Download MINDS-ON 3: Fuel Combustion

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Minds-On 3: Page 1 of 4
MINDS-ON 3: Fuel Combustion
Purpose:
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To observe the complete and incomplete combustion of a hydrocarbon
To write and balance chemical equations for complete combustion reactions
To calculate heat of combustion from heat of formation for hydrocarbons used as fuels
Overview:
In this two-part activity SENIOR CHEMISTRY & TECHNOLOGY EDUCATION students will (1) learn
about the complete and incomplete combustion of hydrocarbons which occur in vehicle engines as well
as (2) determine and compare the heat of combustion for a variety of fuels (thermochemistry).
Suggested Timing:
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40-50 minutes class time
Prior Knowledge and Skills:
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Familiarity with writing basic chemical formulae
Familiarity with basics of balancing chemical equations
Familiarity with the concept of molar enthalpy
Materials:
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Bunsen burner and lighter
Ceramic plate or piece of glassware
BLM M4: Heat of Combustion – 1/student
Heat of Combustion – Answer Page (for teacher use)
Access to calculators – 1/student
Equipment to view a video (optional)
Instructions:
Part 1: Complete and Incomplete Combustion Reactions
 Light the Bunsen burner and ask students to observe and identify what indicates to them that a
chemical reaction is taking place (i.e., heat and light).
 Demonstrate what happens to the appearance of the flame as you move the collar to open and
close the air hole. Students could record their observations during the various demonstrations
using a chart such as the one below (suggested responses are in blue):
Air hole
Colour of flame
Presence of carbon
Type of combustion
Open
Blue
No
Complete
Closed
Yellow
Yes
Incomplete
Students should notice the blue flame when the air hole is open and the yellow flame when it is
closed. Ask students for their explanation of their observations. Ensure that they understand that
in the presence of sufficient amounts of oxygen (from air), the hydrocarbon combusts completely
in a reaction called a complete combustion reaction; this results in a blue flame. When there is
insufficient oxygen, the hydrocarbon does not combust completely in a reaction called an
incomplete combustion reaction; this results in a yellow flame.
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Minds-On 3: Page 2 of 4
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Introduce/review ways in which to describe the combustion of a hydrocarbon, beginning with a
word equation. A simple example is the combustion of methane (main component of natural gas).
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Explain/review that in a complete combustion reaction of a hydrocarbon (hydrocarbon with
oxidizing agent oxygen) the products of the reaction are carbon dioxide and water.
Methane + oxygen → carbon dioxide + water
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Ask students whether they think this reaction is endothermic (requires a net input of heat from its
surroundings) or exothermic reaction (involves a release of heat from the system to its
surroundings). Students will likely know that combustion reactions produce heat (are exothermic).
Have students tell you where to add the word “heat” to the word equation (right side).
Methane + oxygen → carbon dioxide + water + heat
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Next, have students write, using chemical formulae, the reactants and products for the
combustion of methane. Have students pair up and share/discuss their responses. Confirm that
they have identified the correct reactants and products.
CH4 (gas) + 02 (gas) → C02 (gas) + H20 (liquid)
Have the students balance the equation.
CH4 (gas) + 2 02 (gas) → C02 (gas) + 2 H20 (liquid)
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Next, have the students predict the products and then write out the word and balanced chemical
equation (using whole numbers and including states) for the combustion of butane (C4H10).
Butane + oxygen → carbon dioxide + water + heat
2C4H10 (gas) + 13 02 (gas) → 8 C02 (gas) + 10 H20 (liquid)
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Explain that many car engines use gasoline which is made from hydrocarbons with seven to 11
carbon atoms. Assign each student one of the hydrocarbons from the list below. Have each
student write the balanced chemical equations for the complete combustion reaction of the
hydrocarbon. Review the responses and check for understanding.
C7H16 + 11 O2 → 7CO2 + 8H2O
Heptane (C7)
Octane (C8)
2C8H18 + 25 O2 → 16CO2 + 18H2O
Nonane (C9)
C9H20 + 14 O2 → 9CO2 + 10H2O
Decane (C10)
2C10H22 + 31 O2 → 20CO2 + 22H2O
Undecane (C11)
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If you wish, you can have students look
for the pattern and determine a general
formula for the combustion of alkanes.
CxHy + zO2 → xCO2 + y/2H2O
where z = x + 1/4y
C11H24 + 17 O2 → 11CO2 + 12H2O
The reality is that in a car’s engine, these hydrocarbons also undergo incomplete combustion. To
demonstrate that an incomplete reaction can produce solid carbon, hold a ceramic plate or piece
of glassware close to the blue Bunsen burner flame (no carbon produced) and then again close to
the yellow flame (carbon produced). If you are unable to do this demonstration, students could
watch the video Combustion of Alkanes (4:02 min.) on CurioCity.
