Describe the alkanes as a family of saturated hydrocarbons, write the combustion of a simple alkane, and compare complete with incomplete combustion

What this dot point is asking

The syllabus wants you to describe the alkanes as a family of saturated hydrocarbons, to write the combustion of a simple alkane, and to compare complete with incomplete combustion. The key ideas are that a hydrocarbon contains only hydrogen and carbon, that alkanes have only single bonds (so they are saturated), and that how completely they burn depends on how much oxygen is available.

The answer

What a hydrocarbon is

A hydrocarbon is a compound made of hydrogen and carbon only. Crude oil is a mixture of many hydrocarbons, and most of our fuels come from it.

The alkane family

The alkanes are a family of hydrocarbons with similar properties. The first three are:

• methane, $\text{CH}_4$,
• ethane, $\text{C}_2\text{H}_6$,
• propane, $\text{C}_3\text{H}_8$.

Alkanes have only single bonds between their atoms, which is why they are called saturated: every carbon is bonded to as many atoms as it can hold, with no double bonds.

Why alkanes are good fuels

Alkanes burn easily in air, and burning them is exothermic (it gives out a lot of heat). That makes them useful fuels for heating, cooking, and powering vehicles.

Complete combustion

When there is plenty of oxygen, an alkane undergoes complete combustion. It burns fully to give carbon dioxide and water, with a clean blue flame:

Incomplete combustion

When there is not enough oxygen, the alkane undergoes incomplete combustion. As well as water, it can produce carbon monoxide (a poisonous gas) and carbon (soot), and the flame is yellow and smoky. Carbon monoxide is dangerous because it stops the blood carrying oxygen, which is why gas heaters must have a good air supply.

Examples in context

Example 1. Natural gas for cooking. The gas piped to homes for cooking is mostly methane, an alkane. A well-adjusted burner shows a blue flame (complete combustion); a yellow, smoky flame is a warning that the burner needs cleaning so it burns fully and safely.

Example 2. Carbon monoxide alarms. Homes with gas heaters often have carbon monoxide alarms, because incomplete combustion of alkane fuels can release this poisonous, odourless gas. The chemistry of incomplete combustion is exactly why these alarms save lives.

Try this

Q1. State what is meant by a saturated hydrocarbon. [2 marks]

• Cue. A compound of hydrogen and carbon only that has just single bonds between its atoms (no double bonds).

Q2. Write the word equation for the complete combustion of ethane. [1 mark]

• Cue. Ethane + oxygen $\rightarrow$ carbon dioxide + water.

Q3. Explain why a gas heater can produce carbon monoxide if it is used in a poorly ventilated room. [2 marks]

• Cue. Poor ventilation means too little oxygen, so the fuel burns incompletely and produces poisonous carbon monoxide instead of only carbon dioxide.