# 10.8: Combustion Reactions: Review


Learning Objectives
• Define combustion reaction.
• Write a balanced chemical equation that represents the combustion of an organic molecule.

The previous six sections of this chapter identified and defined the five types of structural notations in which an organic molecule can be symbolized and, furthermore, described the compositional characteristics of a substance that can be determined by analyzing its corresponding molecular formula and expanded, VSEPR, condensed, and bond-line pictures.  Subsequently, multiple molecular structures were provided and used as reference images from which alternative structural representations were derived.  While the symbols of heteroatoms or multiple bonds were incorporated into a small number of these structures, the molecules that are represented by these pictures are largely comprised of carbon/carbon and carbon/hydrogen single bonds and, therefore, are classified as alkanes.  As stated in Section 10.1, because alkanes contain a limited combination of elements and bonds, the utility of this class of organic molecules is severely restricted.  Specifically, due to their distinctive electron distributions, these hydrocarbons are immiscible in water and, consequently, are often used as solvents in which other water-insoluble organic molecules can be dissolved.

Furthermore, the types of elements and bonds that are present in an organic molecule dictate not only the physical properties of that substance, but also its chemical reactivity.  Consequently, alkanes, which are comprised of only two elements that are singly-bonded to one another, participate in very few chemical reactions.  As a result, hydrocarbons are primarily used as fuels and, therefore, are burned to produce energy.  This type of chemical change is classified as a combustion reaction, which is defined as the process of burning a chemical in the presence of molecular oxygen, O2, to form carbon dioxide, CO2, water, H2O, and energy, E.  As the only variable component of a combustion reaction is the chemical that is being burned, this type of reaction is highly-specific, relative to the other types of reactions that have been discussed, which were best represented using generic patterns.

For example, write a balanced chemical equation that represents the combustion of isooctane, C8H18, which is the organic molecule that is used as a reference substance for determining the "octane rating" of gasoline.  (States of matter are not required.)

As stated above, a combustion reaction is defined the process of burning a chemical in the presence of molecular oxygen to form carbon dioxide, water, and energy.  This definition can be divided into short words or phrases that each have a corresponding symbolic representation that can, in turn, be incorporated into a reaction equation.  The chemical that is being burned in this reaction is isooctane, C8H18.  The phrase "in the presence of" indicates that an additional chemical must be present as a reactant.  Therefore, a "+" must be written on the left side of the reaction arrow, $$\rightarrow$$, which is, in turn, verbally indicated by the words "to form."  The identity of the remaining reactant, as specified in the definition of combustion, is molecular oxygen, O2.  Finally, the symbolic representations of carbon dioxide, CO2, water, H2O, and energy, E, must each appear on the right side of the reaction arrow, separated by plus signs, as shown below, in order to signify that each of these chemicals is generated as an individual product.

___ $$\ce{C_8H_18}$$ + ___ $$\ce{O_2}$$ $$\rightarrow$$ ___ $$\ce{CO_2}$$ + ___ $$\ce{H_2O}$$ + $$\ce{E}$$

Coefficients are incorporated into a chemical equation in order to account for any relative differences between the formulas of the reactants and products that are involved in the corresponding chemical reaction.  Since, energy, E, is not a chemical material, there is no need to associate a balancing coefficient with the energy that is produced in a combustion reaction.

None of the elemental components of this reaction are balanced.  Oxygen is present in both of the chemical products that are generated during this reaction and, therefore, should be the final element that is considered in the balancing process.  In order to balance carbon, C, and hydrogen, H, coefficients of 8 and 9, respectively, are written in the "blanks" that correspond to these elements on the right side of the reaction arrow, as shown below.

___ $$\ce{C_8H_18}$$ + ___ $$\ce{O_2}$$ $$\rightarrow$$ 8 $$\ce{CO_2}$$ + $$\ce{H_2O}$$ + $$\ce{E}$$

Then, in order to balance oxygen, O, a coefficient of 12.5 is written in the "blank" that corresponds to molecular oxygen, O2, on the left side of the reaction arrow, as shown below.

___ $$\ce{C_8H_18}$$ + 12.5 $$\ce{O_2}$$ $$\rightarrow$$ 8 $$\ce{CO_2}$$ + $$\ce{H_2O}$$ + $$\ce{E}$$

Because a fractional coefficient, 12.5, is written in the equation that is shown above, all of the coefficients in this equation, including the unwritten "1" that is understood to occupy the first blank, must be multiplied by 2, in order to cancel this half-fraction.  The doubled coefficient values are reflected in the chemical equation that is shown below.

