By Polly Dornette
Product Manager

Introduction

A growing number of people are producing their own biodiesel fuel by transesterification of various triglycerides. With pretzel sticks, marshmallows, and gumdrops, your students can model the steps of this chemical process. This activity, based on one developed by the Educational Division of the North Carolina Zoological Park, can serve to introduce a biodiesel wet-lab activity, to reinforce the chemistry afterward, or it can be presented independently.

Biodiesel is among many potential supplements or replacements for fossil fuel. Some benefits of biodiesel are that it is nontoxic, nonflammable, biodegradable, and it can be produced from waste materials. Although biodiesel can be made from a wide variety of vegetable oils and animal fats, currently more than 90% of the biodiesel produced in the US is from soybean oil. The starting material for production of biodiesel is called the feedstock. An efficient biodiesel feedstock provides a good source of triglycerides without a lot of free fatty acids.

In transesterification, the 3 fatty acid chains (esters) are separated from a triglyceride molecule and bound to the methyl groups from 3 methanol molecules in the presence of a catalyst, producing 3 methyl esters (the biodiesel). The hydroxyl (OH) groups from the methanol replace the esters removed from the triglyceride, producing glycerol, a by-product of biodiesel production.

This activity is appropriate for high school students working independently or in small groups and can be completed in 15 min. Depending on the treatment and context, it might also be used with middle school students.

National Science Education Standards

Physical Science

Grades 5–8

• Properties and changes of properties in matter

Grades 9–12

• Structure and properties of matter
• Chemical reactions

Science and Technology

Grades 5–12

• Understandings about science and technology

Science in Personal and Social Perspectives

Grades 9–12

• Natural resources
• Science and technology in local, national, and global challenges

Safety

Students should not eat in the science laboratory. If you want your students to be able to eat their models at the conclusion of the lab, conduct the activity outside the laboratory.

Materials per student lab group

• Paper Plate
• Napkin or Paper Towel
• 3 Jumbo Marshmallows
• 9 Pretzel Sticks
• 9 Gumdrops (preferably of the same color)

Preparation

1. Determine appropriate group sizes for this activity.
2. Gather materials.

Procedure

Write the transesterification reaction on the board, in the form that best fits the ability level of your students and your learning objective. Distribute the materials and provide students the following instruction:

1. Begin the activity with all of the marshmallows, pretzels, and gumdrops on your napkin, representing the various reactants. The marshmallows represent glycerol, the pretzels represent fatty acids, and the gumdrops represent methanol. The paper plate is to be the reaction vessel.
2. Assemble your triglyceride molecules by inserting 3 pretzel sticks into each of your 3 jumbo marshmallows. (Remember to limit steric strain in your models.)
3. Place your 3 triglyceride molecules (your feedstock) into the reaction vessel (the paper plate).
4. Add your methanol gumdrops to the reaction vessel.
5. This reaction requires a catalyst, potassium hydroxide, which will be represented by your hands.
6. Remove the fatty acid (pretzels) from the triglyceride molecules and attach 1 pretzel to each methanol (gumdrop) to form a mono-alkyl ester, a biodiesel molecule.
7. In transesterification, the glycerol, which is dense, sinks to the bottom of the reaction vessel, from where it is separated from the biodiesel layer above. Model this separation by moving the glycerol (marshmallows) to the portion of your plate closest to you and the biodiesel farther back (to the top of your plate).
8. The stoichiometry of the reaction indicates that you will produce 3 biodiesel molecules for each triglyceride molecule in your feedstock. Note that your hands, as catalysts, participated in the reaction but were not changed by it.

Extension activities

1. Prepare biodiesel in your science lab. Carolina offers a classroom kit for the production of biodiesel.
2. Use a molecular model set to emphasize the atoms and study the transesterification reaction in greater detail.
3. Have students investigate the availability of commercial biodiesel or other biofuels in your area. Is there a facility nearby that produces biodiesel? Many biofuel producers allow tours of their facilities or provide speakers for school presentations.