Materials Included In Kit

Additional Materials Required

Prelab Preparation

Methyl alcohol with 2.5% KOH
You need 150 mL of solution for 15 student groups.

1. In a fume hood, add 2.5 g of potassium hydroxide to a beaker containing approximately 70 mL of methyl alcohol and stir until KOH is dissolved.
2. Add the solution to a 100-mL graduated cylinder and dilute to the 100-mL mark with methyl alcohol.
3. Transfer to an appropriate bottle and cap. Label as methyl alcohol with 2.5% KOH.

Safety Precautions

Methyl alcohol is a flammable liquid and a dangerous fire risk—keep away from flames and heat. It is toxic by ingestion and inhalation and is rapidly absorbed by the skin, eyes and mucous membranes. The biodiesel fuel is a flammable liquid. Potassium hydroxide solution in methyl alcohol is corrosive. Avoid contact of all chemicals with eyes and skin and perform this experiment in a hood or well-ventilated lab. Use only borosilicate glass Petri dish and check the glassware for cracks and scratches before using. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Any unused canola oil may be saved for future use or disposed of in the regular trash according to Flinn Suggested Disposal Method #26a. Any unused biodiesel fuel may be disposed of according to Flinn Suggested Disposal Method #18b. Unused potassium hydroxide may be disposed of according to Flinn Suggested Disposal Method #10. Unused methyl alcohol may be disposed of according to Flinn Suggested Disposal Method #18a. The methyl alcohol, KOH solution and the unreacted portion from Part 1 may be disposed of according to Flinn Suggested Disposal Method #18a.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period if a lab microwave is available. If not, the lab will require two lab sessions. The prelaboratory assignment should be completed before coming to lab, and the data compilation and calculations can be completed the day after the lab.
• Dissolving potassium hydroxide in methyl alcohol may generate potassium methoxide, a corrosive and reactive substance. It is harmful if swallowed, inhaled or absorbed through the skin.
• The microwave heating significantly increases the reaction rate for the transesterification reaction. A lab microwave is not simply a microwave used in the lab that is not for food. It is a highly specialized piece of equipment that is completely different from a regular microwave. If no lab microwave is available, the reaction will take overnight to produce a good yield of biodiesel fuel. Because of the hazards associated with methyl alcohol, no burner or hot plate should be used. Do not use a store bought microwave that is designed for food!
• The wick flame should be close but not touching the bottom of the can. If the flame is too close to the can, it may extinguish due to lack of oxygen.
• A lab station can be set up in the hood for students to obtain their cooking oil and methyl alcohol solutions.
• An alcohol burner may be substituted for the Petri dish–paper clip assembly.

Teacher Tips

• The “real world” application in this experiment makes it an effective learning exercise for students who tend to lose interest in purely chemically oriented applications of enthalpy changes and heats of combustion.
• This experiment also provides an excellent opportunity to discuss chemical potential energy—the energy stored in compounds due to their composition and structure. What is the source of energy when biodiesel fuel burns?
• Expect student answers to be only 40 to 60% of the actual heat of combustion value, 42.5 kJ/g. A major source of error in the experiment is heat loss through the calorimeter and the air. The metal calorimeter can get very warm throughout the calorimetry experiment, which means that the calorimeter itself, and not just the water, absorbs some of the heat given off by the burning biodiesel fuel. Heat loss through the calorimeter reduces the measured temperature change for the water surroundings, which in turn decreases the calculated value of the heat absorbed by the water. The calculated energy content values in kJ/g are likely to be lower than their actual values as a result.

Answers to Prelab Questions

A heat of combustion determination yielded the following data.

Mass of water in calorimeter 101.5 g
Initial mass of Petri dish and fuel 45.47 g
Initial temperature of the water 20.5 ºC
Final temperature of the water 39.8 ºC
Final mass of Petri dish and fuel 45.22 g

1. Calculate the change in the temperature of the water and the mass of fuel consumed.

   The change in temperature is 19.3 ºC and the mass of fuel consumed is 0.25 g.

2. Calculate the amount of energy absorbed by the water in the can, where

   Change in Energy, q = mass of water (g) x specific heat of water (4.184 J/gram•ºC) x ΔT
   Δq = 101.5 g x 4.184 J/g ºC x 19.3 ºC = 8200 J x 1 kJ/1000 J = 8.20 kJ

3. Calculate the heat of combustion of the fuel from the change in energy of the water and the mass of fuel consumed in kilojoules/gram.

