Thermal Decomposition Vs Combustion

Thermal decomposition, or thermolysis, is a chemical process of breaking down caused by heat. At high temperatures, the reactants absorb a lot of energy. The temperature at which a substance chemically decomposes is termed its decomposition temperature.

Combustion is a chemical process in which a substance reacts rapidly with oxygen or in the presence of oxygen and gives off heat and light in the form of flame.

Table of Contents

• An Overview of Thermal Decomposition
  • Equations for Thermal Decomposition
  • Examples
  • Practical Applications
• An Overview of Combustion
  • Physical and Chemical Aspects of Combustion
  • Classification of Combustion Phenomena
  • Applications of Combustion
• Difference between Thermal Decomposition and Combustion
• Frequently Asked Questions – FAQs

An Overview of Thermal Decomposition

The term “thermal” refers to the presence of heat. Decomposition is the process of breaking down.

Thermal decomposition, or thermolysis, is a chemical process of breaking down due to high temperatures. The temperature at which a substance chemically decomposes is called its decomposition temperature.

Heat is required to break chemical bonds in the substance being decomposed; hence the reaction is frequently endothermic.

At high temperatures, thermal decomposition reactions take place. While breaking down into products, the reactants absorb a lot of energy. The reactant is the starting chemical. It decomposes into simpler entities.

Further Reading:

Decomposition Reaction

Equations for Thermal Decomposition

There is just one reactant in a thermal decomposition reaction, but two or more products.

AB → A + B

The parent molecule (reactant) is AB, while the resultant molecules are A and B.

For example, when copper carbonate is heated, it decomposes into copper oxide and carbon dioxide.

Copper carbonate is a green substance, while copper oxide is black. During the reaction, the colour changes from green to black. Limewater, which changes from clear to muddy, can be used to identify the carbon dioxide created. The word equation for the same is:

Copper carbonate → Copper oxide + Carbon dioxide

The symbol equation for this reaction is:

CuCO₃ (s) → CuO (s) + CO₂ (g)

Not all thermal decomposition reactions lead to a change in colour. There is no colour change because magnesium carbonate and magnesium oxide are white solids. When magnesium carbonate is heated, magnesium oxide and carbon dioxide are produced. The equation for this reaction is as follows:

Magnesium carbonate → Magnesium oxide + Carbon dioxide

MgCO₃ (s) → MgO (s) + CO₂ (g)

Examples

• Some oxides, particularly weakly electropositive metals, decompose when heated to a high temperature. The decomposition of mercuric oxide to produce oxygen and mercury metal is a typical example. Joseph Priestley employed the process to prepare gaseous oxygen samples.
• When calcium carbonate (limestone or chalk) is heated, it breaks down into calcium oxide and carbon dioxide. The following is the chemical reaction:

CaCO₃ → CaO + CO₂

• A small percentage of water decomposes into OH, monatomic oxygen, monatomic hydrogen, O₂, and H₂ when heated to over 2000°C.

Practical Applications

Thermal decomposition impacts a variety of events in the real world. Fingerprints are one of the things that are affected. When someone touches something, they leave a residue on their fingers. If your fingers are sweaty or have oil on them, the residue will include a lot of chemicals. De Paoli and her colleagues researched the thermal deterioration of specific fingerprint components. When exposed to heat, the amino acid and urea samples began to decompose at 100°C, while the lactic acid samples began to decompose at roughly 50°C. Decomposition of fingerprints is important in the forensics profession since these components are required for further testing.

An Overview of Combustion

Combustion, often known as burning, is a high-temperature exothermic redox chemical reaction that creates oxidised, frequently gaseous compounds in a mixture known as smoke. Combustion is frequently a complex chain of fundamental, radical processes. Solid fuels, such as wood and coal, go through endothermic pyrolysis to form gaseous fuels, subsequently burned to provide the heat needed to produce more.

The rate at which the reactants combine is high, partly due to the nature of the chemical reaction and partly because more energy is generated than can escape into the surrounding medium, causing the reactants’ temperature to speed up the reaction even more.

A lighted match is a common illustration of a combustion reaction. When a match is struck, friction heats the head to the point where the chemicals react and produce more heat than can escape into the air, resulting in a flame. The match will go out if the heat is blown away by the wind or if the chemicals are moist and friction is insufficient to boost the temperature.

Further Reading:

Combustion and Flames

Physical and Chemical Aspects of Combustion

External variables such as heat, light, and sparks start the combustion process. As the combustible mixture reaches the ignition temperature, the reaction begins. When the total heat energy of the reactants and the total heat energies of the products reach equilibrium, combustion ends.

To form hydroxyl radicals, hydrogen combustion involves intricate branched-chain processes involving the interaction of hydrogen and oxygen atoms with oxygen and hydrogen molecules, respectively. The final reaction result is water, which is created when hydroxyl and hydrogen molecules combine.

Physical processes such as diffusion and convection transfer mass and energy in gaseous combustion and chemical reactions. The rate of component diffusion is determined by the concentration of constituents, pressure, temperature variations, and diffusion coefficients in the absence of external forces (a measure of the speed of diffusion). The latter are either measured or estimated in terms of the kinetic theory of gases.

Classification of Combustion Phenomena

Premixed flames and flames that burn without premixing are the two types of flames.

Premixed flames: Fuels that have been premixed with an oxidant, either oxygen or a chemical that provides oxygen, have the most prominent flame burning.

Diffusion flames: Diffusion flames, whether smooth (laminar) or turbulent, are a type of flame in which the components are not combined before entering the burning zone.

Oxidising and reducing flame: A diffusion flame appears when a premixed flame burns in the open air with excess fuel. This is accounted for by atmospheric oxygen diffusion, as in the Bunsen flame produced by a burner to which the air intake can be regulated, thereby changing the flow from an intensely hot flux to a relatively low-temperature flux in which most of the fuel gas is only partially oxidised.

Applications of Combustion

Combustion encompasses many phenomena with widespread use in industry, science, professions, and at home. Its application depends on understanding physics, chemistry, and mechanics; their interaction becomes clearer when handling flame propagation.

The applications of combustion and flame phenomena can be divided into five categories. They are as follows:

1. In heating devices
2. In explosives
3. In internal-combustion engines
4. In rocket propulsion
5. In chemical reactions

Difference between Thermal Decomposition and Combustion

Decomposition and combustion are chemical reactions that break down complex materials into smaller molecules.

One reactant produces two or more products in decomposition reactions. Energy sources such as heat, light, or electricity break down the reactant. Combustion reactions occur when compounds react quickly with oxygen and require an ignition source, such as a flame.

The heat breakdown of molecules into free radicals is the first stage in a combustion reaction. The free radicals then proceed to react with molecules in what is known as propagation steps, which produce new free radicals and continue the reaction.

Finally, there are termination steps, which include the formation of stable molecules such as water by combining free radicals. So, in a combustion reaction, molecules undergo thermal decomposition into free radicals, which keeps the reaction going, as well as a combination of free radicals, which produces stable molecules and brings the reaction to a close.