Combustion Reactions

This is part of Year 11 HSC Chemistry course under the topic of Chemical Reactions

HSC Chemistry Syllabus

  • Conduct investigations to predict and identify the products of a range of reactions, for example:
    • Combustion

    Combustion Reactions

    This video discusses combustion reactions.

    What are combustion Reactions? 

    Combustion is an important class of reactions in chemistry which is explored throughout most of the modules. The core of combustion is oxygen reacting with a fuel (generally an organic substance), ultimately releasing energy. 

    There are two predominant types of the combustion of organic substances: 

    • Complete combustion
    • Incomplete combustion

    Importantly, combustion reactions are exothermic; they release heat energy into the surroundings

    Organic substance: carbon containing substance


    Complete vs Incomplete Combustion

    Complete combustion:

    A combustion reaction is considered to be a complete combustion when an organic substance reacts with an abundant supply of oxygen, result in the formation of only carbon dioxide and water. Complete combustions are represented by the general equation

    $$\text{hydrocarbon} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$$

    Achieving complete combustion outside controlled environments, such as laboratories is challenging due to the precise oxygen requirements. 

    The blue roaring flame of a bunsen burner, when its air hole is open, is an example of complete combustion. A practical example includes the reaction of the octane hydrocarbon with oxygen to produce carbon dioxide and water. 


    hydrocarbon: organic substance which consists of hydrogen and carbon atoms. 


    Incomplete combustion:

    Incomplete combustion occurs when an organic substance has a limited oxygen supply to react with. The result of this is the formation of carbon monoxide and/or carbon soot. Incomplete combustion is generally undesirable because:

    1. Carbon monoxide is a colourless, odourless, flammable, and toxic gas.
    2. Carbon soot contains carcinogens.
    3. Incomplete combustion releases less energy than complete combustion

    In incomplete combustion of octanes the products can include varying amounts of any combination of carbon dioxide, carbon monoxide, soot (which is mostly elemental carbon), and/or water.

    The yellow safety flame of a bunsen burner, when its air hole is closed is an example of incomplete combustion.  

    $$\textbf {hydrocarbon} + \textbf {oxygen} \rightarrow \textbf {carbon dioxide } \text {and/or } \textbf {carbon monoxide } \text {and/or } \textbf {carbon soot } \text {and/or } \textbf {water}$$ 



    Example Problem: 

    Given a hydrocarbon with four carbons and ten hydrogens, deduce the equation for its complete combustion reaction: 

    The combustion reaction, on interaction with oxygen is represented as:

    $$2C_4H_{10}(g) + 3O_2(g) \rightarrow 8CO_2(g) + 10H_2O(l)$$


    Metals and Combustion

    Certain metals can undergo combustion. A common metal which combusts is magnesium. The combustion of magnesium is not only highly exothermic, but also produces an exceptionally bright light. On combustion magnesium forms the compound magnesium oxide according to the following equation:

    $$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$$