Materials Included In Kit

Additional Materials Required

Prelab Preparation

Methyl alcohol with 2.5% KOH solution: In a fume hood, add 5 g of potassium hydroxide (KOH) to a beaker containing approximately 100 mL of methyl alcohol and stir until the KOH is dissolved. Add the solution to a 250-mL graduated cylinder and dilute to the 200-mL mark with methyl alcohol. Transfer to an appropriately labeled bottle with a secure cap. Label as methyl alcohol containing 2.5% KOH—corrosive liquid.

Safety Precautions

Methyl alcohol is a flammable liquid and a dangerous fire risk—keep away from heat, sparks and open flames. It is toxic by ingestion and inhalation and is rapidly absorbed by the skin, eyes and mucous membranes. Potassium hydroxide solution in methyl alcohol is a corrosive liquid and will cause severe skin burns and eye damage. The biodiesel fuel is a flammable liquid. Avoid contact of all chemicals with eyes and skin and perform this experiment in a hood or well-ventilated lab. Make sure that there are no open bottles or other containers of methyl alcohol present when doing the calorimetry activity. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Remind students to wash their 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 vegetable oil may be saved for future use. The biodiesel product may be disposed of according to Flinn Suggested Disposal Method #18b. Excess potassium hydroxide solution may be neutralized according to Flinn Suggested Disposal Method #10. The glycerol layer remaining after the preparation of biodiesel contains unreacted KOH. It may be neutralized according to Flinn Suggested Disposal Method #10.

Lab Hints

• Both parts of this laboratory activity can reasonably be completed in a typical 2-hour lab period.
• 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. This solution should be prepared by the instructor prior to lab.
• Heating the vegetable oil significantly increases the reaction rate for the transesterification reaction. Because of the hazards associated with methyl alcohol, no burners or open flames may be used. Do not overheat the oil. Use the lowest setting only on a hot plate or place the oil in a hot water bath.
• The burner wick flame should be close but not touching the bottom of the calorimeter can. If the flame is too close to the can, it may extinguish due to a lack of oxygen.
• A lab station can be set up in the hood to dispense the methyl alcohol solution.
• 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 results to be only 40–60% of the actual value for the heat of combustion of biodiesel, which is reported to be 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.
• For best results, carry out the calorimetry experiments in a fume hood. Make sure that any remaining methyl alcohol has been removed from the hood before lighting the burners. Return the methyl alcohol/KOH solution to a flammables cabinet after students complete the first part of this experiment.
• The burning of biodiesel produces a black, sooty flame due to incomplete combustion.
• Large test tubes (25 x 200 mm) may be substituted for the separatory funnels if a classroom set of funnels is not available. Simply pour the contents of the biodiesel reaction flask into a large test tube and allow to settle. The glycerol layer will separate out at the bottom of the test tube, leaving a cloudy top layer that will gradually clarify upon standing (20 min). Remove the biodiesel using a pipet.

Answers to Prelab Questions

1. What are the physical and health hazards of the potassium hydroxide/methyl alcohol solution? What safety precautions are necessary to protect against these hazards?

   Methyl alcohol is a flammable liquid that is toxic by ingestion and inhalation. The presence of potassium hydroxide makes the solution corrosive as well. Do not use near any sources of ignition and wear goggles, gloves and a lab coat or apron.

2. What is the purpose of potassium hydroxide in this solution?

   Potassium hydroxide acts as a catalyst for the transesterification reaction of vegetable oil to produce biodiesel.

3. A calorimetry experiment to determine the heat of combustion of a fuel yielded the following data:

   Mass of water in calorimeter: 101.5 g
   Initial mass of burner and fuel: 45.47 g
   Initial temperature of the water: 20.5 °C
   Final temperature of the water: 39.8 °C
   Final mass of burner and fuel: 45.22 g

   1. Calculate the change in the temperature (ΔT) of the water and the mass of fuel consumed when it burned.

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

   2. Calculate the amount of heat energy (q) absorbed by the water in the calorimeter, where:
      Change in energy, q = mass of water (g) x specific heat of water (4.18 J/g • °C) x ΔT

      q = 101.5 g x 4.18 J/g • °C x 19.3 °C = 8190 J x 1 kJ/1000 J = 8.19 kJ

   3. Calculate the heat of combustion of the fuel by dividing the heat absorbed by the water by the mass of fuel consumed. Report the result in kilojoules per gram.

