Describe the alkanes as a homologous series of saturated hydrocarbons from crude oil, write their formulae and combustion equations, and explain complete and incomplete combustion

What this dot point is asking

SEAB wants you to describe the alkanes as a homologous series of saturated hydrocarbons obtained from crude oil, write their formulae and combustion equations, and explain the difference between complete and incomplete combustion, including why incomplete combustion is dangerous. This is the starting point of organic chemistry and links to the energetics topic through combustion.

The answer

Hydrocarbons and crude oil

A hydrocarbon is a compound containing only hydrogen and carbon. Crude oil is a mixture of hydrocarbons formed from the remains of ancient organisms, and it is separated by fractional distillation into useful fractions such as petrol, kerosene and diesel, each used as a fuel. The alkanes are the simplest family of hydrocarbons in crude oil.

The alkanes as a homologous series

The alkanes are a homologous series: a family of compounds with the same general formula and similar chemical properties, where each member differs from the next by $\text{CH}_2$. The alkane general formula is:

So methane is $\text{CH}_4$, ethane $\text{C}_2\text{H}_6$, propane $\text{C}_3\text{H}_8$ and butane $\text{C}_4\text{H}_{10}$. Members of a homologous series share these features: the same general formula, a difference of $\text{CH}_2$ between neighbours, similar chemical reactions, and a gradual trend in physical properties (boiling point rises as the molecules get larger).

Saturated hydrocarbons

The alkanes are saturated: they contain only single carbon-to-carbon bonds, so each carbon is bonded to as many hydrogen atoms as possible. Because the single bonds are strong and the molecules are otherwise unreactive, alkanes are fairly unreactive apart from burning. (This contrasts with the alkenes, which are unsaturated.)

Complete combustion

When an alkane burns in a plentiful supply of oxygen, it undergoes complete combustion to carbon dioxide and water, releasing a lot of heat (an exothermic reaction). For methane:

This clean blue flame is why alkanes are valuable fuels.

Incomplete combustion

When there is a limited supply of oxygen, combustion is incomplete, producing carbon monoxide ($\text{CO}$) and (or) carbon (soot), along with water. Incomplete combustion is dangerous and undesirable because:

• Carbon monoxide is toxic: it is a colourless, odourless gas that prevents the blood from carrying oxygen, so it can poison people without warning.
• Soot blackens surfaces and can block appliances.
• Less energy is released than in complete combustion.

This is why fuel-burning appliances need a good air supply and proper ventilation.

Examples in context

Example 1. Carbon monoxide from a faulty heater. A gas heater burning in a poorly ventilated room can produce carbon monoxide through incomplete combustion. Because the gas is colourless and odourless, victims may not notice it, which is why carbon monoxide detectors and good ventilation are vital, a direct safety consequence of this chemistry.

Example 2. Why a Bunsen burner has an air hole. Opening the air hole lets in plenty of oxygen, giving complete combustion and a hot blue flame; closing it limits oxygen, giving incomplete combustion, a cooler yellow sooty flame. The Bunsen burner demonstrates the difference between complete and incomplete combustion in the lab.

Try this

Q1. State the general formula of the alkanes. [1 mark]

• Cue. $\text{C}_n\text{H}_{2n+2}$.

Q2. Write the balanced equation for the complete combustion of ethane, $\text{C}_2\text{H}_6$. [2 marks]

• Cue. $2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}$.

Q3. Explain why incomplete combustion of a fuel is dangerous. [2 marks]

• Cue. It produces carbon monoxide, a toxic, colourless, odourless gas that stops the blood carrying oxygen, so it can poison people without warning.