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.
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.
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.
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.