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Combustion of Hydrocarbons Alkanes is the process that involves breaking of bonds between the atoms either with plenty or limited oxygen.
- Combustion is the process of burning some substances at high temperature.
- It is an exothermic reaction that occurs between fuel and oxygen, resulting in the production of a gaseous material (smoke).
- Hydrocarbons are compounds made only with the elements of hydrogen and carbon.
- Hydrocarbons are broadly classified as alkanes, alkenes, and alkynes.
- Combustion of alkanes with oxygen-generated heat converts an alkane into CO2, water, and heat energy.
- Alkanes are saturated hydrocarbons with four additional atoms surrounding the core carbon atom (or groups).
Read More: NCERT Solutions for Class 11 Chemistry Hydrocarbons
Key Terms: Combustion of Hydrocarbons Alkanes, Combustion, Alkanes, Complete Combustion, Incomplete Combustion, Spontaneous Combustion
Combustion of Hydrocarbons Alkanes
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Combustion of Hydrocarbons Alkanes is a chemical reaction that takes place with the release of heat, light, and water vapour.
- It is the process of breaking down alkanes into hydrogen and carbon dioxide.
- It takes place either with plenty of oxygen known as Complete Combustion or limited Oxygen, known as Incomplete Combustion.
- Combustion is an exothermic reaction and the heat released can provide warmth or energy for cooking or other purposes.
Combustion of Hydrocarbons Alkanes
What is Alkane?
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Alkane is generally less reactive, as its C-C bonds are stable and are not easily broken. Alkanes' reactivity is quite low because of the saturation. Saturated hydrocarbons were previously referred to as paraffin, which means "little affinity." These alkanes burn, though, quite quickly.
While gaseous alkanes are very explosive in air, liquid alkanes are extremely flammable. The most common reactions occurring with alkanes are combustion, and free radical halogenation reactions.
When hydrocarbons burn completely:
- Carbon is oxidised to carbon dioxide.
- Water is formed when hydrogen oxidises.
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Testing for Alkanes
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Due to the presence of the C=C double bond, alkenes can react in ways that alkanes cannot. This enables us to distinguish alkenes from alkanes using a simple chemical test.
Bromine water is an orange solution of bromine. When it is shaken with an alkene, it loses its colour. Bromine water can be decoloured by alkenes, but not by alkanes. The reaction between Bromine and Alkanes is an example of a type of reaction called an addition reaction. Bromine is decolourized because a colourless dibromo compound forms.
For example,
Ethene + Bromine \(\to\) Dibromoethane
C2H4 + Br2 \(\to\) C2H4Br2
Types of Hydrocarbons Combustion
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Combustion can be classified into several categories based on the amount of energy required and the reaction’s byproducts. The five different forms of combustion are as follows.
Complete Combustion
Alkanes are called saturated hydrocarbons (having single bonds between carbon atoms). They burn with a non-luminous flame and an insufficient supply of air to release energy; hence they are used as fuels. Carbon dioxide and water vapour are produced when alkanes are completely burned.
Alkanes react with oxygen in a combustion reaction. The general equation for complete combustion is
2CnH2n+2 + (3n+1) O2 \(\to\) 2(n+1) H2O + 2n CO2
Here are the equations for the complete combustion of methane:
Methane + Oxygen \(\to\) Carbon dioxide + Water
CH4 (g) + 2O2 (g) \(\to\) CO2 (g) + 2H2O (l)
Incomplete Combination
In the lack of sufficient oxygen, incomplete combustion occurs, resulting in the creation of carbon monoxide and soot, as illustrated here for methane:
2 CH4 + 3 O2 \(\to\) 2 CO + 4 H2O
CH4 + O2 \(\to\) C + 2 H2O
In general, the equation for incomplete combustion is:
Hydrocarbon + Oxygen \(\to\) Carbon Monoxide + Carbon + Water
Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide:
Propane + Oxygen \(\to\) Carbon + Water
C3H8 + 2O2 \(\to\) 3C + 4H2O
Read More: Hydrocarbon Important Questions
Rapid Combustion
Combustion in which a substance burns rapidly and produces heat and flame is known as rapid combustion. This is usually attained by introducing external heat. It is also known as fast combustion. Substances that undergo rapid combustion have lower ignition temperatures—for example, the combustion of LPG, CNG, petrol, etc.
Spontaneous Combustion
Spontaneous combustion occurs when a substance unexpectedly begins to burn without the assistance of any external factor, such as heating. Substances that have relatively low ignition temperatures generally burn by spontaneous combustion.
