Combustion Reactions

Name: Wade Green

Title of lesson: Combustion Reactions

Date of lesson: TBD

Length of lesson: 1 hour

Description of the class:

Name of course: Chemistry

Grade level: 11

Honors or regular:

Source of the lesson:

General Chemistry 6^th Ed., Whitten, Davis, and Peck

TEKS addressed:

112.45 (C.)

(2)  Scientific processes. The student uses scientific methods during field and laboratory investigations. The student is expected to:

(C)  express and manipulate chemical quantities using scientific conventions and mathematical procedures such as dimensional analysis, scientific notation, and significant figures.

(8)  Science concepts. The student knows how atoms form bonds to acquire a stable arrangement of electrons. The student is expected to:

(A)  identify characteristics of atoms involved in chemical bonding;

(11)  Science concepts. The student knows that balanced chemical equations are used to interpret and describe the interactions of matter. The student is expected to:

(A)  identify common elements and compounds using scientific nomenclature;

(B)  demonstrate the use of symbols, formulas, and equations in describing interactions of matter such as chemical and nuclear reactions; and

(C)  explain and balance chemical and nuclear equations using number of atoms, masses, and charge.

I. Overview

A combustion reaction occurs when an organic compound is oxidized in an exothermic reaction that produces CO₂ and H₂O. This lesson is a lecture/demo to introduce the Chemistry student to the characteristics of a combustion reaction and balancing its components.

II. Performance or learner outcomes

Students will be able to:

1. Recognize a combustion reaction by its reactants and products.
2. Balance a combustion reaction equation given only the organic reactant.

III. Resources, materials and supplies needed

1. 1 gallon container of water

2. 2.5 gallon container of water

3. Bunsen burner or propane torch

IV. Supplementary materials, handouts.

1. Combustion reactions worksheet (1 per student)

Five-E Organization

Teacher Does Probing Questions Student Does

Engage: Using a gallon container of water as a reference, ask the students to make an educated guess as to the weight of a gallon of gasoline. Pass around a sealed gallon container of water. For comparison, hand the students a container with 20 lbs. of water (a little less than 2 ½ gallons).

How much do you think a gallon of gasoline weighs? Does gasoline weigh more or less than water? If students are unable to correctly relate the weight of water to gasoline, relate this to the densities of each and which floats on which. The weight of gasoline depends on the blend of hydrocarbons and additives in a particular sample as well as atmospheric conditions, but the range is about 5.8 to 6.5 lbs. Would it surprise you to learn that the government says that for every gallon of gasoline burned, 19.6 pounds of CO2 are released into the atmosphere? How is it possible that roughly 6.5 lbs. of gasoline makes almost 20 lbs. of CO2?

Some students will probably know that a gallon of water weighs about 8 lbs. (actually 8.33 lbs.) Students will likely know that gasoline is less dense than water and therefore weighs less. Students will likely be surprised by the weight disparity. Advanced students may correctly guess that the greater mass of CO2 is due to the oxygen contribution to the combustion reaction.

