Biff Cake
Mr. Simpson
Maroon 5
march 13th, 1928
The effect on energy density of increasing the
percentage of ethanol in an 85-octane
gasoline/ethanol fuel mixture
Research Question:
How does the energy density of a fuel mixture consisting of 85-octane gasoline and
ethyl alcohol vary as the percentage of ethanol is increased?
Hypothesis:
For this investigation, the energy density of a fuel will be defined as the amount of heat energy
released (in kJ) per cm3 of fuel. This is appropriate because fuels are often sold per unit volume and determining if there
is a significant difference in the amount of energy released when ethanol is added to gasoline is the primary goal of this
investigation. Ethanol is typically added to octane-based gasoline as an oxygenate. Incomplete combustion of the
hydrocarbons present in gasoline releases harmful pollutants such as carbon monoxide, nitrogen dioxide, and carbon, all
of which can pollute the air leading to a host of environmental concern. Oxygenation of the fuel helps to decrease the
amounts of these pollutants by making the combustion reaction more complete. However, as ethanol is a two-carbon
alcohol, there is less chemical potential energy stored in the bonds than there is in octane, with an eight-carbon chain.
Because of this, it is predicted that increasing the percentage of ethanol in a gasoline-based fuel will lead to a decrease
in the amount of heat energy released, and thus a lower energy density. This decrease in energy density would
presumably lower the fuel efficiency.
Variables:

Independent: The percentage of ethanol in an 85-octane gasoline/ethanol fuel mixture will be manipulated. A
100% gasoline mixture and a 100% ethanol mixture will be tested, followed by mixtures of 75% gasoline and
25% ethanol, 50% gasoline and 50% ethanol, and 25% gasoline and 75% ethanol. It must be noted, however,
that the gasoline purchased from the local station has already been mixed with a certain amount of ethanol, and
100% octane-based gasoline was not available. For the purposes of this investigation, this mixture purchased
directly from the station will be defined as “100% gasoline.”

Dependent: The energy density of the fuel mixture, measured in kJ/cm3 will be determined by monitoring the
temperature change of a sample of water in a calorimeter as the fuel mixture is combusted beneath.

Controlled:
o
The total volume of fuel (2.0 cm3) will be kept constant among trials. Altering the amount of fuel would
result in an increase or decrease in temperature change that might not necessarily be due to a different
energy density in the fuel mixture.
o
The amount of water in the calorimeter (50.0 cm3) will be kept constant among trials for the same
reason that the volume of fuel will be kept constant.
o
A crucible will be used to contain the combusting fuel, and although the crucible itself will absorb a
large amount of the heat released during combustion, using the same crucible among trials will
eliminate this potential variable. As the goal is not to calculate the actual enthalpy of combustion of the
fuel mixture, it will not be necessary to compare the values obtained in this investigation with literature
values.
o
The mass, shape, composition, and setup of the calorimeter will be kept constant among trials. The
amount of heat absorbed by the calorimeter setup will not be accounted for in this investigation, as it
will be consistent among trials and the goal is not to calculate actual values for enthalpy of combustion.
Only relative differences in energy density are considered here.
o
In terms of calorimeter setup, the following dimensions will be maintained:

the distance between the top of the crucible and the bottom of the aluminum can calorimeter
will be 4.0 cm.

The thermometer will be submerged in the water in the calorimeter so that a minimum of 2.0
cm of the thermometer tip is covered.
o

The flame will be positioned in the center of the bottom of the calorimeter.

The calorimeter will rest on a ring clamp with a diameter of 10 cm.

The crucible will be porcelain with a total capacity of 15.0 cm3.
The experiment will be conducted in a fume hood to limit the amount of exposure to the harmful
products of incomplete combustion. The position of the calorimeter in the fume hood will be kept
constant to limit the effects of differences in air flow on the behavior of the flame.
o
The initial temperature of water in the calorimeter will be kept constant among trials.
Equipment needed:

85-octane gasoline*

½ aluminum can (calorimeter)

99% pure ethyl alcohol (denatured)

Digital thermometer (± 0.1 °C)

15 cm3 porcelain crucible

1 cm3 pipette (± 0.1 cm3)

