- Combustion is a chemical reaction that involves burning the organic compounds
- This chemical reaction converts organic compound into carbon dioxide and water with the release of heat energy.
- Heat of combustion is the amount of energy released when one mole of a substance is burned.
- Combustion reaction is an important process in our daily life as we use hydrocarbons as source of energy for domestic and industrial processes.
- Combustion of natural gas and gasoline combustion are typical examples of this chemical reaction.
Alkanes are saturated hydrocarbons with central carbon atom attached to four other atoms (or groups). This saturation leads to a relatively low reactivity of alkanes. Traditionally in old times, saturated hydrocarbons were known as paraffins meaning “little affinity”. However, these alkanes burn very rapidly. The combination of alkanes with oxygen generating heat is known as combustion. More precisely, combustion is defined as “a chemical reaction with oxygen in which alkane is converted into carbon dioxide and water with the release of heat energy”.
Heat of Combustion or Enthalpy
A chemical reaction is a mean for breaking the old bonds and forming new bonds. The breaking of old bonds requires some energy. This is called energy of activation. The reactant molecule must acquire or possess this energy to start the chemical. In case of combustion, slight ignition or electric spark can provide this initial energy. The heat generation in combustion reaction is because: the energy required breaking old bonds (bonds of CH4 & O2) is less than the energy released by the formation of new bonds (H2O & CO2). Hence the system releases the energy in the form of heat. The heat released on complete combustion of one mole of a substance is called heat of combustion and the reaction is called Exothermic.
ΔHo = Hoproducts - Horeactants
Here H is the heat content or enthalpy of a substance in its standard state (at room temperature and at atmospheric pressure). In an exothermic reaction, enthalpy of product is less than starting reactants and thus reaction generates heat energy and ΔHo is negative. SI unit for heat energy & enthalpy is Kilojoule/mole (kj/mol).
Combustion of larger hydrocarbons
Data about heat of combustion for various saturated hydrocarbon is available suggesting that with increasing number of carbon atoms in a molecule, heat of combustion increases. This is obviously due to more carbon available for burning and more number of bonds undergoing changes. Equation 1 & 2 demonstrates this where methane, a single carbon molecule generates less heat energy compared to butane with four carbon atoms generating more heat energy.
However, the larger hydrocarbons with increasing carbon chain are harder to ignite. This is because the bigger molecules don't vaporize easily. The combustion reaction is facilitated if the oxygen and the hydrocarbon are well mixed as gases. Bigger molecules have greater Vander-Waals attractions that make it difficult for them to break away from their neighbors and turn to a gas.
Colour of flame
The burning of hydrocarbon-oxygen mixture can be complete or incomplete. The two cases depend upon the size of hydrocarbon and oxygen supply. In case of a complete combustion, hydrocarbons will burn with a blue flame while yellow flame shows incomplete combustion. Also, the combustion will be less complete as the number of carbon atoms in the molecules increases. That means that the bigger the hydrocarbon, the more likely you are to get a yellow, smoky flame. Incomplete combustion (where there is not enough oxygen present) can lead to the formation of carbon particles or carbon monoxide. As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets the left over. The presence of carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke.
Relative stability of hydrocarbons
The values of heat of combustion offer an important assessment in understanding the relative stability of isomers. Different compounds with exactly same molecular mass but different structures are called isomers. This structural difference means they have different physical and chemical properties. They might have same number of carbon, hydrogen or oxygen (or other element) atoms; however, these atoms are attached in a different way or at different locations. Figure 1 shows different isomers of octane molecules. These isomers have same molecular mass but different structures because methyl groups are attached in different way in each molecule. Heat of combustion for each isomer of the same octane compound is slightly different. The amount of heat energy released is decreasing with increasing branching where the highly branched isomer 2,2,3,3-tetramethyl butane generates low energy. We may put this heat of combustion in a potential energy diagram. Potential energy is an exclusive energy that molecules possess for its kinetic motion. Since these isomers undergo same combustion reaction and produces same number of moles of water and carbon dioxide, the difference in heat generated can be translated to their different potential energy. A molecule with more potential energy is less stable while the molecule with less potential energy (or less heat of combustion generated) is more stable. Hence in the example of octane, unbranched octane is less stable than its branched isomers.
Extra stability of branched molecule is due to increased intramolecular forces where more methyl groups are in vicinity and are interacting with each other via dipole-dipole interactions. These interactions stabilizes more branched molecule compared to a straight chain molecule where no such dipole-dipole interaction is in action.
Practical scenarios of Combustion reaction
Combustion reaction is an important feature in our everyday life. Our domestic cookers, heating homes and engine moving vehicles all involve combustion of hydrocarbons. We need heat energy for our daily works and hydrocarbons are the raw material used to supply us the required heat energy. Combustion of natural gas in our domestic cooker is a simple example of where methane gas is ignited with an electric spark or phosphorus stick. Similarly other gaseous hydrocarbons e.g. ethane, propane or butane are used for different operations.
Crude oil provides us gasoline or petrol, kerosene and diesel to move our vehicles. Gasoline chemically a mixture of octane, isooctane and other hydrocarbons used for light transport vehicles (LTV).
Internal Combustion engine
Combustion of gasoline-air mixture (or other hydrocarbon fuel) inside the engine generates gases. These hot gases drive the piston of engine as they expand with increasing temperature. This is called internal combustion ignition. The reaction of this reaction is highly complex and seems to follow a free radical mechanism. Octane is an eight carbon chain molecule and its combustion is highly exothermic. However, the explosion of this reaction in the cylinder generates knocking which reduces the engine power. To overcome this problem, octane is replaced or mixed with more branched isomers e.g. isooctane. Knocking can also be solved by adding anti-knocking agent. Tetraethyl lead [(C2H5)4Pb] has been in use for years as a popular anti-knocking agent until recently environmental concerns have been raised about increasing lead accumulations. Leaded Gasoline is known fuel at gas stations. During combustion reaction, lead is oxidized to lead oxide and it is believed that series of chain breaking reactions take place at surface of these tiny lead particles hence lowering the knocking problem and improving the engine performance.
Combustion reaction for a chemical analysis
Combustion reaction is also used for the detection and quantification of different elements present in a molecule. The unknown compound is burnt under in a controlled chamber and the gases evolved are analyzed. Sodium fusion test is popular to detect other elements e.g. halogens, sulfur & nitrogen. The combustion with sodium metal converts halogens (or S & N) into their inorganic salts that can be detected in subsequent chemical test to confirm the presence of halogen, sulfur & nitrogen.
Amount of carbon & hydrogen can be quantified using combustion train. This analytical tool measures the amount of carbon dioxide and water produced by burning organic compound. The experiment is carried out in closed chamber fitted with traps containing strong base (e.g. sodium hydroxide) and drying agent (e.g. magnesium perchlorate). The water formed as a result of combustion reaction is absorbed by drying agent while strong base absorbs carbon dioxide. The difference in initial and final weights of collecting tubes gives an accurate amount of carbon dioxide and water produced by burning the organic compounds under study. Similarly nitrogen trap can also be placed. This is called elemental composition analysis. Other elements e.g. halogens can also be analyzed in a similar way.
Books for further study
- Morrison, R. T., and R. N. Boyd. "Organic chemistry 5th edition." (1987).
- Cary, A, F. Organic Chemistry, 3rd edition, (1996).
- Volhardt K, P, C. Organic Chemistry, (1987).
- Smith M, B and March, J. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th edition (2001).