Fire is a chemical chain reaction which takes place with the evolution of heat and light. In order for a fire to take place there are 3 main ingredients that must be present: Oxygen, Heat and Fuel.
In chemistry we call the type of reaction that produces fire a combustion reaction. Combustion is a high-temperature exothermic (heat releasing) redox (oxygen adding) chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.
Whenever we complete a combustion reaction a hydrocarbon (compound of C and H) there are generally the same products formed: CO2 and H2O.
The fuel you burn in your car's engine contains octane, C8H18. When octane is burned, the products are CO2 and H2O.
2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(g)
The key ingredient to the process is the availability of oxygen. Combustion cannot take place in an atmosphere devoid of oxygen.
So if you have a bottle of gasoline (octane) sitting around and open to the atmosphere which contains oxygen, why doesn�t it just burst into flames?
The answer to this question is the need to overcome the activation energy of the reaction, which means that it requires energy at first to "jump start" the process. In your car, the distributor and battery provide this starting energy by creating an electrical "spark". Other sources of initial energy can come from the Sun, matches, friction, etc.
The combustion reaction itself is quite exothermic.
When heat is produced in the process of a chemical reaction this is known as an Exothermic Reaction.
N2 + 3H2 → 2NH3 + Heat
C + O2 → CO2 + Heat
When heat is absorbed from the reacting substances this is known as an Endothermic Reaction.
2C + H2 - Heat → C2H2
3O2 - Heat → 2O3
But remember, whether endothermic or exothermic, both types of reactions still require an Activation Energy to begin.
Combustion is the chemical reaction that releases energy trapped in fossil fuels. Overall, combustion is an exothermic reaction given off or exiting , which means that energy is released. Usually, heat and light are released during a combustion reaction. During exothermic reactions (like combustion), bonds are broken, which allows the energy trapped in the bonds to be released and do work. During combustion of fossil fuels, the carbon compound in the fossil fuel reacts with oxygen in the presence of a heat source to form carbon dioxide (CO2) and water.
Energy is also needed for the fossil fuel to react with oxygen, so the energy use portion of combustion is an endothermic reaction into or absorbing .
In summary, combustion is a "burning" of fossil fuels in order to release heat energy, and the primary byproduct of combustion is carbon dioxide (CO2).
The following is an example of combustion of a methane molecule. Methane is the major molecule in natural gas.
CH4 + 2O2 → CO2 + 2H2O + Energy
This chemical equation says that 1 molecule of methane reacts with 2 molecules of oxygen to give off 1 molecule of carbon dioxide, 2 molecules of water, and energy.
Remember that the compounds on the left of the arrow are the reactants and the compounds on the right side of the arrow are the byproducts. Because of the its chemical make up being only carbon and hydrogen, methane (natural gas) is a much cleaner burning fossil fuel than coal and oil. The only products released are CO2 and water.
Coal's chemical makeup has atoms of carbon, oxygen, hydrogen, nitrogen, sulfur, and other minerals, such as mercury. When coal is burned, the main byproduct released into the atmosphere is CO2. However, when coal goes through combustion, it also releases nitrogen and sulfur. The nitrogen is in the form of nitrogen gas (N2) or Nitrogen dioxide (NO2), and the sulfur is in the form of sulfur dioxide (SO2), which is a gas that causes acid rain. Also, if coal or wood products are burned with an insufficient oxygen supply, carbon monoxide is produced. Carbon monoxide is a poisonous gas that replaces the oxygen molecules in the hemoglobin molecules in the blood supply of the lungs causing suffocation.
When petroleum products (oil) are burned, CO2 is the primary byproduct as with the other fossil fuels. Like coal, petroleum combustion releases nitrogen oxides (NO, NO2) and sulfur dioxides (SO2). Also, the combustion of oil-based products can release the following byproducts: carbon monoxide, particulate matter (PM), and lead.
Poisonous carbon monoxide (CO) from an exhaust pipe
View Animated Engines:Four Stroke Engine to see how combustion of gasoline (a petroleum product) drives a car or truck.
When looking at the amount of carbon dioxide produced in the combustion of fossil fuels, coal produces the most CO2 followed by oil/petroleum with methane/natural gas producing the least amount of carbon dioxide. Natural gas releases almost 30% less CO2 than oil and 43% less CO2 than coal. Fossil fuels tend to be more efficient and cost effective in producing energy. Alternate sources of energy tend to be cleaner and safer for the environment, but their technologies are not as advanced as those for fossil fuels, causing them to be less efficient and more expensive.
Go to PBS: On Fire and go through the virtual lab. Complete 4.03 Combustion Lab. Submit your completed work to the 4.03 Combustion Lab Dropbox.