[Chemistry Class Notes] on Combustion Reaction Pdf for Exam

A combustion reaction is usually a reaction that produces fire. Combustion takes place at an elevated temperature. It is a heat releasing (exothermic), redox chemical reaction that usually occurs between a fuel and oxidizing agent (mostly oxygen of the atmosphere). The products released are often gaseous, and the resultant mixture is commonly called smoke. The products of combustion are called oxides.

Examples of Combustion Reactions

• During combustion of methane, CH4(g) reacts with  2 molecules of O2(g) to give  CO2 (g) and 2 molecules of H2O(g)

• During the naphthalene burning process, C10H8 reacts with 12 molecules of O2 and gives 10 CO2 + 4 H2O.

• Combustion of ethane includes reacting between 2 C2H6 and 7 O2 to give 4 CO2 + 6 H2O as the resultant. 

• In lighters, the reaction of butane combustion is commonly seen. Here, 2C4H10(g) reacts with 13O2(g) and after combustion gives 8CO2(g) +10H2O(g) as the resultant.

• In grills, fireplaces etc. combustion of propane takes place. In this, 6CO2(g) + 8H2O(g) is produced after the combustion between 2C3H₈(g) and 7O2(g).

Oxygen is the main ingredient for the process of combustion to combustion cannot happen in the absence of oxygen.

If you leave a can of gasoline open in the atmosphere (containing oxygen), why does it not start burning?

This happens because, to initiate a reaction, we have to overcome its activation energy. This is done by heating the fuel so that it reaches its ignition temperature after which the combustion proceeds smoothly in the sufficient supply of oxygen.

Types of Combustion Reactions

1. Complete Combustion: Complete combustion takes place when a fuel burns completely to produce carbon dioxide and heat with a sufficient supply of oxygen. 

2. Incomplete Combustion: In the case of a limited or partial supply of oxygen, the fuel burns incompletely, thereby producing carbon monoxide in place of carbon dioxide.

Examples of Combustion Reactions

• Our vehicles such as cars, bikes, etc. run on fossil fuels. These fruits undergo a combustion reaction with oxygen to run the engine. 

• Burning of wood or other solid fuels such as coal in winters: The carbon content present in wood or coal reacts with oxygen in the atmosphere to release heat and form other gaseous products.

• The liquefied petroleum gas is released from the cylinders through the holes in the burner. On complete combustion, it produces a clear blue flame. If the supply of oxygen is limited, the same flame turns yellowish and sooty.

• Fireworks are tough to light during winters, it’s all due to the absence of oxygen.

• The burning of wax candles can also be taken as an example of the combustion reaction.

Why is a Combustion Reaction an Oxidation Reaction?

As we already know, a combustion reaction cannot proceed in the absence of oxygen. For burning or combustion to take place, oxygen should be essentially present because it is the only gas that supports burning. A combustion reaction will always occur when a substance reacts with oxygen to release energy. For example, when methane burns in oxygen, it releases carbon dioxide and water. The formation of carbon dioxide shows the carbon present in the methane has undergone oxidation.

Combustion Equations

CH4 + 2O2 ——-> CO2+ 2H2O+ energy

2C2H6 + 7O2 ——-> 4CO2 + 6H2O + energy 

• The food that we eat gets converted into glucose by the body. In the process of respiration, glucose is oxidized to produce carbon dioxide, water and energy. This energy is utilized for carrying out life processes. 

C6H12O6 + 6O2 ——> 6CO2+ 6H2O + energy 

How is Combustion different from Burning?

Burning is the physical phenomenon of setting some object on fire whereas combustion is just a chemical reaction. Combustion involves oxidation of the fuel by chemical means which is not seen during burning. Burning always creates a flame but combustion does not necessarily create a flame. Burning produces a low amount of heat whereas combustion results in large amounts of heat production. Burning can produce light energy but combustion may not form light as an energy form in some cases. Also, burning and combustion can be done naturally as well as manually. But, most of the time, burning results in the emission of less harmful gas as compared to combustion.

Applications of the Combustion Reaction

Humans have been using the process of combustion for several millenniums. From cooking food to keeping the home warm, there are many major domestic and industrial applications of combustion.

• In the steam engine, originally coal was burnt to heat water and produce steam. The jet of steam moved pistons that operated the machinery of the engine. Today, this same concept is used in operating thermal power plants.

• Modern vehicles make use of internal combustion engines in which hydrocarbon fuels such as diesel, petrol, etc. are used to move the vehicle.

• The process of combustion is widely used in explosives.

• Rocket propulsion is based on the principle of combustion. A liquid or gaseous propellant is filled in the engine of the rocket. On combustion, several products are ejected from the combustion chamber at very high velocities. This provides a thrust for the rocket to take off.

• In the metallurgy industry, the process of combustion is used in furnaces for purifying metals. 

Harmful Effects of Combustion

In the process of combustion, the chemical energy stored in fuel is converted to heat and light. Combustion is used to move locomotives, vehicles, machinery in industries and for a variety of other purposes. The smoke released from burning pollutes the atmosphere. The number of industries is increasing day by day. As a result, the continuous burning of fuels and the release of smoke from the chimneys and vehicular exhausts are adding up to the carbon dioxide in the atmosphere, thereby disturbing its original composition.

Carbon dioxide and other products of combustion are greenhouse gasses that trap solar radiation and increase the temperature of the earth, leading to a serious problem called global warming. If combustion does not take place efficiently, harmful products such as carbon monoxide and particulate matter are also released. Oxides of sulfur, nitrogen and other organic compounds lead to the formation of ozone and smog on the ground level. The burning of fossil fuels is the biggest contributor to air pollution. The efficiency of internal combustion engines can be increased by installing catalytic converters. Chimneys should be fitted with electrostatic precipitators so that the emanating smoke does not cause as much pollution as it otherwise would.