Combustion is a rapid chemical reaction of a carbon fuel with oxygen and is usually complemented by the production of light and heat in the form of a flame. It is one of the most fundamental chemical reactions even considered as a culminating step in the oxidation of a variety of substances. Combustion incorporates a great range of phenomena with extensive applications in industry, the sciences, and professions. Given the significance of combustion for energy supply and transport, an improved understanding of combustion is strongly desired and thus, this process will be thoroughly investigated. However, combustion systems are challenging to simulate due to the interaction of complex chemical reactions, transport phenomena, turbulence and radiation effects. Therefore, during this experimental investigation, a narrower perspective of combustion is explored where four variant fuels (differing in the number of carbon atoms) are tested on their enthalpy of combustion to determine the most effective fuel through a combination of experimentation and theoretical enthalpy mathematics.
The fuels used in this investigation are of the alkanol group which are known for their high melting and boiling points. Alkanols, commonly known as alcohols, are hydrocarbon-derived organic compounds (compounds comprising of hydrogen and carbon atoms) that feature a hydroxy functional group (-OH). This hydroxy functional group is bonded to saturated carbon atoms in these organic compounds.
These alcohols are produced from reacting water with the alkenes – called hydration – or through anaerobic respiration. These substances are specifically branched under biofuels, a fuel derived from living matter (biomass) through contemporary biological processes such as hydration, rather than through geological processes such as those involved in fossil fuel formation. Bioenergy is energy derived from biofuels, which are subdivided into two categories: primary and secondary biofuels. Primary biofuels such as wood chips and organic materials are used in an unprocessed form for cooking, heating or electricity production. Secondary biofuels, in which this investigation will be primarily focused on, result from the processing of biomass and include the liquid biofuels that can be used in vehicles and industrial processes such as ethanol and biodiesel. The energy sources particularly used in Australia are biodiesel, bioethanol and biogas.
Figure 1 A biofuel, biodiesel.
Figure 1 shows a biodiesel with 16 carbons and an ester group. An ester group is a chemical compound in which at least one -OH group is replaced by an O-alkyl group, which are derived from carboxylic acids.
Conversely, bioethanol does not have an ester group but the hydroxy functional group, -OH (figure 2).
Figure 2 A biofuel, bioethanol.
The difference between the two compounds can be seen in table 1.
Property Ethanol Butanol Biodiesel
Octane (average) 99.5 97 (cetane) 50-65
Energy Density (MJ/kg) 20 30 38
Table 1 The octane and energy density of ethanol, butanol and biodiesel.
The density of fuel indicates the amount of energy stored in a region of space per unit volume (power output), thus a high energy density for a fuel is preferable. Octane rating is the standard measure of the performance of an engine. More specifically, it is the measure of a fuel’s ability to resist “pinging” or “knocking” during combustion where the higher the octane number, the greater the fuel’s resistance to pinging during combustion. Knocking or pinging is undesirable and occurs when there is a premature combustion in the cylinder in an internal-combustion engine. Petrol used now can be classified with different octane ratings, with unleaded gasoline having regular octane ratings of 87, midgrade having an octane rating of 88 to 89 and premium with octane levels of 91 – 94. Using lower octane fuel than what is recommended causes damage to the engine and the emissions control system causing the car to run poorly. Ethanol has a much higher-octane rating as shown by table 1 with a rating of about 99.5. Thus, refiners blend ethanol with gasoline to help boost gasoline’s octane rating, usually using a blend of 10% ethanol.
It is shown that as the carbon chain increases, the octane rating decreases. However, the density of the fuel increases. Thus, researching compounds with longer carbon chains may be beneficial due to their high densities as shown by research and will be shown through this investigation. A conclusion to this will be made in the discussion.
The specific chemicals used in this investigation are constituents of a homologous series of linear alcohols where they all share similar chemical structures and properties. The ten consecutive members of this homologous series are methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanal and decanal.
In an excess supply of oxygen, the alkanols undergo complete combustion to form the products carbon dioxide and water as shown through equation :
Equation I The chemical equation of the combustion of octanol.
Incomplete combustion occurs in a restricted oxygen supply where along with producing carbon dioxide and water, carbon monoxide and carbon (soot) is also generated (Eqn ).
Equation I The chemical reaction of the incomplete combustion of octanol.
Providing the right conditions with the sufficient supply of air and optimal hydrocarbon-air mixing ratio, complete combustion is achieved. A blue flame indicates complete combustion and a yellow or mauve flame denotes incomplete combustion. Incomplete combustion is unfavourable due to it releasing less energy than complete combustion and produces the poisonous gas, carbon monoxide.