Some examples of spontaneous combustion are as follows:
- Phosphorus and Sulphur start burning instantaneously at room temperature.
- Because of the increase in temperature, haystacks, linseed oil, coal, pyrite, and other materials can suddenly burst into flame.
Explosion
When combustion is accompanied by the sudden production of heat, sound, and a large amount of gas, it is called an explosion. This form of combustion occurs at a rapid speed. Explosions of firecrackers and bombs occur. When a spark ignites the fuse, the fireworks explode, producing heat, light, and sound.
The standard enthalpy changes of combustion, for alkanes, increase by about 650 kJ/mol per CH2 group.
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Steps in Combustion of Hydrocarbons Alkanes
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The process of combustion in Alkanes occurs in a two-step reaction.
- First, the alkane must be heated to its ignition temperature.
- This is the point at which the molecules break down into more reactive fragments and then react with oxygen molecules to produce heat and light.
- The second step is called oxidation where the heat and light produced by the first step cause further reactions that consume more oxygen and release more heat.
Combustion of Hydrocarbons Alkanes occurs at a high temperature because it requires oxygen to be present for the reaction to occur. This makes it easier for the reaction to produce energy while also making it more dangerous because there is always a risk of fire or explosion when oxygen is present.
Things to Remember
- Hydrocarbon is a chemical composed of only two elements: carbon and hydrogen atoms.
- Combustion includes burning the organic compound, releasing carbon dioxide and water, and releasing a lot of heat energy.
- Combustion is a critical process, and hydrocarbons constitute the primary energy source for both residential and commercial processes.
- Alkane is referred to as a saturated open-chain hydrocarbon consisting of a single carbon-carbon bond.
- Combustion of Hydrocarbons Alkanes is a chemical reaction that involves breaking of bonds between the atoms either with plenty or limited oxygen.
Previous Years’ Questions
- The alkane that gives only one mono-chloro product on chlorination… (NEET - 2019)
- The distance between two adjacent carbon atoms is largest in… (NEET - 1994)
- The correct order of boiling points of 2, 2−dimethylpropane… (KEAM)
- When methane is heated with dioxygen in the presence of… (KEAM)
- Which of the following is not an endothermic reaction… (JKCET - 2005)
- The reaction of propene with HOCl proceeds via the addition of… (JEE Advanced - 2001)
- The major constituent of natural gas is…
- The reagent which can do the conversion… (KCET - 2021)
- The organic compound with two asymmetric carbon atoms is… (KEAM)
- The 4th higher homologue of ethane is… (WBJEE - 2014)
Sample Questions
Ques. Explain Incomplete Combustion. (3 Marks)
Ans. Incomplete combustion takes place in the absence of sufficient oxygen, hence leading to the formation of carbon monoxide and soot.
2 CH4 + 3 O2 \(\to\) 2 CO + 4 H2O
CH4 + O2 \(\to\) C + 2 H2O
The equation for Incomplete Combustion is
Hydrocarbon + Oxygen \(\to\) Carbon Monoxide + Carbon + Water
Ques. What is Combustion of Hydrocarbons Alkanes? (5 Marks)
Ans. Combustion of Hydrocarbons Alkanes is a chemical reaction that involves the release of heat, light, and water vapour on burning the compound. It is defined as the process of breaking down alkanes into hydrogen and carbon dioxide.
Combustion of Alkanes has two types which are as follows.
Complete Alkane Combustion
It is the process of burning an alkane in a sufficient amount of air or oxygen, resulting in carbon dioxide, water, and a large amount of heat.
The complete reaction for Propane (C3H8) is as follows:
C3H8 + O2 \(\to\) 3CO2 + 4H2
It is commonly utilised as a fuel since it generates a large quantity of heat.
Incomplete Alkane Combustion
It is the process of burning alkanes without sufficient air or oxygen. It creates carbon and carbon monoxide as byproducts, with carbon monoxide being a colourless toxic gas.
The incomplete reaction for methane (CH4) is as follows:
CH4 + O2 \(\to\) C + 2H2O
The carbon black produced as a byproduct of the combustion process is employed in the production of inks.
