What Are the Physical and Chemical Changes During Combustion
Combustion is a chemical reaction in which a substance reacts with the oxygen molecule to give off energy in the form of heat, which is known as combustion. In the combustion, process light is given off during the process either in a form of flame or as a glow. Some substances that are combustible are fuel, coal, and organic compounds. Combustion is an exothermic reaction. The energy is released during this process, therefore, called the exothermic process. The characteristics features of the combustion process are the emission of heat and light.
The lowest temperature at which a substance catches fire is known as the ignition temperature. The combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature. In this article, we have discussed the chemical and physical aspects of combustion.
Types of Combustion Reactions
The combustion process can be classified into two types, depending on the nature of the product formed.
Complete combustion
Incomplete combustion
Complete Combustion
In this type of combustion process the reactant burns in the presence of oxygen or in an open environment. The final product obtained in the complete combustion is carbon dioxide and a water molecule. In this process hydrocarbon generally burns in the presence of oxygen to yield carbon dioxide and water. The reaction to this type of combustion process is given below:
C₂H₆ + O₂ → CO₂ + H₂O
However, the combustion process is not always very efficient. Sometimes the reactant undergoes an inefficient process and releases harmful products and byproducts. Generally, fossil fuels burn with the inefficient process of combustion.
Incomplete Combustion
In this type of combustion process, the reactant burns in a limited amount of oxygen or in a sealed container. The final product obtained in this type of reaction is carbon monoxide and the other harmful products in the environment. This type of combustion leads to the major contribution of air pollutants. The major stable product of this type of reaction is carbon, carbon monoxide and hydroxyl radicals.
In the Combustion Process, the Flames Can be Classified as:
Premixed flames
Diffusion flames
Oxidizing and reducing flames
Physical Aspects of Combustion
Let us discuss the physical aspect of combustion, in the combustion process the transfer of mass and energy takes place. This change takes place with the help of the diffusion process and the convection process occurs in the gases combustion. The rate of combustion is dependent on the pressure, temperature, and diffusion coefficient. The diffusion phenomena play an important role in the combustion process. Physical changes like sublimation and evaporation contribute to the ease of the combustion process.
Chemical Aspects of Combustion
Let us discuss the chemical aspects of combustion, combustion is a complex chemical reaction process. It involves various steps. These steps are dependent on the chemical aspects of combustion undergoing substance. These steps are affected by the environmental surrounding factors like ignition temperature, heat, and light.
Steps involved in the chemical aspects of the combustion process
Attainment of the ignition temperature by the substance.
Spreading the heat from the ignition source to the adjacent layer of the substance.
This in turn leads to the ignition source for the next adjacent layer.
This process continues and the combustion gets completed.
Did you know that?
The engines of the car and the engines of the rocket both work by the combustion process.
Do you know that during the combustion process the fuel reacts with the oxygen molecule present in the atmosphere and the heat to release energy.
FAQs on Physical and Chemical Aspects of Combustion in Chemistry
1. What is combustion in chemistry?
Combustion is a rapid exothermic oxidation reaction in which a substance reacts with oxygen to release heat and usually light. In most combustion reactions, a fuel reacts with O2 to form oxides.
- General form: Fuel + O2 → Oxides + heat + light
- Example: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
- It involves breaking and forming chemical bonds, releasing energy due to formation of stable products.
2. What are the chemical and physical aspects of combustion?
The chemical aspect of combustion involves oxidation reactions, while the physical aspect involves heat, light, and flame formation.
- Chemical aspects:
- Reaction with oxygen
- Formation of new substances such as CO2 and H2O
- Bond breaking and bond formation
- Physical aspects:
- Release of heat energy
- Emission of light (flame)
- Expansion of gases and temperature rise
3. What is complete and incomplete combustion?
Complete combustion occurs when a fuel burns in excess oxygen to form CO2 and H2O, while incomplete combustion occurs in limited oxygen and produces CO or carbon (soot).
- Complete combustion example:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) - Incomplete combustion example:
2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(g) - Incomplete combustion may also produce solid carbon (C).
4. Why is combustion an exothermic reaction?
Combustion is an exothermic reaction because more energy is released in forming product bonds than is absorbed in breaking reactant bonds.
- Bond breaking in fuel and O2 requires energy.
- Bond formation in products like CO2 and H2O releases a larger amount of energy.
- The net energy change (ΔH) is negative.
5. What is the general equation for hydrocarbon combustion?
The general equation for complete combustion of a hydrocarbon CxHy is CxHy + (x + y/4)O2 → xCO2 + (y/2)H2O.
- Balance carbon atoms first to form CO2.
- Balance hydrogen atoms to form H2O.
- Finally balance oxygen atoms.
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g).
6. What is the ignition temperature in combustion?
The ignition temperature is the minimum temperature at which a substance starts burning in air.
- Below this temperature, combustion does not occur.
- It depends on the nature of the fuel and surface area.
- For example, paper has a lower ignition temperature than wood.
7. What are the stages of combustion?
The main stages of combustion are preheating, ignition, and flame propagation.
- Preheating: Fuel is heated to its ignition temperature.
- Ignition: Rapid oxidation begins with release of heat and light.
- Flame propagation: The reaction sustains itself as heat keeps supplying activation energy.
8. What are the products of combustion reactions?
The products of combustion are usually oxides, such as CO2 and H2O, depending on the fuel and oxygen supply.
- Hydrocarbons: Produce CO2 and H2O (complete combustion).
- Metals: Form metal oxides, e.g., 2Mg(s) + O2(g) → 2MgO(s).
- Incomplete combustion: Produces CO and C (soot).
9. How do you balance a combustion reaction step by step?
To balance a combustion reaction, balance carbon first, then hydrogen, and finally oxygen.
- Step 1: Write the unbalanced equation (e.g., C2H6 + O2 → CO2 + H2O).
- Step 2: Balance carbon: C2H6 + O2 → 2CO2 + H2O.
- Step 3: Balance hydrogen: C2H6 + O2 → 2CO2 + 3H2O.
- Step 4: Balance oxygen: 2C2H6 + 7O2 → 4CO2 + 6H2O.
10. What is the difference between combustion and oxidation?
Combustion is a rapid oxidation reaction that releases heat and light, while oxidation is any reaction involving loss of electrons or gain of oxygen.
- All combustion reactions are oxidation reactions.
- Not all oxidation reactions are combustion (e.g., rusting of iron).
- Example of slow oxidation: 4Fe(s) + 3O2(g) → 2Fe2O3(s).