The general structural formula for alcohols is as follows: CnH2n+1OH where n is the number of carbon atoms. As the number of carbons increases down the homologous chain of linear primary alcohols, a CH3 group is being added into the alcohol chain. As a result, the boiling points, heat of combustion and other characteristics increase. Table 2 shows the overview of C1 to C8 alcohols.
Table 2 The chemical composition and oxygen content of alcohols.
Seen by the graph, it is important to note that there is a decreasing percentage of oxygen content as the chain of carbons increase down the linear alcohols in relation to its mass.
The combustion of these alkanols is known to be an exothermic process as opposed to an endothermic reaction where instead of heat taken in, heat is released from the reaction. By convention, exothermic reactions (such as combustion reactions that give out heat) are assigned negative enthalpy (?H) values. This can be seen in figure 3.
Figure 3 Two graphs showing the potential energy against the reaction progress.
Both graphs show the reaction progress against enthalpy, the internal energy of the system. Graph A represents an endothermic reaction, such that heat is absorbed into the system shown by the energy of the products being higher than the reactants. However, the processes in this investigation is best shown through Graph B, where energy has left the system making it an exothermic reaction. Here, enthalpy change, denoted by H, is negative as opposed to graph A with a positive enthalpy change. The change in enthalpy will equal to the heat consumed or released by the system calculated through the formula:
where Q is the amount of heat energy (J)
m is the mass of water (g)
c is the specific heat capacity of water = 4.18 J/g oC
T is the temperature change of the water (oC)
The endothermic breaking of bonds begins with the ignition of the lighted spirit of the spirit burner (figure 4), providing the activation energy, the energy which must be available to a chemical system with potential reactant to result in a chemical reaction.
This is a self-perpetuated combustion which is followed by the exothermic formation of both water and carbon dioxide as shown by the products of equation I.
When a hydrocarbon fuel such as the ones listed above combusts, the C-C, C-H, C-O, O-H and O=O bonds are broken and C=O and H-O bonds are then formed. Figure 5 shows the combustion reaction of ethanol illustrating the bonds involved.
Figure 5 The combustion reaction of ethanol in bond form.
In a chemical bond, bond energy (E) or bond enthalpy (H) is the measure of bond strength, particularly, the amount of energy stored in a bond between atoms. The breaking and making of bonds are involved in all chemical reactions, in which the bonds break (figure 6) in reactants and reform to make the products.
Figure 6 The breaking of bonds.
The higher the bond enthalpy, the more energy is required to break the bond due to its greater strength. This also correlates with bond order and bond length, where the higher the bond order, and thus, the shorter the bond length, the stronger the electric attraction between the atoms. Bond order is the number of bonding pairs of electrons between two atoms, where a single bond has an order of one, a double bond has a bond order of two and a triple bond has a bond order of three etc. A high bond order is indicative of a greater attraction between the atoms, and thus the shorter the bond length – the distance between the centers of two covalently bonded atoms. This all correlates with bond energy.
These values are quite useful and thus, average bond enthalpies for common bond types are readily available as shown in reference table 3.
The amount of energy released to form the reactants: H-O and C=O bonds surpasses the amount of energy it takes to break the products: C-C, C-H, C-O, O-H and O=O bonds as shown by the table of bond energies. This implies that heat is given off to the surroundings calculating enthalpy (?H) to be negative, and therefore indicates that the reaction is exothermic as mentioned above.
Average Bond Energies (kJ/mol)
Single Bonds Multiple Bonds
C-C 348 O=O 495
C-H 413 C=O 799
Table 3 The table of bond energies used for this investigation.
As chain length increases, it is known that the number of C-C bonds increases proportionality with the number of C-H bonds. This results in more carbon dioxide and water to be formed in relation to the increasing amount of C-C and C-H bonds. The bond formation of C=O bonds in carbon dioxide releases much more energy specifically having an average bond enthalpy of 799. Through the general molecular equation for a reaction:
As number of carbons increase, the CO2 bonds made also increase in a direct proportional relationship to achieve a balanced equation, as indicated by subscript . Thus, longer chain length alcohols will release more energy and thus have a lower enthalpy value than an alcohol containing one carbon. This will be caused by this subsequent increased formation of carbon dioxide and water molecules increasing the negativity of the enthalpy change.
Due to the wide variety of alcohols to use, the consecutive even members that is – ethanol, butanol, hexanol and octanol – are used to study the effect of the length of the carbon chain on the enthalpy of combustion.
The aim of the investigation was to determine to what extent increasing the length of the carbon chain of alcohol fuels would affect their respective enthalpy of combustion.
Hypothesis: It was hypothesised that as the number of carbon atoms in a linear alcohol increases, the enthalpy of combustion will also increase. Thus, octanol, with the longer carbon-chain is expected to have the highest enthalpy of combustion value due to the greater energy yield from the increased formation of carbon dioxide and water compared to ethanol which should have the lowest enthalpy of combustion.