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4.18: Classifying Chemical Reactions: Combustion Reactions

  • Page ID
    217479
  • Learning Objectives
    • Define combustion reaction.
    • Identify the unique characteristic of a combustion reaction.

    combustion reaction is defined as the process of burning a chemical in the presence of molecular oxygen to form carbon dioxide, water, and energy.  The unique characteristic of a combustion reaction is that energy, which is abbreviated as "E," is produced during the reaction.  Furthermore, a combustion reaction requires that molecular oxygen, O2, participate in the reaction as a reactant and that carbon dioxide, CO2, and water, H2O, be generated as products.  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 previously-discussed reaction classifications, which were best represented using generic patterns. 

    The following equation represents the combustion of heptane, C7H16, an organic solvent that is commonly-utilized to dissolve grease and other carbon-based materials.  The definition of combustion can be divided into short words or phrases that each have a corresponding symbolic representation in the reaction that is shown below.  The chemical that is being burned in this reaction is heptane, C7H16.  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, in order to signify their generation as individual products.

    \(\ce{C_7H_1_6} \left( l \right) + \ce{11 O_2} \left( g \right) \rightarrow \ce{7 CO_2} \left( g \right) + \ce{8 H_2O} \left( g \right) + \ce{E}\)

    Finally, the balancing coefficients that are indicated in this equation are written in order to uphold the Law of Conservation of Matter, which, as stated in Section 4.12, mandates that particles cannot be created or destroyed in the course of a chemical reaction.  The process through which these coefficients are determined will be described in a later section of this chapter.