The Heats of Combustion of Fuels

Introduction: Gasoline,the primary automotive fuel in the U.S., is a hydrocarbon obtained from petroleum. The realization that the world supply of petroleum is limited and will eventually be depleted has led to the development of alternative fuels. Two of these are methanol, CH3OH, and ethanol, C2H5OH.

One advantage of methanol over gasoline is that methanol can be made from coal. World reserves of coal are vastly greater than those of oil, natural gas and uranium combined.

Unlike petroleum, ethanol is a renewable resource since it can be made by the fermentation of plant materials. In the U.S. and Canada, ethanol is used in a fuel called gasohol consisting of 10% ethanol and 90% gasoline. In Brazil, 100% ethanol, is used as a fuel in automobiles with specially designed engines.

An additional advantage of methanol and ethanol is that they are "oxygenated" fuels. Their molecules contain oxygen, and their use as fuels reduces the emission of certain pollutants

The combustion reactions of methanol and ethanol are:

2CH3OH(l) + 3O2(g) 2CO2(g) + 4H2O(l)

C2H5OH(l) + 3O2(g) 3H2O(l) + 2CO2(g)

The combustion of lamp oil cannot be represented by an analogous equation because lamp oil itself is a mixture of hydrocarbons rather than a pure compound. The reaction, in words is :

lamp oil + oxygen carbon dioxide + water

A convenient unit of energy is the calorie (cal), defined as the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius. A kilocalorie (kcal) is equal to 1000 calories.

Purpose: One purpose of this experiment is to compare lamp oil, a hydrocarbon similar to gasoline, and ethanol with respect to the amount heat energy produced per gram of fuel burned. This energy will be referred to as the heat of combustion per gram. Another purpose is to compare the appearance of the flame produced when each of the fuels methanol, ethanol and lamp oil are burned.

Method:The measurement of the heat evolved by a chemical reaction is measured using a device called a calorimeter. Calorimeters vary in design. The one to be used in this experiment is shown in Figure 1. The calorimeter consists of a 250 mL Erlenmeyer flask containing 200 mL of water, a thermometer and a draft shield.

A burner containing a pre-weighed amount of a liquid fuel, such as ethanol, is placed under the Erlenmeyer flask . The burner flame causes the temperature of the water to rise. When the water temperature has risen by a sufficient amount, the burner is extinguished, and the weight of the fuel is re-measured. Thus, the temperature rise produced by the burning of a known mass of fuel is determined, and from this information, the heat produced by the reaction can be calculated.

Figure 1

Calibration of the calorimeter: The conversion of the temperature rise of the water into an amount of heat requires that the calorimeter be calibrated. This is accomplished replacing the burner containing the fuel to be tested with a burner containing a reference fuel with a known heat of combustion per gram. The reference fuel to be used here is methanol. Its heat of combustion per gram is 5,420 cal/g. That is, the burning of 1 gram of methanol releases 5,420 cal of heat energy.

If methanol is burned in the calorimeter, the total heat released is equal to the mass of the methanol burned times 5,420 cal/g.

heat of combustion of methanol (cal) = mass burned x 5,420 cal/g

Next, calculate the number of calories used to raise the calorimeter temperature by 1C.

calibration value = heat of combustion of the methanol(cal)/ temperature change of the calorimeter (C)

Do this for each trial and calculate the average value.

The next step is to calculate the heat of combustion of the unknown fuel (either ethanol or lamp oil). After measuring the mass of the unknown fuel burned and the temperature change, calculate the number of calories released by the burning of the fuel. This is done by multiplying the temperature change (in C) by your average calibration value, the number of calories needed to raise the temperature of the calorimeter by 1C.

heat of combustion of fuel (cal) = temperature change (C) x average calibration value (cal/C)

Finally, since we are interested in the amount of heat released by the burning of one gram of the fuel, calculate the number of calories per gram by dividing the heat of combustion of the fuel by the number of grams of fuel burned.

heat of combustion per gram = heat of combustion of fuel (cal) / mass of fuel (g) burned

Usually, the average value for several trials is then calculated and reported.

Procedure:

Obtain from the stockroom three burners containing methanol, ethanol and lamp oil, respectively.

Clamp a 250-ml Erlenmeyer flask to a ring stand, place one of the burners under it, and adjust the height of the flask so that the bottom of the flask is about 2. cm above the top of the burner wick. Remove the flask from the clamp, but do not change the height of the clamp on the ring stand. This height must remain constant throughout the experiment. Insert the neck of the flask through the hole in the bottom of the coffee can, and re-clamp the flask to the ring stand. Your apparatus should look like the one in Figure 1.

Calibration of the Calorimeter

Determine the mass of the methanol burner by weighing it on a balance. Record the result in your notebook.

Using a graduated cylinder, transfer exactly 200 ml of cold tap water to the Erlenmeyer flask. Stir the water thoroughly with the thermometer, read the temperature to the nearest 0.2 C and record it in your notebook.

 

Light the methanol burner and quickly place it under the flask centering the flame under the bottom of the flask. The easiest way to do this is to tip the ring stand back so that the burner can be slipped under the coffee can, and then lower the ring stand to its original position. Stir the water with the thermometer (be careful, it is fragile), and when the temperature of the water in the flask has risen about 20 C, blow out the flame. Continue to stir the water, recording the highest temperature reached. Remove the burner and when it has cooled to room temperature, re-weigh it and record the mass. While the lamp is cooling, empty the water from the flask, clean the soot, if any, off the bottom, and refill the flask with 200 ml of cold tap water as before. Repeat this calibration.

Observe and record the appearance of the flame.

 

Heats of Combustion of ethanol and lamp oil

Measure the heats of combustion of both ethanol and lamp oil. The procedure is the same as that used in the calibration step except that the methanol burner is replaced by the burner containing the fuel of interest. Perform each measurement twice.

Observe and record the appearance of the flames produced by ethanol and lamp oil.


Results

Use the table below as a guide for organizing your calculations. All data and calculations should be in the "Results" section of your report.

Calibration by Combustion of Methanol

Data Trial 1 Trial 2
initial mass of burner (g)    
final mass of burner (g)    
initial temperature of water (C)    
final temperature of water (C)    

 

Calculations Trial 1 Trial 2
mass of methanol burned(g)    
temperature change of the water (C)

(final temperature-initial temperature)

   
heat of combustion of methanol (cal)

(mass x 5,420 cal/g)

   
calibration value (cal/C)

(heat of combustion / temperature change)

   
average calibration value (cal/C)  

 

Heat of Combustion of Ethanol

(A similar table will also be required for the combustion of lamp oil)

Data Trial 1 Trial 2
initial mass of burner (g)    
final mass of burner (g)    
initial temperature of water (C)    
final temperature of water (C)    

 

Calculations Trial 1 Trial 2
mass of ethanol burned (g)    
temperature change of the water (C)

(final temperature-initial temperature)

   
actual heat of combustion of ethanol (cal)

(calibration value x temperature change)

   
heat of combustion per gram of ethanol burned

(actual heat of combustion / mass of oil burned)

   
average heat of combustion per gram of ethanol  

Conclusion:

Answer the following questions.

1. Report the heats of combustion per gram of ethanol and lamp oil.

2. Compare the appearance of the flames produced by methanol, ethanol and lamp oil. Which fuel burns most cleanly?

3. What are some advantages and disadvantages of methanol and ethanol compared to lamp oil?