Key Differences Between Complete and Incomplete Combustion
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:
With Propane ( C3H8 ), the complete reaction is given as follows:
C3H8 + O2 → 3CO2 + 4H2
With Butane (C4H10), the complete combustion reaction is given as follows:
C4H10 + O2 → 4CO2 + 5H2
The generalised form of this reaction is as follows:
CnH2n+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.
(Image to be added soon)
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
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.
FAQs on Combustion of Hydrocarbons: Complete vs Incomplete
1. What is the combustion of hydrocarbons in chemistry?
The combustion of hydrocarbons is a chemical process that involves a rapid reaction between a hydrocarbon (a compound of hydrogen and carbon) and an oxidant, typically oxygen, to produce heat and light. This exothermic reaction is the primary principle behind the use of fossil fuels for energy. The nature of the products depends on the amount of oxygen available for the reaction.
2. What are the products of the complete combustion of a hydrocarbon?
During complete combustion, which occurs when there is an excess supply of oxygen, a hydrocarbon burns to produce primarily carbon dioxide (CO₂) and water (H₂O). For example, the complete combustion of propane (C₃H₈), used in BBQ grills, is represented by the balanced equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + Heat.
3. What is incomplete combustion and what harmful products does it create?
Incomplete combustion occurs when the supply of oxygen is insufficient. Instead of completely oxidizing to carbon dioxide, the carbon atoms form other substances. The main products are:
- Carbon Monoxide (CO): A colourless, odourless, and highly toxic gas that is dangerous to inhale.
- Carbon (C): Released as fine black particles, commonly known as soot.
- Water (H₂O): Is still produced from the hydrogen atoms in the fuel.
This process releases less energy compared to complete combustion.
4. Why is the combustion of hydrocarbons always an exothermic reaction?
Combustion is an exothermic reaction because the chemical bonds formed in the product molecules (CO₂ and H₂O) are significantly stronger and more stable than the bonds broken in the reactant molecules (hydrocarbon and O₂). The overall process releases the net difference in energy, primarily in the form of heat and light. This energy release is what makes hydrocarbons valuable as fuels.
5. What is the general formula to balance a complete hydrocarbon combustion reaction?
For any hydrocarbon with the formula CₓHᵧ, the general balanced equation for its complete combustion is:
CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O
Here, 'x' represents the number of carbon atoms and 'y' represents the number of hydrogen atoms in the hydrocarbon molecule. This formula is a fundamental tool for stoichiometric calculations in chemistry.
6. How does the combustion of unsaturated hydrocarbons like alkenes differ from saturated ones like alkanes?
While both saturated (alkanes) and unsaturated (alkenes, alkynes) hydrocarbons produce CO₂ and water on complete combustion, there is a key difference in their burning characteristics. Due to a higher carbon-to-hydrogen ratio, unsaturated hydrocarbons are more likely to undergo incomplete combustion. This results in them typically burning with a sootier, more luminous yellow flame compared to the cleaner, blue flame of alkanes under similar conditions.
7. What are some real-world examples of hydrocarbon combustion?
The combustion of hydrocarbons is central to modern life. Common examples include:
- The burning of LPG (Propane/Butane) in a kitchen stove for cooking.
- The combustion of petrol (Gasoline) or diesel in an internal combustion engine to power vehicles.
- The use of natural gas (Methane) in furnaces to heat homes and generate electricity.
- The burning of kerosene in lamps for light.
8. What are the major environmental impacts associated with the large-scale burning of hydrocarbons?
The widespread combustion of hydrocarbons as fossil fuels poses significant environmental challenges. The key problems arise from the reaction products:
- Global Warming: The release of carbon dioxide (CO₂), a potent greenhouse gas, traps heat in the atmosphere, leading to climate change.
- Air Pollution: Incomplete combustion releases toxic carbon monoxide (CO) and particulate matter (soot), which cause respiratory illnesses and smog.
- Acid Rain: Impurities in fuels, like sulphur, can produce sulphur dioxide (SO₂) upon combustion, which dissolves in rainwater to form acid rain, harming ecosystems and infrastructure.