2 $$\ce{C_8H_18}$$ + 25 $$\ce{O_2}$$ $$\rightarrow$$ 16 $$\ce{CO_2}$$ + 18 $$\ce{H_2O}$$ + $$\ce{E}$$

By multiplying all of the coefficients in this equation by 2, the quantities in which carbon, C, hydrogen, H, and oxygen, O, are present in the equation have changed, but their relative ratios have not.  Therefore, all of the components of this equation are still balanced.  Finally, these coefficients cannot be divided, as they do not all share a common divisor that would result in the calculation of four whole number coefficients.  Therefore, the final equation that is presented above is the chemically-correct representation of the combustion of isooctane, C8H18.

Exercise $$\PageIndex{1}$$

Write a balanced chemical equation that represents the combustion of the organic molecule that is shown below.  (States of matter are not required.)

The given structure, which explicitly shows all of the stick bonds that are located between consecutive pairs of elemental symbols, reflects the two-dimensional connectivity of the atoms that are present in the corresponding organic molecule and, therefore, is drawn using expanded structural notation.  However, because chemical formulas are incorporated into chemical equations, the molecular formula of the molecule that is represented above must be determined before a combustion equation can be developed.  A molecular formula is an abbreviation that summarizes both the identities and quantities of elements that are present in an organic chemical.  In order to determine this information for a molecule that is represented using expanded structural notation, the elemental symbols that are shown, which indicate the types of elements that are found in the corresponding substance, can be counted.  Since the elemental symbol "C" appears twice in the given structure, "H" is written six times, and a single "O" is present, the corresponding molecule contains two carbon atoms, six hydrogens, and one oxygen atom.  Because an organic molecule must be comprised of carbon, C, and usually contains hydrogen, H, these elemental symbols, as well as their corresponding subscripts, are written first and second, respectively, when developing a molecular formula.  Since oxygen, O, is classified as a heteroatom, its elemental symbol is written last.  Therefore, because values of "1" are usually implicitly-understood in chemistry, C2H6O is a chemically-correct molecular formula of the organic substance that is represented by the given expanded structure.

As stated above, a combustion reaction is defined the process of burning a chemical, which, in this Exercise, is symbolized as C2H6O, in the presence of molecular oxygen, O2, to form carbon dioxide, CO2, water, H2O, and energy, E.  Writing the formulas of these molecules on the appropriate sides of a reaction arrow generates the unbalanced equation that is shown below.

___ $$\ce{C_2H_6O}$$ + ___ $$\ce{O_2} \rightarrow$$ ___ $$\ce{CO_2 }$$ + ___ $$\ce{H_2O}$$ + $$\ce{E}$$

Coefficients are incorporated into a chemical equation in order to account for any relative differences between the formulas of the reactants and products that are involved in the corresponding chemical reaction.  Since, energy, E, is not a chemical material, there is no need to associate a balancing coefficient with the energy that is produced in a combustion reaction.

None of the elemental components of this reaction are balanced.  Oxygen is present in both of the reactants that are consumed in, as well as both of the chemical products that are generated during, this reaction and, therefore, should be the final element that is considered in the balancing process.  In order to balance carbon, C, and hydrogen, H, coefficients of 2 and 3, respectively, are written in the "blanks" that correspond to these elements on the right side of the reaction arrow, as shown below.

___ $$\ce{C_2H_6O}$$ + ___ $$\ce{O_2} \rightarrow$$ 2 $$\ce{CO_2 }$$ + 3 $$\ce{H_2O}$$ + $$\ce{E}$$

In order to balance oxygen, O, a coefficient should be written in a "blank" on the left side of the equation, as fewer oxygens are present on this side of the reaction arrow.  However, as stated above, oxygen, O, is present in both of the reactants in this equation.  As the first reactant, C2H6O, also contains carbon, C, and hydrogen, H, which have already been balanced, a coefficient should not be placed in the "blank" that is associated with this molecule.  Instead, a coefficient should be written in the "blank" that corresponds to molecular oxygen, O2.  The value of this coefficient, 3, is determined by first subtracting 1 from the total reactant-side quantity of this element, 7, in order to account for the oxygen, O, that is present in C2H6O, and then dividing the resultant quantity, 6, by the subscript that is present in the chemical formula of molecular oxygen, 2.  This coefficient value is reflected in the chemical equation that is shown below.

