Balancing combustion equations

 

What is a hydrocarbon?

Hydrocarbons are compounds that contain the elements hydrogen and carbon. They are obtained by the fractional distillation of crude oil.

Hydrocarbons can be divided into three main groups, alkanes, alkenes and alkynes.

Examples of hydrocarbons are propane, C₃H₈ and octane, C₈H₁₈

Carbon atoms	Alkane, CnH2n+2	Alkene, CnH2n	Alkyne, CnH2n-2
1	methane, CH4	NA	NA
2	ethane, C2H6	ethene, C2H4	ethyne, C2H2
3	propane, C3H8	propene, C3H6	propyne, C3H4
4	butane, C4H10	butene, C4H8	butyne, C4H6
5	pentane, C5H12	pentene, C5H10	pentyne, C5H8
6	hexane, C6H14	hexene, C6H12	hexyne, C6H10
7	heptane, C7H16	heptene, C7H14	heptyne, C7H12
8	octane, C8H18	octene, C8H16	octyne, C8H14
9	nonane, C9H20	nonene, C9H18	nonyne, C9H16
10	decane, C10H22	decene, C10H20	decyne, C10H18

 

What is combustion?

Combustion is a self-sustaining chemical reaction in which heat is produced by the burning of a fuel with oxygen.

Note: In Senior Chemistry fuels can be burnt in other substances like chlorine. These substances act in the same way as oxygen and are called oxidizing agents.

 

How to balance combustion equations

A helpful aid in balancing combustion equations is to balance each element in alphabetical order, C H O

Balance the carbon atoms first, hydrogen atoms second and oxygen atoms last, C H O

 

Examples

1. Pentane + oxygen carbon dioxide + water

C₅H₁₂ + O₂ CO₂ + H₂O

Solution

Balance C first	C5H12 + O2 5CO2 + H2O	5 C atoms are needed
Balance H second	C5H12 + O2 5CO2 + 6H2O	12 H atoms are needed
Balance O third	C5H12 + 3O2 5CO2 + 6H2O	6 O atoms are needed

The balanced equation for the complete combustion of pentane is

C₅H₁₂ + 3O₂ 5CO₂ + 6H₂O

 

2. Butane + oxygen carbon dioxide + water

C₄H₁₀ + O₂ CO₂ + H₂O

Solution

Balance C first	C4H10 + O2 4CO2 + H2O	4 C atoms are needed
Balance H second	C4H10 + O2 4CO2 + 5H2O	10 H atoms are needed
Balance O third	C4H10  + 5.5O2 4CO2 + 6H2O	11 O atoms are needed

The balanced equation can be left as a fraction as chemists deal with quantities called moles.

C₄H₁₀  + 5.5O₂ 4CO₂ + 6H₂O

The 5.5 O₂ is not 5½ molecules of O₂. It represents 5.5 moles of oxygen.

If you don't like working in halves then double each number in front  the formula

2C₄H₁₀ + 11O₂ 8CO₂ + 12H₂O

 

3. Ethanol + oxygen carbon dioxide + water

C₂H₅OH + O₂ CO₂ + H₂O

Solution

Balance C first	C2H5OH + O2 2CO2 + H2O	5 C atoms are needed
Balance H second	C2H5OH + O2 2CO2 + 3H2O	6 H atoms are needed
Balance O third	C2H5OH + 3O2 2CO2 + 3H2O	7 O atoms are needed

The balanced equation for the combustion of ethanol is

C₂H₅OH + 3O₂ 2CO₂ + 3H₂O

 

What is complete combustion?

Complete combustion is the burning of hydrocarbons in sufficient oxygen to produce carbon dioxide and water as products.

A common example of complete combustions is the blue flame of a Bunsen burner. The air hole is open.

Complete Combustion Equation

Hydrocarbon + Oxygen Carbon dioxide + water

Example

Propane + oxygen carbon dioxide + water

C₃H₈ + 5O₂ 3CO₂ + 4H₂O

 

What is incomplete combustion?

Incomplete combustion is the burning of a hydrocarbon in insufficient oxygen producing carbon (soot), carbon monoxide and water as products.

A common example of the incomplete combustion of hydrocarbons is the yellow flame of a Bunsen burner. The air hole is closed.

a) Incomplete Combustion Equation - Multiple carbon products

Hydrocarbon + Oxygen Carbon + Carbon monoxide + water

When writing equations with incomplete combustion it is advisable to include only one carbon product otherwise there will be multiple solutions to the equation.

 

Multiple carbon products produce balanced equations with multiple solutions

 

Example

The incomplete combustion of propane, C₃H₈ produces two possible balanced equations.

C₃H₈ + 3 O₂ 2CO + C + 4H₂O

C₃H₈ + 2.5 O₂ CO + 2C + 4H₂O

 

b) Incomplete Combustion Equation - Single carbon product

Hydrocarbon + Oxygen Carbon monoxide + water

and

Hydrocarbon + Oxygen Carbon + water

 

When there is only one carbon product there is only one solution to each equation.

 

Example

If the product is only carbon monoxide CO, there is only one solution to the balanced equation.

C₃H₈ + 4 O₂ 3 CO + 5H₂O

and

If the product is only soot C, there is only one solution to the balanced equation.

C₃H₈ + 2 O₂ 3C + 4H₂O

 

Balancing Equations Worksheet with answers