Background

Redox reactions, especially those involving elemental oxygen, are often highly energetic. Oxidation of fuels, referred to as combustion, can be quite dramatic under appropriate conditions.

Resources:

1. M4 Fuels and Energy: This page contains links to two PowerPoint slides: one on Energy Changes and the other on Fuels and Energy.
2. M5 Redox I: Definitions: Definitions of Redox Reactions.
3. M5 Redox II: Oxidation State: Description of oxidation state and how it can be used to identify redox reactions.

Discrepant Event

Oxidation of sugar is a very simple and easy reaction to conduct but produces some very dramatic results. All you need, in addition to sugar, is an oxidizing agent and an initiator.

The video below is that of six different mixtures of sugar and an oxidizing agent set off in a fume hood. The girls were competing against the boys. The reaction in this instance was initiated by adding a drop of concentrated sulfuric acid. Watch the movie for the results.

Here are the questions:

1. What was the oxidizing agent?
2. There are two chemical reactions taking place in the process. The first one involves the initiator concentrated sulfuric acid and sugar and the other between the oxidizing agent and suger. Can you suggest chemical equations for those reactions?
3. What was the competition? In other words, what was the criteria on which their mixture was being judged?
4. What factors affect the duration for which the flame lasts and the height of the flame? In other words, what determines the efficiency of the oxidation? How could one determine the duration and height of the flame using the video?
5.  What other measurements could be made to determine the relative efficiency of the oxidation of the different mixtures?
6. If you had to prepare a 5.0g mixture of the oxidizing agent and sugar so that it produced as high a flame as possible, what would be the composition?
7. If you had to prepare a 5.0g mixture so that it burned for as long as possible, releasing energy steadily, what would be the composition?

Concepts involved:

• reduction, oxidation, complete combustion, incomplete combustion, dehydration, rate of reaction, oxidizing agent, reducing agent.
• stoichiometry, molar ratio, stoichiometric ratio, limiting regent, excess reagent.

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