      Heat of combustion = q/mass of biodiesel fuel consumed = 8.19 kJ/0.25 g = 33 kJ/g

Sample Data

Preparation of Biodiesel

Heat of Combustion

Answers to Questions

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

   Trial 1 ΔT = 38.4 °C – 19.6 °C = 18.8 °C Mass of fuel consumed = 105.35 g – 104.78 g = 0.57 g
   Trial 2 ΔT = 39.5 °C – 19.7 °C = 19.8 °C Mass of fuel consumed = 104.77 g – 104.37 g = 0.40 g

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

   Trial 1 q = (170.33 g – 13.80 g) x 4.18 J/g • °C x 18.8 °C

   q = 156.53 g (4.18 J/g • °C)(18.8 °C)
   q = 12,300 J

   Trial 2 q = (110.65 g – 13.95 g) x 4.18 J/g • °C x 19.8 °C

   q = 96.7 g (4.184 J/g • °C)(19.8 °C)
   q = 8003 J

3. Calculate the heat of combustion of the fuel by dividing the energy absorbed by the water by the mass of fuel consumed. Report the result in kilojoules per gram.
   {14052_Answers_Equation_1}
4. Calculate the average heat of combustion value for the two trials. The reported value for the heat of combustion for biodiesel is 42.5 kJ/gram. Describe at least two sources of experimental error that might account for the difference between your value and the expected value.

   The average value of the heat of combustion was 21 kJ/g, considerably less than the reported value. The calorimeter (soda can) is not insulated—a lot of heat was lost to the surroundings. The biodiesel burned with a black, sooty flame due to carbon produced by incomplete combustion. Incomplete combustion occurs when there is not a stoichiometric amount of oxygen for the amount of fuel. This results in a lower heat of combustion value.

5. Would the measured heat of combustion for biodiesel be higher or lower than the reported value if the fuel continued to burn after it was moved away from the colorimeter?

   If the fuel continued to burn after it was removed from the calorimeter, the mass of fuel that burned would be greater than the amount that burned and released heat to the calorimeter. The heat of combustion would be lower than its actual value.

Introduction

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

Concepts

• Fats and fatty acids
• Esterification
• Biodiesel
• Heat of combustion
• Calorimetry
• Alternative energy

Background

Natural fats and oils, known as triglycerides, are organic esters containing three fatty acid groups attached via ester linkages 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 in Equation 1 represent any combination of carbon and hydrogen atoms and are called alkyl groups. The C and H atoms in the alkyl group may be arranged in either chain or ring structures. Fatty acids are naturally occuring carboxylic acids with a long, unbranched hydrocarbon chain attached to the –CO₂H functional groups. Common fatty acids have an even number of carbon atoms, C₁₂–C₁₈, and may be saturated, unsaturated or polyunsaturated.

If the alcohol is 1,2,3-propanetriol (glycerol), the esterification reaction results in the production of a triester called a triglyceride. The triesters formed in this way constitute fats and oils. Most fats and oils contain a mixture of fatty acid residues of different chain lengths, with 14–18 carbon atoms being most common. Unsaturated and polyunsaturated fatty acids contain one or more C═C double bonds, respectively, in their structures, while saturated fatty acids do not contain any C═C double bonds.

Biodiesel is also an ester, but it results from the substitution reaction, called transesterification, of a light-weight alcohol such as methyl alcohol with a triglyceride fat or oil. This reaction is carried out in the presence of a basic catalyst, potassium hydroxide (Equation 3). Typical biodiesel products might be methyl stearate, C₁₇H₃₅COOCH₃, and methyl oleate, C₁₇H₃₃COOCH₃. There are several reasons biodiesel is a better fuel than vegetable oil. Biodiesel esters are smaller molecules than triglycerides. Smaller molecules require less oxygen for complete combustion than larger molecules and therefore burn much cleaner in combustion engines, producing less soot and lower carbon monoxide emissions. Smaller molecules also 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 resulting biodiesel will be determined by calorimetry. Calorimetry is the measurement of the amount of heat energy produced in a reaction. Calorimetry experiments are carried out by measuring the temperature change in a container of water (called a calorimeter) that is in contact with or surrounds the reactants and/or products. In this experiment, the biodiesel will be burned in a glass alcohol burner placed underneath a “sodacan” calorimeter. The calculations involved in a calorimetry experiment are reviewed in the Prelaboratory Assignment.

