Fire in Terms of Chemistry and Thermodynamics
A fire has formed an important ingredient of societal cultures and religions, from historic times to our present world, and has played a key role in the advancement towards civilization. The fire has taken different trends all through the history of our world.
For thousands of years, people have used fire in several ways, such as cooking, keeping warm during cold days, and driving machines. However, many people know what fire can do, but few know what it is. This paper will prove that fire is an uncontrolled combustion involving chemistry, thermodynamics, fluid mechanics, and heat transfer.
For a fire to start, it needs three elements: heat, fuel, and an oxidizing agent (oxygen gas or another oxygen-rich compound), and it takes place uncontrollably when they are combined in the right proportion (University of Maryland University College, n.d.).
This is usually referred to as the fire triangle. For preventing or extinguishing a combustion reaction, one of these elements has to be removed. The three elements have to be present for fire to start. For instance, a combustible fluid will only commence burning when the fuel and oxygen are adequately available in the correct amounts.
The heat source serves the purpose of getting rid of the moisture near the fuel and increasing the temperature of the neighboring air. Some reactions may require a catalyst for speeding up the combustion process.
After the reaction has been ignited by raising the temperature above the flashpoint for the fuel and oxidizer mix, a chain reaction then takes place in which the fire is able to sustain its own heat through the further discharge of heat energy. The fire can spread when the delivery of the oxidizer and fuel are not stopped.
Combustion is an oxidation-reduction chemical reaction whereby the structures of complex substances are reduced in size. They then form more stable molecules through the reorganization of atomic bonds. It has been noted that a major aspect of the chemistry of combustion at high temperature entails radical reactions; nonetheless, combustion reactions can also take place as a single overall reaction. For example:
H3C-CH2-CH3 + 5O2 → 3CO2 + 4H2O
In the above oxidation-reduction reaction, the resulting products, carbon dioxide, and water are more stable than oxygen and propane. Similar to any other chemical reaction, a catalyst can be used to speed up the combustion process. Since the reaction will be at a high activation energy level, the incorporation of a catalyst makes it possible to operate at a lower temperature, which leads to complete combustion.
As pertains to solid burnable materials, the activation energy makes it possible for vaporization or pyrolysis of the burnable to take place, and the gas generated will then combine with an oxidizer giving yield to a burnable mixture. The reaction is usually self-sustainable when the energy generated by the burning process exceeds or is equivalent to the amount of energy needed for the burning to take place.
Thermodynamics, as the branch of science that deals with the nature of heat and its conversion to other types of energy, can be used to describe fire. Generally, a thermodynamic system is defined by variables such as temperature, pressure, as well as other properties, and it considers both the system and its environment.
When there is uncontrolled combustion, the environment within the vicinity of the combustible material will have heat sources with limitless heat capacity. This makes it possible for the environment to give and receive heat while maintaining its temperature constant.
If the condition alters, the thermodynamic system reacts by altering its state. That is, the temperature, pressure, and volume will alter accordingly so as to maintain the original state of equilibrium. A changing system can adhere to any of the three laws of thermodynamics.
For a system to receive energy, the surrounding has to make it available. On the other hand, a system giving out energy provides it to the surrounding. The energy is simply converted from one form to another as the transfer occurs. However, the energy can never be created nor destroyed; therefore, this means that as the combustion takes place, the products of combustion, such as heat and light, are simply being converted from their innate state.
The first law of thermodynamics states that “energy cannot be destroyed or created, but can be transformed from one state to the other. This law is also known as the law of conservation of energy” (Fermi, 1956, p.11). For example, when gasoline (a combustible) is consumed in the engine of a vehicle, an equivalent quantity of work and heat are given out as the energy is generated.
The heat generated from the engine, which is converted into other forms of energy, raises the temperature of the surroundings as well as the parts of the vehicle, and the change in the quantity of all the energies produced from the burnt gasoline would be equivalent to the change in the energy between the reactants and the products.
The second law of thermodynamics states, “Since heat energy exhibits entropy, it cannot spontaneously move from a region of lower temperature to a region of higher temperature” (Fermi, 1956, p.29).
Therefore, the heat generated by a fire will always move from a hotter region to a colder region. The third law makes this to be clearer. It states that all processes stop as the temperature nears absolute zero; therefore, fire cannot take place at this temperature since the production of energy is not possible.
Heat transfer alludes to the physical process through which thermal energy is conveyed from an area at a higher temperature to an area at a reduced temperature, and this difference in temperature is what necessitates the transfer of heat (DeHaan, 2007). The heat transfer takes place depending on the capability of the material to exchange heat energy.
This is called the thermal conductivity of an object. In general, a material having a higher thermal conductivity exchanges heat at a faster rate. It is important to note that the exchange occurs until the material and its surroundings have attained the same thermal conditions. Heat exchange mainly takes place in three ways: “conduction, convection, and radiation” (Zielenkiewicz & Margas, 2002, p.10).
Fluids are made up of molecules that interact with each other; that is, the discrete molecules are always in motion. This elastic impact in fluids allows the fire to transfer its heat energy from one place to the other. The transfer of heat through conduction occurs when heat moves due to the direct contact of molecules in a fluid.
It is important to note that the process of free electron dissemination, which is most common in solid materials, can also result in the exchange of heat energy. In fluids, the transfer of heat through conduction takes place when neighboring atoms collide with one another (Quintiere, 1998).
The movement of electrons from one atom to the subsequent one can also result in this. Conduction is the main method of heat exchange from the point of origin of the fire to the materials that surround it.
As the burning process continues and fire spreads through fluid circulation, convection then becomes the main mode of heat exchange — density differences between the fluid cause the medium to flow. The less-dense gases are able to rise through the buoyant flow.
Consequently, as the fire grows, the hot plume gaseous substance coming from the fire rises because it is less dense, while the colder air sinks and gets into close contact with the fire that enables it to gain heat and become less dense.
This process, in which the hot air rises and the cooler air sinks, leads to the development of a convective current within the vicinity of the fire. Transfer of heat is achieved when hot fluids release their energy through an encounter with other materials.
Radiation, as a process in which electromagnetic waves travel from a heat source to an absorbing material, makes the impact of a completely grown fire to be felt as it exerts heat energy all around a hot material. The transfer of heat through this method does not require any medium of transmission.
Fluid mechanics is also applicable here. The layer of hot air expands across the upper borders of a compartment. At the same time, the energy in that layer also radiates outward. This process has an effect on the fluids below the gas layer and away from the spot where the fire commenced.
DeHaan, J. D. (2007). Kirk’s fire investigation (6th ed.). Upper Saddle River, NJ: Pearson Education.
Fermi, E. (1956). Thermodynamics. New York: Dover.
Quintiere, J. G. (1998). Principles of fire behavior. Albany, NY: Delmar Publishers.
University of Maryland University College. (n.d.). Fire analysis.
Zielenkiewicz, W., & Margas, E. (2002). Theory of calorimetry. Dordrecht : Kluwer.
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