Thermal Decomposition Vs Combustion 

1.0Introduction to Thermal Decomposition and Combustion

In Chemistry, reactions involving heat and energy changes are highly significant. For JEE-level students, two fundamental types are:

• Thermal decomposition – a reaction where a compound breaks down into simpler substances when heated.
• Combustion – a chemical process where a substance reacts with oxygen to produce heat and light.

2.0What is Thermal Decomposition?

Thermal decomposition is a chemical reaction in which a single compound splits into two or more simpler substances when heated. It is also known as thermolysis.

General Reaction

$\text{Compound}\stackrel{\Delta }{\to }\text{Simpler Substance}+\text{Simpler Substance}$

Here, Δ denotes heating.

Characteristics of Thermal Decomposition

• Requires continuous heat supply.
• Reaction is endothermic (absorbs heat).
• No oxygen is required.
• Products formed are simpler compounds or elements.

Common Examples of Thermal Decomposition

1. Calcium Carbonate (limestone):

${\text{CaCO}}_3(s)\stackrel{\Delta }{\to }\text{CaO}(s)+{\text{CO}}_2(g)$

2. Potassium Chlorate:

$2{\text{KClO}}_3(s)\stackrel{\Delta }{\to }2\text{KCl}(s)+3{\text{O}}_2(g)$

3. Copper (II) Sulphate Pentahydrate:

${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}(s)\stackrel{\Delta }{\to }{\text{CuSO}}_4(s)+5{\text{H}}_2\text{O}(g)$

4. Metal Carbonates and Hydroxides:
   Most carbonates and hydroxides decompose into oxides on heating.

Industrial and Everyday Applications

• Lime production from limestone (cement industry).
• Extraction of metals (e.g., Zn, Fe) from ores.
• Laboratory preparation of gases like O₂.
• Baking – decomposition of baking soda releases CO₂.

3.0What is Combustion?

Combustion is a high-temperature chemical reaction between a substance (fuel) and oxygen, releasing heat, light, and energy.

General Reaction

$\text{Fuel}+{\text{O}}_2\stackrel{\text{heat}}{\to }\text{Products}+\text{Energy}$

Types of Combustion

1. Complete Combustion – occurs with excess oxygen.
   Example:

${\text{CH}}_4(g)+2{\text{O}}_2(g)\to {\text{CO}}_2(g)+2{\text{H}}_2\text{O}(l)$

2. Incomplete Combustion – occurs with limited oxygen, forming carbon monoxide or soot.
   Example:

$2{\text{CH}}_4(g)+3{\text{O}}_2(g)\to 2\text{CO}(g)+4{\text{H}}_2\text{O}(l)$

3. Spontaneous Combustion – occurs without external ignition (e.g., white phosphorus in air).
4. Explosive Combustion – extremely fast combustion releasing energy in the form of an explosion (e.g., firecrackers).

Characteristics of Combustion

• Always requires oxygen.
• Reaction is exothermic (releases heat).
• Produces CO₂, H₂O, CO, soot, and energy.
• Can be complete or incomplete depending on oxygen supply.

Common Examples of Combustion

• Burning of hydrocarbons (LPG, petrol, diesel).
• Combustion of coal and natural gas.
• Hydrogen combustion to produce water.

Industrial and Everyday Applications

• Power generation in thermal plants.
• Internal combustion engines in vehicles.
• Domestic heating and cooking (LPG, wood, kerosene).
• Explosives and fireworks industry.

4.0Differences Between Thermal Decomposition and Combustion

Thermodynamics:

Thermal Decomposition:

• Endothermic process.
• ΔH (enthalpy change) is positive.
• Energy is absorbed to break chemical bonds.

Combustion:

• Exothermic process.
• ΔH (enthalpy change) is negative.
• Energy is released as strong CO₂ and H₂O bonds form.

5.0Importance of Thermal Decomposition and Combustion 

• Helps understand thermochemistry, kinetics, and energetics.
• Important in inorganic reactions (carbonates, nitrates, hydroxides).
• Combustion reactions are key in organic chemistry for hydrocarbons.
• Builds conceptual clarity on endothermic vs exothermic reactions.

6.0Real-Life Applications in Industry and Daily Life

Thermal Decomposition Applications

• Cement industry (lime production).
• Baking industry (release of CO₂).
• Metallurgy (ore decomposition).

Combustion Applications

• Energy production in power plants.
• Running automobiles and engines.
• Household cooking fuel (LPG, wood, coal).
• Fireworks and explosives.