Hydrocarbons

Hydrocarbon Types

Hydrocarbons are compounds made up of only carbon (C) and hydrogen (H) atoms. They are the main components of fuels and are classified mainly into two types:

• Alkanes: Saturated hydrocarbons, meaning they contain only single bonds between carbon atoms.
• Alkenes: Unsaturated hydrocarbons, meaning they contain at least one carbon-carbon double bond.

General formulae:

• Alkanes: $C_nH_{2n+2}$ (e.g., methane $CH_4$, ethane $C_2H_6$)
• Alkenes: $C_nH_{2n}$ (e.g., ethene $C_2H_4$, propene $C_3H_6$)

Isomerism occurs when compounds have the same molecular formula but different structural formulas. This means the atoms are connected in different ways, resulting in different properties.

For example, butane and methylpropane both have the formula $C_4H_{10}$, but their structures differ:

• Butane: a straight chain of 4 carbon atoms.
• Methylpropane: a branched chain with 3 carbon atoms in the main chain and one attached as a branch.

Isomerism is important because it affects physical properties like boiling points and chemical reactivity.

For instance, the boiling point of straight-chain alkanes is generally higher than that of their branched isomers due to stronger intermolecular forces.

Properties of Hydrocarbons

Physical properties:

• Boiling points: Increase with the size of the hydrocarbon molecule because larger molecules have stronger intermolecular forces (van der Waals forces) that require more energy to overcome.
• Volatility: Smaller hydrocarbons are more volatile (evaporate easily) because they have lower boiling points.

Chemical properties:

• Combustion: Hydrocarbons burn in oxygen to produce carbon dioxide and water if combustion is complete.
• Alkanes are less reactive than alkenes because alkenes have a double bond that can open up and react more easily.

Alkenes react with bromine water in an addition reaction, which is a test to distinguish them from alkanes. Alkenes decolourise bromine water from orange to colourless, while alkanes do not.

For example, ethene reacts with bromine water:

$$C_2H_4 + Br_2 \rightarrow C_2H_4Br_2$$

Crude Oil and Fractional Distillation

Crude oil is a complex mixture of hydrocarbons formed from the remains of ancient sea creatures. It contains hydrocarbons of different chain lengths and types.

Fractional distillation is the process used to separate crude oil into useful fractions based on their boiling points.

The crude oil is heated until it vaporises and enters a fractionating column, which is cooler at the top and hotter at the bottom. Hydrocarbons condense at different heights depending on their boiling points:

• Gases (e.g., LPG) condense near the top (lowest boiling points).
• Petrol (gasoline) condenses higher up.
• Kerosene condenses further down.
• Diesel oil condenses lower still.
• Fuel oil condenses near the bottom.
• Bitumen is left at the bottom as a thick residue.

Uses of fractions:

• Gases: Used for heating and cooking (e.g., bottled gas).
• Petrol: Fuel for cars.
• Kerosene: Jet fuel and heating.
• Diesel oil: Fuel for diesel engines.
• Fuel oil: Used in ships and power stations.
• Bitumen: Road surfacing and roofing.

Cracking and Uses

Cracking is the process of breaking down long-chain hydrocarbons into shorter, more useful hydrocarbons.

This is important because shorter hydrocarbons are more valuable as fuels (e.g., petrol) and alkenes produced are useful feedstocks for making plastics and chemicals.

Types of cracking:

• Thermal cracking: Uses high temperature (about 700°C) and high pressure to break bonds.
• Catalytic cracking: Uses a catalyst (usually zeolite) at a lower temperature (about 450°C) and atmospheric pressure, making the process more efficient.

Products of cracking:

• Shorter-chain alkanes (used as fuels).
• Alkenes (used as feedstocks for making polymers and other chemicals).

For example, cracking decane ($C_{10}H_{22}$) might produce octane ($C_8H_{18}$) and ethene ($C_2H_4$):

$$C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4$$