   Heat of combustion (ΔH) = Δq/mass of biodiesel fuel consumed = 8.20 kJ/0.25 g = 33 kJ/g

Sample Data

Part A. Data Table

Part B. Data Table

Answers to Questions

1. Calculate the change in the temperature of the water and the mass of fuel consumed.

   #1    ΔT = 43.1 °C – 23.0 °C = 20.1 °C Mass of fuel consumed = 42.70 g – 42.35 g = 0.35 g
   #2    ΔT = 38.8 °C – 23.3 °C = 15.5 °C Mass of fuel consumed = 42.35 g – 42.08 g = 0.27 g

2. Calculate the amount of energy absorbed by the water in the can.

   #1    Δq = (112.90 g – 14.20 g) x 4.184 J/g °C x 20.1 °C

   Δq = 98.70 g (4.184 J/g °C)(20.1 °C)
   Δq = 8300 J

   #2    Δq = (112.75 g – 14.20 g) x 4.184 J/g °C x 15.5 °C

   Δq = 98.55 g (4.184 J/g•°C)(15.5 °C)
   Δq = 6390 J

3. Calculate the heat of combustion of the fuel from the change in energy of the water and the mass of fuel consumed, in kilojoules/gram.
   {12566_Answers_Equation_4}
4. Average the heat of combustion values for the two samples and enter this value in the data table. The accepted value for the heat of combustion for most biodiesel fuels is 42.5 kJ/gram. How do you account for the difference between your value and the expected value?

   Any explanation involving loss of heat to the environment (can, air) or other sources.

5. List some potential sources of error if
   1. The heat of combustion value was too small.

      Any reasonable answer that explains either a measured value of q being too low such as heat loss to the can or a higher measured value of biodiesel fuel such as a loss of biodiesel fuel by spillage.

   2. The heat of combustion value was too large.

      When run with no experimental errors, the value is always lower than the accepted value, due to various heat losses to the can and surroundings other than the water. Values that are too high would only result from errors in performing the procedure, such as misreading the temperatures or the mass of the biodiesel fuel.

References

Special thanks to Walter Rohr, West High School, Middleton, IL, for providing the idea and the instructions for this activity to Flinn Scientific.

Introduction

Biodiesel is an alternative, processed fuel for cars and trucks that is obtained from biological sources, usually vegetable oils. There is currently a great deal of interest in alternative fuels such as biodiesel or bioethanol because of concerns about global warming and the depletion of energy sources. The purpose of this activity is to prepare biodiesel fuel and to investigate the amount of energy it releases when burned.

Concepts

• Fats and fatty acids
• Heat of combustion
• Esterification
• Calorimetry

Background

Natural fats and oils, known as triglycerides, are organic esters containing three fatty acid groups attached via ester linkage to a glycerol backbone (see Figure 1).

Esters are considered derivatives of carboxylic acids—they may be prepared by the reaction of a carboxylic acid with an alcohol in the presence of a strong acid catalyst such as sulfuric acid (Equation 1). The “R” groups represent any combination of carbon and hydrogen atoms and are called alkyl groups. The C and H atoms in the alkyl group may be bonded together to form either chain or ring structures. Fatty acids are carboxylic acids with a long chain structure alkyl group.

If the alcohol is 1,2,3-propanetriol (glycerol), the esterification reaction results in the production of a triester (also called a triglyceride). The triesters formed this way constitute fats and oils. Most fats and oils contain a mixture of fatty acid residues of different chain lengths. The most common fatty acids have 12–18 carbon atoms and may be saturated or unsaturated. Unsaturated and polyunsaturated fatty acids contain one or more C=C double bonds, respectively, in their structures while saturated fatty acids contain no C=C double bonds.

Biodiesel fuel is also an ester, but results from the substitution reaction, called transesterification, of a light weight alcohol, such as methyl alcohol or ethyl alcohol, with a triglyceride fat or oil. Typical biodiesel compounds might be methyl stearate, C₁₇H₃₅COOCH₃, or ethyl stearate, C₁₇H₃₅COOC₂H₅. There are several reasons biodiesel is a better fuel than vegetable oil. Biodiesel molecules are much smaller. Smaller molecules require less oxygen for complete combustion than larger molecules and therefore burn much cleaner in combustion engines, producing less soot and carbon monoxide emissions. Smaller molecules have weaker intermolecular attractive forces, giving biodiesel fuel a lower resistance to flow, or viscosity, than cooking oil. This allows biodiesel to be injected more freely into an automobile engine.

Experiment Overview

The purpose of this experiment is to convert cooking (vegetable) oil to a methyl ester by transesterification with methyl alcohol and a strong base. Once produced, the heat of combustion of the biodiesel fuel will be determined by calorimetry.

Materials

Prelab Questions

Carefully read the entire procedure for determining the heat of combustion and then answer the following questions.

A calorimetric experiment using a synthetic fuel yielded the following data.

