Burning Does Not Always Produce CO2 and H2O

What students typically write

“When any hydrocarbon burns, it produces carbon dioxide and water.” Or students write only the complete combustion equation when the question specifies limited air.

Students learn the complete combustion equation first and apply it universally, ignoring the oxygen supply conditions stated in the question.

Why it is wrong

Combustion of hydrocarbons has two outcomes depending on the oxygen supply:

Complete combustion (excess oxygen): hydrocarbon + oxygen goes to carbon dioxide + water. Example: CH4 + 2O2 goes to CO2 + 2H2O

Incomplete combustion (limited oxygen): hydrocarbon + limited oxygen goes to carbon monoxide and/or carbon (soot) + water. Example: 2CH4 + 3O2 goes to 2CO + 4H2O Or: CH4 + O2 goes to C + 2H2O

The products depend entirely on how much oxygen is available. In a well-ventilated Bunsen burner with an open air hole, complete combustion occurs (blue flame). In a Bunsen burner with a closed air hole, incomplete combustion occurs (yellow flame, soot).

Incomplete combustion is a major safety concern because carbon monoxide (CO) is toxic. It is colourless and odourless, so it cannot be detected without a CO alarm. It binds to haemoglobin in the blood, preventing it from carrying oxygen, which can be fatal. This is why gas appliances must be properly ventilated and serviced.

What the mark scheme actually wants

A typical question: “Methane burns in a limited supply of air. State the products of this reaction.” [2]

Marking points:

1. Carbon monoxide / CO (or carbon / soot) (1 mark)
2. Water / H2O (1 mark)

An answer that gives CO2 and H2O scores 0 for the first mark because the question specifies limited air.

Worked example: wrong vs right

Question: Write the equation for the incomplete combustion of propane (C3H8) to produce carbon monoxide and water. [2]

Wrong answer: C3H8 + 5O2 goes to 3CO2 + 4H2O. This scores 0/2. It is the complete combustion equation.

Right answer: 2C3H8 + 7O2 goes to 6CO + 8H2O (1 for correct products, 1 for balancing). This scores 2/2.

How to avoid this mistake

Read the question carefully for oxygen supply clues:

• “Excess air/oxygen”, “plenty of air”, “complete combustion” = CO2 and H2O
• “Limited air/oxygen”, “insufficient oxygen”, “incomplete combustion” = CO and/or C, plus H2O

If the question does not specify, and asks you to write a combustion equation without qualification, complete combustion (CO2 and H2O) is the default. But if the question mentions limited air, soot, yellow flames, or carbon monoxide, you must give the incomplete combustion products.

For the full treatment of combustion and fuel chemistry, see fuels and alkanes.