Physical Science
Name_____________________________________________________ Date__________
Modeling the Combustion of Fuels
Objectives
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Balance chemical equations by rearranging the atoms of reactants to make the products.
Demonstrate the concept of conservation of mass in chemical reactions.
Explain how one gallon of gasoline (6 lbs.) can produce 19 lbs. of carbon dioxide.
Introduction
Combustion is a chemical reaction that produces heat and light. The most common form of combustion is fire. Most
forms of combustion happen when O2 oxygen gas joins with a carbon based substance. For example, when wood burns,
oxygen in the air joins with carbon in wood.
Fuels are chemicals called hydrocarbons – substances composed mostly of hydrogen and carbon. Fuels react with O2
oxygen to produce CO2 carbon dioxide and H2O water. Familiar examples of fuels include propane and octane (gasoline).
Fuel emissions are the products of combustion reactions and are released in to the atmosphere. The main human
activity that emits CO2 is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation. Annually
the United States emits 543,3000 lbs. of CO2 into the atmosphere. In fact for every gallon of gasoline burned 19 lbs. of
CO2 is produced. How can 6 lbs. of gasoline (one gallon) produce 19 lbs. of carbon dioxide?
1. What is a combustion reaction reaction?__________________________________________________________
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2. What are the reactants in a combustion reaction? __________________________ +_______________________
3. What are the products in a combustion reaction? ___________________________+_______________________
4. Make a prediction: Does the statement: “One 6 lbs. (1 gallon) can produce 19 lbs. of carbon dioxide” violate the
law of conservation of mass? Why or why not?
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Modeling Activity
Equipment:
A molecular modeling kit with the following color atoms:
__________________ with 4 holes (carbon)
__________________ with 2 holes (oxygen)
__________________ with 1 hole (hydrogen)
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For single bonds (one bond connecting 2 atoms), use the wooden pegs.
For double or (two or three bonds connecting the same 2 atoms), use the bendy springs.
All holes (bonds) in each atom need to be filled.
You can add single, or double bonds between atoms.
Every stick, or spring represents a pair of electrons shared in a covalent bond.
Hydrogen only needs two electrons, not a full octet, and no lone pair electrons
Building Models of Molecules and Balancing Combustion Equations.
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You will be provided the balanced formula for the reactants.
Use the formula to build models of the reactants.
Use the models of the reactants to build the products.
Use only the atoms from the reactants to build the products.
Complete the chemical equation for each type of fuel.
Model the following combustion reactions. Count the product molecules to complete the balanced equations.
Combustion of Methane (fuel)
CH4 + 2O2  ______ + ______
Combustion of Ethane (fuel)
2C2H6 + 7O2  ______ + ______
Combustion of Propane (fuel)
2C3H8 + 5O2  ______ + ______
Combustion of Pentane (fuel)
C5H8 + 8O2  ______ + ______
Combustion of Methanol (alcohol)
CH3OH + 5O2  ______ + ______
Combustion of Ethanol (alcohol)
C2H5OH + 5O2  ______ + ______
Bonus Section
Combustion of Glucose (sugar)
C2H5OH + 5O2  ______ + ______
Combustion of TNT (High Explosive)
C3H6N6O6  ______ + ______ + ______
Analysis Questions
It seems impossible that a gallon of gasoline, which weighs about 6.3 pounds, could produce 19 pounds of carbon
dioxide (CO2) when burned. However, most of the weight of the CO2 doesn't come from the gasoline itself, but the
oxygen in the air. When gasoline burns, the carbon and hydrogen separate. The hydrogen combines with oxygen to form
water (H2O), and carbon combines with oxygen to form carbon dioxide (CO2).
1. Where did the carbon atoms in the CO2 come from?_______________________________________________
2. Where did the oxygen atoms in the CO2 come from? _______________________________________________
3. Where did the hydrogen atoms in the H2O come from?_____________________________________________
2
Burning gasoline
+ 25
 18
+ 16
2C8H18 + 25O2  18H2O + 16 CO2
How is it possible that 6 lbs. of gasoline (one gallon) of can produce 19 lbs. of carbon dioxide? Gasoline consists mostly
of octane C8H18. Octane reacts with oxygen O2 to produce water and carbon dioxide. A carbon atom has an atomic mass
of 12 (6 protons and 6 neutrons), and each oxygen atom has a weight of 16 (8 protons and 8 neutrons). This means that
each single molecule of CO2 has an atomic mass of 44 (12 from carbon and 32 from oxygen). Where is most of the CO2
mass coming from?
4. How is it possible that 6lbs (one gallon) of gasoline can produce 19 lbs. of carbon dioxide?
5. Does the statement: “One gallon of gasoline (6 lbs.) can produce 19 lbs. of carbon dioxide” violate the law of
conservation of mass? Why or why not?
6. How do balanced chemical equations demonstrate the law of conservation of mass?
7. Why is it important to represent chemical reactions as balanced equations?