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ACTION PROJECT
Minds-On 3: Page 3 of 4
Develop students’ understanding that in an incomplete combustion reaction, a number of different
products can form which include carbon, carbon monoxide, carbon dioxide, and water. Explain
that which products are actually formed depend on the amount of oxygen present. Carbon
monoxide (CO), a toxic gas, is a particularly undesirable product of incomplete combustion and
together with carbon dioxide are primary vehicle pollutants.
Part 2: Heat of Combustion of Fuels
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Explain that in an internal combustion engine, when the fuel is ignited, a large amount of energy
is released as gases (CO2 and H2O) are formed. These gases are able to exert force against a
piston, thus transforming the chemical energy of fuel into mechanical energy.
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Explain that for each type of fuel, it is possible to calculate the Heat of Combustion (∆H˚c) for the
fuel. Heat of combustion is the energy released as heat when a compound undergoes complete
combustion with oxygen under standard conditions. It is measured in kJ/mol. Heat of combustion
can be measured experimentally (in a bomb calorimeter) as well as calculated using the
difference between the heat of formation (∆H˚f) of the products and reactants.
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The combustion of methane can be used as an example. The standard molar enthalpy (total
energy resulting from the formation of one mole of a substance at constant pressure) of
combustion for methane can be expressed in terms of the standard molar enthalpies of formation
(∆H˚f) of the reactants (methane and oxygen) and products (carbon dioxide and water).
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Show students the overall equation for heat of combustion. Ask them what they notice about the
products and reactants. Ensure that they understand that the products are added whereas the
reactants are subtracted.
∆H˚c = Σ∆H˚f (products) - Σ∆H˚f (reactants)
= [∆H˚f carbon dioxide + ∆H˚f water] - [∆H˚f hydrocarbon + ∆H˚f oxygen]
Products
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Reactants
Next, use the balanced chemical equation to identify the coefficients. These will be required to
calculate the heat of combustion. Add them to the ∆H˚c equation.
CH4 (gas) + 2 02 (gas) → C02 (gas) + 2 H20 (liquid)
∆H˚c (methane) = (∆H˚f carbon dioxide + 2∆H˚f water) - (∆H˚f hydrocarbon + 2∆H˚f oxygen)
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Show how ∆H˚f values can be inserted in the equation. ∆H˚f values can be obtained from tables
such as this table of alkanes from Wikipedia. Keep in mind that values in these tables have
been experimentally derived, so the values on tables from different sources may not match.
(Note: For this exercise students will have the values provided to them.)
∆H˚c (methane) = [-393.5 + 2(-285.8)] – [(-74.9) + 2(0)]
= -393.5 – 571.6 + 74.8
= -890.3 kJ/mol
Ask students what they notice. They should notice that the
value is negative.
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Name
Formula
∆fH (kJ/mol)
Methane (g)
CH4
-74.9
Water (l)
H2O
-285.8
Carbon
dioxide (g)
CO2
-393.5
Oxygen (g)
O2
0
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Minds-On 3: Page 4 of 4
A negative value indicates that the reaction is exothermic (energy is released). They should also
be aware that the standard enthalpy of formation for an element in its most stable state is defined
by scientists as being zero.
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Provide students with BLM M4: Heat of Combustion. Using the same method as above and the
balanced chemical equations determined in Part A have the students calculate the ∆H˚c values
for the fuels given. Suggested responses can be found Heat of Combustion – Answer Page.
Assessment Suggestions:
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Responses to questions can provide insight into students’ current level of understanding and
could indicate where further discussions about combustion reactions might be needed.
Discuss the responses on BLM M4: Heat of Combustion as a class or assess individually and
provide feedback.
Connecting to Other Resources on CurioCity:
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Combustion of Alkanes
This video (4:02 minutes, 2013) explains the different types of combustion of alkanes using
laboratory examples.
Experiments with a candle flame
This video (3:52 minutes, 2013) demonstrates a number of simple experiments that can be done
with a candle to illustrate concepts about combustion.
Extensions:
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Students can compare the combustion of the butane lighter to that of a candle and answer
questions such as:
o How are the flames similar and different?
o Where is the hottest part of each flame? (Have students use a thermocouple to determine
this.)
o What is the source of fuel in a candle?
Students could research a given type of fuel used in transportation and answer questions such
as:
o What is the most common method of producing the fuel?
o What are some advantages of the fuel? Some disadvantages?
o What energy efficiency does the fuel have?
o Answer their own questions from Minds-On 1 if related (or just answer a question that they
might have.
Have students determine the heat of combustion for alternative fuels such as biodiesel, ethanol
and natural gas and compare these values to those of petroleum-based fuels.
Additional Information:
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http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/carbon_chemistry/carbon
_fuelsrev1.shtml (Accessed Dec. 17, 2013)
This set of pages, from BBC Schools Bitesize Science, talks about the complete and incomplete
combustion of fuels.
http://science.howstuffworks.com/gasoline1.htm (Accessed Dec. 17, 2013)
How Stuff Works explains what gasoline is, where it comes from and its combustion.
http://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table)
(Accessed Dec. 17, 2013)
This page is the source of ∆Hf (kJ/mol) values for alkanes used by the students.
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