Combustion refers to a high-energy chemical reaction in which fuel is oxidized and converted into a mixture of often gaseous products. Combustion is an exothermic reaction, in that it involves the release of energy in the form of light and heat. The most common oxidizing agent in combustion reactions is atmospheric oxygen (O), but other oxidizing agents include: chlorine (Cl), fluorine (F), and nitrous oxide (N₂O) Combustion reaction occur in many places in nature and were among the first chemical reactions that humans harnessed control over (fire).
Combustion reactions are extremely useful technologically, as they can be exploited to produce high amounts of energy that can be used to perform physical work. Several technologies exist for converting heat energy generated during combustion into mechanical energy, such as the internal combustion engine of a car, or into electrical energy, such as an electric power plant.
Basics Of Combustion
Combustion reactions require three main ingredients: reactants (fuel), an oxidizing agent, and heat. By far, the most common kinds of fuel for combustion reactions are hydrocarbon compounds, like methane (CH₄), propane (C₃H₈) or octane (C₈H₁₈). The most common oxidizing agent is atmospheric oxygen. Thus, the majority of examples of combustion we will consider involve the combustion of hydrocarbon compounds in an oxygenated atmosphere.
For example, methane is a common combustible fuel. The general form of the chemical equation for the combustion of methane in oxygen is:
CH₄ + 2O₂ → CO₂ + 2H₂0
Since combustion is an exothermic reaction, one of the products of the reaction is energy in the form of heat. The amount of heat released by a chemical reaction is called the reaction’s enthalpy of change. The enthalpy of change is a measure of how much heat a chemical reaction emits or absorbs from the environment. The standard notation for representing enthalpy is ΔH. The standard SI unit used to measure enthalpy of change is the joule (J). If a reaction is endothermic (absorbs heat), the ΔH will be positive. If a reaction gives off energy (exothermic) then ΔH will have a negative value.
Combustion is always exothermic, so the enthalpy of change for any combustion reaction will always be negative. For example, the enthalpy of change for a methane combustion reaction is ΔH = -891 per kJ/mol. A methane combustion reaction releases 891 kilojoules of heat energy per mole of methane. The energy produced by a combustion reaction comes from the energy stored in the fuel’s chemical bonds that is released when the reactants split apart and are rearranged into the products. Combustion of hydrocarbon reactants is energetic in particular due to the high energy bonds in hydrocarbon compounds oxygen molecules.
Combustion occurs with many other hydrocarbon products, not just methane. The products of a hydrocarbon reaction are always carbon dioxide (or carbon monoxide) and water. For example, the formula for the combustion reaction of propane is:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (ΔH = -2043.455 kJ/mol)
Propane has a higher enthalpy of change due to the presence of more hydrocarbon bonds in the initial reactant. Likewise, the formula for the combustion of ethane (C2H6) is:
2C2H6 + 7O₂ → 4CO₂ + 6H₂O (ΔH = -3120 kJ/mol)
In general, the formula for a hydrocarbon combustion reaction in oxygen is:
CₓHᵧ + zO₂ → xCO₂ + (y/2)H₂O
where z = x + (y/4).
In most cases of combustion, the source of oxygen is normal atmospheric air. Atmospheric air also contains large quantities of nitrogen (N). In our atmosphere, oxygen is outnumbered by nitrogen in a ratio of about 3.77 nitrogen molecules per mole of oxygen molecules. So most combustion reactions that use plain air also produce nitrogenous products. Nitrogen is not normally considered a combustible material, but combustion reactions in air produce a small amount of nitrous oxides (compounds in the form of NOₓ)
Combustion requires the presence of oxygen, otherwise it cannot occur. Thus, one way to stop a combustion reaction is to remove is oxygen source. This is the main principle behind the design of fire extinguishers. Fire extinguishes spray carbon dioxide near a fire. Carbon dioxide is heavier than atmospheric air, so it will displace any air around the fire, cutting off its oxidizing source. This is also why fires are particularly dangerous in space. The high oxygen content of artificial habitats in space allows fires to spread very quickly and violently. Even a single spark can be devastating in a high oxygen environment.
