What is a Combustion Reaction?

A combustion reaction is an exothermic chemical reaction between substances, usually including oxygen gas and accompanied by the generation of heat, energy, and light (flame). The products of a combustion reaction depend on the combusted substance. When metals and nonmetals burn in the presence of oxygen, they will give off their corresponding oxides. On the other hand, hydrocarbons give off carbon dioxide and water upon combustion.

A hydrocarbon is a compound made up solely of carbon and hydrogen. Hydrocarbons are the primary constituent of fossil fuels, namely natural gas, petroleum, and coal. They are used as fuel because their combustion releases enormous amounts of energy. Since different hydrocarbons have different ratios of hydrogen to carbon, they produce different ratios of water to carbon dioxide ^[1-6].

General Equation

The general equation for combustion reaction of metals and nonmetals is:

x M + (y/2) O₂ → M_xO_y

The general equation for the combustion of hydrocarbon is ^[1]:

C_xH_y + (x+y/4) O₂ → x CO₂ + (y/2) H₂O

Complete and Incomplete Combustion Reactions

Complete combustion reactions, also known as clean combustion reactions, is the complete oxidation of the fuel. Such reactions often liberate only water and carbon dioxide as the products, apart from heat and light. An example could be the combustion of propane in the presence of oxygen.

Incomplete combustion reactions are the combustion reactions that involve the formation of by-products such as ash and soot. This reaction happens when the supply of oxygen is inadequate, and carbon and carbon monoxide are produced. An example could be the combustion of propane in an environment where the oxygen is deficient ^[7].

How to Identify a Combustion Reaction?

A combustion reaction is a reaction in which one of the reactants is oxygen, and the other reactant is usually a hydrocarbon. The products are carbon dioxide and water, accompanied by heat and light (flame). The products are either complete or incomplete, in which case, carbon and carbon monoxide results. Inorganic substances can also burn in the presence of oxygen to give vapors of oxides ^[6].

Balancing Combustion Reaction ^[7]

Method 1: Balancing it directly.

Consider the combustion of the hydrocarbon propane (C₃H₈) in the presence of oxygen (O₂), resulting in carbon dioxide (CO₂) and water (H₂O).

C₃H₈ (g) + O₂ (g) → CO₂ (g) + H₂O (g)

We start with the carbon atom. We note that there are three atoms on the left-hand side of the equation and one on the right. So, we multiply CO₂ by 3.

C₃H₈ (g) + O₂ (g) → 3 CO₂ (g) + H₂O (g)

Then, we move to the hydrogen atom. We note that there are eight atoms on the left and two on the right. So, we multiply H₂O by 4.

C₃H₈ (g) + O₂ (g) → 3 CO₂ (g) + 4 H₂O (g)

Finally, we focus on oxygen. We find that there are ten atoms on the right and two on the left. So, we multiply O₂ by 5.

C₃H₈ (g) + 5 O₂ (g) → 3 CO₂ (g) + 4 H₂O (g)

This equation is now a balanced equation representing the combustion of propane.

Method 2: Using the general equation

We can also use the general equation to balance the combustion reaction equation.

C_xH_y + (x+y/4) O₂ → x CO₂ + (y/2) H₂O

Here, x = 3 and y = 8.

C₃H₈ (g) + 5 O₂ (g) → 3 CO₂ (g) + 4 H₂O (g)

Examples of Combustion Reaction

There are a few examples of combustion reactions of metals, nonmetals, and hydrocarbons. The reactions, along with the formulas of the compounds, are given below ^[1-10].

Combustion of Metals and Nonmetals

Metals and nonmetals combine with oxygen to give their corresponding oxides. These oxides are released in the atmosphere as gases. Some examples are as follows:

• Hydrogen (H₂) reacts with oxygen (O₂) to give water vapor (H₂O).

2 H₂ (g) + O₂ (g) → 2 H₂O (g)

• Carbon or coal (C) burns with oxygen to give and carbon monoxide (CO) and carbon dioxide (CO₂)

2 C (s) + O₂ (g) → 2 CO (g)

C (s) + O₂ (g) → CO₂ (g)

• Sulfur (S) burns with oxygen to give carbon dioxide (SO₂)

S (s) + O₂ (g) → SO₂ (g)

• Magnesium (Mg) burns in oxygen (O₂) to give magnesium oxide (MgO)

2 Mg (s) + O₂ (g) → MgO (g)

• Combustion of nitrogen (N₂) with oxygen results in nitric oxide (NO) and nitrogen dioxide (NO₂)

N₂ (g) + O₂ (g) → 2 NO (g)

N₂ (g) + 2 O₂ (g) → 2 NO₂ (g)

Combustion of Hydrocarbons

Hydrocarbons are an organic compound primarily made up of carbon and hydrogen. They are an excellent source of fuel as they ignite easily in the presence of oxygen to give carbon dioxide (CO₂) and water (H₂O).

• Methane (CH₄)

CH₄ (g) + 2 O₂ (g) → CO₂ (g) + 2 H₂O (g)

• Ethane (C₂H₆)

2 C₂H₆ (g) + 7 O₂ (g) → 4 CO₂ (g) + 6 H₂O (g)

• Butane (C₄H₁₀)

2 C₄H₁₀ (g) + 13 O₂ (g) → 8 CO₂ (g) + 10 H₂O (g)

• Hexane (C₆H₁₄)

2 C₆H₁₄ (g) + 13 O₂ (g) → 12 CO₂ (g) + 3 H₂O (g)

• Octane (C₈H₁₈)/gasoline

2 C₈H₁₈ (g) + 25 O₂ (g) → 16 CO₂ (g) + 18 H₂O (g)

• Methanol

2 CH₃OH (l) + 3 O₂ (g) → 2 CO₂ (g) + 4 H₂O (g)

• Ethylene

C₂H₄ (g) + 3 O₂ (g) → 2 CO₂ (g) + 2 H₂O (g)

• Acetone

C₃H₆O (l) + 4 O₂ (g) → 3 CO₂ (g) + 3 H₂O (g)

• Naphthalene

C₁₀H₈ (s) + 12 O₂ (g) → 10 CO₂ (g) + 4 H₂O (g)

• Benzoic acid

2 C₆H₅CO₂H (s) + 15 O₂ (g) → 14 CO₂ (g) + 6 H₂O (g)

Examples and Uses of Combustion Reaction in Everyday Life

There are a few examples and uses of combustion reactions in real life ^[10].

• Heating homes using wood or coal
• Running vehicles using petrol or diesel
• Lighting candles, which is made from paraffin wax, a hydrocarbon fuel
• Cooking food on a stovetop using natural gas or liquefied petroleum gas (LPG)
• Lighting a lighter, which consists of butane, a highly flammable hydrocarbon fuel
• Lighting a matchstick due to the presence of sulfur
• Production of energy from coal, oil, and natural gas in thermal power plants generates electricity
• Bursting of firecrackers, which contain many flammable chemicals
• Combustion of rocket propellent, which contains many hydrocarbons and its derivatives

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