuric acid. When sulfuric acid dissolves in water a large quantity of heat is released. This is why chemists are told to ‘do as you oughta, add acid to watah’. When you add the dense acid to water it sinks to the bottom and any boiling of the water that may occur does not lead to spattering hot acid all over the chemist. In this demonstration your teacher poured about 40 mL of 98% sulfuric acid into the sugar. How much heat is released when this much sulfuric acid is diluted with water? The density of 98% sulfuric acid is 1.84 g/mL.
Combining your calculations so far, how much heat is released in the classroom demonstration you witnessed?
Water has an enthalpy of vaporization of +40.7 kJ/mol. Given the amount of water produced in the overall chemical change in the demonstration determine whether enough heat has been released in order to vaporize all of the water. Prove your answer with a calculation and if it was not enough heat then calculate what amount of water actually can be vaporized by the heat released during the demonstration.
This demonstration is known as the ‘carbon soufflé’. Using your best writing, and referencing the relevant scientific details, write a description of what happens in the demonstration. Explain what you saw using what you know about thermochemistry. Also, explain why it has been given a nickname referring to a food preparation.
In the demonstration as shown in class the use of 40 g of sugar results in the release of –104 kJ of heat. The heat of dilution for 98% sulfuric acid is –41 kJ/mol. If 40 mL are used in the demonstration then the amount of heat released will be –30 kJ. Thus the total heat released ought to be about 134 kJ. The water that evolves during the dehydration is turned to steam by this heat (the amount of water produced is 1.3 mol and requires 52.3 kJ to vaporize) and this is what inflates what is known as the ‘carbon soufflé’.
In case you missed the classroom demonstration there is a video I made which you can watch: https://youtu.be/xV3d94FrjzE. This demonstration is based on demonstration 1.32 on pg. 77 of Chemical Demonstrations, Vol. 1 by Bassam Z. Shakhashiri.