Properties of Hydrocarbons

Physical Properties of Hydrocarbons

Hydrocarbons are compounds made of hydrogen and carbon atoms only. Their physical properties depend mainly on the size (chain length) of the hydrocarbon molecules.

Boiling Points and Chain Length

As the length of the hydrocarbon chain increases, the boiling point also increases. This happens because longer chains have more surface area, leading to stronger intermolecular forces (van der Waals forces) between molecules. These stronger forces require more energy (higher temperature) to break.

For example, methane (CH₄) is a gas at room temperature with a very low boiling point, while octane (C₈H₁₈) is a liquid with a much higher boiling point.

For instance, propane (C₃H₈) boils at about −42°C, but hexane (C₆H₁₄) boils at about 69°C, showing how boiling point rises with chain length.

Solubility in Water

Hydrocarbons are non-polar molecules, so they do not dissolve in water, which is polar. This means hydrocarbons are insoluble in water. Instead, they tend to float on water because they are less dense.

Viscosity and Size

Viscosity is a measure of how thick or runny a liquid is. Hydrocarbons with short chains are runny (low viscosity), while longer chains are thicker (high viscosity). For example, petrol (mainly short-chain hydrocarbons) is runny, but bitumen (long-chain hydrocarbons) is very thick and sticky.

Chemical Properties of Hydrocarbons

Hydrocarbons undergo chemical reactions that are important for their use as fuels and feedstocks. Their chemical behaviour depends on whether they are saturated (alkanes) or unsaturated (alkenes).

Combustion Behaviour

Hydrocarbons burn in oxygen to release energy. This combustion produces carbon dioxide and water if complete. The amount of energy released depends on the hydrocarbon's size and structure.

Alkanes generally burn well, producing a clean flame, while alkenes can be more reactive and may burn with a smokier flame due to incomplete combustion, producing carbon monoxide and soot which are harmful to health and the environment.

Reactivity Differences Between Alkanes and Alkenes

Alkanes are saturated hydrocarbons, meaning they only have single bonds between carbon atoms. This makes them relatively unreactive.

Alkenes contain at least one carbon-carbon double bond, making them unsaturated. This double bond is more reactive, allowing alkenes to undergo addition reactions (e.g., adding bromine water, which decolourises from orange to colourless).

For example, alkenes react with bromine water quickly, while alkanes do not.

Cracking Process

Cracking is the process of breaking down long-chain hydrocarbons into shorter, more useful hydrocarbons. It is important because shorter hydrocarbons are more useful as fuels and feedstocks.

Cracking uses heat and a catalyst to break the long chains into smaller alkanes and alkenes. The alkenes produced are valuable for making polymers and other chemicals.

Types of Hydrocarbons

Hydrocarbons are mainly classified into two types relevant here: alkanes and alkenes.

Alkanes (Saturated Hydrocarbons)

Alkanes have only single bonds between carbon atoms. They are saturated because they contain the maximum number of hydrogen atoms per carbon.

The general formula for alkanes is:

$$\text{C}_n\text{H}_{2n+2}$$

Alkenes (Unsaturated Hydrocarbons)

Alkenes contain at least one carbon-carbon double bond, making them unsaturated.

The general formula for alkenes is:

$$\text{C}_n\text{H}_{2n}$$

The double bond in alkenes makes them more chemically reactive than alkanes.

Uses of Hydrocarbons

Hydrocarbons are vital in everyday life, mainly as fuels and as raw materials for making other chemicals.

Fuels

Hydrocarbons are used as fuels because they release energy when burned. Different fractions of crude oil contain hydrocarbons of different chain lengths, which are suited to different uses:

• Short-chain hydrocarbons (like petrol) are used in cars.
• Medium-chain hydrocarbons (like diesel) are used in lorries and trains.
• Long-chain hydrocarbons (like fuel oil) are used for ships and power stations.

Feedstocks for Chemicals

Hydrocarbons, especially alkenes, are used as feedstocks in the chemical industry. They provide the starting materials to make plastics, solvents, and other important chemicals.

Fractional Distillation Relevance

Fractional distillation separates crude oil into fractions based on boiling points. The crude oil is heated, and hydrocarbons evaporate at different temperatures. Each fraction contains hydrocarbons with similar chain lengths and properties, making them suitable for specific uses as fuels or feedstocks.

Learning Example

For example, consider why petrol (mainly hydrocarbons with 5–8 carbon atoms) is a liquid at room temperature, but methane (1 carbon atom) is a gas.

Methane has a very low boiling point (−161°C) because it is a small molecule with weak intermolecular forces. Petrol hydrocarbons have longer chains, stronger intermolecular forces, and so higher boiling points, making them liquids at room temperature.