4.5: Classifying Chemical Reactions - Combustion Reactions
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- Scott Van Bramer
- Widener University
Learning Objectives
- Define combustion reactions
- Identify when a combustion reaction will occur
- Write a balanced chemical equation that describes what happens when a combustion reaction occurs
- Solve stoichometry problems with combustion reactions
Humans interact with one another in various and complex ways, and we classify these interactions according to common patterns of behavior. When two humans exchange information, we say they are communicating. When they exchange blows with their fists or feet, we say they are fighting. Faced with a wide range of varied interactions between chemical substances, scientists have likewise found it convenient (or even necessary) to classify chemical interactions by identifying common patterns of reactivity. This module will provide an introduction to combustion reactions, one of the most prevalent types of chemical reactions.
Combustion Reactions
Combustion reactions are another common class of reactions. These reactions provide the energy to drive your car, heat your home, and cook your food. Anytime something burns, it is a combustion reaction. These reactions are critical to our economy but they are also responsible for emitting 35 gigatons of CO 2 into the atmosphere in 2020. In a combustion reaction a fuel, like methane, reacts with oxygen to produce carbon dioxide and water. Almost all combustion reactions require additional oxygen, which comes from the atmosphere, with the reactants. In a combustion reaction all the carbon in the reactants is converted into carbon dioxide and all the hydrogen in the reactants is converted into water.
Example \(\PageIndex{1}\): Writing Equations for Combustion Reactions
Write balanced chemical equations for the combustion reaction described here:
- the combustion of methane, CH 4
- the combustion of ethane, C 2 H 6
Solution
(a) In addition to methane, CH 4 , the combustion reaction also requires oxygen from the atmosphere. The products of the combustion reaction will be H 2 O and CO 2 .
\[\ce{CH4}(g)+\ce{2O2}(g)\rightarrow \ce{CO2}(g)+\ce{2H2O}(l) \nonumber \]
(b) In addition to ethane, C 2 H 6 , the combustion reaction also requires oxygen from the atmosphere. The products of the combustion reaction will be H 2 O and CO 2 .
\[\ce{2C2H6}(g)+\ce{7O2}(g)\rightarrow \ce{4CO2}(g)+\ce{6H2O}(l) \nonumber \]
Notice that balancing the reaction requires doubling the number of ethane molecules so that the coefficient for oxygen is a whole number.
Exercise \(\PageIndex{1}\)
Write the balanced equation for the combustion of methanol, CH 3 OH
- Answer
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\[\ce{2CH3OH}(l)+\ce{3O2}(g)\rightarrow \ce{2CO2}(g)+\ce{4H2O}(l) \nonumber \]
Combustion in Biological Systems
Similar reactions also occur in biological systems. The overall chemical reaction that occurs in aerobic respiration is basically the same as the combustion of glucose. In biological systems these reactions are broken down into small steps to control the release of energy. The overall reaction for the combustion of glucose is:
\[\ce{C6H12O6}(s)+\ce{6O2}(g)\rightarrow \ce{6CO2}(g)+\ce{6H2O}(l) \nonumber \]
Combustion Analysis
The elemental composition of hydrocarbons and related compounds may be determined via a gravimetric method known as combustion analysis . In a combustion analysis, a weighed sample of the compound is heated to a high temperature under a stream of oxygen gas, resulting in its complete combustion to yield gaseous products of known identities. The complete combustion of hydrocarbons, for example, will yield carbon dioxide and water as the only products. The gaseous combustion products are swept through separate, preweighed collection devices containing compounds that selectively absorb each product (Figure \(\PageIndex{1}\)). The mass increase of each device corresponds to the mass of the absorbed product and may be used in an appropriate stoichiometric calculation to derive the mass of the relevant element.
Example \(\PageIndex{2}\): Combustion Analysis
Polyethylene is a hydrocarbon polymer used to produce food-storage bags and many other flexible plastic items. A combustion analysis of a 0.00126-g sample of polyethylene yields 0.00394 g of CO 2 and 0.00161 g of H 2 O. What is the empirical formula of polyethylene?
Solution
The primary assumption in this exercise is that all the carbon in the sample combusted is converted to carbon dioxide, and all the hydrogen in the sample is converted to water:
\[\mathrm{C_xH_y}(s)+\ce{excess\: O2}(g)\rightarrow x\ce{CO2}(g)+ \dfrac{y}{2} \ce{H2O}(g) \nonumber \]
Note that a balanced equation is not necessary for the task at hand. To derive the empirical formula of the compound, only the subscripts x and y are needed.
First, calculate the molar amounts of carbon and hydrogen in the sample, using the provided masses of the carbon dioxide and water, respectively. With these molar amounts, the empirical formula for the compound may be written as described in the previous chapter of this text. An outline of this approach is given in the following flow chart:
\[\mathrm{mol\: C=0.00394\:g\: CO_2\times\dfrac{1\:mol\: CO_2}{44.01\: g/mol}\times\dfrac{1\:mol\: C}{1\:mol\: CO_2}=8.95\times10^{-5}\:mol\: C} \nonumber \]
\[\mathrm{mol\: H=0.00161\:g\: H_2O\times\dfrac{1\:mol\: H_2O}{18.02\:g/mol}\times\dfrac{2\:mol\: H}{1\:mol\: H_2O}=1.79\times10^{-4}\:mol\: H} \nonumber \]
The empirical formula for the compound is then derived by identifying the smallest whole-number multiples for these molar amounts. The H-to-C molar ratio is
\[\mathrm{\dfrac{mol\: H}{mol\: C}=\dfrac{1.79\times10^{-4}\:mol\: H}{8.95\times10^{-5}\:mol\: C}=\dfrac{2\:mol\: H}{1\:mol\: C}} \nonumber \]
and the empirical formula for polyethylene is CH 2 .
Exercise \(\PageIndex{2}\)
A 0.00215-g sample of polystyrene, a polymer composed of carbon and hydrogen, produced 0.00726 g of CO 2 and 0.00148 g of H 2 O in a combustion analysis. What is the empirical formula for polystyrene?
- Answer
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CH
Summary
Chemical reactions are classified according to similar patterns of behavior. A large number of important reactions are included in three categories: precipitation, acid-base, and combustion. Precipitation reactions involve the formation of one or more insoluble products. Acid-base reactions involve the transfer of hydrogen ions between reactants.
Glossary
- combustion analysis
- gravimetric technique used to determine the elemental composition of a compound via the collection and weighing of its gaseous combustion products
- combustion reaction
- vigorous redox reaction producing significant amounts of energy in the form of heat and, sometimes, light
Contributors and Attributions
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Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110 ).