[Chemistry Class Notes] on Combustion of Hydrocarbons Pdf for Exam

Combustion is the process of burning some substances at high temperatures. It is an exothermic reaction that happens between fuel and oxygen, producing a gaseous substance (smoke) as a product. Combustion is of two types, one is complete combustion and another is incomplete combustion. Hydrocarbons are compounds made only with the elements of hydrogen and carbon. Hydrocarbons are broadly classified as alkanes, alkenes, and alkynes.

Complete Combustion of Alkanes

It is the process of burning the alkane in the presence of sufficient air or oxygen; it produces carbon dioxide, water, and a huge amount of heat as a product. For example: 

1. With Propane ( C3H8 ), the complete reaction is given as follows:

             C₃H₈ + O₂ → 3CO₂  + 4H₂ 

2. With Butane (C₄H₁₀), the complete combustion reaction is given as follows: 

            C₄H₁₀ + O₂ → 4CO₂  + 5H₂ 

The generalised form of this reaction is as follows:

C_nH_2n+2 + ((3n + 1)/2) O₂ → nCO₂  + (n+1)H₂O

In general, it is used as a fuel since it produces a huge amount of heat.

Incomplete Combustion of Alkanes

It is the process of burning alkane in the absence of sufficient air or oxygen. It produces carbon and carbon monoxide as a product whereas carbon monoxide is a by-product that is a colorless poisonous gas. For example:

With methane (CH₄), the incomplete reaction is given as follows:

CH₄ + O₂ → C  + 2H₂O

The carbon black which is formed as a product of the combustion process is used in the manufacturing of inks. 

Combustion of Hydrocarbons (Alkene)

Complete Combustion of Alkene

Like the same as an alkane, alkene also undergoes complete combustion. It occurs in the presence of excess air or oxygen for combustion. For example:

   C₂H₄ + 3O₂ → 2CO₂  + 2H₂O

Incomplete Combustion of Alkene

It happens in the absence of oxygen during the combustion process and carbon monoxide is formed as a product instead of carbon dioxide. For example:

 C₂H₄ + 2O₂ → 2CO  + 2H₂O

Combustion of Hydrocarbons  

It is the process of burning the hydrocarbons which leads to breaking the bonds either in the presence or in the absence of excess oxygen.

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Complete Combustion of Hydrocarbons

It is the process of burning hydrocarbons in excess of oxygen and yields carbon dioxide and water as a product. Oxygen should be present in excess and hydrocarbon is used as a limiting reagent to achieve this process. 

Incomplete Combustion of Hydrocarbons

It is a process of burning hydrocarbons in the absence of excess oxygen and produces the most oxidized form of carbon which is carbon dioxide as a product. We should have oxygen as a limiting reagent and hydrocarbons as an excess reagent.

The “sooty” flame is produced by the incomplete combustion of a hydrocarbon, due to the presence of carbon ( C ). 

Hydrocarbon Formula of Complete Combustion

The general form of this combustion reaction is as follows:

Methane  +  oxygen gas  →  carbon dioxide gas + water vapour

Hydrocarbon Formula of Incomplete Combustion

The general form of this incomplete reaction is given as follows:

methane  +  oxygen gas  →  solid carbon  +  water vapour

Types of Combustion

Different types of combustion are as follows:

Rapid combustion: Rapid combustion is a type of combustion when quick heat energy is needed for the reaction to take place. A large amount of heat and light energy is produced in this type of reaction. The combustion occurs as long as the fuel is available. For example, when we light a candle, it will burn until the wax burns out. 

Spontaneous combustion: This type of combustion occurs spontaneously. This means that this type of reaction does not need any external energy for the combustion to begin. It occurs due to self-heating. This type of combustion takes place in substances with low-ignition temperatures. The process starts as soon as the temperature rises above the ignition point. The combustion will take place in the presence of oxygen. 

Explosive combustion: This is a type of combustion in which the reaction occurs very rapidly. This combustion occurs when something is ignited to produce heat, light, and sound energy as in firecrackers. 

Solved Examples

1. Ethanol is a fuel source in an alcohol lamp. The formula for ethanol is given by C₂H₅OH. Write the balanced equation for the process of combustion of ethanol.

Solution:

Step 1: Think of the given problem. The question is given on ethanol which is a reactant and also with oxygen. Carbon dioxide and water are the products.

Step 2: Write the skeleton equation and solve:

C₂H₅OH (l) + O₂ (g) →  CO₂ (g) + H₂O (g)

Now balance the equation.

C₂H₅OH (l) + 3O₂ (g) →  2CO₂ (g) + 3H₂O (g)

Evaluate the number of each element present on the reactant and product side.

Interesting Facts:

Nearly 21% of the air in the atmosphere is filled with oxygen. To get complete combustion, it is necessary to have plenty of air, mainly oxygen in it. Natural gas and petrol are such fuels that have hydrocarbons.

A hydrocarbon is a compound made of only 2 elements namely carbon and hydrogen atoms.

The interesting thing is, that it is found in crude oil and can be separated by fractional distillation. The bond between them is non-polar covalent bonds.

Summary

Combustion includes the burning of organic substances and it is a chemical reaction. 

Combustion includes burning of the organic compound and releasing carbon dioxide and water and releasing a lot of heat energy.

Combustion is an important process and hydrocarbons are the main source of energy for domestic and industrial processes. 

Combustion of fossil fuels such as natural gas is an example of such a chemical reaction.