Explore: Combustion is an exothermic reaction where a carbon compound is oxidized (reacted with O2(g)) where the products are CO2 and H2O. “If we say that the carbon-containing compound (in the case of gasoline, mostly C8H18) is the fuel, the oxygen in the form of O2 gas is acting as the other reactant in the reaction.” “The heat in the fire triangle provides the energy of activation to get the reaction started. Once it is started, the reaction is VERY exothermic, hence the heat output from a fire.” Light a Bunsen burner or propane blow-torch and adjust for a hot, blue flame. “The flame you see isn’t producing any smoke that we can see unless we adjust the flame for a yellow flame.” Adjust the flame for a smoky, yellow flame. “To make the burner create smoke, I have to adjust the oxygen/fuel mixture so that not enough oxygen is available for the fuel present. This produces a rough, low-temperature flame and smoke. The smoke is a sign of incomplete combustion. It is particles of unburned fuel. In this case, the fuel is natural gas, CH4 (or propane, C3H8 as appropriate).” If the combustion reaction is perfectly balanced, the products are CO2 and H2O, both in gas form and invisible. “The chemical reaction for combustion or burning of methane or natural gas is: CH4 + 2O2 →CO2 + 2H2O” Write this on the board. Because the oxygen comes from the surrounding atmosphere, it is said to be in excess. This means that when writing a balanced combustion reaction, the oxygen will usually not be given. You must look at the carbon compound in the reactants and figure out how much O2 is needed for complete combustion. This can be a little tricky, but there are a few hints to use. Look at the number of carbons in the reactant compound first. The CO2 product is the only place for these carbons to go, so the coefficient for the CO2 needs to be the same or a multiple of the number of carbons in the reactant compound. Next, balance the hydrogens. There is only one place for hydrogen on either side of the equation. Finally, balance the oxygens. If necessary, readjust the other components to maintain balance. You may find references in books or on the internet showing ½ O2 as a reactant. This is acceptable as long as you recognize that you could also simply double the amounts of everything else and get a whole number coefficient for oxygen.

What do you already know about combustion? What is required to make a fire? If we look at fire as a chemical reaction, what is the product? What comes out of a fire? Can you describe how to adjust the burner for optimum flame? What is actually being adjusted? You may be asking, “but doesn’t water put out a fire?” How does water put out a fire? What is it doing? Note that the equation is balanced, but where does the oxygen come from for most fires? Give an example on the board: _C2H6+_O2→_CO2+_H2O Have the students lead through the balancing steps. 2C2H6+7O2→4CO2+6H2O

Students should remember the fire triangle and know that fuel, oxygen, and heat are required. Some students will think that smoke is the product of a combustion reaction. Students should know that the adjustment changes the color of the flame and that a blue cone represents the hottest flame. Students may recall that the oxygen/fuel ratio is being changed. Students should know that water smothers the fire and removes the oxygen supply. To a lesser extent, it also cools the fire to remove the heat. Students should recognize that the oxygen needed comes from the surrounding atmosphere. Students should be able to balance this equation.

Explain: In class worksheet: Combustion Reactions Worksheet

What if you were asked to write a combustion reaction for ethanol, C2H5OH?

Students may not recognize that the products are the same for any complete combustion reaction containing only C, H, and O and that the inclusion of O2 in the reactants is implied by the condition of a combustion reaction.

Extend / Elaborate: Back to the gasoline example. As a class example, balance the combustion of octane, C8H18. For homework, assign the students to write a balanced equation for the combustion of octane and then perform dimensional analysis converting 1 lb. of octane into __ lbs. of CO2. The answer is provided at the end of this lesson and should be reviewed with the class the next day.

Now, for our earlier example using gasoline, can you explain how 6.5 lbs. of gasoline becomes ~20 lbs of CO2?

Students should respond that the O2 reacting with gasoline to form CO2 adds the additional mass.

Evaluate:

Students will be given an end of unit test where they must recognize types of reactions and balance them accordingly.

Name_________________________

Date__________________________

Combustion Reactions Worksheet

Balance the following combustion reactions in the presence of excess oxygen.

1. Propane

_____C₃H₈ + _____O₂ → ______CO₂ + _______H₂O

2. Butane

_____C₄H₁₀ + _____O₂ → ______CO₂ + _______H₂O

3. Methanol

_____CH₂OH + _____O₂ → ______CO₂ + _______H₂O

4. Cyclohexane

_____C₆H₁₂ + _____O₂ → ______CO₂ + _______H₂O

Combustion Reactions Worksheet Answer Key

Balance the following combustion reactions in the presence of excess oxygen.

1. Propane

1 C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

2. Butane

2 C₄H₁₀ + 13 O₂ → 6 CO₂ + 10 H₂O

3. Methanol

1 CH₂OH + 1 O₂ → 1 CO₂ + 1 H₂O

4. Cyclohexane

1 C₆H₁₂ + 9 O₂ → 6 CO₂ + 6 H₂O