Materials

Prelab Questions

1. What are the physical and health hazards of the potassium hydroxide/methyl alcohol solution? What safety precautions are necessary to protect against these hazards?
2. What is the purpose of potassium hydroxide in this solution?
3. A calorimetry experiment to determine the heat of combustion of a fuel yielded the following data:

   Mass of water in calorimeter: 101.5 g
   Initial mass of burner and fuel: 45.47 g
   Initial temperature of the water: 20.5 °C
   Final temperature of the water: 39.8 °C
   Final mass of burner and fuel: 45.22 g

   1. Calculate the change in the temperature (ΔT) of the water and the mass of fuel consumed when it burned.
   2. Calculate the amount of heat energy (q) absorbed by the water in the calorimeter, where:
      Change in energy, q = mass of water (g) x specific heat of water (4.18 J/gram • °C) x ΔT
   3. Calculate the heat of combustion of the fuel by dividing the heat absorbed by the water by the mass of fuel consumed. Report the result in kilojoules per gram.

Safety Precautions

Methyl alcohol is a flammable liquid and a dangerous fire risk—keep away from heat, sparks and open flames. It is toxic by ingestion and inhalation and is rapidly absorbed by the skin, eyes and mucous membranes. Potassium hydroxide solution in methyl alcohol is a corrosive liquid and will cause severe skin burns and eye damage. The biodiesel fuel is a flammable liquid. Carefully inspect the workplace area before doing the calorimetry experiment. Make sure there are no open methyl alcohol containers. Avoid contact of all chemicals with eyes and skin and perform this experiment in a hood or well-ventilated lab. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Preparation of Biodiesel

1. Using a clean, 25-mL graduated cylinder, add 25 mL of canola or corn oil to a 125-mL Erlenmeyer flask. Gently heat the oil to 45–50 °C in a hot water bath or on a hot plate at the lowest setting. Turn off the heat when the oil reaches the desired temperature.
2. Transfer 10 mL of methyl alcohol/KOH solution to the flask.
3. Add a spin bar to the flask, stopper, and mix rapidly on a magnetic stirrer for 5 minutes.
4. Stop stirring and check the mixture. The mixture should be an emulsion, which will gradually separate into two layers.
5. Continue stirring the mixture for 20 minutes.
6. Obtain a separatory funnel. Making sure the stopcock at the bottom of the funnel is closed, carefully decant the reaction mixture from the Erlenmeyer flask into the separatory funnel. Note: If a separatory funnel is not available, pour the mixture into a large test tube.
7. Place the separatory funnel in a ring and secure the ring to the ring stand (see Figure 1).
   {14052_Procedure_Figure_1}
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. The lower or more dense layer contains glycerol and methyl alcohol.
10. Close the stopcock when the lower layer has been removed. Pour the remaining liquid from the separatory funnel into a flask or graduated cylinder. This is the crude biodiesel.
11. Measure the volume of biodiesel in a 50-mL graduated cylinder. Record the volume of fuel produced and its color and appearance.
12. Clean all glassware and dispose of the bottom layer, which is a mixture of glycerol, methyl alcohol and potassium hydroxide, as directed by the instructor.

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 empty soda can.
2. Add 100–125 mL of tap water (15–20 °C) to the can. Measure and record the combined mass of the can and water.
3. Bend the top tab on the can up and slide a stirring rod through the tab opening. Suspend the can on a support stand using a ring clamp (see Figure 2).
   {14052_Procedure_Figure_2}
4. Pour the biodiesel fuel into an empty glass alcohol burner.
5. Place the wick into the burner and allow the liquid to saturate the wick.
6. Measure and record the combined mass of the alcohol burner, biodiesel fuel and wick.
7. Insert a digital thermometer into the soda can. Measure and record the initial temperature of the water. Inspect your workplace bench and the surrounding area before proceeding to steps 8–13. Make sure there are no open containers or glassware with methyl alcohol in the area before lighting the burner.
8. Place the alcohol burner under the can and carefully adjust the height of the ring so the can is about 1 cm above the wick (see Figure 3).
   {14052_Procedure_Figure_3}
9. Center the wick under the can and light the top of the wick.
10. Stir the water using the digital thermometer until the temperature of the water has increased about 20 °C. Immediately place the metal cap on top of the burner to extinguish the flame. Record the final temperature of the water in the soda can.
11. Allow the glass alcohol burner and its contents to cool to room temperature. Measure and record the final mass of the burner and its contents.
12. Repeat steps 6–11 to obtain a second set of calorimetry data (trial 2).
13. Dispose of the biodiesel fuel as directed by the instructor.

Student Worksheet PDF

14052_Student1.pdf