Ques. Write down the equation for the complete Combustion of Methane. (1 Mark)
Ans. The equations for the complete combustion of methane is as follows:
Methane + Oxygen \(\to\) Carbon dioxide + Water
CH4 (g) + 2O2 (g) \(\to\) CO2 (g) + 2H2O (l)
Ques. What is meant by Rapid Combustion? (3 Marks)
Ans. Combustion is a chemical reaction that occurs when chemicals react with oxygen in the air to produce heat and light. Combustion in which a substance burns rapidly and produces heat and flame is known as rapid combustion. This is usually done by introducing external heat. It is also known as fast combustion. Substances that undergo rapid combustion have lower ignition temperatures.
Examples of rapid combustion include
- Lighting a candle, which burns continuously until all of the wax is consumed.
- The combustion of kerosene oil in a kerosene stove produces heat and light.
- Cooking gas burns instantly, providing heat and light.
Rapid combustion is distinct from other kinds of combustion in the fact that it lasts as long as the fuel.
Ques. What are Hydrocarbons? (3 Marks)
Ans. Hydrocarbons are organic molecules composed completely of two types of atoms: carbon and hydrogen.
- Hydrocarbons are typically colourless gases with relatively mild odours.
- They can have simple or complicated structures and are categorised into four subcategories: alkanes, alkenes, alkynes, and aromatic hydrocarbons.
- Hydrocarbons such as propane and butane are employed in the commercial fuel Liquefied Petroleum Gas (LPG).
- Benzene, one of the most basic aromatic hydrocarbons, is used to create several pharmaceutical drugs.
Ques. What is meant by the Explosion Combination? (2 Marks)
Ans: When combustion is accompanied by the sudden production of heat, sound, and a large amount of gas, it is called an explosion. This type of combustion occurs at an extremely rapid speed. Firecrackers and bombs undergo an explosion. The fireworks burst when a spark ignites the fuse, producing heat, light, and sound.
Ques. What is meant by Complete Combustion of Alkanes? Write an Example. (5 Marks)
Ans. The complete combustion of alkanes produces carbon dioxide and water vapour. Alkanes react with oxygen in a combustion reaction. The general equation for complete combustion is
2 CnH2n+2 + (3n+1) O2 \(\to\) 2(n+1) H2O + 2nCO2
For example:
Methane + oxygen \(\to\) carbon dioxide + water
CH4 (g) + 2O2 (g) \(\to\) CO2 (g) + 2H2O (l)
Ques. What is the Combustion of Hydrocarbons? (3 Marks)
Ans. Hydrocarbons Combustion is the chemical reaction in which a hydrocarbon combines with oxygen to produce carbon dioxide, water, and heat. This process happens when there is a sufficient supply of oxygen to react with the hydrocarbons.
- Hydrocarbons are molecules that are made up of both hydrogen and carbon.
- They are well known for being the main component of fossil fuels such as natural gas, petroleum, and coal. As a result, fossil fuel resources are frequently referred to as hydrocarbon resources.
- If a hydrocarbon completely ignites, it burns with a blue flame. As the molecular mass of a hydrocarbon grows, it begins to burn with a yellow flame, indicating incomplete combustion. With rising molecular weights, burning hydrocarbons becomes more challenging.
Ques. How is the Testing of Alkanes done? (3 Marks)
Ans. The presence of the C=C double bond allows alkenes to react in ways alkanes cannot. This allows us to tell alkenes apart from alkanes using a simple chemical test.
- Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene.
- Alkenes decolourise bromine water, but alkanes cannot.
- The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction.
- The bromine is decolourised because a colourless dibromo compound forms.
For example:
Ethene + Bromine → Dibromoethane
C2H4 + Br2 \(\to\) C2H4Br2
Ques. What are the environmental risks related to the Combustion of Hydrocarbons? (5 marks)
Ans. Carbon monoxide pollution is also caused by the incomplete combustion of hydrocarbons. Carbon monoxide, an odourless and colourless gas, causes headaches and difficulties in persons with heart disease.
- When hydrocarbons are burned, carbon dioxide is always released. It is a major contributor to global climate change and ocean acidification.
- The combustion of oil and coal, in particular, produces nitrogen oxides and sulphur dioxide. These oxides react with water and oxygen in the atmosphere to form nitric and sulfuric acids, which then fall to the Earth's surface as acid deposition, sometimes known as "acid rain." Acid deposition damages trees and harms aquatic life.
- It affects ecological and farm production by making some nutrients, such as calcium and phosphorus, less available to plants.
- Another issue with nitrogen oxides is that they, like hydrocarbon pollutants, contribute to creating tropospheric ozone, which is a key component of smog.
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