Ring stand with 10 cm diameter ring clamp

Water kept at room temperature
* This gasoline had previously been mixed with ethyl alcohol as an oxygenate. For the purposes of this
investigation, this fuel will be defined as 100% gasoline.
Method:
1. Prepare the calorimeter setup as shown in the diagram. The thermometer tip
should be covered by a minimum of 2.0 cm of water. The distance between the
top of the crucible and the bottom of the aluminum can calorimeter should be 4.0
cm. The calorimeter should be filled with 50.0 cm3 of room temperature water.
(A beaker of water should be maintained at this temperature).
2. Place the calorimeter setup under the fume hood. Be sure that the setup is in a similar position throughout the
experiment to minimize the effect of differences in air flow in the behavior of the flame.
3. Record the initial temperature of the water in the calorimeter.
4. Use a 1 cm3 pipette to measure 2.0 cm3 of 100% gasoline. Dispense the gasoline into the crucible and place the
crucible directly under the calorimeter so that it is in the center of the can.
5. As gasoline is a high-volatility liquid, ignite the fuel immediately to minimize the effect of volume loss.
6.
Closely monitor the temperature of the water as the fuel combusts. The temperature may fluctuate slightly; be
sure and record the maximum temperature reached until the fuel has been completely consumed.
7. Reset the apparatus by disposing of the water in the calorimeter and completely drying between trials. Allow the
thermometer to return to room temperature before proceeding. Clean and dry the crucible.
8. Repeat steps 5, 6, and 7 for a total of three trials.
9. Prepare a mixture of 75% gasoline and 25% ethanol by adding 1.5 cm3 of 100% gasoline and 0.5 cm3 of 99% ethyl
alcohol to the crucible. Repeat steps 5 – 8 for this mixture.
10. Continue repeating steps 5 – 8 for 50% gasoline and 50% ethanol mixtures (1.0 cm3 of each fuel), 25% gasoline and
75% ethanol mixtures (0.5 cm3 gasoline and 1.5 cm3 ethanol), and 100% ethanol mixtures.
Data:
Trial 1
% Gasoline
Trial 2
Trial 3
% Ethanol
100
0
Ti (± 0.1°C)
21.0
Tf (± 0.1°C)
56.0
Ti (± 0.1°C)
21.0
Tf (± 0.1°C)
47.0
Ti (± 0.1°C)
21.0
Tf (± 0.1°C)
59.0
75
25
21.0
62.0
21.0
57.0
21.0
61.0
50
50
21.0
58.0
21.0
50.0
21.0
54.0
25
75
21.0
49.0
21.0
43.0
21.0
52.0
0
100
21.0
54.0
21.0
58.0
21.0
56.0
Volume of water in calorimeter: 50.0 cm3 ± 0.1 cm3
Volume of fuel combusted: 2.0 cm3 ± 0.1 cm3
(The motion of the flame, which was quite pronounced, was not consistent among trials)
Trial 1
Trial 2
Trial 3
Average
∆ T (± 0.2 °C)
∆ T (± 0.2 °C)
∆ T (± 0.2 °C)
∆ T (± 0.2 °C)
0.0
35.0
26.0
38.0
33.0
75
25
41.0
36.0
40.0
50
50
37.0
29.0
33.0
33.0
25
75
28.0
22.0
31.0
27.0
0
100
33.0
37.0
35.0
35.0
%
Gasoline
% Ethanol
100
39.0
To calculate the energy density of each fuel mixture, the average amount of heat energy released (in kJ) will be
calculated. This quantity will then be divided by the volume of fuel combusted.
Ex: 100% gasoline, 0% ethanol:
The amount of heat absorbed by the water will be assumed to be equal to the amount of heat released
by the combusting fuel. Although a significant portion of this released heat energy was lost to the
surroundings, this was a controlled variable as the setup of the apparatus was identical for each trial.
Qwater = (mwater)(cwater)(∆Twater)
Qwater = -Qfuel
Qwater = (50.0 g)(4.18 J/g° C)(33°C) = 6900 J = 6.9 kJ
6.9 kJ / 2.0 cm3 of fuel = 3.5 kJ / cm3 released
The random uncertainty for each energy density measurement must also be calculated:
Qwater = (50.0 cm3)(4.18 J/g° C)(33°C)
Volume of water: (0.1 cm3/50.0 cm3)100 = 0.2 % uncertainty
Temperature change of water : (0.2 cm3/33 °C)100 = 0.6 % uncertainty
Volume of fuel: (0.1 cm3 / 2.0 cm3)100 = 5%
Total 5.8 % uncertainty
Summary: Energy densities of various mixtures of 85-octane gasoline and ethanol
% Gasoline
% Ethanol
Energy Density
100
0
3.5 kJ/cm3 ± 5.8%
75
25
4.1 kJ/cm3 ± 5.7%
50
50
3.5 kJ/cm3 ± 5.8%
25
75
2.8 kJ/cm3 ± 5.9%
0
100
3.7 kJ/cm3 ± 5.8%
The trend line displays a general trend of decreasing energy density as the amount of ethanol in the mixture is
increased. However, the value for r2 (coefficient of determination) is quite low, indicating a very weak
correlation. This is indicative of the lack of precision and consistency in the investigation. The error bars clearly
display that random uncertainty alone cannot account for the variance in the data points from the linear trend.
This imprecision is apparently not directional in nature. While some data points lie above the trend line, others