___ $$\ce{C_2H_6O}$$ + 3 $$\ce{O_2} \rightarrow$$ 2 $$\ce{CO_2 }$$ + 3 $$\ce{H_2O}$$ + $$\ce{E}$$

Finally, these coefficients cannot be divided because of the unwritten "1" that is understood to occupy the first "blank" in this equation.  Therefore, the final equation that is presented above is the chemically-correct representation of the combustion of C2H6O.
Exercise $$\PageIndex{2}$$

Write a balanced chemical equation that represents the combustion of the organic molecule that is shown below.  (States of matter are not required.)

The given structure, which uses "zig-zag" lines to represent the bonds that exist between consecutive carbon atoms, does not explicitly show the elemental symbols of carbon or hydrogen and, therefore, is drawn using bond-line notation.  However, because chemical formulas are incorporated into chemical equations, the molecular formula of the molecule that is represented above must be determined before a combustion equation can be developed.  A molecular formula is an abbreviation that summarizes both the identities and quantities of elements that are present in an organic chemical.  In bond-line notation, a carbon atom is understood to be located at every point at which two or more bonds meet and on the end of every line that is not explicitly-associated with a heteroatom.  While the given structure is cyclic and, therefore, has no discernable "ends," since there are ten internal places at which consecutive bonds connect with one another, the molecule that is represented above contains a total of ten carbon atoms.  Furthermore, each of these carbons is assumed to be bonded to enough hydrogens to achieve its preferred four-bond configuration.  Because each internal carbon that is represented in this bond-line structure is shown to be bonded to two additional atoms, two hydrogens are also present on each of these carbons.  Therefore, a total of 20 hydrogens are present in the organic molecule that is symbolized by the image that is shown above.  Because an organic molecule must be comprised of carbon, C, and usually contains hydrogen, H, these elemental symbols, as well as their corresponding subscripts, are written first and second, respectively, when developing a molecular formula.  Therefore, C10H20 is the chemically-correct molecular formula of the organic substance that is represented by the given bond-line structure.  These subscripts should not be divided by 10, even though it is mathematically-possible to do so, as the resultant formula, CH2, would not be consistent with the image that is shown above.

As stated above, a combustion reaction is defined the process of burning a chemical, which, in this Exercise, is symbolized as C10H20, in the presence of molecular oxygen, O2, to form carbon dioxide, CO2, water, H2O, and energy, E.  Writing the formulas of these molecules on the appropriate sides of a reaction arrow generates the unbalanced equation that is shown below.

___ $$\ce{C_{10}H_{20}}$$ + ___ $$\ce{O_2} \rightarrow$$ ___ $$\ce{CO_2 }$$ + ___ $$\ce{H_2O}$$ + $$\ce{E}$$

Coefficients are incorporated into a chemical equation in order to account for any relative differences between the formulas of the reactants and products that are involved in the corresponding chemical reaction.  Since, energy, E, is not a chemical material, there is no need to associate a balancing coefficient with the energy that is produced in a combustion reaction.

None of the elemental components of this reaction are balanced.  Oxygen is present in both of the chemical products that are generated during this reaction and, therefore, should be the final element that is considered in the balancing process.  In order to balance carbon, C, and hydrogen, H, coefficients of 10 are written in both of the "blanks" that correspond to these elements on the right side of the reaction arrow, as shown below.

___ $$\ce{C_{10}H_{20}}$$ + ___ $$\ce{O_2} \rightarrow$$ 10 $$\ce{CO_2 }$$ + 10 $$\ce{H_2O}$$ + $$\ce{E}$$

Then, in order to balance oxygen, O, a coefficient of 15 is written in the "blank" that corresponds to molecular oxygen, O2, on the left side of the reaction arrow, as shown below.

___ $$\ce{C_{10}H_{20}}$$ + 15 $$\ce{O_2} \rightarrow$$ 10 $$\ce{CO_2 }$$ + 10 $$\ce{H_2O}$$ + $$\ce{E}$$

Finally, these coefficients cannot be divided because of the unwritten "1" that is understood to occupy the first "blank" in this equation.  Therefore, the final equation that is presented above is the chemically-correct representation of the combustion of C10H20.

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