Types Of Combustion
Combustion reaction can be divided into two main types, complete or incomplete, each based on the amount of oxygen available for the reaction. Whether a combustion reaction is complete of incomplete depends upon the efficiency of the combustion reaction.
During a complete chemical reaction, the reactants are completely converted into water and an oxide product (product in the form XO2). In the case of hydrocarbon reactants, the oxide product is carbon dioxide. Essentially, a complete reaction occurs when there are enough oxygen molecules so that 2 oxygen atoms can be matched to each carbon atom. In the case of other fuel sources besides hydrocarbons, the products are common dioxides of that element; e.g. sulfur dioxide (SO2) for sulfur (S), iron(III) dioxide (Fe2O3) for iron.
A complete combustion reaction is the most ideal and hypothetically would continue until all the fuel is used up. In the real world, chemical reactions are never 100% efficient. Some released energy dissipates into the environment so not all 100% of the reactant is converted into product. The efficiency of a combustion reaction can be related to impurities in fuel source, impurities in the oxidizing agent, and the temperature. In general, the higher the temperature, the more efficient the combustion reaction.
In contrast, an incomplete combustion reaction occurs when there is not enough oxygen present to completely convert the fuel into products. Due to the low amount of oxygen molecules, there are not enough oxygen atoms to pair two with each carbon atom. The result is that incomplete combustion of hydrocarbons produces carbon and carbon monoxide products, along with water. For example, an incomplete combustion reaction of ethane might look like:
2C2H6 + 5O₂ → 4CO + 6H₂O + heat
In this case, there is not enough oxygen present to match 2 oxygen atoms with each carbon atom. So instead, the gaseous product carbon monoxide forms. Carbon monoxide is a very poisonous gas so incomplete combustion in living space can be extremely dangerous.
2C2H6 + 3O₂ → 4C + 6H₂O + heat
In this case, ethane combusts to form gaseous carbon molecules. These carbon molecules then depose on nearby solid surfaces, forming soot.
Examples Of Combustion
By far the most common combustion reaction in everyday life is fire. Fire is the result of a high energy combustion reaction that occurs due to the heating of organic reactants in atmospheric air. The visible flame of a fire is composed of heated gases such as carbon dioxide, oxygen, nitrogen, and water vapor. In some fires, the flames get hot enough that they convert the gas into plasma, a state of super-heated ionized matter.
It has not always been possible to have a fire on Earth. During the Precambrian era, the oxygen content of Earth’s atmosphere was too low to allow combustion that produced an open flame. Fire only became possible on Earth about 460 million years ago, when the oxygen content of the Earth reached a bit more than 13%.
Modern rocket technology typically utilizes the combustion of hydrogen and oxygen into water vapor as a source of propulsion. Modern rockets use liquid hydrogen that combusts with oxygen according to the reaction:
2H₂ + O₂ → 2H₂O (ΔH = -242 kJ/mol)
Rockets prefer to use hydrogen as fuel for combustion because it is abundantly available and the only product of the reaction (besides heat energy) is water vapor. Hydrogen combustion reactions are very energetic and have a high mass-to-energy ration, so it requires less amount of fuel per unit of energy output than traditional methods of rocket propellant.
Most modern cars operate on some kind of internal combustion engine, which works by combusting gasoline in the presence of oxygen. Inside of a car engine, combustion reactions between gasoline and oxygen produce large quantities of heat. This heat pushes on the pistons, whose movement causes the wheels to move. So an internal combustion engine works by converting the thermal energy produced by a combustion reaction into mechanical energy (via the pistons) which moves the wheels.
Combustion reactions require a steady source of oxygen to propagate. Without enough oxygen, a combustion reaction will be incomplete, or may not be able to occur at all. This is why cars have carburetors. Carburetors work by injecting air into the fuel of an engine, giving it a steady source of oxygen. Without a carburetor, an internal combustion engine would not have an air supply, and any combustion reaction would quickly use up all the available oxygen.