lie below. If all points were above or below the line, it would indicate that a directional systematic error had
occurred, for example consistently measuring too much or too little fuel. Instead, the lack of consistency is likely
due to differences in the behavior of the flame among trials. While every attempt was made to control the
variables, including controlling the distance of the flame from the calorimeter, utilizing the same calorimeter
between trials, controlling the amount of water in the calorimeter, fuel volume, etc., there was no practical way
to control the way the flame behaved or “danced.” It was observed that the flame movement was actually quite
pronounced during some trials, which limited the amount of heat concentrated toward the water in the
calorimeter. During other trials, the flame was actually quite calm, concentrating the heat released directly into
the calorimeter. This resulted in different amounts of heat being lost to the environment, and was beyond the
practical control of the method. This heat lost to the environment represents the primary source of error in this
investigation, despite exhaustive attempts to control it. The amount of random uncertainty caused by the
measuring tools averaged only about 5.8%, and was not enough to explain the deviation of the data points from
the trend line (as displayed by the error bars).
Conclusion
The data collected do display a general trend of decreasing energy density as the percentage of ethanol in a 85-octane
gasoline/ethanol fuel mixture increases. However, the correlation of the data points to the trend is a very weak one.
Most striking is the apparent truth that a fuel of 100% ethanol has a higher energy density than a fuel of 100% gasoline
(it must be noted, however, that the gasoline purchased from the station had already been mixed with a small amount
of ethanol, thus 100% gasoline was not available for this investigation). This discrepancy can best be explained by the
inconsistency in the amount of heat lost to the environment between trials, as two of the three trials conducted show
that 100% gasoline does in fact have a higher energy density than 100% ethanol. In fact, the data collected in trial 2
shows that 100% ethanol has a significantly larger energy density than 100% gasoline. The impact of outlying data
points such as this would be greatly reduced by increasing the number of trials conducted in a future experiment. The
overall trend in decreasing energy density with increasing amounts of ethanol does support the initial prediction, albeit
weakly due to the low value of the correlation coefficient, r.
As described earlier, the primary source of error in this investigation was the inconsistency in the amount of heat lost to
the environment among trials. The average percent random uncertainty was approximately 5.8%, an amount that alone
could not account for the deviation of the data points from the trend displayed. There was no evidence that this
imprecision was directional in nature, as data points lie above and below the trend line. If the experiment were to be
performed again, it would be necessary to implement several improvements directed at limiting this difficult-to-control
error. Foremost among these improvements would be a technique that limits the amount of motion displayed by the
flame. Performing the reaction outside of a fume hood, where the air flow would be less pronounced, would help to
limit this motion (although use of a fume hood was necessary to remove the harmful products given off by the
incomplete combustion of the hydrocarbons). Protecting the flame inside of an enclosed container would also serve to
limit this motion, although it would be necessary to provide the reaction with a steady source of oxygen.
Another technique for limiting the variance among trials would be to move the calorimeter closer to the flame, helping
to concentrate the heat released into the water in the calorimeter. However, care must be taken not to smother the
flame. Perhaps using an alcohol lamp in place of the crucible would also help to concentrate the flame, although alcohol
lamps were not available for this experiment.
In summary, the data collected display a weak trend of decreasing energy density with increasing percentage of ethanol
in a gasoline/ethanol fuel mixture. The weakness of the trend can primarily be attributed to inconsistencies in heat lost
to the surroundings among trials, although increasing the number of trials and employing several simple modifications
could help to increase the consistency among data points and thus strengthen the conclusion.