# Environmental Science and International Politics: Acid Rain in Europe, 1979-1989, and Climate Change in Copenhagen 2009

By David E. Henderson; Susan K. Henderson | Go to book overview

Appendix 3. Quantitative Look at Combustion Reactions
When organic chemicals containing only the elements carbon, oxygen, and hydrogen react completely with oxygen, there are only two products formed: carbon dioxide (CO2) and water (H2O). If you have access to a model kit, build a model for ethane (C2H6). Also, take eight oxygen atoms from your model kit and connect them together with bonds to represent the oxygen molecules. The reaction of ethane with oxygen is described by the general equation:

C2H6+ O2 → CO2 + H2O

Take the model of ethane and the oxygen molecules. Take the ethane apart, and use the parts to construct CO2 and H2O. Now count the number of molecules of products you have made. You should have two molecules of carbon dioxide and three molecules of water. We write the number of each type of atom or molecule to the left of the formula to indicate how many are used in the reaction. These are called the coefficients of the reactants and products.

C2H6+ 4O2 → 2CO2 + 3H2O + O

The problem in the reaction we have just created using our model is that only seven oxygen atoms are needed by the reaction. Because oxygen exists as O2 molecules, this means that we left a halfmolecule of oxygen behind. We would say this reaction is unbalanced because we have some spare atoms that we did not use. This occurs because oxygen always exists as the O2 molecule. We can solve this problem by using a second molecule of ethane. Now the reaction will look like this:

2C2H6+ 7O2 → 4CO2 + 6H2O

When we write the equation for any chemical reaction, we must do so in such a way that all of the molecules on each side come out even. Fractions are not normally allowed. Using models to study this reaction is particularly informative because it make it clear that chemical reactions occur on discrete molecules that we can count and must keep track of.To write a balanced chemical equation for a combustion reaction, follow these three steps:
1. Add a coefficient to the CO2 equal to the total number of carbon atoms in the hydrocarbon.
2. Add a coefficient to the H2O equal to half of the total hydrogens in the hydrocarbon.
3. Count the total oxygen atoms on the right side of the equation. Subtract any oxygen atoms in the hydrocarbon.
 a. If the resulting number is even, divide by 2 and use this as the coefficient of the O2. b. If the number is odd, double the coefficients on all molecules and use the number as the coefficient for the O2.

These three simple steps will allow you to balance any combustion reaction for compounds of carbon, hydrogen, and oxygen. Because we cannot use half-molecules of oxygen, the third step is designed to ensure that the oxygens come out even.

Examples

Example 1: Balancing the Reaction for the Combustion of Decane

Write the reactants and products:

C10H22 + O2 → CO2 + H2O

Balance the carbon:

C10H22 + O2 → 10CO2 + H2O

-172-

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Environmental Science and International Politics: Acid Rain in Europe, 1979-1989, and Climate Change in Copenhagen 2009

• Title Page iii
• Contents v
• Note to Instructors viii
• How to Play These Games 1
• Contents 8
• Figures and Tables 11
• 1 - Historical Background 14
• 2 - The Game 35
• 3 - Roles and Factions 41
• 4 - Core Texts and Supplemental Readings 44
• Bibliography 106
• Acknowledgments 109
• Appendix 1- Introduction to Environmental Philosophy 110
• Appendix 2- Introduction to Environmental Economics 116
• Appendix 3- Using Numbers to Make Arguments 119
• Appendix 4- Study Questions for Reading Assignments 121
• Contents 124
• Figures and Tables 127
• 1 - Historical Background 129
• 2 - The Game 153
• 3 - Roles and Factions 159
• 4 - Core Texts and Supplemental Readings 162
• Acknowledgments 164
• Appendix 1- Green House Gases 165
• Appendix 2- Chemicals in Fossil Fuels 168
• Appendix 3- Quantitative Look at Combustion Reactions 172
• Appendix 4- Leaked Draft Document by Danish Delegates 180
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