Introduction to Alkanes and Fractional Distillation Revision Notes | A Level AQA Chemistry

1. Why Alkanes Matter: The Starting Point of Organic Chemistry

Every time you fill a car with petrol, fly in an aircraft, or use a plastic bottle, you are interacting with alkanes. These molecules are the backbone of the petrochemical industry and the first organic compound class you will study at A Level.

Understanding alkanes is not just about memorising a formula. It is about grasping why certain molecules burn well, why some liquids evaporate faster than others, and how a single crude oil deposit can be turned into dozens of different products. That understanding starts here.

Saturated does not mean a saturated solution here. In organic chemistry, saturated means the molecule contains only single C-C bonds and no C=C or C≡C bonds.

Alkanes belong to a class of compounds called hydrocarbons — molecules containing only carbon and hydrogen. The defining feature of an alkane is that it is saturated, meaning every carbon atom forms only single bonds. There are no double or triple bonds anywhere in the molecule.

The word 'saturated' is borrowed from the idea that carbon is 'full up' — it cannot accept any more hydrogen atoms without breaking existing bonds. Compare this to an unsaturated fat in food science: the same principle applies. A carbon-carbon double bond means there is room to add more hydrogen across it.

At A Level, you need to be precise: alkanes are saturated and they are hydrocarbons. Both conditions must be stated. A molecule could be saturated but contain oxygen (like an alcohol), so the hydrocarbon label is equally important.

2a. The General Formula and Homologous Series

Alkanes form a homologous series — a family of compounds that share the same general formula, differ by a $CH_2$ unit from one member to the next, and show a gradual trend in physical properties. The general formula for alkanes is $C_nH_{2n+2}$, where $n$ is the number of carbon atoms.

Notice how each successive member adds one carbon and two hydrogen atoms. This $CH_2$ increment is the hallmark of any homologous series. The gradual change in state — from gases to liquids to waxy solids as chain length increases — is a direct consequence of increasing intermolecular forces, which you will explore shortly.

That retrieval check is exactly the kind of two-part question AQA uses. Giving the formula alone is worth one mark; the explanation of saturation earns the second. Always address both parts when the question asks you to 'state and explain'.

3. Petroleum: A Mixture of Alkanes

Crude oil, or petroleum, is not a single compound. It is a complex mixture consisting mainly of alkane hydrocarbons with different chain lengths, ranging from short-chain gases like methane through to long-chain waxy solids. Because it is a mixture, its components are not chemically bonded to one another and can be separated by physical means.

The key physical property that makes separation possible is boiling point. Different alkanes have different boiling points, and those boiling points increase systematically with chain length. This trend arises because longer chains have greater surface area, which means stronger London dispersion forces (also called van der Waals forces) between molecules. More energy is needed to overcome those forces, so the boiling point is higher.

4. Fractional Distillation of Petroleum

Because petroleum is a mixture with components of different boiling points, it can be separated using fractional distillation. This is a physical separation process carried out in a fractionating column at an oil refinery. The principle is straightforward: heat the mixture so that components vaporise at different temperatures, then condense each vapour at the appropriate level of the column.

Worked example: How Fractional Distillation of Petroleum Works

Describe, in sequence, how petroleum is separated into fractions in a fractionating column.

1. Petroleum is heated to approximately 350-400 °C in a furnace, converting most of it to vapour. — Heating provides enough energy to overcome the intermolecular forces in the liquid mixture, converting components to the gaseous state.
2. The hot vapour mixture is fed into the base of the fractionating column, which has a temperature gradient: hottest at the bottom (~350 °C), coolest at the top (~25 °C). — The temperature gradient is essential. It means different alkane fractions will condense at different heights in the column.
3. As vapour rises up the column, it cools. When a component reaches the level where the temperature equals its boiling point, it condenses back to liquid and is collected on a tray. — Short-chain alkanes (low boiling points) condense near the top; long-chain alkanes (high boiling points) condense near the bottom. Very short chains (e.g. methane, ethane) remain as gases and exit at the top.
4. Each tray collects a fraction — a mixture of alkanes with similar chain lengths and therefore similar boiling points. Fractions are drawn off at different heights. — A fraction is not a pure compound; it is still a mixture, but a narrower one than the original petroleum. Each fraction has a characteristic boiling point range.
5. Very long-chain alkanes that do not vaporise at the furnace temperature remain as a liquid residue at the base of the column. — These heavy residues are used as bitumen for road surfacing or further processed by cracking.

Answer: Petroleum is heated to vapour, fed into the base of a fractionating column, and components condense at different heights according to their boiling points. Short-chain fractions collect near the top; long-chain fractions collect near the bottom. Each fraction is a mixture of alkanes with similar boiling points.

The chart above illustrates the key trend: as you move from the top to the bottom of the fractionating column, the boiling point range of each fraction increases. This directly reflects the increasing chain length and the stronger London dispersion forces in those longer molecules.

You do not need to memorise exact carbon numbers for every fraction, but you should be able to describe the general trend and explain it in terms of intermolecular forces. AQA questions frequently ask you to 'explain why' a fraction has a higher boiling point than another, and the London dispersion force argument is the answer they are looking for.

5. Comparing Fractions: Properties and Uses

The physical properties of each fraction are directly linked to chain length, and those properties determine what each fraction is useful for. Three properties change systematically across the fractions: boiling point, viscosity (resistance to flow), and flammability.

• Boiling point increases with chain length — stronger London dispersion forces require more energy to overcome
• Viscosity increases with chain length — longer molecules become more entangled and flow less freely
• Flammability decreases with chain length — shorter chains ignite more readily, making them better fuels for engines requiring rapid combustion
• Colour tends to darken with chain length — short-chain fractions are colourless liquids or gases; long-chain fractions are dark, viscous oils

These trends explain why petrol (a short-chain fraction) is used in car engines — it vaporises readily and ignites easily — while bitumen (a very long-chain residue) is used for road surfaces, where its high viscosity and resistance to flow are advantageous rather than problematic.

6. Exam Technique and Mark-Winning Strategies

AQA questions on this topic tend to fall into three categories: recall of definitions, explanation of trends, and evaluation of the separation process. Each requires a different approach, and the command word tells you exactly which one is needed.

For 'describe' questions about fractional distillation, give a sequential account of what happens — crude oil is heated, vapour rises, components condense at different heights, fractions are collected. Do not explain why at this stage; the command word 'describe' asks for what, not why.

For 'explain' questions about boiling point trends, you need a three-step chain: chain length increases → surface area increases → London dispersion forces are stronger → more energy is needed to separate molecules → boiling point is higher. Stopping at 'longer chains have higher boiling points' will not earn the explanation mark.

Notice how the model answer above follows the three-step chain precisely and uses the term 'London dispersion forces' rather than the vague phrase 'intermolecular forces' alone. Naming the specific force type is what separates a three-mark answer from a four-mark answer at A Level.

That recall question models the 'evaluate a claim' style that appears in AQA Paper 1. The key move is to state the correct classification, define both terms, and then use evidence (variable composition, physical separation) to justify your conclusion. A one-line answer will not earn full marks.

To consolidate the two specification requirements covered in this lesson: alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$, and petroleum is a mixture of mainly alkane hydrocarbons that is separated into fractions by fractional distillation, exploiting differences in boiling point that arise from differences in chain length and London dispersion force strength.