Hydrocarbons: Question 1

Worked solution

Why the products depend on the oxygen supply

Butane, like any alkane, releases energy by reacting with oxygen, but exactly which products form depends on how much oxygen is available per mole of fuel.

Complete combustion (plentiful air)

With a plentiful supply of oxygen, every carbon atom is fully oxidised to carbon dioxide, and every hydrogen atom ends up in water:

$2\text{C}_4\text{H}_{10} + 13\text{O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O}$

This is the reaction occurring in a stove burning with a clean blue flame.

Incomplete combustion (limited air)

If the air supply is restricted, there is not enough oxygen to oxidise every carbon atom all the way to $\text{CO}_2$. Some carbon may instead end up as carbon monoxide:

$2\text{C}_4\text{H}_{10} + 9\text{O}_2 \rightarrow 8\text{CO} + 10\text{H}_2\text{O}$

or, if oxygen is even more restricted, as fine solid carbon particles (soot), with no gaseous carbon oxide at all:

$2\text{C}_4\text{H}_{10} + 5\text{O}_2 \rightarrow 8\text{C} + 10\text{H}_2\text{O}$

Check (atom balance for the soot equation): C: $2 \times 4 = 8$ on the left, $8$ on the right. H: $2 \times 10 = 20$ on the left, $10 \times 2 = 20$ on the right (all in water). O: $5 \times 2 = 10$ on the left, $10 \times 1 = 10$ on the right (all from water, since elemental carbon carries no oxygen). All three elements balance.

In practice, whenever the air supply is only partially restricted, a mixture of $\text{CO}_2$, $\text{CO}$ and carbon (soot) can all form together, alongside water. A poorly ventilated gas appliance is a real hazard for exactly this reason, since the colourless, odourless carbon monoxide produced is toxic.

Why the other options are wrong

• A swaps the two definitions: complete combustion is the one that gives only $\text{CO}_2$ and water, not carbon monoxide.
• C ignores that the amount of available oxygen genuinely changes the products formed.
• D is incorrect: saturation (only single C-C bonds) affects an alkane’s addition chemistry, not whether its combustion can be incomplete. Incomplete combustion depends only on the oxygen supply, not on the presence or absence of C=C bonds.

Final answer

B, complete combustion of butane forms only carbon dioxide and water; a restricted air supply can additionally produce carbon monoxide and/or soot.