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Complete vs. Incomplete Combustion of Alkanes

This page deals briefly with the combustion of alkanes and cycloalkanes. In fact, there is very little difference between the two.

Complete combustion

Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water. It is quite important that you can write properly balanced equations for these reactions, because they often come up as a part of thermochemistry calculations. Some are easier than others. For example, with alkanes, the ones with an even number of carbon atoms are marginally harder than those with an odd number!

Example 1: Propane Combustion

For example, with propane (C3H8), you can balance the carbons and hydrogens as you write the equation down. Your first draft would be:

\[ C_3H_8 + O_2 \rightarrow 3CO_2 + 4H_2O\]

Counting the oxygens leads directly to the final version:

\[ C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O\]

Example 2: Butane Combustion

With butane (C4H10), you can again balance the carbons and hydrogens as you write the equation down.

\[ C_4H_{10} + O_2 \rightarrow 4CO_2 + 5H_2O\]

Counting the oxygens leads to a slight problem - with 13 on the right-hand side. The simple trick is to allow yourself to have "six-and-a-half" O2 molecules on the left.

\[ C_4H_{10} + 6\dfrac{1}{2}\, O_2 \rightarrow 4CO_2 + 5H_2O\]

If that offends you, double everything:

\[ 2C_4H_{10} + 13 O_2 \rightarrow 8CO_2 + 10 H_2O\]

The hydrocarbons become harder to ignite as the molecules get bigger. This is because the bigger molecules don't vaporize so easily - the reaction is much better if the oxygen and the hydrocarbon are well mixed as gases. If the liquid is not very volatile, only those molecules on the surface can react with the oxygen. Bigger molecules have greater Van der Waals attractions which makes it more difficult for them to break away from their neighbors and turn to a gas.

Provided the combustion is complete, all the hydrocarbons will burn with a blue flame. However, combustion tends to be less complete as the number of carbon atoms in the molecules rises. That means that the bigger the hydrocarbon, the more likely you are to get a yellow, smoky flame.

Incomplete combustion

Incomplete combustion (where there is not enough oxygen present) can lead to the formation of carbon or carbon monoxide. As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over! The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke. Carbon monoxide is produced as a colorless poisonous gas.

Why carbon monoxide is poisonous: Oxygen is carried around the blood by hemoglobin, which unfortunately binds to exactly the same site on the hemoglobin that oxygen does. The difference is that carbon monoxide binds irreversibly (or very strongly) - making that particular molecule of hemoglobin useless for carrying oxygen. If you breath in enough carbon monoxide you will die from a sort of internal suffocation.