Mass of water in calorimeter 101.5 g
Initial mass of Petri dish and fuel 45.47 g
Initial temperature of water in calorimeter 20.5 ºC
Final temperature of the water in calorimeter 39.8 ºC
Final mass of Petri dish and fuel 45.22 g

1. Calculate the change in the temperature of the water in the calorimeter and the mass of fuel consumed.
2. Calculate the amount of heat energy (Δq) absorbed by the water in the can, where

   Δq = mass of water (g) x specific heat of water (4.184 J/gram•ºC) x ΔT

3. Divide the heat absorbed by the water by the mass of fuel consumed to calculate the heat of combustion of the fuel in kilojoules/gram.

Safety Precautions

Methyl alcohol is a flammable liquid and a dangerous fire risk—keep away from flames and heat. It is toxic by ingestion and inhalation and is rapidly absorbed by the skin, eyes, and mucous membranes. The biodiesel fuel is a flammable liquid. Potassium hydroxide solution in methyl alcohol is corrosive. Avoid contact of all chemicals with eyes and skin and perform this experiment in a hood or well-ventilated lab. Use only borosilicate glass Petri dish and check the glassware for cracks and scratches before using. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. Preparation of Biodiesel Fuel

1. Using a clean, 25-mL graduated cylinder, add 25 mL of cooking oil to a 125-mL Erlenmeyer flask, followed by 10 mL of the methyl alcohol containing 2.5% KOH.
2. Add a spin bar to the flask, stopper and mix rapidly on a magnetic stirrer 2–3 minutes.
3. Stop stirring and check the mixture. The mixture should be an emulsion. If two distinct layers remain, continue stirring for one or two minutes until an emulsion is formed.
4. Remove the stir bar and add two to three boiling chips to the flask.
5. If a lab microwave is available, place the flask in the oven and heat for two minutes at 50% power. If no microwave is available, allow the emulsion to sit overnight.
6. Obtain a separatory funnel. Making sure the stopcock at the bottom of the funnel is closed, carefully decant the reaction mixture into a separatory funnel.
7. Place the separatory funnel in a ring and secure the ring to the ring stand (see Figure 2).
   {12566_Procedure_Figure_2}
8. Allow the mixture to sit until two distinct liquid layers are observed.
9. Place a 50-mL beaker under the funnel and slowly open the stopcock. Collect the lower layer in a small beaker. Note: The lower or more dense layer contains glycerol and unreacted methyl alcohol solution.
10. Close the stopcock when the lower layer has been completely drained. Pour the remaining liquid from the separatory funnel into the 125-mL flask. This is the crude biodiesel fuel.
11. Cool the biodiesel fuel, if necessary, by gently swirling the flask under running water.
12. Measure the volume of biodiesel fuel obtained in the reaction in a 50-mL graduated cylinder. Record the volume of fuel produced and its color and appearance in the data table.
13. Clean all glassware and dispose of the bottom layer containing glycerol, methyl alcohol and potassium hydroxide mixture as directed by your instructor.

Part B. Determination of the Heat of Combustion

1. Obtain a clean, empty soda can with its opening tab still attached. Measure and record the mass of the soda can in the data table.
2. Add 100–125 mL of tap water (15–20 ºC) to the can. Measure and record the combined mass of the can and water in the data table.
3. Bend the top tab on the can up and slide a stirring rod through the tab opening. Suspend the can on a ring stand using a metal ring (see Figure 3).
   {12566_Procedure_Figure_3}
4. Take a large paper clip and bend one loop so that the loop is perpendicular to the rest of the paper clip (see Figure 4).
   {12566_Procedure_Figure_4}
5. Prepare a wick by rolling a 3-cm square of paper towel into a very tight and narrow tube.
6. Wet the paper towel with the biodiesel fuel and secure the tube to the paper clip with a small piece of aluminum foil. Leave only a small portion of the wick exposed to minimize the size of the flame (see Figure 5).
   {12566_Procedure_Figure_5}
7. Pour a 4–5 mm layer of the biodiesel fuel into a borosilicate glass Petri dish.
8. Place the paper clip wick assembly into the Petri dish.
9. Measure the combined mass of the Petri dish, biodiesel fuel and wick assembly and record this value in the data table.
10. Insert a thermometer into the soda can. Measure and record the initial temperature of the water in the data table.
11. Place the Petri dish under the can and carefully adjust the height of the ring so that the can is about 1 cm or less above the wick (see Figure 6).
    {12566_Procedure_Figure_6}
12. Center the wick under the can and light the top of the wick.
13. If using a digital thermometer, stir the water while heating until the temperature of the water has increased about 20 ºC. Blow out the flame and record the final temperature of the water in the data table.
14. Allow the Petri dish and its contents to cool to room temperature. Measure and record the final mass of the Petri dish and its components in the data table.
15. Repeat steps 7–14 using a second biodiesel fuel sample.
16. Dispose of the biodiesel fuel as directed by your instructor.

Student Worksheet PDF

12566_Student1.pdf