Part 1: Petroleum, fractional distillation and cracking

Connect molecular structure to boiling temperature, then distinguish separating an existing mixture from making new molecules.

Why alkane size matters

Alkanes are saturated hydrocarbons. Acyclic alkanes have general formula CₙH₂ₙ₊₂; cycloalkanes with one ring have CₙH₂ₙ. Both contain only carbon–carbon and carbon–hydrogen single bonds. The relatively strong, almost non-polar bonds help explain why alkanes resist many common reagents at room temperature. They still burn once ignition supplies the activation energy.

As a straight-chain alkane gets larger, its electron cloud becomes more polarisable and intermolecular London attractions become stronger. More energy is needed to separate the molecules, so boiling temperature rises. Volatility decreases and, broadly, viscosity increases with chain length. The smallest members are gases under ordinary room conditions, intermediate members liquids and larger members solids.

For isomers with the same molecular formula, increased branching usually lowers boiling temperature because compact molecules make less effective contact with their neighbours. Melting points are less regular because crystal packing matters. Alkanes mix poorly with water: they cannot replace water’s hydrogen-bonding network with comparably favourable interactions.

Crude oil is separated into fractions

Petroleum is a complex mixture, mainly hydrocarbons. A fraction is itself a mixture of substances with a similar boiling range, not one pure compound and not molecules with identical boiling temperatures.

In a refinery, heated crude oil enters a column that is hottest near the bottom and cooler towards the top. Vapour rises, cools and condenses in regions appropriate to its boiling range. High-boiling, larger hydrocarbons are collected lower down; lower-boiling, smaller hydrocarbons reach higher levels. The lightest gases can leave the top without condensing. Residue remains at the bottom.

Distillation changes physical state and separates substances already present. It overcomes intermolecular attractions without breaking the C–C bonds inside those molecules. For heavy residues, reducing pressure lowers boiling temperatures, allowing further distillation with less unwanted thermal decomposition.

Laboratory fractionation: explain each piece of apparatus

Repeated condensation and vaporisation in the fractionating column enrich the rising vapour in the more volatile component. This improves separation of miscible liquids with relatively close boiling temperatures. It does not guarantee a perfectly pure first fraction.

Position the thermometer bulb at the side-arm entrance so that it measures the vapour entering the condenser. Cooling water enters the bottom of the condenser jacket and leaves at the top, helping the jacket stay full. The condenser removes thermal energy from vapour to collect liquid; it does not perform the main fractionation.

For a supervised practical involving flammable organic liquids, use an appropriate electrical heater or bath, not a naked flame. Add anti-bumping granules before heating and keep the apparatus open to the atmosphere. Change the receiver over the temperature intervals instructed, and record the range collected.

Cracking changes the molecules

Cracking breaks carbon–carbon bonds in larger hydrocarbons and produces smaller molecules. It is a chemical change. Large fractions may be available in excess of demand; shorter fuel molecules and alkene feedstocks often have greater commercial value. An economic explanation should connect the surplus feedstock with demand for specific products.

The AQA comparison is thermal cracking at high temperature and high pressure versus catalytic cracking at high temperature, slight pressure and a zeolite catalyst. Numerical conditions vary between processes; do not assume one temperature describes every industrial plant. Mechanisms are not required for cracking in this section.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources