C3.2 Explain complete and incomplete combustion

Ontario Grade 11 Chemistry

Chemical Reactions

How the oxygen supply affects the products of burning

A flame can look different when it does not receive enough air. A flame that produces little visible smoke may be undergoing complete combustion. A yellow, smoky flame may leave black soot. These observations point to differences in the products formed as a fuel burns. Appearance is a clue, not a complete test. We can explain the difference by considering which products form and how many oxygen atoms are available to form them.

What you will learn

Describe combustion as a fuel reacting with oxygen.
Explain how the oxygen supply distinguishes complete from incomplete combustion.
Identify common products of complete and incomplete combustion.
Read and balance simple combustion equations.

1. Prerequisite bridge: reactants, products, and formulas

A chemical reaction changes starting substances, called reactants, into new substances, called products. A chemical equation represents that change with formulas. A formula shows which atoms are in a substance. In a balanced equation, each kind of atom has the same total count on both sides.

Combustion is a reaction in which a fuel reacts with oxygen. A fuel is a substance that can burn. The fuels used in this lesson contain carbon and hydrogen. Oxygen in air is represented by the formula

\mathrm{O_2}

. The subscript

2

means that each oxygen particle contains two oxygen atoms.

A coefficient is a number placed before a formula. It tells how much of that substance is represented. When balancing an equation, change coefficients, not subscripts. Changing a subscript changes the substance itself.

Combustion is a fuel reacting with oxygen.
Reactants are the starting substances; products are the substances formed.
Balance equations by changing coefficients, not subscripts.

2. Observable clues and the particle model

A flame that burns with little visible smoke may be undergoing complete combustion. A flame that produces smoke or black deposits may be undergoing incomplete combustion. Soot is the common name for fine black particles of solid carbon. These visible clues can suggest what is happening, but they do not identify every product.

At the particle level, fuel particles react with oxygen particles. When enough oxygen is available, carbon atoms in a fuel can form carbon dioxide. Hydrogen atoms in a fuel that contains hydrogen can form water. Carbon dioxide is a compound containing carbon and oxygen. Water is a compound containing hydrogen and oxygen.

When oxygen is insufficient for full conversion, some carbon may form carbon monoxide instead of carbon dioxide. Carbon monoxide contains one oxygen atom for each carbon atom. In some cases, solid carbon also forms. Those solid carbon particles can appear as soot. Incomplete combustion can therefore produce carbon monoxide, soot, or both. Water may also form when the fuel contains hydrogen.

Carbon monoxide is poisonous. It has no colour or smell, so people cannot reliably detect it by sight or smell. Visible smoke is not a dependable warning that carbon monoxide is present. The key distinction is the oxygen supply and the products formed, not just the appearance of a flame.

Enough oxygen allows carbon dioxide to form from the fuel's carbon.
Limited oxygen can lead to carbon monoxide and sometimes solid carbon.
A fuel containing hydrogen can form water in either type of combustion.
Flame appearance is a clue, not a reliable way to identify every product.

3. Representing complete combustion

Methane is a fuel containing carbon and hydrogen. In the complete-combustion equation below, its carbon forms carbon dioxide and its hydrogen forms water vapour. Check the atom counts: there is one carbon atom and four hydrogen atoms on each side. There are four oxygen atoms on each side as well.

This equation represents complete combustion because enough oxygen is available for the carbon-containing product to be carbon dioxide. The equation shows the particular products for methane; it is not a claim that every fuel has the same formula.

\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)}

For the carbon-and-hydrogen fuels in these examples, complete combustion forms carbon dioxide and water.
A balanced equation has the same count of each kind of atom on both sides.

4. Comparing incomplete combustion

Methane can also burn with insufficient oxygen to form carbon monoxide and water vapour. The equation below is balanced: it has two carbon atoms, eight hydrogen atoms, and six oxygen atoms on each side. It represents incomplete combustion because oxygen is limited and carbon monoxide forms.

Incomplete combustion does not always produce only carbon monoxide. It may produce solid carbon, or both carbon monoxide and solid carbon. A chemical equation must match the products named for that reaction and conserve each kind of atom.

State symbols describe the physical state of a substance. The symbol

(g)

means gas, and

(s)

means solid. Water is shown as water vapour in these methane examples. State symbols add information about the substances but do not change how atoms are counted. When explaining an equation, name both the oxygen condition and the product pattern.

\mathrm{2CH_4(g) + 3O_2(g) \rightarrow 2CO(g) + 4H_2O(g)}

Limited oxygen can produce carbon monoxide and/or solid carbon.
State symbols show physical state and do not affect atom counting.
Explain the oxygen condition and the products together.

Combustion at a glance

Feature	Complete combustion	Incomplete combustion
Oxygen supply	Enough for the fuel to react fully	Insufficient for full conversion
Carbon-containing products shown	Carbon dioxide	Carbon monoxide and/or solid carbon
Product when fuel contains hydrogen	Water	Water may also form
Possible visible clue	Often little visible smoke	Smoke or black soot may appear

Worked example

Balance and classify an incomplete-combustion equation

Propane burns with limited oxygen to form carbon monoxide and water vapour. Balance the equation and explain why it represents incomplete combustion.

Write the product pattern
The reactants are propane and oxygen. The products are carbon monoxide and water. Write each substance's formula first. Keep the subscripts fixed while balancing.
$\mathrm{C_3H_8 + O_2 \rightarrow CO + H_2O}$
Balance carbon and hydrogen
One propane molecule contains three carbon atoms, so place a coefficient of three before carbon monoxide. Propane contains eight hydrogen atoms, so place a coefficient of four before water. Carbon and hydrogen now match on both sides.
$\mathrm{C_3H_8 + O_2 \rightarrow 3CO + 4H_2O}$
Balance oxygen
The products contain seven oxygen atoms in total: three in carbon monoxide and four in water. Oxygen gas contains two oxygen atoms per particle, so use seven halves as the oxygen coefficient temporarily. Multiply every coefficient by two to obtain whole-number coefficients without changing the atom ratio.
$\mathrm{2C_3H_8 + 7O_2 \rightarrow 6CO + 8H_2O}$
Classify and check
The stated oxygen supply is limited, and carbon monoxide forms. This is incomplete combustion. Count the atoms on both sides to check the equation.
$\mathrm{C: 6=6,\quad H: 16=16,\quad O: 14=14}$

Answer: The balanced equation is

\mathrm{2C_3H_8(g) + 7O_2(g) \rightarrow 6CO(g) + 8H_2O(g)}

. It represents incomplete combustion because oxygen is limited and carbon monoxide forms.

Check: Each side has 6 carbon atoms, 16 hydrogen atoms, and 14 oxygen atoms. The atom counts match.

Common mistakes and how to avoid them

Calling any burning reaction complete combustion because oxygen is present.

Correction: Oxygen must be available in sufficient amount for the complete-combustion product pattern. Limited oxygen can lead to incomplete combustion.

Writing carbon monoxide as the product of complete combustion.

Correction: For the carbon-and-hydrogen fuels in this lesson, complete combustion forms carbon dioxide and water. Carbon monoxide can form during incomplete combustion.

Changing a subscript to balance an equation.

Correction: Keep each formula unchanged. Adjust coefficients in front of formulas so atom counts match.

Treating soot as a gas or assuming every incomplete-combustion reaction makes soot.

Correction: Soot is solid carbon. Incomplete combustion can produce carbon monoxide, soot, or both, depending on the products formed.

Lesson summary

Combustion is a reaction between a fuel and oxygen.
Complete combustion occurs when enough oxygen is available. For the carbon-and-hydrogen fuels in this lesson, it forms carbon dioxide and water.
Incomplete combustion occurs when oxygen is insufficient for full conversion. It can form carbon monoxide and sometimes solid carbon, as well as water when hydrogen is in the fuel.
Balanced equations show the products and conserve every kind of atom.

Check your understanding

Question 1

A fuel containing carbon and hydrogen burns with enough oxygen for complete combustion. Which products should its equation show?

Carbon monoxide and water
Carbon dioxide and water
Carbon dioxide only
Solid carbon and oxygen

Show answer and explanation

Carbon dioxide and water

For the carbon-and-hydrogen fuels in this lesson, complete combustion forms carbon dioxide and water.

Question 2

A balanced equation shows a fuel forming carbon monoxide and water while oxygen is limited. How should this combustion be classified?

Complete combustion
Incomplete combustion
Not combustion because water forms
Complete combustion because oxygen is a reactant

Show answer and explanation

Incomplete combustion

Limited oxygen and carbon monoxide as a product identify the example as incomplete combustion.

Question 3

What does a coefficient change when balancing a chemical equation?

The number of atoms in each particle's formula
The identity of the substance
The amount of a substance represented
The charge on every atom

Show answer and explanation

The amount of a substance represented

A coefficient changes the amount of a substance represented. It does not change that substance's formula.

Key terms

Combustion: A chemical reaction in which a fuel reacts with oxygen.
Fuel: A substance that can burn in a combustion reaction.
Complete combustion: Combustion with enough oxygen for the fuel to form the complete-combustion products represented in the equation.
Incomplete combustion: Combustion with insufficient oxygen for full conversion, which can form carbon monoxide and/or solid carbon.
Soot: Fine black particles of solid carbon.
Coefficient: A number placed in front of a chemical formula to show the amount of that substance represented in an equation.

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C3.2 · Explain complete and incomplete combustion

Ontario Grade 11 Chemistry

How the oxygen supply affects the products of burning

What you will learn

1. Prerequisite bridge: reactants, products, and formulas

2. Observable clues and the particle model

3. Representing complete combustion

4. Comparing incomplete combustion

Combustion at a glance

Balance and classify an incomplete-combustion equation

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SCH3U

About this lesson and its review