A Simple Guide to Combustion in Thermodynamics

High-temperature exothermic chemical reaction between “reductant” (fuel) and “oxidant” (oxygen from the air) is known as “combustion.” “Smoke” is the mixture of oxidized, gaseous products that result from this process. Burning may not always result in flames, but when it does a flame serves as a clear indicator of the chemical process that took place. When you light a match to start a fire, you need to overcome the activation energy to begin the process of combustion. In some cases, the heat from a flame may be sufficient to sustain the reaction.

Combustion

Combustion, also called “burning,” is a high-temperature exothermic redox chemical reaction between a fuel (the “reductant”) and a “oxidant,” which is usually oxygen from the air. This reaction creates oxidized, often gaseous products, which are mixed together to make a substance called “smoke.” Fire doesn’t always come from combustion because a flame can only be seen when the substances being burned turn into gas, but when it does, a flame is a clear sign of the reaction. To start combustion (like when you use a lit match to start a fire), you have to overcome the activation energy. However, the heat from a flame may provide enough energy to keep the reaction going on its own.

Most of the time, combustion is a complicated chain of simple radical reactions. Solid fuels like wood and coal go through endothermic pyrolysis to make gaseous fuels, which are then burned to get the heat needed to make more solid fuels. When something burns, it often gets hot enough to make a flame or glow that gives off incandescent light. A simple example is when hydrogen and oxygen burn to make water vapor, which is a process that is often used to power rocket engines. At the same temperature and pressure, this reaction gives off 242 kJ/mol of heat and lowers the enthalpy by the same amount:

 2H2(g) + O2(g) → 2H2O(g)

When an organic fuel burns in air, energy is always released. This is because the double bond in O2 is much weaker than other double bonds or pairs of single bonds. The stronger bonds in the products of combustion, CO2 and H2O, cause energy to be released.

 The bond energies in the fuel don’t have much of an effect because they are similar to those in the products of combustion. For example, the sum of the bond energies in CH4 is almost the same as that of CO2. The elemental makeup of the fuel can be used to estimate that the heat of combustion is about 418 kJ per mole of O2 used up in the combustion reaction. 

For combustion to happen in air without a catalyst, temperatures have to be pretty high. Stoichiometry says that the fuel is completely burned when there is no more fuel and, ideally, no more oxidant. From a thermodynamic point of view, the chemical equilibrium of combustion in air is mostly on the side of the things that are left over. But it’s almost impossible to get complete combustion because the chemical equilibrium isn’t always reached and there may be unburned products like carbon monoxide, hydrogen, and even carbon (soot or ash). So, the smoke that is made is usually dangerous because it contains unburned or partially burned products. When nitrogen is burned at high temperatures in the air, which is 78 percent nitrogen, small amounts of nitrogen oxides, or NOx, are also made. This is because nitrogen is thermodynamically easier to burn at high temperatures than at low temperatures. Since burning is rarely clean, laws may require fuel gas cleaning or catalytic converters.

Different Kinds of Biochemical Reactions

Even though there are a lot of possible biochemical reactions, they can be broken down into just a few types:

Oxidation and reduction, like when an alcohol and an aldehyde change into each other.

Functional groups can move within or between molecules. Like when phosphate groups move from one oxygen to another.

Adding and taking away water. For example, hydrolysis is the removal of water from a link between an amine and a carboxyl group.

There are reactions that break bonds, like when a carbon-carbon bond breaks.

Life is complicated not because there are a lot of different kinds of reactions, but because these simple reactions happen in a lot of different ways. So, adding water to a carbon-carbon double bond is one way to break down compounds like sugars, lipids, and amino acids.

Keeping Biochemical Reactions Under Control

If you mix gasoline and oxygen, your car engine can run or an explosion can happen. The difference between the two situations is how much gasoline can flow. When you put your foot on the accelerator, you can control how much gasoline goes into the combustion chamber. Like this process, it’s important that biochemical reactions don’t happen too quickly or too slowly and that the right reactions happen when they’re needed to keep the cell working.

Large Molecules Tell Cells What to Do

The genetic information in the DNA of a cell is the most important thing for controlling biochemical reactions. This information is expressed in a controlled way so that the enzymes that carry out the chemical reactions in the cell are released when the cell needs to make energy, copy itself, or do other things. The information is made up of long strings of subunits, and each subunit is one of the four nucleotides that make up nucleic acid.

Weak Connections and a Stable Structure

Biochemical systems are often broken by heat. When a slice of liver is cooked at just slightly above 100°F, the enzymes are killed. This isn’t enough heat to break a covalent bond, so why aren’t these enzymes stronger? The answer is that the structure and activity of enzymes depend on weak interactions that have much less energy than a covalent bond. All of these weak interactions add up to make up the stability of biological structures.

The Direction of Biochemical Reactions Is Downhill

In the end, life on Earth depends on energy sources that are not alive. The sun is the most obvious of these. Its energy is captured on Earth through photosynthesis (the use of the light energy to carry out the synthesis of biochemicals especially sugars). The way the Earth is put together is another source of energy. Microorganisms that live in deep water, soil, and other places without sunlight can get their energy from chemosynthesis, which is the oxidation and reduction of inorganic molecules to make biological energy.

The goal of these energy-storing processes is to make organic compounds with carbon whose carbon is more reduced (has more electrons) than the carbon in CO2. The reduced carbon is oxidized by metabolic processes that produce energy in the process. Again, energy is used to make complex structures out of the organic compounds made by these processes. All of these things use the original source of energy, which is light from the sun, to keep living things like humans alive and help them grow and reproduce.

The amount of energy that comes out of these reactions is always less than what went into them. This is another way of saying that living systems follow the Second Law of Thermodynamics, which says that spontaneous reactions go “downhill,” leading to more disorder in the system. (For example, glucose, which is made up of six carbons that are linked together, is more organized than the six molecules of CO2 that are made when it breaks down in the body.

Conclusion

From the following article we can conclude that Combustion is a high-temperature exothermic redox chemical reaction between a fuel (the “reductant”) and a “oxidant,” which is often oxygen from the air. This process generates oxidized, frequently gaseous byproducts, which are combined to form “smoke.” Fire is not usually the result of combustion, as a flame is only visible when the chemicals being burned transform into gas, but when it is, it is a clear indication of the reaction. To initiate combustion (such as when a lit match is used to ignite a fire), it is necessary to overcome the activation energy. Nevertheless, the heat from a flame may provide sufficient energy to sustain the reaction on its own.